HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (31) Citing Articles via Google Scholar Google Scholar Articles by Adair, T. H. Search for Related Content PubMed PubMed Citation Articles by Adair, T. H.

INVITED REVIEW

Growth regulation of the vascular system: an emerging role for adenosine

Angiogenesis Research Laboratories, Department of Physiology and Biophysics, Center for Excellence in Cardiovascular-Renal Research, University of Mississippi Medical Center, Jackson, Mississippi

	ABSTRACT

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
The importance of metabolic factors in the regulation of angiogenesis is well understood. An increase in metabolic activity leads to a decrease in tissue oxygenation causing tissues to become hypoxic. The hypoxia initiates a variety of signals that stimulate angiogenesis, and the increase in vascularity that follows promotes oxygen delivery to the tissues. When the tissues receive adequate amounts of oxygen, the intermediate effectors return to normal levels, and angiogenesis ceases. An emerging concept is that adenosine released from hypoxic tissues has an important role in driving the angiogenesis. The following feedback control hypothesis is proposed: AMP is dephosphorylated by ecto-5'-nucleotidase, producing adenosine under hypoxic conditions in the extracellular space adjacent to a parenchymal cell (e.g., cardiomyocyte, skeletal muscle fiber, hepatocyte, etc.). Extracellular adenosine activates A₂ receptors, which stimulates the release of vascular endothelial growth factor (VEGF) from the parenchymal cell. VEGF binds to its receptor (VEGF receptor 2) on endothelial cells, stimulating their proliferation and migration. Adenosine can also stimulate endothelial cell proliferation independently of VEGF, which probably involves modulation of other proangiogenic and antiangiogenic growth factors and perhaps an intracellular mechanism. In addition, hemodynamic factors associated with adenosine-induced vasodilation may have a role in the development and remodeling of the vasculature. Once a new capillary network has been established, and the diffusion/perfusion capabilities of the vasculature are sufficient to supply the parenchymal cells with adequate amounts of oxygen, adenosine and VEGF as well as other proangiogenic and antiangiogenic growth factors return to near-normal levels, thus closing the negative feedback loop. The available data indicate that adenosine might be an essential mediator for up to 50–70% of the hypoxia-induced angiogenesis in some situations; however, additional studies in intact animals will be required to fully understand the quantitative importance of adenosine.

vascular endothelial growth factor; angiogenic growth factors; angiogenesis; adenosine receptors; negative feedback

An emerging concept is that adenosine also has a long-term role in maintaining tissue oxygenation by stimulating the growth of blood vessels. Physiological concentrations of adenosine produced under hypoxic conditions stimulate a concentration-dependent proliferation and migration of endothelial cells (ECs) obtained from both large and small blood vessels (46, 60, 61, 64, 94, 104, 133, 149). EC proliferation and migration are key steps in the angiogenesis process necessary for establishing capillary sprouts in the microenvironment of a hypoxic tissue where adenosine levels are highest. Numerous studies have shown that adenosine or adenosine uptake inhibitors can stimulate blood vessel growth in vivo (5, 7, 18, 32, 44, 100, 112, 153–155, 165, 169).

Although the mechanism by which adenosine stimulates angiogenesis is poorly understood, many studies have shown that the administration of adenosine (50–52, 54, 61, 63, 69, 145) as well as the upregulation of endogenous adenosine (64) can induce the expression of vascular endothelial growth factor (VEGF) in a variety of cell types. VEGF is a vital mediator of angiogenesis released from hypoxic tissues in both physiological and pathological settings and is a target for clinical therapy in several pathological conditions (53). Adenosine can also stimulate EC proliferation independently of VEGF, which may involve the modulation of other proangiogenic and antiangiogenic growth factors (50, 51, 61, 120) or perhaps an intracellular mechanism. Hemodynamic factors associated with adenosine-induced vasodilation (e.g., increased shear rate in downstream capillaries) may also have a role in the development and remodeling of the vasculature (2, 75, 78, 169).

The purposes of this review are as follows: 1) to understand how adenosine is produced in hypoxic tissues and how hypoxia itself can modulate adenosine transporters and enzymes of adenosine metabolism, 2) to provide evidence for adenosine-induced angiogenesis in vitro and in vivo, 3) to examine the mechanism of adenosine-induced angiogenesis and the importance of VEGF in the growth regulation process, 4) to explore the possibility that adenosine provides a maintenance factor function for the vasculature, and 5) to consider the potential for adenosine-based angiogenic therapies.

	ADENOSINE METABOLISM

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
Adenosine production and consumption. The concentration of adenosine in the interstitial fluids may be an important determinant of the angiogenic activity in a tissue because adenosine can induce angiogenic growth factors and initiate hemodynamic events that lead to vascular growth. The major pathway for adenosine formation in most tissues is stepwise dephosphorylation of ATP. Adenosine accumulates in the heart and other tissues when oxygen demand exceeds oxygen supply, e.g., during exercise, vascular deficiencies, and other conditions associated with tissue hypoxia/ischemia and/or increased ATP turnover (38, 77, 137). An increase in ATP turnover in tissues with relatively high metabolic rates may increase AMP levels and subsequently also adenosine levels independent of hypoxia (13, 83, 110, 161), i.e., tissues with high metabolic rates are expected to have high adenosine concentrations even under normoxic conditions, as discussed later.

At least four enzymes and a membrane carrier are involved in controlling the interstitial adenosine concentration, as shown in Fig. 1. Adenosine is produced by dephosphorylation of AMP and hydrolysis of S-adenosylhomocysteine (SAH). The hydrolysis of SAH to adenosine (and homocysteine) by SAH hydrolase is thought to be a constitutive pathway that contributes marginally to adenosine production (83, 161). Dephosphorylation of AMP is the major source of adenosine under hypoxic/ischemic conditions: this reaction occurs intracellularly by cytosolic-5'-nucleotidase and extracellularly by cd73/ecto-5'-nucleotidase. The relative contribution of the cytosolic and ecto pathways for adenosine production has been the subject of much debate (22, 39, 70, 86, 128, 131); however, recent studies in mice show that targeted disruption of cd73/ecto-5'-nucleotidase can modulate basal coronary vascular tone as well as other adenosine-mediated events, suggesting an important role for adenosine produced extracellularly (82). This latter finding is consistent with the notion that physiologically significant amounts of adenosine are produced extracellularly by cardiomyocytes (36, 39) and skeletal muscle fibers (97) and that both cell types are a sink rather than a source for adenosine. Histochemical and/or functional studies have demonstrated the existence of ecto-5'-nucleotidase on many different cell types including cardiomyocytes (127), skeletal muscle fibers (71), Muller cells of the retina (95), astrocytes (171), various cell types in the kidney (87), and fibroblasts (87, 109, 129). Two enzymes can utilize adenosine and thus decrease its concentration. Adenosine is either deaminated to form inosine via adenosine deaminase or rephosphorylated into AMP via adenosine kinase, using ATP as the phosphate donor. The nucleoside transporter shown in Fig. 1 represents a bidirectional equilibrative nucleoside transporter (ENT1) that translocates adenosine down its concentration gradient by facilitated diffusion (16, 30).

View larger version (17K): [in this window] [in a new window]   Fig. 1. Metabolic pathways for adenosine (Ado) production and consumption in intracellular and extracellular fluids. SAH, S-adenosylhomocysteine; 5'N, ecto-5'-nucleotidase.

AMP hydrolysis via nucleotidase is the dominant pathway for adenosine production under normoxic conditions, and more than 90% of the adenosine produced under normoxic conditions is thought to be rephosphorylated to AMP via adenosine kinase (this is called the salvage pathway) (13, 83, 161). Because of the large flux through this AMP-adenosine substrate cycle, small changes in the activities of adenosine kinase or nucleotidase can produce large changes in adenosine concentration. For example, Gu and associates (64) have shown that pharmacological inhibition of adenosine kinase can raise adenosine levels sufficiently to induce VEGF mRNA and protein expression in rat myocardial myoblasts. Modulation of adenosine deaminase activity, on the other hand, is expected to have a minimal effect on adenosine concentration under basal conditions because the Michaelis-Menten constant of the deaminase is far greater compared with that of the kinase (13).

Recent studies indicate that exposing cells to a hypoxic environment can increase the activity of nucleotidase and decrease the activity of adenosine kinase, thereby causing a net increase in the production of adenosine and hence an increase in the interstitial adenosine concentration (90). Decking and associates (35) perfused isolated guinea pig hearts with hypoxic perfusate and used a mathematical model to determine that hypoxia decreased adenosine kinase activity to 6% of basal levels. Although the mechanism by which hypoxia inhibits adenosine kinase activity is poorly understood, studies by Gorman et al. (59) indicate that cytosolic levels of inorganic phosphate achieved under hypoxic conditions in the heart are capable of inhibiting adenosine kinase activity. Other studies (86) have shown that exposing aortic ECs to an anoxic environment (0% oxygen, 18 h) induced a twofold increase in cd73/ecto-5'-nucleotidase activity and increased cell surface expression of the enzyme but had no effect on its synthesis. The hypoxic induction of cd73/ecto-5'-nucleotidase activity can also occur in the ischemic heart (107) and brain (24) of intact animals, possibly by way of a hypoxia inducible factor-1 (HIF-1)-dependent regulatory pathway (143). Hypoxia can also downregulate (albeit transiently) the gene expression of a dipyridamole-sensitive ENT (mENT1) in mouse cardiomyocytes and thereby decrease [³H]adenosine uptake by cells (31). The hydrolysis of SAH to adenosine (and homocysteine) by SAH hydrolase is probably not upregulated under hypoxic conditions (37, 81, 161).

Hypoxia not only regulates the activity of enzymes and transporters but may also increase the availability of the primary substrate for adenosine, AMP. Hellsten and associates (72) found that knee extensor exercise in humans increased interstitial AMP levels by 20-fold. Others (110) have confirmed that muscle contraction can increase interstitial AMP levels in perfused dog skeletal muscle; however, interstitial levels of AMP did not increase when resting muscles were perfused under hypoxic conditions. Using NMR spectroscopy, Pucar and associates (123) showed that hypoxia induced a 2.5-fold increase in AMP levels in the Langendorff-perfused rat heart. Also, Kuzmin and associates (84) found that ischemia increased interstitial ATP levels by 10-fold in the Langendorff-perfused rat heart and that adenine nucleotides were sequentially dephosphorylated in the interstitial space by a chain of separate ectoenzymes. Therefore, it appears that hypoxia/ischemia can increase AMP levels in the interstitial fluid.

Hypoxic modulation of nucleoside transporters and enzymes of adenosine metabolism may require hours to days for full adaptation. Kobayashi and associates (81) found that prolonged exposure (but not acute exposure) of rat pheochromocytoma (PC12) cells to a hypoxic environment (5% oxygen, 48 h) caused the cells to shift toward an adenosine-producing phenotype. The adaptations consisted of decreased gene expression of the rENT1/nucleoside transporter, downregulation of adenosine-metabolizing enzymes (adenosine kinase, adenosine deaminase), and upregulation of adenosine-producing enzymes (cytosolic and cd73/ecto-5'-nucleotidase). It is likely that prolonged exposure to a hypoxic environment will prove necessary to determine the quantitative importance of adenosine in the angiogenesis process.

