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Am J Physiol Regul Integr Comp Physiol 291: R1669-R1676, 2006. First published July 20, 2006; doi:10.1152/ajpregu.00183.2006
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RENAL HEMODYNAMICS AND CARDIORENAL INTEGRATION

ANG II type 2 receptors and neural control of intrarenal blood flow

Niwanthi W. Rajapakse,1 Gabriela A. Eppel,1 Robert E. Widdop,2 and Roger G. Evans1

Departments of 1Physiology and 2Pharmacology, Monash University, Victoria, Australia

Submitted 14 March 2006 ; accepted in final form 19 July 2006


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
We tested the hypothesis that activation of angiotensin type 2 (AT2) receptors, by both exogenous and endogenous ANG II, modulates neurally mediated vasoconstriction in the renal cortical and medullary circulations. Under control conditions in pentobarbital-anesthetized rabbits, electrical stimulation of the renal nerves (RNS; 0.5–8 Hz) reduced renal blood flow (RBF; –88 ± 3% at 8 Hz) and cortical perfusion (CBF; –92 ± 2% at 8 Hz) more than medullary perfusion (MBF; –67 ± 6% at 8 Hz). Renal arterial infusion of ANG II, at a dose titrated to reduce RBF by ~40–50% (5–50 ng·kg–1·min–1) blunted responses of MBF to RNS, without significantly affecting responses of RBF or CBF. Subsequent administration of PD123319 (1 mg/kg plus 1 mg·kg–1·h–1) during continued renal arterial infusion of ANG II did not significantly affect responses of RBF or CBF to RNS but enhanced responses of MBF, so that they were similar to those observed under control conditions. In contrast, administration of PD123319 alone blunted responses of CBF and MBF to RNS. Subsequent renal arterial infusion of ANG II in PD123319-pretreated rabbits restored CBF responses to RNS back to control levels. In contrast, ANG II infusion in PD123319-pretreated rabbits did not alter MBF responses to RNS. These data indicate that exogenous ANG II can blunt neurally mediated vasoconstriction in the medullary circulation through activation of AT2 receptors. However, AT2-receptor activation by endogenous ANG II appears to enhance neurally mediated vasoconstriction in both the cortical and medullary circulations.

kidney medulla; renal circulation; renin-angiotensin system; sympathetic nervous system

THE RENAL MEDULLARY MICROCIRCULATION (7–10, 20), the renal sympathetic nerves (11), and the renin-angiotensin system (37) all play key roles in long-term regulation of arterial pressure. Renal sympathetic nerves innervate juxtamedullary afferent and efferent arterioles and outer medullary descending vasa recta (17, 18), vascular elements likely important in control of medullary perfusion (20). Angiotensin type 2 (AT2) and/or AT1 receptors are also localized to these vascular elements (32). Activation of the renal nerves (18, 24) and AT receptors (13, 14) can alter renal medullary blood flow. However, medullary blood flow appears to be less sensitive than cortical blood flow to vasoconstriction evoked by either neural activation or ANG II (18, 20).

We recently found that renal arterial infusion of ANG II, at doses that reduce baseline total renal blood flow (RBF) and cortical laser Doppler flux (CLDF; an index of cortical blood flow) by >30% but do not significantly alter baseline medullary laser Doppler flux (MLDF), blunt reductions in MLDF induced by electrical stimulation of the renal nerves (RNS) (21). In contrast, ANG II infusion did not significantly affect responses of RBF or CLDF to RNS (21). This raised the interesting possibility that ANG II might act specifically within the medullary circulation to blunt vasoconstrictor responses to RNS. This concept would be consistent with the observation that ANG II can increase nitric oxide (NO) concentrations in the medulla (44), possibly through release from tubular sites (8, 12). Furthermore, ANG II can cause vasodilatation in the medullary circulation via activation of AT1 receptors, and this action can be abolished by blockade of NO synthase (13, 14, 35, 36, 41). However, this concept is not supported by our recent finding that the AT1 receptor antagonist candesartan blunted RNS-induced reductions in MLDF (as well as RBF and CLDF) in anesthetized rabbits (35). Indeed, these data suggest that endogenous ANG II acting at AT1 receptors acts to enhance neurally mediated vasoconstriction throughout the renal circulation.