Interstitial adenosine concentrations under normoxic and hypoxic/ischemic conditions. Adenosine can increase, decrease, or have no effect on the proliferation and migration of ECs, depending on the concentration to which the cells are exposed. For this reason, it is important to determine the full range of interstitial adenosine concentrations that occur under normoxic and hypoxic/ischemic conditions in the intact animal. Interstitial fluid is typically collected using a flow-through microdialysis technique (157, 158) in which microdialysis tubing with a low-molecular-weight cutoff is implanted into a tissue and then perfused at a slow rate with a physiological electrolyte solution. The adenosine concentration in the effluent is then measured using HPLC. Adenosine measured in the blood greatly underestimates the interstitial concentration because of rapid metabolism by cellular elements in the blood and also because the vascular endothelium has a high capacity to metabolize adenosine (111, 114).

The interstitial adenosine concentration measured using the microdialysis technique typically ranges from 0.5 to 2 µM under basal conditions and from 10 to 15 µM under ischemic conditions in the left ventricle of dogs (158), swine (66, 130), and rats (118). In skeletal muscle, the interstitial adenosine concentration typically ranges from 0.1 to 0.4 µM under basal conditions and from 0.5 to 1 µM after hypoxic-hypoxia, exercise, or muscle stimulation in humans (72, 99) and rats (91). In the frontal cortex of newborn piglets, the interstitial adenosine concentration was 0.7 µM under basal conditions and 1.6 µM after hypoxic-hypoxia (117). Adenosine increased from 2 to 40 µM in the rat striatum after 15 min of ischemia (65). Adenosine concentrations in the extracellular fluids of solid tumors in mice ranged from 0.2 to 2.4 µM (mean, 0.5 µM), which is 10- to 20-fold higher compared with the concentrations in adjacent subcutaneous tissues (21). Adenosine levels in tumors increased to as high as 13 µM (mean, 9 µM) after simultaneous treatment with inhibitors of adenosine deaminase and adenosine kinase (21).

The possibility that adenosine is generated in close proximity to its receptors would limit the usefulness of interstitial adenosine concentrations to determine physiological relevance because interstitial values would underestimate the local concentration at the receptor (26). However, based on the available data, the upper limit of interstitial adenosine concentration under steady-state conditions would appear to be much less than 50 µM, even in severely ischemic tissues. It is important to understand the limits of interstitial adenosine concentrations in living animals to distinguish between physiological/pathological mechanisms and pharmacological effects.

	EVIDENCE FOR ADENOSINE-INDUCED ANGIOGENESIS

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
Adenosine induces EC proliferation and migration in vitro. The proliferation and migration of ECs are key steps in the angiogenesis process. Many investigators have shown that adenosine or adenosine analogs/agonists can stimulate EC proliferation and/or migration at physiological concentrations (42, 46, 47, 60, 61, 64, 94, 104, 133, 149).

The results of EC proliferation studies in which cells were actually counted are summarized in Fig. 2. Ethier and associates (46) found that adenosine caused a concentration-dependent increase in human umbilical vein EC (HUVEC) proliferation over a physiological range of 1 nM to 10 µM (Fig. 2). Proliferation was blocked by the nonselective adenosine receptor antagonist 8-phenyltheophylline (8-PT), indicating mediation by an external membrane receptor. Physiological concentrations of adenosine had no effect on the proliferation of human embryonic lung fibroblasts in the Ethier et al. study (46). Meininger and associates (104) found that adenosine was chemotactic and mitogenic for bovine aortic and coronary venular ECs. Adenosine caused a concentration-dependent proliferation of coronary venular ECs that could be blocked with 8-PT, again indicating mediation by an external membrane receptor. Grant and associates (61) found that the nonselective adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA) caused a concentration-dependent proliferation of human retinal ECs mediated by A_2B receptors. In other studies, they found a concentration-dependent stimulation of EC migration (60). Dubey and associates (42) found that the nonselective adenosine analog 2-chloroadenosine caused a concentration-dependent proliferation of growth arrested porcine coronary artery ECs mediated by A_2B receptors (Fig. 2). The nonselective adenosine receptor agonists 5'-N-methylcarboxamidoadenosine and NECA also caused a concentration-dependent increase in EC proliferation as well as an increase in EC migration (42). Lutty and associates (94) found that adenosine did not stimulate the proliferation of canine retinal ECs; however, physiological concentrations (10 µM) caused a threefold increase in EC migration into culture wounds and a fourfold increase in EC tube length. Both effects were mediated by A₂ receptors. Gu and associates (64) found that both adenosine and the adenosine kinase inhibitor GP-515 increased the proliferation of HUVECs, but neither compound affected the growth of rat myocardial myoblasts. Therefore, physiological concentrations of adenosine can stimulate the proliferation and migration of ECs obtained from both large and small blood vessels in a concentration-dependent manner.

View larger version (34K): [in this window] [in a new window]   Fig. 2. Adenosine or adenosine agonists cause a concentration-dependent stimulation of endothelial cell (EC) proliferation. HUVEC, human umbilical vein ECs; BCVEC, bovine coronary venular ECs; PCAEC, porcine coronary artery ECs; HREC, human retinal ECs; CREC, canine retinal ECs. The shaded area indicates the physiological/pathological range of adenosine concentrations in vivo.

Adenosine induces blood vessel growth in vivo. Numerous studies have shown that adenosine, adenosine agonists, and/or adenosine transport inhibitors can stimulate blood vessel growth in mammalian skeletal muscle (7, 8, 151, 152, 169), the mammalian heart (18, 100, 102, 153, 154, 165, 169), the body (5) and chorioallantoic membrane (CAM) of chick embryos (43, 44), the rabbit cornea (48), the optic tectum of developing amphibians (79), and wounds (32, 112). Several review articles address the role of adenosine in the regulation of blood vessel growth (2, 3, 29, 32, 75, 101).

Prolonged administration of dipyridamole (Persantin) can stimulate angiogenesis in skeletal muscle (7, 8, 151, 152, 169) and the heart (18, 100, 102, 153–155, 169) in several mammalian species, including humans, with evidence of significant growth after only 2 wk of treatment in some studies. Dipyridamole inhibits ENT1, which translocates adenosine down its concentration gradient by facilitated diffusion (16, 30), and thus raises adenosine levels in the interstitial fluid and blood during both normoxic and hypoxic conditions (80, 115, 163). Definitive proof that dipyridamole-induced angiogenesis is actually mediated by adenosine has not been forthcoming. Nevertheless, the adenosine antagonist 8-PT can inhibit dipyridamole-induced vasodilation in perfused guinea pig hearts (159), and theophylline can antagonize cardiovascular responses to dipyridamole in humans, e.g., changes in heart rate and blood pressure (136), indicating that acute cardiovascular responses to dipyridamole are mediated by adenosine. The possibility must also be considered that dipyridamole stimulates angiogenesis by its phosphodiesterase-inhibiting effect (103, 150), which can enhance the proangiogenic effects of the nitric oxide/cGMP system (113, 167, 170).

Interestingly, dipyridamole stimulated blood vessel growth in the body of the chick embryo (5) but not in the CAM (44). Yet, adenosine alone caused the CAM vasculature to proliferate, and the angiogenic effects of adenosine in the CAM were potentiated by dipyridamole and attenuated by the adenosine antagonist isobutylmethylxanthine (44). These results indicate that adenosine receptors are present in the CAM vasculature but that adenosine levels are low in CAM tissues under basal conditions. This could be expected because the CAM is a respiratory organ that facilitates oxygen delivery to the chick embryo body rather than local membrane tissues.

Prolonged administration of adenosine itself stimulates blood vessel growth in the intact animal. Ziada and associates (169) found that a continuous intravenous infusion of adenosine (42 µmol/h) for 3–5 wk in rabbits caused a 27% increase in myocardial capillary density with no change in the heart-to-body weight ratio, suggesting that adenosine stimulated angiogenesis in the heart. Adenosine also caused a 26% increase in the capillary-to-muscle fiber ratio in the skeletal muscles of the same animals, indicating that adenosine stimulated capillary growth (169). Wothe and associates (165) found that adenosine caused an approximately twofold increase in structural coronary conductance (i.e., the conductance of the maximally dilated vasculature) when adenosine was infused (12 h/day) into the circumflex coronary artery of near-term fetal sheep for 4 days, suggesting adenosine-induced growth of the coronary vasculature.

Dusseau et al. (44) found that adenosine applied locally to the chick CAM caused a concentration-dependent stimulation of angiogenesis that was inhibited with the adenosine antagonist isobutylmethylxanthine. Adair and associates (5) found that prolonged administration of adenosine into the air space of the chick egg caused a concentration-dependent increase in the structural vascular conductance in the embryo. Dipyridamole caused a similar concentration-dependent increase in vascularity, and the normal increase in vascularity that occurs in the developing embryo was attenuated in a concentration-dependent manner by the adenosine antagonist aminophylline. These latter findings suggest that adenosine could have a physiological role in the development of the vasculature (5).

	MECHANISM OF ADENOSINE-INDUCED ANGIOGENESIS

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
Although the mechanism by which adenosine induces angiogenesis is poorly understood, many studies have shown that the administration of adenosine or adenosine agonists/analogs (50–52, 54, 61, 63, 69, 145) as well as the upregulation of endogenous adenosine (64) can modulate the expression of VEGF and other angiogenic factors (50, 51, 61, 112) in a variety of cell types.

Adenosine receptors. Adenosine exerts most of its physiological actions by way of cell surface G protein-coupled receptors termed adenosine A₁, A_2A, A_2B, and A₃. All four receptors have been cloned and characterized pharmacologically using a variety of agonists and antagonists in several mammalian species (58). A₁ and A_2A are high-affinity receptors activated by submicromolar concentrations of adenosine, whereas A_2B and A₃ are low-affinity receptors activated when adenosine levels rise to micromolar concentrations (27, 49, 58). Adenosine receptors are also differentiated by signal transduction pathways. A_2A and A_2B receptors interact with the G_q and G_s protein families to stimulate adenylate cyclase, whereas A₁ and A₃ receptors interact with the G_i and G_o protein families to inhibit adenylate cyclase (57, 58). A₂ receptor activation is classically known to increase oxygen supply, whereas A₁ receptor activation (and possibly A₃ receptor activation) (15) is known to decrease oxygen demand. For example, in the cardiovascular system, A₁ receptors mediate the antiadrenergic actions of adenosine, whereas A₂ receptors mediate coronary vasodilation (26, 134). The physiological role of A₂ receptor activation to induce angiogenesis is a major focus of this review.

Adenosine induces VEGF and other angiogenic growth factors in vitro. VEGF is an EC-specific mitogen in vitro and an angiogenic inducer in a variety of animal models (53). As a key mediator of angiogenesis, VEGF is released from hypoxic tissues in both physiological and pathological settings (53) and is a target for clinical therapy in several pathological conditions (53). VEGF has a major physiological role to induce angiogenesis in exercising muscle (25, 28, 67, 125, 162) and is a survival factor (or maintenance factor) for capillary ECs in resting skeletal muscle (147) as well as in other tissues (53). Also, it is likely that VEGF is subject to negative feedback regulation in the exercising muscles of intact animals (67).

Many cell types respond to hypoxia by increasing VEGF release (53, 62, 85, 135), which stimulates angiogenesis, ensuring adequate oxygenation. Hypoxia induces both increased transcription and decreased degradation of VEGF mRNA. Transcriptional regulation is mediated by HIF-1, which accumulates under hypoxic conditions and activates VEGF transcription by binding to specific promoter sequences (56, 132). In addition to the stabilization of mRNA (76, 146), hypoxia can induce VEGF mRNA by other HIF-1-independent pathways (73, 121, 164). The mechanism by which adenosine induces VEGF is poorly understood, but recent studies indicate mediation by a HIF-1-independent pathway (52).

Adenosine or adenosine agonists/analogs cause a concentration-dependent increase in VEGF expression (Fig. 3) (50, 61, 63, 64, 69, 88, 124, 145). Gu and associates (63) found that as little as 1 µM adenosine caused a significant increase in VEGF protein release from dog myocardial vascular smooth muscle cells; high, pharmacological concentrations of adenosine suppressed VEGF expression (Fig. 3). Grant and associates (61) found that the adenosine analog NECA increased VEGF protein release by nearly 16-fold in human retinal ECs when the cells were cultured in serum-free media. However, this was an unusually dramatic response, i.e., other data from the same study ( 61) show an 2-fold increase in VEGF protein release from a different primary culture of human retinal ECs after treatment with 10^–5 M NECA. The addition of anti-human VEGF antibody inhibited EC proliferation in the Grant study (61), providing evidence for an adenosine mechanism mediated by VEGF. Leibovich and associates (88) found that a concentration-dependent VEGF response to the A_2A agonist CGS-21680 was amplified severalfold in the presence of interferon and lipopolysaccharide, reaching a >10-fold increase in VEGF release from mouse macrophages (88). The adenosine kinase inhibitor GP-515 caused a concentration-dependent increase in VEGF protein release from rat myocardial myoblasts, indicating that endogenous adenosine can mediate the VEGF response (64). Clearly, adenosine has the potential to cause dramatic increases in VEGF expression in a variety of cell types.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Adenosine or adenosine agonists/analogs cause a concentration-dependent stimulation of VEGF expression (protein or mRNA) in cell culture. NECA, 5'-N-ethylcarboxamidoadenosine; NECA-SF, NECA in serum-free media; CGS, 2-p-(carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680). The shaded area indicates the physiological/pathological range of adenosine concentrations in vivo.

Recent reports from several laboratories indicate that adenosine can modulate the expression of other angiogenic growth factors. The adenosine analog NECA increased the expression of the proangiogenic factors insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF) in a time-dependent manner in human retinal ECs (61). NECA also increased the expression of the proangiogenic factors interleukin-8 (IL-8) and angiopoietin-2 (Ang-2) mRNA in human mast cells (51) as well as bFGF in human microvascular ECs (50). A_2B receptors mediated the IL-8 and bFGF responses and A₃ receptors may have mediated the Ang-2 response (50, 51). Other studies have shown that adenosine and A_2A agonists can downregulate the antiangiogenic factor tumor necrosis factor (TNF)- in mouse macrophages (112). TNF- inhibits the proliferative response of ECs through inactivation of VEGFRs (141). Therefore, adenosine can modulate proangiogenic and antiangiogenic factors in a manner that promotes angiogenesis.

What is the functional importance of adenosine-induced VEGF release from ECs compared with other cell types? Although the induction of VEGF expression by adenosine has been studied in ECs more than any other cell type (see Table 3), the functional importance of VEGF released from ECs in the stimulation of angiogenesis must be questioned for the following reason: capillary sprouts grow toward hypoxic areas of a tissue where there are no capillaries and hence no ECs. These hypoxic areas have parenchymal cells that use oxygen in proportion to their metabolic requirements. When the metabolic requirements of the parenchymal cells exceed the perfusion/diffusion capabilities of the local capillary network, interstitial levels of adenosine as well as intracellular levels of HIF-1 increase greatly, causing the cells to release greater amounts of VEGF. VEGF, in turn, diffuses in all directions, establishing a concentration gradient for EC migration toward the hypoxic parenchymal cells.

Once new functional capillaries have been established and tissues have adequate amounts of oxygen, HIF-1 should then decrease to basal levels. Although adenosine levels will also decrease when tissue oxygenation returns to normal, it is likely that adenosine levels will remain elevated to some extent, in proportion to the metabolic requirements of the local tissues. It is well known that the concentration of adenosine in the interstitial fluid is higher in tissues with a higher metabolic rate even under basal conditions, e.g., adenosine levels are higher in the heart compared with skeletal muscle, as discussed previously.

For these reasons, it is likely that the adenosine-induced release of VEGF from ECs is far less important for stimulating angiogenesis compared with parenchymal cell types located away from capillaries where hypoxia can be most severe and there are no ECs.

Adenosine induces VEGF expression in chick embryos and humans. The possibility that adenosine can induce VEGF expression in whole animals has received relatively little attention (1, 6). Table 1 shows results from studies in chick embryos in which adenosine (4 µmol/day) or saline vehicle was administered into the air space of the egg for 5 days, and VEGF mRNA was determined by Northern blot analysis on day 14 using a quail cDNA probe (6). Note that VEGF mRNA was more than threefold higher in cardiac muscle compared with skeletal muscle under basal conditions and that adenosine caused an approximately fourfold increase in skeletal muscle VEGF mRNA and an approximately threefold increase in cardiac muscle VEGF mRNA. These results are consistent with studies showing that adenosine can stimulate blood vessel growth in chick embryos (5), as discussed previously.

Other studies suggest that A_2A and A_2B receptors have opposing effects on the expression of VEGF and other proangiogenic factors in some cell types. Feoktistov and associates (50) found that only A_2B receptors mediated adenosine-induced transcription of IL-8 and VEGF genes in human microvascular ECs after cotransfection of IL-8 and VEGF reporters with adenosine receptors (50). Overexpression of A_2A receptors attenuated the stimulation of IL-8 and VEGF mediated by native A_2B receptors (50). Other studies have shown that A_2A receptor activation downregulated IL-8 in HUVECs (23) as well as VEGF in rat pheochromocytoma PC12 cells (116). But again, both pharmacological (145) and gene manipulation (88) studies provide strong evidence that A_2A receptor activation can induce VEGF expression in some cell types. Also, Taomoto and associates (148) used immunohistochemical techniques to show that A_2A receptors were upregulated at the edge of the proliferating vasculature in a canine model of oxygen-induced retinopathy (in which angiogenesis is induced in hypoxic retina after hyperoxic obliteration of retinal blood vessels), implicating the importance of these receptors in the angiogenesis process. The possibility that different cell types within a given species can modulate the expression of VEGF and other angiogenic growth factors via different receptor subtypes has not been confirmed nor refuted at the writing of this review.

Hypoxic induction of an angiogenic phenotype by modulating adenosine A₂ receptor expression. Prolonged exposure to a hypoxic environment can create an angiogenic phenotype that promotes adenosine-induced VEGF expression. Feoktistov and associates (52) found that exposure to a hypoxic environment upregulates A_2B receptors and downregulates A_2A receptors in HUVECs and human smooth muscle cells. Both cell types normally express A_2A and A_2B receptors, but A_2A receptors predominate under normoxic conditions both in terms of mRNA expression and functional coupling to agonists. Because adenosine-induced VEGF expression is mediated by A_2B receptors in both cell types, exposure to hypoxia was found to amplify the VEGF response to adenosine. NECA did not stimulate VEGF release from either cell type under normoxic conditions; however, NECA caused a 0.5- to 2.5-fold increase in VEGF release from cells preconditioned in a hypoxic environment, indicating that A_2B receptors were functionally coupled to the upregulation of VEGF.

Studies from other laboratories confirm that exposure to hypoxia (45, 168) or simulation of hypoxia by cobalt ions (166) can upregulate A_2B receptor expression. Treatment with cobalt ions caused a time-dependent increase in A_2B receptor expression in U87MG human glioma cells, increasing mRNA levels by more than threefold at 24 h (166). Eltzschig and associates (45) found that exposing confluent human microvascular ECs to a hypoxic environment (PO₂ = 20 mmHg) caused a four- to six-fold increase in A_2B receptor mRNA expression after 6–24 h of exposure as well as a 75% reduction in A_2A receptor expression. Other studies suggest that cerebral ischemia can upregulate A_2B receptor expression in rats (168). Even so, Gu and associates (63) found that an adenosine A₂ antagonist could not block more than 20% of the VEGF expression induced by exposing myocardial vascular smooth muscle cells to a hypoxic environment, suggesting a relatively minor role for adenosine under hypoxic conditions. Other studies (56) indicate that VEGF mRNA is induced only slightly under hypoxic conditions in mutant mouse hepatoma cells that do not express the HIF-1 (ARNT) subunit, which also suggests a minor role for adenosine independent of any HIF-1-mediated pathway.

Nevertheless, it could be that adenosine mediates a level of compensatory angiogenesis in hypoxic tissues that cannot be predicted from studies performed under normoxic conditions, especially because adenosine can stimulate EC proliferation independently of VEGF, as discussed in Can adenosine induce angiogenesis independently of VEGF?. Also, adenosine is a potent vasodilator in many vascular beds, and mechanical events associated with vasodilation of larger blood vessels and increased shear rate in capillaries is thought to stimulate angiogenesis as well as vascular remodeling (2, 75, 78, 169).

Interestingly, A_2B receptors are more abundant in oxidative fibers (type I) compared with glycolytic fibers (type II) in human skeletal muscle (96), and the activation of adenylate cyclase by adenosine in rat skeletal muscle is mediated mainly by the A_2B receptor (98). Given that VEGF-induced angiogenesis occurs in hypoxic tissues and given that oxidative fibers have a more extensive capillary network compared with glycolytic fibers (2, 74, 75), one can speculate that hypoxia in the microenvironment of an oxidative fiber can induce the expression of A_2B receptors. This increase in A_2B receptor expression would promote angiogenesis in the vicinity of the fiber by adenosine induction of VEGF expression. In other words, the A_2B receptors could have an important role in stimulating the release of extra amounts of VEGF from the oxidative fiber, not only to maintain the relatively extensive capillary network associated with the fiber but also to stimulate the growth of new capillaries in the vicinity of the fiber when metabolic demand increases. This speculation is supported by the findings that oxidative skeletal muscle fibers have higher levels of VEGF protein in the adjacent interstitial spaces (12) as well as higher VEGF mRNA levels within the fibers themselves during basal conditions (28) compared with glycolytic fibers. The possibility that adenosine is a vascular maintenance factor is discussed in the next major section.

Can adenosine induce angiogenesis independently of VEGF? The actions of adenosine to induce VEGF and stimulate EC proliferation and migration have been attributed to the activation of A_2A or A_2B receptors; however, it is possible that adenosine can stimulate EC proliferation by mechanisms independent of VEGF. For example, adenosine caused a concentration-dependent proliferation of HUVECs under normoxic conditions (Fig. 2) (46), but the adenosine analog NECA did not induce VEGF in HUVECs under normoxic conditions (Table 3) (50). The induction of VEGF by NECA in HUVECs preconditioned in a hypoxic environment can be attributed to upregulation of the A_2B receptor subtype (52), as discussed previously.