How then, can we reconcile the ability of exogenous ANG II to selectively blunt responses of medullary perfusion to RNS, with the strong evidence that the dominant effect of AT1-receptor activation by endogenous ANG II is to enhance neurally mediated vasoconstriction across the entire renal circulation? One possibility is that the effects of exogenous ANG II on neural control of medullary blood flow are mediated by AT2 receptors. This hypothesis is consistent with the evidence that renal AT2-receptor activation can stimulate phospholipase A2-dependent signaling (23) and increase production of epoxyeicosatrienoic acids (2), bradykinin, and cyclic guanosine 5-monophosphate (38). Our observations might also reflect the different biophases within which exogenous and endogenous ANG II likely act. When infused into the renal artery, ANG II will likely reach high concentrations within vascular and tubular lumens, where it could induce release of NO and other paracrine factors (8). However, the vascular endothelium likely acts as a barrier to retard diffusion of ANG II to vascular smooth muscle (25–27), where it could directly modulate renal neurovascular function. In contrast, endogenous ANG II reaches high concentrations within the renal interstitium and so would have direct access to sites such as vascular smooth muscle (30, 43). Therefore, in the current study, we tested the effects of the AT2 receptor antagonist PD123319, both on the ability of exogenous ANG II to modulate intrarenal blood flow responses to RNS, and on responses to RNS in the absence of exogenous ANG II. Our results indicate that the AT2-receptor antagonist PD123319 can reverse the action of renal arterial infusion of ANG II to blunt RNS-induced reductions in MLDF. However, this appears not to reflect the role of endogenous ANG II in neural control of intrarenal blood flow, since PD123319 alone reduced responses of RBF, CLDF, and MLDF to RNS, as did candesartan in our previous study (35). Thus endogenous ANG II appears to act through both AT1 and AT2 receptors to enhance neurally mediated vasoconstriction throughout the renal circulation.


   METHODS
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 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
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Animals

Twenty-five male New Zealand white rabbits (mean weight 2.83 ± 0.06 kg) were used. They were meal fed and provided with water ad libitum (19). All experiments were conducted in accordance with the American Physiological Society's "Guiding Principles for Research Involving Animals and Human Beings" (1) and were approved in advance by the Animal Ethics Committee of the Department of Physiology, Monash University, Victoria, Australia.

Surgical Preparations

The surgical preparations used in this study have been described in detail previously (21, 24, 35), so will only be described briefly here. Catheters were placed in ear arteries and veins for measurement of arterial pressure and intravenous infusions, respectively. Rabbits were anesthetized with pentobarbital sodium (90–150 mg plus 30–50 mg/h Nembutal; Rhone Merieux, Pinkenba, QLD, Australia) and artificially ventilated. An intravenous infusion of Hartmann's solution (0.18 ml·kg–1·min–1 compound sodium lactate; Baxter Healthcare, Toongabbie, NSW, Australia) then commenced and continued until all surgical preparations were completed. The left kidney was exposed via a left flank incision and placed in a stable cup. A catheter was placed in the side branch of the renal artery (suprarenolumbar artery) for infusion of ANG II or saline. The renal nerves running parallel to the left renal artery were carefully isolated and placed across a stimulating electrode. The nerves were then sectioned proximally. A transit-time ultrasound flow probe (type 2SB, Transonic Systems, Ithaca, NY) was placed around the renal artery to measure RBF. MLDF was measured using a needle-type laser-Doppler flow probe (26 gauge, DP4s; Moor Instruments, Millwey, Devon, UK) advanced ~9 mm into the kidney. A standard plastic laser-Doppler flow probe (DP2b; Moor Instruments) was placed on the dorsal surface of the kidney to measure CLDF. After completion of all surgical preparations, the infusion of Hartmann's solution was replaced with a solution containing four parts Hartmann's solution and one part 10% vol/vol polygeline (Haemaccel; Hoechst, Melbourne, VIC, Australia). The experimental protocol commenced after a 30- to 60-min equilibration period.