Ethier and Dobson (47) found that adenosine stimulation of [³H]thymidine uptake by HUVECs cultured in serum-free medium could not be mimicked by adenosine receptor agonists or inhibited by adenosine receptor antagonists, suggesting a mechanism independent of A₁, A₂, and A₃ receptors. Yet, the high-molecular-weight nontransportable adenosine analog Poly(A) also increased [³H]thymidine uptake in a concentration-dependent manner, suggesting that extracellular (not intracellular) adenosine stimulated DNA synthesis. The stimulation of DNA synthesis did not appear to involve the cAMP, PKC, or tyrosine kinase signaling pathways. Inhibition of Na⁺/H⁺ exchange and phospholipase A₂ quelled the effect, suggesting that cell alkalinization and arachidonic acid metabolites may contribute to an adenosine-induced signaling pathway leading to DNA synthesis. Others have shown that adenosine stimulation of DNA synthesis in bovine aortic ECs cultured in serum-free medium was neither inhibited by the adenosine antagonist 8-PT nor mimicked by A₁- or A₂-selective agonists (156).

It is important to point out that neither adenosine nor adenosine agonists/analogs can stimulate HUVEC proliferation in serum-free medium (47) and that the adenosine-induced stimulation of HUVEC proliferation (which occurs in the presence of fetal calf serum) could be inhibited by 8-PT in other studies (46), implicating a receptor-dependent mechanism. In any case, the Ethier and Dobson study (47) provides another example in which [³H]thymidine uptake may not provide a reasonable estimate of cell proliferation.

Gu and associates (64) found that the adenosine kinase inhibitor GP-515 caused a greater increase in cell proliferation and DNA synthesis in HUVECs compared with equimolar amounts of exogenous adenosine even though exogenous adenosine was more effective than GP-515 on an equimolar basis in stimulating VEGF release from rat myocardial myoblasts. It is conceivable that adenosine kinase inhibition raises intracellular levels of adenosine to a greater extent than can be achieved by exogenous adenosine and that the high intracellular concentration of adenosine somehow stimulated EC growth. Although it is not clear whether adenosine can induce EC growth by way of an intracellular mechanism, the available data indicate that adenosine can stimulate the proliferation of HUVECs without first stimulating the release of VEGF from cells.

What is the quantitative importance of adenosine-induced angiogenesis under hypoxic conditions? Few studies have assessed quantitatively the importance of adenosine as a mediator of hypoxia-induced angiogenesis in the intact animal. Dusseau and Hutchins (43) found that hypoxia-induced angiogenesis in the chick CAM could be attenuated by 70% when adenosine receptors were blocked with methylisobutylxanthine. Adair and associates (3–5) found that the maximum increase in structural vascular conductance in chick embryos caused by administration of adenosine was 50% of that achieved with prolonged exposure to a hypoxic environment. Neovascularization was reduced by 50% in the retina of the eye after intraocular administration of an anti-A_2B receptor ribozyme in a mouse model of oxygen-induced retinopathy (10). Therefore, the available data indicate that adenosine might be an essential mediator for 50–70% of the hypoxia-induced angiogenesis in some situations; however, additional studies in intact animals will be required to fully understand the quantitative importance of adenosine.

Mechanism of adenosine-induced angiogenesis. Figure 4 shows a first approximation mechanism of adenosine-induced angiogenesis under hypoxic conditions. AMP is dephosphorylated by ecto-5'-nucleotidase, producing adenosine in the extracellular space adjacent to a parenchymal cell, which is the major important source of adenosine under hypoxic/ischemic conditions. The parenchymal cell could be a cardiomyocyte, skeletal muscle fiber, hepatocyte, or any other functional element of an organ or tissue that is subjected to a hypoxic environment. Extracellular adenosine then stimulates the release of VEGF from the parenchymal cell by activating A_2A or A_2B receptors. The receptor subtype (A_2A or A_2B) that leads to the induction of VEGF may depend on species and cell type; however, the available data suggest that the A_2B receptor is a likely mediator in human cells. VEGF released from parenchyma cells binds to its receptor (VEGFR-2) on ECs, stimulating EC proliferation and migration. VEGF is also a survival factor or maintenance factor for ECs that may be regulated by adenosine under basal conditions, as discussed in DOES ADENOSINE PROVIDE A MAINTENANCE FACTOR FUNCTION FOR THE VASCULATURE? Adenosine can stimulate EC proliferation independently of VEGF, which probably involves modulation of other proangiogenic and antiangiogenic growth factors or perhaps an intracellular mechanism. In addition, hemodynamic factors associated with adenosine-induced vasodilation may have a role in development and remodeling of the capillaries as well as larger blood vessels. Once a new capillary network has been established and the diffusion/perfusion capabilities of the vasculature can supply the parenchymal cells with adequate amounts of oxygen, adenosine and VEGF as well as other proangiogenic and antiangiogenic growth factors return to near-normal levels, thus closing the negative feedback loop.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Summary of adenosine-induced angiogenesis under hypoxic conditions. HIF-1, hypoxia-inducible factor-1; VEGFR-2, VEGF receptor 2; A2A and A2B, adenosine receptors.

	DOES ADENOSINE PROVIDE A MAINTENANCE FACTOR FUNCTION FOR THE VASCULATURE?

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

That a basal level of VEGF is essential for maintaining capillary integrity has been demonstrated by Tang and associates (147). Targeted skeletal muscle inhibition of VEGF expression in VEGFloxP mice through viral delivery of cre recombinase caused a 70% decrease in both capillary density and the capillary-to-fiber ratio in the VEGF-inactivated regions of the muscle. Furthermore, capillary regression was accompanied by TdT-mediated dUTP nick-end labeling (TUNEL)-positive apoptotic ECs. Additional evidence that VEGF is a vascular maintenance factor is provided in a recent review (53).

The possibility that adenosine is important for establishing basal levels of VEGF under normoxic conditions and therefore provides an essential maintenance factor function for the vasculature is partially based on the following findings: 1) >60% of the VEGF released from cultured myocardial vascular smooth muscle cells can be inhibited by adenosine A₂ antagonist during normoxic conditions (63), and 2) adenosine deaminase (which metabolizes adenosine to inosine) can decrease basal levels of VEGF by 60% in the media of myocardial myoblasts (64). Also, oxidative muscle has a relatively high metabolic rate, high basal level of adenosine, high basal level of VEGF, and high capillarity compared with glycolytic muscle (2, 12, 28), and, when a glycolytic muscle is converted to an oxidative muscle by prolonged electrical stimulation of a motor nerve, an approximately twofold increase in capillarity (34, 74) is associated with an approximately twofold increase in basal VEGF expression when full adaptation has occurred (67).

Is HIF-1 important for establishing basal levels of VEGF? HIF-1 is detectable in normoxic tissues (106, 140), and HIF-1 levels increase under hypoxic conditions. However, it is not clear whether HIF-1 levels are proportional to the basal metabolic activity of a tissue under normoxic conditions, which would appear necessary to account for differences in the basal levels of VEGF. Stroka and associates (140) found that basal levels of HIF-1 were higher in skeletal muscle compared with the heart, which argues against a maintenance factor role for HIF-1 considering that basal levels of VEGF are higher in the heart compared with skeletal muscle. Additional studies will be required to determine the possible role of HIF-1 in establishing the basal levels of VEGF.

For these reasons, it is conceivable that adenosine plays an important role in establishing the basal levels of VEGF in a tissue under normoxic conditions. If this is true, and if adenosine serves to "fine-tune" the levels of VEGF in a tissue in accordance with the metabolic requirements of the tissue, then adenosine can be considered a vascular maintenance factor.

	ADENOSINE MODULATION OF ANGIOGENESIS COULD HAVE THERAPEUTIC VALUE

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
Mounting evidence suggests that modulating adenosine levels in the tissues or adenosine receptor function could provide a basis for the treatment of cancer (105, 122, 138, 139, 166), wounds (32, 88, 112, 120), vasoproliferative retinopathies (10, 60, 61, 94, 148), coronary artery disease (9, 18, 92, 119, 142), and other angiogenic diseases characterized by too much or too little angiogenesis. Chronic treatment with dipyridamole (which increases interstitial adenosine levels) can decrease myocardial infarct size in rats (9) and improve coronary collateralization and left ventricular systolic performance in humans (18). This latter finding suggests that the angiogenic actions of dipyridamole may have clinical relevance (119). Also, dipyridamole can increase the survival of experimental skin flaps in rats (14). Several laboratories seek to prevent inappropriate angiogenesis in the retina of the eye through modulation of adenosine and its receptors (60, 61, 93, 94, 148). In particular, Afzal and associates (10) found that ribozymes that cleave A_2B receptor mRNA caused a substantial reduction in preretinal neovascularization in a neonatal mouse model of oxygen-induced retinopathy.

Cronstein and associates (32, 105, 112) provide convincing evidence that adenosine can promote neovascularization in wounds. Recent studies (112) indicate that topical application of the adenosine A_2A agonist CGS-21680 increases neovascularization in full-thickness excisional wounds in mice by increasing both local vessel sprouting and recruitment of endothelial progenitor cells from the bone marrow. Clinical studies are currently underway to determine the utility of topical adenosine A_2A receptor agonists in the therapy of diabetic foot ulcers (32).

Adenosine stimulation of angiogenesis is a natural phenomenon that occurs in the microenvironment of tissues in accordance with local metabolic needs. Amplification of this natural process through modulation of the enzymes of adenosine metabolism or genetic or pharmacological induction of A_2A or A_2B receptors could, theoretically, lead to stimulation of angiogenesis in those areas of a tissue where hypoxia is most severe and adenosine levels are highest. This type of physiological therapy can be viewed as a departure from classical attempts to stimulate angiogenesis in a global manner, in which many or most cells of an organ or tissue are made to produce greater amounts of VEGF. In contrast to adenosine-based therapies, global induction of VEGF may not be an effective means to build capillary networks in the microenvironment of a tissue where hypoxia is most severe.

	GRANTS

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 
Some of the work reported herein was supported by National Heart, Lung, and Blood Institute Grant HL-51971.

	ACKNOWLEDGMENTS

 
The author appreciates the helpful comments of Janelle Pryor, Preston McDonnell, and Leslie Spencer.

	FOOTNOTES

 

Address for reprint requests and other correspondence: T. H. Adair, Dept. of Physiology and Biophysics, Univ. of Mississippi Medical Center, 2500 N. State St., Jackson, MS 39216-4505 (E-mail: tadair{at}physiology.umsmed.edu)

	REFERENCES

TOP ABSTRACT ADENOSINE METABOLISM EVIDENCE FOR ADENOSINE-INDUCED... MECHANISM OF ADENOSINE-INDUCED... DOES ADENOSINE PROVIDE A... ADENOSINE MODULATION OF... GRANTS REFERENCES

 

1. Adair TH, Cotton R, McMullan MR, Li W, and Gu JW. Adenosine increases plasma levels of VEGF in humans (Abstract). FASEB J 15: A277, 2001.
2. Adair TH, Gay WJ, and Montani JP. Growth regulation of the vascular system: evidence for a metabolic hypothesis. Am J Physiol Regul Integr Comp Physiol 259: R393–R404, 1990.[Abstract/Free Full Text]
3. Adair TH, Gay WJ, Hester RL, and Montani JP. Does adenosine have a regulatory role in the growth of blood vessels? In: Role of Adenosine and Adenine Nucleotides in the Biological System, edited by Imai S and Nakazawa M. Amsterdam: Elsevier, 1991, p. 443–455.
4. Adair TH, Guyton AC, Montani JP, Lindsay HL, and Stanek KA. Whole body structural vascular adaptation to prolonged hypoxia in chick embryos. Am J Physiol Heart Circ Physiol 252: H1228–H1234, 1987.[Abstract/Free Full Text]
5. Adair TH, Montani JP, Strick DM, and Guyton AC. Vascular development in chick embryos: a possible role for adenosine. Am J Physiol Heart Circ Physiol 256: H240–H246, 1989.[Abstract/Free Full Text]
6. Adair TH, Li W, McIntire T, and Gu JW. Adenosine stimulates angiogenesis and induces VEGF expression in chick embryos (Abstract). FASEB J 14: A709, 2000.
7. Adolfsson J. The time dependence of training-induced increase in skeletal muscle capillarization and the spatial capillary to fibre relationship in normal and neovascularized skeletal muscle of rats. Acta Physiol Scand 128: 259–266, 1986.[Web of Science][Medline]
8. Adolfsson J. Time dependence of dipyridamole-induced increase in skeletal muscle capillarization. Arzneimittelforschung 36: 1768–1769, 1986.[Medline]
9. Adolfsson J, Tornling G, Unge G, and Ljungqvist A. The prophylactic effect of dipyridamole on the size of myocardial infarction following coronary artery occlusion. Acta Pathol Microbiol Immunol Scand 90: 273–275, 1982.
10. Afzal A, Shaw LC, Caballero S, Spoerri PE, Lewin AS, Zeng D, Belardinelli L, and Grant MB. Reduction in preretinal neovascularization by ribozymes that cleave the A_2B adenosine receptor mRNA. Circ Res 93: 500–506, 2003.[Abstract/Free Full Text]
11. Agrawal K, Majors AR, Pryor JS, McDonnell PB, and Adair TH. Muscle paralysis caused by botulinum toxin type A (BOTOX) downregulates VEGF in mouse skeletal muscle. FASEB J 19: 693.12, 2005.
12. Annex BH, Torgan CE, Lin P, Taylor DA, Thompson MA, Peters KG, and Kraus WE. Induction and maintenance of increased VEGF protein by chronic motor nerve stimulation in skeletal muscle. Am J Physiol Heart Circ Physiol 274: H860–H867, 1998.[Abstract/Free Full Text]
13. Arch JR and Newsholme EA. Activities and some properties of 5'-nucleotidase, adenosine kinase and adenosine deaminase in tissues from vertebrates and invertebrates in relation to the control of the concentration and the physiological role of adenosine. Biochem J 74: 965–977, 1978.
14. Arnander C, Jurell G, Tornling G, and Unge G. Effect of dipyridamole on the survival of experimental critical skin flaps. Scand J Plast Reconstr Surg Hand Surg 13: 261–262, 1979.
15. Auchampach JA, Ge ZD, Wan TC, Moore J, and Gross GJ. A₃ adenosine receptor agonist IB-MECA reduces myocardial ischemia-reperfusion injury in dogs. Am J Physiol Heart Circ Physiol 285: H607–H613, 2003.[Abstract/Free Full Text]
16. Baldwin SA, Beal PR, Yao SY, King AE, Cass CE, and Young JD. The equilibrative nucleoside transporter family, SLC29. Pflügers Arch 447: 735–743, 2004.[CrossRef][Web of Science][Medline]
17. Belardinelli L, Vogel S, Linden J, and Berne RM. Antiadrenergic action of adenosine on ventricular myocardium in embryonic chick hearts. J Mol Cell Cardiol 14: 291–294, 1982.[CrossRef][Web of Science][Medline]
18. Belardinelli R, Belardinelli L, and Shryock JC. Effects of dipyridamole on coronary collateralization and myocardial perfusion in patients with ischaemic cardiomyopathy. Eur Heart J 22: 1205–1213, 2001.[Abstract/Free Full Text]
19. Berne RM. Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow. Am J Physiol 204: 317–322, 1963.[Abstract/Free Full Text]
20. Berne RM, Knabb RM, Ely SW, and Rubio R. Adenosine in the local regulation of blood flow: a brief overview. Fed Proc 42: 3136–3142, 1983.[Web of Science][Medline]
21. Blay J, White TD, and Hoskin DW. The extracellular fluid of solid carcinomas contains immunosuppressive concentrations of adenosine. Cancer Res 57: 2602–2605, 1997.[Abstract/Free Full Text]
22. Borst MM and Schrader J. Adenine nucleotide release from isolated perfused guinea pig hearts and extracellular formation of adenosine. Circ Res 68: 797–806, 1991.[Abstract/Free Full Text]
23. Bouma MG, van den Wildenberg FA, and Buurman WA. Adenosine inhibits cytokine release and expression of adhesion molecules by activated human endothelial cells. Am J Physiol Cell Physiol 270: C522–C529, 1996.[Abstract/Free Full Text]
24. Braun N, Lenz C, Gillardon F, Zimmermann M, and Zimmermann H. Focal cerebral ischemia enhances glial expression of ecto-5'-nucleotidase. Brain Res 766: 213–226, 1997.[CrossRef][Web of Science][Medline]
25. Breen EC, Johnson EC, Wagner H, Tseng HM, Sung LA, and Wagner PD. Angiogenic growth factor mRNA responses in muscle to a single bout of exercise. J Appl Physiol 81: 355–361, 1996.[Abstract/Free Full Text]
26. Bruns RF. Adenosine receptors. Roles and pharmacology. Ann NY Acad Sci 603: 211–225, 1990.[Web of Science][Medline]
27. Bruns RF, Lu GH, and Pugsley TA. Characterization of the A₂ adenosine receptor labeled by [³H]NECA in rat striatal membranes. Mol Pharmacol 29: 331–346, 1986.[Abstract]
28. Brutsaert TD, Gavin TP, Fu Z, Breen EC, Tang K, Mathieu-Costello O, and Wagner PD. Regional differences in expression of VEGF mRNA in rat gastrocnemius following 1 hr exercise or electrical stimulation. BMC Physiol 2: 8, 2002.[CrossRef][Medline]
29. Burnstock G. Purinergic signaling and vascular cell proliferation and death. Arterioscler Thromb Vasc Biol 22: 364–373, 2002.[Abstract/Free Full Text]
30. Cass CE, Young JD, Baldwin SA, Cabrita MA, Graham KA, Griffiths M, Jennings LL, Mackey JR, Ng AM, Ritzel MW, Vickers MF, and Yao SY. Nucleoside transporters of mammalian cells. Pharm Biotechnol 12: 313–352, 1999.[Medline]
31. Chaudary N, Naydenova Z, Shuralyova I, and Coe IR. Hypoxia regulates the adenosine transporter, mENT1, in the murine cardiomyocyte cell line, HL-1. Cardiovasc Res 61: 780–788, 2004.[Abstract/Free Full Text]
32. Cronstein BN. Adenosine receptors and wound healing. Scientific World Journal 4: 1–8, 2004.
33. Dawicki DD, Chatterjee D, Wyche J, and Rounds S. Extracellular ATP and adenosine cause apoptosis of pulmonary artery endothelial cells. Am J Physiol Lung Cell Mol Physiol 273: L485–L494, 1997.[Abstract/Free Full Text]
34. Dawson JM and Hudlicka O. The effect of long-term activity on the microvasculature of rat glycolytic skeletal muscle. Int J Microcirc Clin Exp 8: 53–69, 1989.[Web of Science][Medline]
35. Decking UK, Schlieper G, Kroll K, and Schrader J. Hypoxia-induced inhibition of adenosine kinase potentiates cardiac adenosine release. Circ Res 81: 154–164, 1997.[Abstract/Free Full Text]
36. Deussen A. Metabolic flux rates of adenosine in the heart. Naunyn Schmiedebergs Arch Pharmacol 362: 351–363, 2000.[CrossRef][Web of Science][Medline]
37. Deussen A, Lloyd HG, and Schrader J. Contribution of S-adenosylhomocysteine to cardiac adenosine formation. J Mol Cell Cardiol 21: 773–782, 1989.[CrossRef][Web of Science][Medline]
38. Deussen A and Schrader J. Cardiac adenosine production is linked to myocardial PO₂. J Mol Cell Cardiol 23: 495–504, 1991.[CrossRef][Web of Science][Medline]
39. Deussen A, Stappert M, Schafer S, and Kelm M. Quantification of extracellular and intracellular adenosine production: understanding the transmembranous concentration gradient. Circulation 99: 2041–2047, 1999.[Abstract/Free Full Text]
40. Dobson JG Jr and Fenton RA. Adenosine A₂ receptor function in rat ventricular myocytes. Cardiovasc Res 34: 337–347, 1997.[Abstract/Free Full Text]
41. Drury A and Szent-Gyorgyi A. The physiological activity of adenine compounds with special reference to their action upon the mammalian heart. J Physiol 68: 213–237, 1929.[Free Full Text]
42. Dubey RK, Gillespie DG, and Jackson EK. A_2B adenosine receptors stimulate growth of porcine and rat arterial endothelial cells. Hypertension 39: 530–535, 2002.[Abstract/Free Full Text]
43. Dusseau JW and Hutchins PM. Hypoxia-induced angiogenesis in chick chorioallantoic membranes: a role for adenosine. Respir Physiol 71: 33–44, 1988.[CrossRef][Web of Science][Medline]
44. Dusseau JW, Hutchins PM, and Malbasa DS. Stimulation of angiogenesis by adenosine on the chick chorioallantoic membrane. Circ Res 59: 163–170, 1986.[Abstract/Free Full Text]
45. Eltzschig HK, Ibla JC, Furuta GT, Leonard MO, Jacobson KA, Enjyoji K, Robson SC, and Colgan SP. Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A_2B receptors. J Exp Med 198: 783–796, 2003.[Abstract/Free Full Text]
46. Ethier MF, Chwander V, and Dobson JG. Adenosine stimulates proliferation human endothelial cells in culture. Am J Physiol Heart Circ Physiol 265: H131–H138, 1993.[Abstract/Free Full Text]
47. Ethier MF and Dobson JG Jr. Adenosine stimulation of DNA synthesis in human endothelial cells. Am J Physiol Heart Circ Physiol 272: H1470–H1479, 1997.[Abstract/Free Full Text]
48. Fenselau A. An angiogenic role for adenine nucleotide catabolites (Abstract). Fed Proc 43: 587, 1984.
49. Feoktistov I and Biaggioni I. Adenosine A_2B receptors. Pharmacol Rev 49: 381–402, 1997.[Abstract/Free Full Text]
50. Feoktistov I, Goldstein AE, Ryzhov S, Zeng D, Belardinelli L, Voyno-Yasenetskaya T, and Biaggioni I. Differential expression of adenosine receptors in human endothelial cells: role of A_2B receptors in angiogenic factor regulation. Circ Res 90: 531–538, 2002.[Abstract/Free Full Text]
51. Feoktistov I, Ryzhov S, Goldstein AE, and Biaggioni I. Mast cell-mediated stimulation of angiogenesis: cooperative interaction between A_2B and A₃ adenosine receptors. Circ Res 92: 485–492, 2003.[Abstract/Free Full Text]
52. Feoktistov I, Ryzhov S, Zhong H, Goldstein AE, Matafonov A, Zeng D, and Biaggioni I. Hypoxia modulates adenosine receptors in human endothelial and smooth muscle cells toward an A_2B angiogenic phenotype. Hypertension 44: 649–654, 2004.[Abstract/Free Full Text]
53. Ferrara N. Vascular endothelial growth factor: basic science and clinical progress. Endocr Rev 25: 581–611, 2004.[Abstract/Free Full Text]
54. Fischer S, Sharma HS, Karliczek GF, and Schaper W. Expression of vascular permeability factor/vascular endothelial growth factor in pig cerebral microvascular endothelial cells and its upregulation by adenosine. Brain Res Mol Brain Res 28: 141–148, 1995.[Medline]
55. Flamme I, Breier G, and Risau W. Vascular endothelial growth factor (VEGF) and VEGF receptor 2 (flk-1) are expressed during vasculogenesis and vascular differentiation in the quail embryo. Dev Biol 169: 699–712, 1995.[CrossRef][Web of Science][Medline]
56. Forsythe JA, Jiang BH, Iyer NV, Agani F, Leung SW, Koos RD, and Semenza GL. Activation of vascular endothelial growth factor gene transcription by hypoxia-inducible factor 1. Mol Cell Biol 16: 4604–4613, 1996.[Abstract]
57. Fredholm BB, Abbracchio MP, Burnstock G, Daly JW, Harden TK, Jacobson KA, Leff P, and Williams M. Nomenclature and classification of purinoceptors. Pharmacol Rev 46: 143–156, 1994.[Web of Science][Medline]
58. Fredholm BB, IJzerman AP, Jacobson KA, Klotz KN, and Linden J. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors. Pharmacol Rev 53: 527–552, 2001.[Abstract/Free Full Text]
59. Gorman MW, He MX, Hall CS, and Sparks HV. Inorganic phosphate as regulator of adenosine formation in isolated guinea pig hearts. Am J Physiol Heart Circ Physiol 272: H913–H920, 1997.[Abstract/Free Full Text]
60. Grant MB, Davis MI, Caballero S, Feoktistov I, Biaggioni I, and Belardinelli L. Proliferation, migration, and ERK activation in human retinal endothelial cells through A_2B adenosine receptor stimulation. Invest Ophthalmol Vis Sci 42: 2068–2073, 2001.[Abstract/Free Full Text]