Experimental Protocol

We compared responses to RNS during renal arterial infusion of ANG II or its saline vehicle, intravenous infusion of the selective AT2-receptor antagonist PD123319 (Sigma Aldrich, Castle Hill, NSW, Australia), or its vehicle, or combinations of these treatments, with the respective control responses within each animal. This within-subject design largely eliminates the potential for between-rabbit variations in the responses of intrarenal blood flow to RNS to confound interpretation of the experiments. Rabbits were randomized to four groups (n = 6 except in group 2, where n = 7) and were each subjected to three stimulation sequences. The first stimulation sequence defined the control response in rabbits of all four groups. Rabbits of group 1 received a renal arterial infusion of 154 mM NaCl (20 µl·kg–1·min–1; vehicle for ANG II) after the first stimulation sequence. Twenty minutes after commencing this infusion, a second stimulation sequence began. After a 10-min equilibration period, following completion of the second RNS sequence, these rabbits received the saline vehicle for PD123319 (1 ml/kg plus 1 ml·kg–1·h–1 154 mM NaCl). Twenty minutes after commencing this infusion, responses to RNS were reevaluated. This treatment regimen allowed us to determine whether, in the absence of active treatments, responses to RNS remain stable across the course of the experiment. The same experimental paradigm was used in rabbits of groups 2–4, except that these rabbits received the relevant active treatments instead of the saline vehicles. Thus rabbits of group 2 received a renal arterial infusion of ANG II (5–15 ng·kg–1·min–1) titrated to reduce basal RBF by ~40%, after the first stimulation sequence, which continued for the rest of the experiment. After the second stimulation sequence, an intravenous infusion of the vehicle for PD123319 was superimposed. Rabbits of group 3 received ANG II after the first stimulation sequence (as for group 2) and PD123319 (1 mg/kg plus 1 mg·kg–1·h–1) after the second stimulation sequence. Comparison of responses during stimulation sequences 1 and 2 in groups 2 and 3 combined allowed us to determine whether renal arterial infusion of ANG II altered responses to RNS. We have previously reported this comparison for 9 of the 13 rabbits in groups 2 and 3 (35), so these data are not novel. However, they are required for interpretation of responses during stimulation sequence 3, data that have not previously been reported. Comparison of responses during stimulation sequences 2 and 3 in these rabbits allowed us to determine whether the effects of ANG II were sustained in the absence of further active treatment (group 2) and whether PD123319 altered responses to RNS during renal arterial infusion of ANG II (group 3). Rabbits in group 4 received the same treatments as those of group 3, but in the opposite order. Thus, after stimulation sequence 1, these rabbits received an intravenous infusion of PD123319 (1 mg/kg plus 1 mg·kg–1·h–1), which continued for the rest of the experiment, and after stimulation sequence 2, a renal arterial infusion of ANG II was superimposed on this. As for groups 2 and 3, the dose of ANG II was titrated to reduce RBF by ~40%. Most of the rabbits in this group required a dose of ANG II between 5 and 20 ng·kg–1·min–1 to achieve this reduction in RBF, but one rabbit required 50 ng·kg–1·min–1. Comparison of responses during stimulation sequences 1 and 2 in group 4 allowed us to determine whether PD123319 alone (in the absence of exogenous ANG II) altered responses to RNS. Comparison of responses during stimulation sequences 2 and 3 in these rabbits allowed us to determine whether PD123319 pretreatment altered the effects of exogenous ANG II on responses to RNS.

Electrical stimulation of the renal nerves. The Labview graphical programming language (National Instruments, Austin, TX) was used to provide computer-generated RNS. The voltage that produced the maximum reductions in RBF, CLDF, and MLDF was determined in each rabbit by stimulating the renal nerves with various voltages (3–10 V) at 5 Hz for 60 s. This predetermined voltage was used in all subsequent stimulation sequences in each rabbit. Each stimulation sequence consisted of five, 3-min stimulus trains (0.5, 1, 2, 4, and 8 Hz; 2-ms pulse duration). These frequencies were applied in random order. Recovery periods of 8–10 min were allowed between each stimulus train.

Recording of hemodynamic variables. Signals were acquired and processed as previously described (24), to provide 2-s averages of mean arterial pressure (MAP; mmHg), heart rate (HR, as determined from the arterial pulse pressure; beats/min), RBF (ml/min), CLDF (perfusion units), and MLDF (perfusion units). The offset values of CLDF (5.0 ± 0.2) and MLDF (8.0 ± 0.5) after the rabbit was humanely killed by overdose with pentobarbital sodium (300 mg), were subtracted from the laser-Doppler flux values recorded during the experiment, before data analysis was performed.