61. Grant MB, Tarnuzzer RW, Caballero S, Ozeck MJ, Davis MI, Spoerri PE, Feoktistov I, Biaggioni I, Shryock JC, and Belardinelli L. Adenosine receptor activation induces vascular endothelial growth factor in human retinal endothelial cells. Circ Res 85: 699–706, 1999.[Abstract/Free Full Text]
62. Gu JW and Adair TH. Hypoxia-induced expression of vascular endothelial growth factor is reversible in myocardial vascular smooth muscle cells. Am J Physiol Heart Circ Physiol 273: H628–H633, 1997.[Abstract/Free Full Text]
63. Gu JW, Brady AL, Anand V, Moore MC, Kelly WC, and Adair TH. Adenosine upregulates VEGF expression in cultured myocardial vascular smooth muscle cells. Am J Physiol Heart Circ Physiol 277: H595–H602, 1999.[Abstract/Free Full Text]
64. Gu JW, Ito BR, Sartin A, Frascogna N, Moore M, and Adair TH. Inhibition of adenosine kinase induces expression of VEGF mRNA and protein in myocardial myoblasts. Am J Physiol Heart Circ Physiol 279: H2116–H2123, 2000.[Abstract/Free Full Text]
65. Hagberg H, Andersson P, Lacarewicz J, Jacobson I, Butcher S, and Sandberg M. Extracellular adenosine, inosine, hypoxanthine, and xanthine in relation to tissue nucleotides and purines in rat striatum during transient ischemia. J Neurochem 49: 227–231, 1987.[CrossRef][Web of Science][Medline]
66. Halejcio-Delophont P, Siaghy EM, Devaux Y, Ungureanu-Longrois D, Richoux JP, Beck B, Burlet C, Villemot JP, and Mertes PM. Increase in myocardial interstitial adenosine and net lactate production in brain-dead pigs: an in vivo microdialysis study. Transplantation 66: 1278–1284, 1998.[Web of Science][Medline]
67. Hang J, Kong L, Gu JW, and Adair TH. VEGF gene expression is upregulated in electrically stimulated rat skeletal muscle. Am J Physiol Heart Circ Physiol 269: H1827–H1831, 1995.[Abstract/Free Full Text]
68. Harrington EO, Smeglin A, Parks N, Newton J, and Rounds S. Adenosine induces endothelial apoptosis by activating protein tyrosine phosphatase: a possible role of p38. Am J Physiol Lung Cell Mol Physiol 279: L733–L742, 2000.[Abstract/Free Full Text]
69. Hashimoto E, Kage K, Ogita T, Nakaoka T, Matsuoka R, and Kira Y. Adenosine as an endogenous mediator of hypoxia for induction of vascular endothelial growth factor mRNA in U-937 cells. Biochem Biophys Res Commun 204: 318–324, 1994.[CrossRef][Web of Science][Medline]
70. Headrick JP, Matherne GP, and Berne RM. Myocardial adenosine formation during hypoxia: effects of ecto-5'-nucleotidase inhibition. J Mol Cell Cardiol 24: 295–303, 1992.[CrossRef][Web of Science][Medline]
71. Hellsten Y. The effect of muscle contraction on the regulation of adenosine formation in rat skeletal muscle cells. J Physiol 518: 761–768, 1999.[Abstract/Free Full Text]
72. Hellsten Y, Maclean D, Radegran G, Saltin B, and Bangsbo J. Adenosine concentrations in the interstitium of resting and contracting human skeletal muscle. Circulation 98: 6–8, 1998.[Abstract/Free Full Text]
73. Hossain MA, Bouton CM, Pevsner J, and Laterra J. Induction of vascular endothelial growth factor in human astrocytes by lead. Involvement of a protein kinase C/activator protein-1 complex-dependent and hypoxia-inducible factor 1-independent signaling pathway. J Biol Chem 275: 27874–27882, 2000.[Abstract/Free Full Text]
74. Hudlicka O. Development of microcirculation: capillary growth and adaptation. In: Handbook of Physiology. The Cardiovascular System. Microcirculation. Bethesda, MD: Am. Physiol. Soc., 1984, sect 2, vol IV, pt 1, chapt 5, p. 165–216.
75. Hudlicka O, Brown M, and Egginton S. Angiogenesis in skeletal and cardiac muscle. Physiol Rev 72: 369–417, 1992.[Free Full Text]
76. Ikeda E, Achen MG, Breier G, and Risau W. Hypoxia-induced transcriptional activation and increased mRNA stability of vascular endothelial growth factor in C6 glioma cells. J Biol Chem 270: 19761–19766, 1996.
77. Imai S, Riley AL, and Berne RM. Effect of ischemia on adenine nucleotides in cardiac and skeletal muscle. Circ Res 15: 443–450, 1964.[Abstract/Free Full Text]
78. Ingber DE. Mechanical signaling and the cellular response to extracellular matrix in angiogenesis and cardiovascular physiology. Circ Res 91: 877–887, 2002.[Abstract/Free Full Text]
79. Jen SC and Rovainen CM. An adenosine agonist increases blood flow and density of capillary branches in the optic tectum of Xenopus laevis tadpoles. Microcirculation 1: 59–66, 1994.[Medline]
80. Knabb RM, Gidday JM, Ely SW, Rubio R, and Berne RM. Effects of dipyridamole on myocardial adenosine and active hyperemia. Am J Physiol Heart Circ Physiol 247: H804–H810, 1984.[Abstract/Free Full Text]
81. Kobayashi S, Zimmermann H, and Millhorn DE. Chronic hypoxia enhances adenosine release in rat PC12 cells by altering adenosine metabolism and membrane transport. J Neurochem 74: 621–632, 2000.[CrossRef][Web of Science][Medline]
82. Koszalka P, Ozuyaman B, Huo Y, Zernecke A, Flogel U, Braun N, Buchheiser A, Decking UK, Smith ML, Sevigny J, Gear A, Weber AA, Molojavyi A, Ding Z, Weber C, Ley K, Zimmermann H, Godecke A, and Schrader J. Targeted disruption of cd73/ecto-5'-nucleotidase alters thromboregulation and augments vascular inflammatory response. Circ Res 95: 814–821 2004.[Abstract/Free Full Text]
83. Kroll K, Decking UK, Dreikorn K, and Schrader J. Rapid turnover of the AMP-adenosine metabolic cycle in the guinea pig heart. Circ Res 73: 846–856, 1993.[Abstract/Free Full Text]
84. Kuzmin AI, Lakomkin VL, Kapelko VI, and Vassort G. Interstitial ATP level and degradation in control and postmyocardial infarcted rats. Am J Physiol Cell Physiol 275: C766–C771, 1998.[Abstract/Free Full Text]
85. Ladoux A and Frelin C. Hypoxia is a strong inducer of vascular endothelial growth factor mRNA expression in the heart. Biochem Biophys Res Commun 195: 1005–1010, 1993.[CrossRef][Web of Science][Medline]
86. Ledoux S, Runembert I, Koumanov K, Michel JB, Trugnan G, and Friedlander G. Hypoxia enhances ecto-5'-Nucleotidase activity and cell surface expression in endothelial cells: role of membrane lipids. Circ Res 92: 848–855, 2003.[Abstract/Free Full Text]
87. Le Hir M and Kaissling B. Distribution and regulation of renal ecto-5'-nucleotidase: implications for physiological functions of adenosine. Am J Physiol Renal Fluid Electrolyte Physiol 264: F377–F387, 1993.[Abstract/Free Full Text]
88. Leibovich SJ, Chen JF, Pinhal-Enfield G, Belem PC, Elson G, Rosania A, Ramanathan M, Montesinos C, Jacobson M, Schwarzschild MA, Fink JS, and Cronstein B. Synergistic up-regulation of vascular endothelial growth factor expression in murine macrophages by adenosine A_2A receptor agonists and endotoxin. Am J Pathol 160: 2231–2244, 2002.[Abstract/Free Full Text]
89. Li W, Gu JW, Makey I, Wang J, Elam J, and Adair T. VEGF expression is decreased in rat skeletal muscle following Achilles tenotomy (Abstract). FASEB J 15: A273, 2001.
90. Linden J. Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection. Annu Rev Pharmacol Toxicol 41: 775–787, 2001.[CrossRef][Web of Science][Medline]
91. Lo SM, Mo FM, and Ballard HJ. Interstitial adenosine concentration in rat red or white skeletal muscle during systemic hypoxia or contractions. Exp Physiol 86: 593–598, 2001.[Abstract]
92. Lombardi F and Rambaldi R. Coronary angiogenesis: dipyridamole is back on the stage? Eur Heart J 22: 1151–1153, 2001.[Free Full Text]
93. Lutty GA and McLeod DS. Retinal vascular development and oxygen-induced retinopathy: a role for adenosine. Prog Retin Eye Res 22: 95–111, 2003.[CrossRef][Web of Science][Medline]
94. Lutty GA, Mathews MK, Merges C, and McLeod DS. Adenosine stimulates canine retinal microvascular endothelial cell migration and tube formation. Curr Eye Res 17: 594–607, 1998.[CrossRef][Web of Science][Medline]
95. Lutty GA, Merges C, and McLeod DS. 5' Nucleotidase and adenosine during retinal vasculogenesis and oxygen-induced retinopathy. Invest Ophthalmol Vis Sci 41: 218–229, 2000.[Abstract/Free Full Text]
96. Lynge J and Hellsten Y. Distribution of adenosine A₁, A_2A and A_2B receptors in human skeletal muscle. Acta Physiol Scand 169: 283–290, 2000.[CrossRef][Web of Science][Medline]
97. Lynge J, Juel C, and Hellsten Y. Extracellular formation and uptake of adenosine during skeletal muscle contraction in the rat: role of adenosine transporters. J Physiol 537: 597–605, 2001.[Abstract/Free Full Text]
98. Lynge J, Schulte G, Nordsborg N, Fredholm BB, and Hellsten Y. Adenosine A 2B receptors modulate cAMP levels and induce CREB but not ERK1/2 and p38 phosphorylation in rat skeletal muscle cells. Biochem Biophys Res Commun 307: 180–187, 2003.[CrossRef][Web of Science][Medline]
99. MacLean DA, Sinoway LI, and Leuenberger U. Systemic hypoxia elevates skeletal muscle interstitial adenosine levels in humans. Circulation 98: 1990–1992, 1998.[Abstract/Free Full Text]
100. Mall G, Schikora I, Mattfeldt T, and Bodle R. Dipyridamole-induced neoformation of capillaries in the rat heart. Quantitative stereological study on papillary muscles. Lab Invest 57: 86–93, 1987.[Web of Science][Medline]
101. Marshall JM. Roles of adenosine and nitric oxide in skeletal muscle in acute and chronic hypoxia. Adv Exp Med Biol 502: 349–363, 2001.[Web of Science][Medline]
102. Mattfeldt T and Mall G. Dipyridamole-induced capillary endothelial cell proliferation in the rat heart–a morphometric investigation. Cardiovasc Res 17: 229–237, 1983.[Web of Science][Medline]
103. McElroy FA and Philip RB. Relative potencies of dipyridamole and related agents as inhibitors of cyclic nucleotide phosphodiesterases: possible explanation of mechansim of inhibition of platelet function. Life Sci 17: 1479–1493, 1975.[CrossRef][Web of Science][Medline]
104. Meininger CJ, Schelling ME, and Granger HJ. Adenosine and hypoxia stimulate proliferation and migration of endothelial cells. Am J Physiol Heart Circ Physiol 255: H554–H562, 1988.[Abstract/Free Full Text]
105. Merighi S, Mirandola P, Varani K, Gessi S, Leung E, Baraldi PG, Tabrizi MA, and Borea PA. A glance at adenosine receptors: novel target for antitumor therapy. Pharmacol Ther 100: 31–48, 2003.[CrossRef][Web of Science][Medline]
106. Milkiewicz M, Pugh CW, and Egginton S. Inhibition of endogenous HIF inactivation induces angiogenesis in ischaemic skeletal muscles of mice. J Physiol 560: 21–26, 2004.[Abstract/Free Full Text]
107. Minamino T, Kitakaze M, Morioka T, Node K, Komamura K, Takeda H, Inoue M, Hori M, and Kamada T. Cardioprotection due to preconditioning correlates with increased ecto-5'-nucleotidase activity. Am J Physiol Heart Circ Physiol 270: H238–H244, 1996.[Abstract/Free Full Text]
108. Miyazaki K, Komatsu S, Ikebe M, Fenton RA, and Dobson JG Jr. Protein kinase Cepsilon and the antiadrenergic action of adenosine in rat ventricular myocytes. Am J Physiol Heart Circ Physiol 287: H1721–H1729, 2004.[Abstract/Free Full Text]
109. Mlodzik K, Loffing J, Le Hir M, and Kaissling B. Ecto-5'-nucleotidase is expressed by pericytes and fibroblasts in the rat heart. Histochem Cell Biol 103: 227–236, 1995.[CrossRef][Web of Science][Medline]
110. Mo FM and Ballard HJ. The effect of systemic hypoxia on interstitial and blood adenosine, AMP, ADP and ATP in dog skeletal muscle. J Physiol 536: 593–603, 2001.[Abstract/Free Full Text]
111. Mohrman DE and Heller LJ. Transcapillary adenosine transport in isolated guinea pig and rat hearts. Am J Physiol Heart Circ Physiol 259: H772–H783, 1990.[Abstract/Free Full Text]
112. Montesinos MC, Shaw JP, Yee H, Shamamian P, and Cronstein BN. Adenosine A_2A receptor activation promotes wound neovascularization by stimulating angiogenesis and vasculogenesis. Am J Pathol 164:1887–1892, 2004.[Abstract/Free Full Text]
113. Morbidelli L, Chang CH, Douglas JG, Granger HJ, Ledda F, and Ziche M. Nitric oxide mediates mitogenic effect of VEGF on coronary venular endothelium. Am J Physiol Heart Circ Physiol 270: H411–H415, 1996.[Abstract/Free Full Text]
114. Nees S, Herzog V, Becker BF, Bock M, Des Rosiers C, and Gerlach E. The coronary endothelium: a highly active metabolic barrier for adenosine. Basic Res Cardiol 80: 515–529, 1985.[CrossRef][Web of Science][Medline]
115. Newby AC. How does dipyridamole elevate extracellular adenosine concentration? Predictions from a three-compartment model of adenosine formation and inactivation. Biochem J 237: 845–851, 1986.[Web of Science][Medline]
116. Olah ME and Roudabush FL. Down-regulation of vascular endothelial growth factor expression after A_2A adenosine receptor activation in PC12 pheochromocytoma cells. J Pharmacol Exp Ther 293: 779–787, 2000.[Abstract/Free Full Text]
117. Park TS, Van Wylen DG, Rubio R, and Berne RM. Increased brain interstitial fluid adenosine concentration during hypoxia in newborn piglet. J Cereb Blood Flow Metab 7: 178–183, 1987.[Web of Science][Medline]
118. Peart J and Headrick JP. Intrinsic A₁ adenosine receptor activation during ischemia or reperfusion improves recovery in mouse hearts. Am J Physiol Heart Circ Physiol 279: H2166–H2175, 2000.[Abstract/Free Full Text]
119. Picano E and Michelassi C. Chronic oral dipyridamole as a "novel" antianginal drug: the collateral hypothesis. Cardiovasc Res 33: 666–670, 1997.[Abstract/Free Full Text]
120. Pinhal-Enfield G, Ramanathan M, Hasko G, Vogel SN, Salzman AL, Boons GJ, and Leibovich SJ. An angiogenic switch in macrophages involving synergy between Toll-like receptors 2, 4, 7, and 9 and adenosine A_2A receptors. Am J Pathol 163: 711–721, 2003.[Abstract/Free Full Text]
121. Pore N, Liu S, Shu HK, Li B, Haas-Kogan D, Stokoe D, Milanini-Mongiat J, Pages G, O’Rourke DM, Bernhard E, and Maity A. Sp1 is involved in Akt-mediated induction of VEGF expression through an HIF-1-independent mechanism. Mol Biol Cell 15: 4841–4853, 2004.[Abstract/Free Full Text]
122. Presta M, Rusnati M, Belleri M, Morbidelli L, Ziche M, and Ribatti D. Purine analogue 6-methylmercaptopurine riboside inhibits early and late phases of the angiogenesis process. Cancer Res 59: 2417–2424, 1999.[Abstract/Free Full Text]
123. Pucar D, Dzeja PP, Bast P, Gumina RJ, Drahl C, Lim L, Juranic N, Macura S, and Terzic A. Mapping hypoxia-induced bioenergetic rearrangements and metabolic signaling by ¹⁸O- assisted ³¹P NMR and ¹H NMR spectroscopy. Mol Cell Biochem 256–257: 281–289, 2004.[CrossRef]
124. Pueyo ME, Chen Y, D'Angelo G, and Michel JB. Regulation of vascular endothelial growth factor expression by cAMP in rat aortic smooth muscle cells. Exp Cell Res 238: 354–358, 1998.[CrossRef][Web of Science][Medline]
125. Richardson RS, Wagner H, Mudaliar SR, Saucedo E, Henry R, and Wagner PD. Exercise adaptation attenuates VEGF gene expression in human skeletal muscle. Am J Physiol Heart Circ Physiol 279: H772–H778, 2000.[Abstract/Free Full Text]
126. Romano FD, Naimi TS, and Dobson JG Jr. Adenosine attenuation of catecholamine-enhanced contractility of rat heart in vivo. Am J Physiol Heart Circ Physiol 260: H1635–H1639, 1991.[Abstract/Free Full Text]
127. Rubio R, Berne RM, and Dobson JG Jr. Sites of adenosine production in cardiac and skeletal muscle. Am J Physiol 225: 938–953, 1973.[Free Full Text]
128. Sala-Newby GB, Freeman NV, Curto MA, and Newby AC. Metabolic and functional consequences of cytosolic 5'-nucleotidase-IA overexpression in neonatal rat cardiomyocytes. Am J Physiol Heart Circ Physiol 285: H991–H998, 2003.[Abstract/Free Full Text]
129. Schmid TC, Loffing J, Le Hir M, and Kaissling B. Distribution of ecto-5'-nucleotidase in the rat liver: effect of anaemia. Histochemistry 101: 439–447, 1994.[CrossRef][Web of Science][Medline]
130. Schulz R, Post H, Vahlhaus C, and Heusch G. Ischemic preconditioning in pigs: a graded phenomenon: its relation to adenosine and bradykinin. Circulation 98: 1022–1029, 1998.[Abstract/Free Full Text]
131. Schutz W, Schrader J, and Gerlach E. Different sites of adenosine formation in the heart. Am J Physiol Heart Circ Physiol 240: H963–H970, 1981.[Abstract/Free Full Text]
132. Semenza GL. Hypoxia-inducible factor 1: oxygen homeostasis and disease pathophysiology. Trends Mol Med 7: 345–350, 2001.[CrossRef][Web of Science][Medline]
133. Sexl V, Mancusi G, Holler C, Gloria-Maercker E, Schutz W, and Freissmuth M. Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary human endothelial cells. J Biol Chem 272: 5792–5799, 1997.[Abstract/Free Full Text]
134. Shryock JC and Belardinelli L. Adenosine and adenosine receptors i n the cardiovascular system: biochemistry, physiology, and pharmacology. Am J Cardiol 79: 2–10, 1997.[Web of Science][Medline]
135. Shweiki D, Itin A, Soffer D, and Keshet E. Vascular endothelial growth factor induced by hypoxia may mediate hypoxia-initiated angiogenesis. Nature 359: 843–845, 1992.[CrossRef][Medline]
136. Sollevi A, Ostergren J, Fagrell B, and Hjemdahl P. Theophylline antagonizes cardiovascular responses to dipyridamole in man without affecting increases in plasma adenosine. Acta Physiol Scand 121: 165–171, 1984.[Web of Science][Medline]
137. Sparks HV Jr and Bardenheuer H. Regulation of adenosine formation by the heart. Circ Res 58: 193–201, 1986.[Abstract/Free Full Text]
138. Spychala J, Lazarowski E, Ostapkowicz A, Ayscue LH, Jin A, and Mitchell BS. Role of estrogen receptor in the regulation of ecto-5'-nucleotidase and adenosine in breast cancer. Clin Cancer Res 10: 708–717, 2004.[Abstract/Free Full Text]
139. Spychala J. Tumor-promoting functions of adenosine. Pharmacol Ther 87: 161–173, 2000.[CrossRef][Web of Science][Medline]
140. Stroka DM, Burkhardt T, Desbaillets I, Wenger RH, Neil DA, Bauer C, Gassmann M, and Candinas D. HIF-1 is expressed in normoxic tissue and displays an organ-specific regulation under systemic hypoxia. FASEB J 15: 2445–2453, 2001.[Abstract/Free Full Text]
141. Sugano M, Tsuchida K, and Makino N. Intramuscular gene transfer of soluble tumor necrosis factor-alpha receptor 1 activates vascular endothelial growth factor receptor and accelerates angiogenesis in a rat model of hindlimb ischemia. Circulation 109: 797–802, 2004.[Abstract/Free Full Text]
142. Symons JD, Firoozmand E, and Longhurst JC. Repeated dipyridamole administration enhances collateral-dependent flow and regional function during exercise. A role for adenosine. Circ Res 73: 503–513, 1993.[Abstract/Free Full Text]
143. Synnestvedt K, Furuta GT, Comerford KM, Louis N, Karhausen J, Eltzschig HK, Hansen KR, Thompson LF, and Colgan SP. Ecto-5'-nucleotidase (CD73) regulation by hypoxia-inducible factor-1 mediates permeability changes in intestinal epithelia. J Clin Invest 110: 993–1002, 2002.[CrossRef][Web of Science][Medline]
144. Takagi H, King GL, Ferrara N, and Aiello LP. Hypoxia regulates vascular endothelial growth factor receptor KDR/Flk gene expression through adenosine A₂ receptors in retinal capillary endothelial cells. Invest Ophthalmol Vis Sci 37: 1311–1321, 1996.[Abstract/Free Full Text]
145. Takagi H, King GL, Robinson GS, Ferrara N, and Aiello LP. Adenosine mediates hypoxic induction of vascular endothelial growth factor in retinal pericytes and endothelial cells. Invest Ophthalmol Vis Sci 37: 2165–2176, 1996.[Abstract/Free Full Text]
146. Tang K, Breen EC, and Wagner PD. Hu protein R-mediated posttranscriptional regulation of VEGF expression in rat gastrocnemius muscle. Am J Physiol Heart Circ Physiol 283: H1497–H1504, 2002.[Abstract/Free Full Text]
147. Tang K, Breen EC, Gerber HP, Ferrara NM, and Wagner PD. Capillary regression in vascular endothelial growth factor-deficient skeletal muscle. Physiol Genomics 18: 63–69, 2004.[Abstract/Free Full Text]
148. Taomoto M, McLeod DS, Merges C, and Lutty GA. Localization of adenosine A2a receptor in retinal development and oxygen-induced retinopathy. Invest Ophthalmol Vis Sci 41: 230–243, 2000.[Abstract/Free Full Text]
149. Teuscher E and Weidlich V. Adenosine nucleotides, adenosine and adenine as angiogenesis factors. Biomed Biochim Acta 44: 493–495, 1985.[Web of Science][Medline]
150. Thomas MK, Francis SH, and Corbin JD. Characterization of a purified bovine lung cGMP-binding cGMP phosphodiesterase. J Biol Chem 265: 14964–14970, 1990.[Abstract/Free Full Text]
151. Tornling G, Adolfsson J, Unge G, and Ljungqvist A. Capillary neoformation in skeletal muscle of dipyridamole-treated rats. Arzneimittelforschung 30: 791–792, 1980.[Medline]
152. Tornling G, Unge G, Adolfsson J, Ljungqvist A, and Carlsson S. Proliferative activity of capillary wall cells in skeletal muscle of rats during long-term treatment with dipyridamole. Arzneimittelforschung 30: 622–623, 1980.[Medline]
153. Tornling G, Unge G, Skoog L, Ljungqvist A, Carlsson S, and Adolfsson J. Proliferative activity of myocardial capillary wall cells in dipyridamole-treated rats. Cardiovasc Res 12: 692–695, 1978.[Web of Science][Medline]
154. Tornling G. Capillary neoformation in the heart of dipyridamole-treated rats. Acta Pathol Microbiol Immunol Scand 90: 269–271, 1982.
155. Torry RJ, O’Brien DM, Connell PM, and Tomanek RJ. Dipyridamole-induced capillary growth in normal and hypertrophic hearts. Am J Physiol Heart Circ Physiol 262: H980–H986, 1992.[Abstract/Free Full Text]
156. Van Daele P, Van Coevorden A, Roger PP, and Boeynaems JM. Effects of adenine nucleotides on the proliferation of aortic endothelial cells. Circ Res 70: 82–90, 1992.[Abstract/Free Full Text]
157. Van Wylen DG, Park TS, Rubio R, and Berne RM. Cerebral blood flow and interstitial fluid adenosine during hemorrhagic hypotension. Am J Physiol Heart Circ Physiol 255: H1211–H1218, 1988.[Abstract/Free Full Text]
158. Van Wylen DG, Willis J, Sodhi J, Weiss RJ, Lasley RD, and Mentzer RM Jr. Cardiac microdialysis to estimate interstitial adenosine and coronary blood flow. Am J Physiol Heart Circ Physiol 258: H1642–H1649, 1990.[Abstract/Free Full Text]
159. Von Beckerath N, Cyrys S, Dischner A, and Daut J. Hypoxic vasodilatation in isolated, perfused guinea-pig heart: an analysis of the underlying mechanisms. J Physiol 442: 297–319, 1991.[Abstract/Free Full Text]
160. Wagatsuma A, Tamaki H, and Ogita F. Capillary supply and gene expression of angiogenesis-related factors in murine skeletal muscle following denervation. Exp Physiol. In press.
161. Wagner DR, Bontemps F, and van den Berghe G. Existence and role of substrate cycling between AMP and adenosine in isolated rabbit cardiomyocytes under control conditions and in ATP depletion. Circulation 90: 1343–1349, 1994.[Abstract/Free Full Text]
162. Wagner PD. Skeletal muscle angiogenesis. A possible role for hypoxia. Adv Exp Med Biol 502: 21–38, 2001.[Web of Science][Medline]
163. Wangler RD, Gorman MW, Wang CY, DeWitt DF, Chan IS, Bassingthwaighte JB, and Sparks HV. Transcapillary adenosine transport and interstitial adenosine concentration in guinea pig hearts. Am J Physiol Heart Circ Physiol 257: H89–H106, 1989.[Abstract/Free Full Text]
164. Wei LH, Kuo ML, Chen CA, Chou CH, Lai KB, Lee CN, and Hsieh CY. Interleukin-6 promotes cervical tumor growth by VEGF-dependent angiogenesis via a STAT3 pathway. Oncogene 22: 1517–1527, 2003.[CrossRef][Web of Science][Medline]
165. Wothe D, Hohimer A, Morton M, Thornburg K, Giraud G, and Davis L. Increased coronary blood flow signals growth of coronary resistance vessels in near-term ovine fetuses. Am J Physiol Regul Integr Comp Physiol 282: R295–R302, 2002.[Abstract/Free Full Text]
166. Zeng D, Maa T, Wang U, Feoktistov I, Biaggioni I, and Belardinelli L. Expression and function of A2B adenosine receptors in the U87MG tumor cells. Drug Dev Res 58: 405–411, 2003.[CrossRef]
167. Zhang R, Wang L, Zhang L, Chen J, Zhu Z, Zhang Z, and Chopp M. Nitric oxide enhances angiogenesis via the synthesis of vascular endothelial growth factor and cGMP after stroke in the rat. Circ Res 92: 308–313, 2003.[Abstract/Free Full Text]
168. Zhou AM, Li WB, Li QJ, Liu HQ, Feng RF, and Zhao HG. A short cerebral ischemic preconditioning up-regulates adenosine receptors in the hippocampal CA1 region of rats. Neurosci Res 48: 397–404, 2004.[CrossRef][Web of Science][Medline]
169. Ziada AM, Hudlicka O, Tyler KR, and Wright AJ. The effect of long-term vasodilatation on capillary growth and performance in rabbit heart and skeletal muscle. Cardiovasc Res 18: 724–732, 1984.[Web of Science][Medline]
170. Ziche M, Morbidelli L, Choudhuri R, Zhang HT, Donnini S, Granger HJ, and Bicknell R. Nitric oxide synthase lies downstream from vascular endothelial growth factor-induced but not basic fibroblast growth factor-induced angiogenesis. J Clin Invest 99: 2625–2634, 1997.[Web of Science][Medline]
171. Zimmermann H and Braun N. Extracellular metabolism of nucleotides in the nervous system. J Auton Pharmacol 16: 397–400, 1996.[Web of Science][Medline]