Statistical Methods

Data are expressed as means ± SE. The computer software package SYSTAT (Version 9, SPSS, Chicago, IL) was used to perform all statistical tests. P ≤ 0.05 was considered statistically significant. Baseline levels were determined by averaging the 30-s control periods immediately before each stimulus train, across all five frequencies in each RNS sequence. Responses to RNS were determined by comparing the levels of each variable during the last 30 s of each stimulus train, with the control values during the 30 s before stimulation. All data were initially subjected to global repeated-measures ANOVA (29), the factors being group, rabbit, treatment (i.e., stimulation sequence), and where appropriate, frequency (of RNS). The group·treatment interaction term from these analyses tested the global hypothesis that the effects of the various treatments differed according to group. Specific contrasts were then performed in a within-group fashion. The Tukey-Kramer multiple-comparison procedure (28) was used to determine whether baseline levels of hemodynamic variables differed during stimulation sequences 2 or 3 (Treat 2 or 3) compared with stimulation sequence 1 (Treat 1), and during stimulation sequence 3 compared with stimulation sequence 2 (PTreat). For responses to RNS, we used F tests to compare responses during stimulation sequence 2 with those during sequence 1, and those during sequence 3 compared with sequence 2 (28). The error mean square values from the global analyses were used as the denominators for all specific contrasts. ANOVA was also used to test whether responses to RNS were frequency-dependent (PFrequency) and whether responses of MLDF to RNS differed from those of CLDF (PRegion).


   RESULTS
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DISCUSSION
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Baseline Hemodynamics

During the initial stimulation sequence, baseline levels of MAP, HR, RBF, CLDF, and MLDF, when averaged across all 25 rabbits, were 69 ± 2 mmHg, 247 ± 4 beats/min, 23 ± 2 ml/min, 272 ± 11 units, and 51 ± 5 units, respectively. In group 1 (vehicle only), baseline levels of all these variables remained stable across the course of the experiment (Table 1, group 1). In group 2, baseline levels of MAP and MLDF remained stable over the course of the experiment. In contrast, renal arterial infusion of ANG II significantly reduced baseline levels of RBF (–42 ± 4%) and CLDF (–30 ± 6%), and HR fell (–32 ± 14%) (Table 1, group 2). Subsequent administration of the vehicle for PD123319 had little or no effect on baseline levels of any of the measured variables. In group 3, renal arterial infusion of ANG II was accompanied by reduced RBF (–36 ± 3%) and CLDF (–29 ± 2%), and slightly reduced HR (–9 ± 2%). As for group 2, there was little or no change in MAP or MLDF (Table 1, group 3). Subsequent infusion of PD123319 had little or no further effect on any of the variables we measured (Table 1, group 3). In group 4, PD123319 infusion alone had no significant effect on any of the variables. During subsequent administration of ANG II, there were significant reductions in RBF (–49 ± 3%) and CLDF (–41 ± 5%) but no significant changes in MAP, HR, or MLDF (Table 1, group 4).


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  		Table 1. Mean baseline levels of systemic and renal hemodynamic variables

 
Responses to RNS

Responses to RNS under control conditions. When averaged across all 25 rabbits, RNS caused frequency-dependent reductions in RBF (–88 ± 3% at 8 Hz; PFrequency < 0.001), CLDF (–92 ± 2% at 8 Hz; PFrequency < 0.001), and MLDF (–67 ± 6% at 8 Hz; PFrequency < 0.001; Fig. 1). As we have found previously (16, 21, 22, 24, 35), RNS caused greater reductions in RBF and CLDF than in MLDF (PRegion ≤ 0.001). RNS caused a slight but significant increase in MAP (6 ± 1% at 8 Hz; PFrequency < 0.001) and a very small yet statistically significant reduction in HR (–0.8 ± 0.3% at 8 Hz; PFrequency = 0.001).


Figure 1
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  		Fig. 1. Effects of renal nerve stimulation on renal (A) and systemic (B) hemodynamic variables during the initial stimulation sequence. Symbols and error bars indicate means ± SE of data from all rabbits in all four groups (n = 25). RBF, renal blood flow; CLDF, cortical laser Doppler flux; MLDF, medullary laser Doppler flux; MAP, mean arterial pressure; HR, heart rate. PFrequency indicates the outcomes of ANOVA, which tested whether the responses to electrical stimulation of the renal nerves were dependent on the frequency at which the nerves were stimulated.

 
Effects of vehicle, ANG II, and PD123319. Global repeated-measures ANOVA revealed a significant group times treatment interaction for responses of MLDF (PGroup·Treat = 0.04), but not RBF (PGroup·Treat = 0.47) or CLDF (PGroup·Treat = 0.14) to RNS. In group 1, responses of RBF, CLDF, and MLDF to RNS remained remarkably stable across the course of the three stimulation sequences (PTreat ≥ 0.42) (Fig. 2). In groups 2 and 3 (combined), responses of MLDF (but not RBF or CLDF) to RNS were blunted after ANG II (PTreat = 0.02). For example, 4 Hz RNS reduced MLDF by –53 ± 10% under control conditions but only by –39 ± 10% during ANG II infusion (Fig. 3). In group 2, responses of RBF, CLDF, and MLDF to RNS during renal arterial infusion of ANG II did not differ after administration of the vehicle for PD123319 compared with responses before its administration (PTreat ≥ 0.21; Fig. 4). In contrast, in group 3, RNS-induced reductions in MLDF (but not RBF or CLDF) were enhanced after subsequent PD123319 treatment (Fig. 4). For example, 4 Hz RNS reduced MLDF by –38 ± 18% during ANG II infusion and by –60 ± 3% when PD123319 infusion was superimposed. In group 4, responses of both CLDF and MLDF to RNS were blunted by PD123319 (Fig. 5). For example, during RNS at 2 Hz, CLDF was reduced by –80 ± 4% during control conditions and by –63 ± 7% during treatment with PD123319. For CLDF, the effects of PD123319 were observed at frequencies of 2 Hz or less. For MLDF, the effects of PD123319 were observed at the higher frequencies, 4 and 8 Hz. RNS at 4 Hz, reduced MLDF by –67 ± 10% during control conditions and only by –37 ± 10% during PD123319 infusion. When ANG II infusion was superimposed on a background of PD123319, RNS-induced responses in CLDF were restored back to control levels. In contrast, MLDF responses to RNS were not significantly altered by ANG II in PD123319-pretreated rabbits.


Figure 2
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  		Fig. 2. Effects of vehicle treatments on renal hemodynamic responses to stimulation of the renal nerves. Symbols and error bars indicate means ± SE of data from rabbits in group 1 (n = 6). PTreat values indicate the outcomes of ANOVA, which tested whether renal hemodynamic responses to stimulation of the renal nerves during vehicle treatment periods (Treat 2 and 3) differed from those observed during the preceding stimulation sequence.

 

Figure 3
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  		Fig. 3. Effects of ANG II (5–15 ng·kg–1·min–1) on renal hemodynamic responses to renal nerve stimulation. Symbols and error bars indicate means ± SE of combined data of rabbits in groups 2 and 3 (n = 13). PTreat values indicate the outcomes of ANOVA testing whether renal hemodynamic responses to stimulation of the renal nerves during ANG II treatment differed from those observed during the initial stimulation sequence. Analogous data for a subset of these animals (9 of the 13 rabbits) were presented in a previous publication (35).

 

Figure 4
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  		Fig. 4. Effects of vehicle for PD123319 (154 mM NaCl, 1 ml/kg plus 1 ml·kg–1·h–1) (A) or PD123319 (1 mg/kg plus 1 mg·kg–1·h–1) (B) on renal hemodynamic responses to renal nerve stimulation in rabbits receiving a renal arterial infusion of ANG II (5–15 ng·kg–1·min–1). Symbols and error bars indicate means ± SE of data during stimulation sequences 2 and 3 of rabbits in groups 2 (n = 7) and 3 (n = 6). PTreat values indicate the outcomes of ANOVA, which tested whether responses to stimulation of the renal nerves during treatment with ANG II plus the vehicle for PD12319 or PD123319 itself differed from those observed during ANG II treatment alone.

 

Figure 5
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  		Fig. 5. Effects of PD123319 (1 mg/kg plus 1 mg·kg–1·h–1), alone and in combination with renal arterial infusion of ANG II (5–50 ng·kg–1·min–1), on renal hemodynamic responses to stimulation of the renal nerves. Symbols and error bars indicate means ± SE (n = 6). PTreat values indicate the outcomes of ANOVA, which tested whether responses during administration of PD123319 (Treat 2) differed from those observed during the initial stimulation sequence (Treat 1) and whether responses during administration of both PD123319 and ANG II (Treat 3) differed from those observed during PD123319 treatment alone (Treat 2).