This article has been cited by other articles:

				K. A. Zwetsloot, L. M. Westerkamp, B. F. Holmes, and T. P. Gavin AMPK regulates basal skeletal muscle capillarization and VEGF expression, but is not necessary for the angiogenic response to exercise J. Physiol., December 15, 2008; 586(24): 6021 - 6035. [Abstract] [Full Text] [PDF]

				P. M. Costello, A. Rowlerson, N. A. Astaman, F. E. W. Anthony, A. A. Sayer, C. Cooper, M. A. Hanson, and L. R. Green Peri-implantation and late gestation maternal undernutrition differentially affect fetal sheep skeletal muscle development J. Physiol., May 1, 2008; 586(9): 2371 - 2379. [Abstract] [Full Text] [PDF]

				J. A. Auchampach Adenosine Receptors and Angiogenesis Circ. Res., November 26, 2007; 101(11): 1075 - 1077. [Full Text] [PDF]

				F. Suhr, K. Brixius, M. de Marees, B. Bolck, H. Kleinoder, S. Achtzehn, W. Bloch, and J. Mester Effects of short-term vibration and hypoxia during high-intensity cycling exercise on circulating levels of angiogenic regulators in humans J Appl Physiol, August 1, 2007; 103(2): 474 - 483. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (31) Citing Articles via Google Scholar Google Scholar Articles by Adair, T. H. Search for Related Content PubMed PubMed Citation Articles by Adair, T. H.