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Our current results indicate that activation of AT2 receptors by endogenous ANG II has very different effects on the neural control of intrarenal blood flow than activation of these receptors by renal arterial infusion of ANG II. As we had found previously (21), renal arterial infusion of ANG II, at doses that reduced RBF and CLDF by ~40%, had little impact on responses of these variables to RNS. However, this ANG II infusion blunted responses of MLDF to RNS, without significantly altering basal MLDF. Subsequent administration of PD123319 abolished this effect of exogenous ANG II on responses of MLDF to RNS. Furthermore, ANG II infusion did not significantly alter responses of MLDF to RNS in rabbits pretreated with PD123319. Collectively, these data indicate that exogenous ANG II diminishes neurally mediated vasoconstriction in the medullary circulation through activation of AT2 receptors. However, when PD123319 was given in the absence of exogenous ANG II, it reduced responses of CLDF and MLDF to RNS. These data support the hypothesis that endogenous ANG II augments neurally mediated vasoconstriction within both the cortical and medullary circulations, through activation of AT2 receptors.

How can we reconcile these apparent differences in the effects of exogenous and endogenous ANG II? A possible explanation relates to the bioavailability of ANG II when infused intravascularly. Under these conditions, ANG II will reach high concentrations in the vicinity of endothelial cells, and because it is freely filtered, within the tubular lumen. Both vascular endothelial cells (40) and tubular epithelial cells (8, 12) release paracrine factors, including NO, in response to ANG II. Indeed, it has been proposed that the phenomenon of tubulovascular NO cross talk mediates ANG II-induced vasodilatation within the medullary circulation (8, 12). According to this concept, NO released from tubular elements such as the thick ascending limb diffuses to neighboring outer medullary descending vasa recta to reduce calcium concentrations within their associated contractile cells (pericytes and vascular smooth muscle cells) (8, 12). Access of exogenous ANG II to vascular smooth muscle and its associated sympathetic nerves, on the other hand, is likely limited by a number of factors. There is good evidence that the vascular endothelium acts as a diffusional barrier for hydrophilic molecules such as ANG II (25–27). It also seems likely that tubular epithelial tight junctions might limit access of intratubular ANG II to the interstitium. Rapid degradation of ANG II (3) within the vascular and tubular lumen likely also limits access of ANG II to vascular smooth muscle. This contrasts with the situation for endogenous ANG II, which is generated within the renal interstitium (30), where it could gain direct access to the adventitial aspect of the renal vasculature. Relatively stable pharmacological agents such as PD123319 would likely equilibrate throughout the vascular, tubular, and interstitial compartments of the kidney and so antagonize actions of ANG II mediated within all of these compartments. Thus the effects on renal neurovascular function of exogenous ANG II might result chiefly from indirect actions mediated by release of paracrine factors, while the effects of endogenous ANG II might be chiefly mediated directly on smooth muscle and its associated sympathetic nerves.

We have previously shown that the AT1-receptor antagonist candesartan (35), like PD123319, blunts responses of RBF, CLDF, and MLDF to RNS. Thus it appears that endogenous ANG II acts via both AT1 and AT2 receptors to enhance renal neurovascular function. This notion is consistent with evidence from in vitro studies, that ANG II-induced facilitation of neural norepinephrine release is mediated by both AT1 and AT2 receptors in the rat caudal artery (6) and human heart (33).

In contrast to the apparent effects of endogenous ANG II, renal arterial infusion of the peptide at doses that reduce RBF by ~30% or more reduce the responses of MLDF to RNS (21, current study). We have indirect evidence that this effect is mediated by NO, since 1) NO synthase (NOS) blockade enhances responses of MLDF to RNS (16, 34, 35), and 2) under conditions of NOS blockade renal arterial infusion of ANG II enhances responses of MLDF to RNS (35). Our current observations suggest that the effects of exogenous ANG II on neural control of medullary blood flow are mediated by AT2 receptors, since they were abolished by PD123319. This concept is consistent with evidence that AT2-receptor activation can induce NO release within the kidney, particularly under conditions of elevated activity of the renin-angiotensin system (4, 5, 38, 42). However, there is also evidence that AT2-receptor activation can mediate renal vasodilatation through activation of pathways of arachidonic acid metabolism (2). Thus further studies are required to elucidate the mechanisms by which AT2-receptor activation by exogenous ANG II blunts neurally mediated vasoconstriction in the medullary circulation.

One of the strengths of our current study was that the effects of exogenous ANG II, and of PD123319, on responses of MLDF to RNS were observed in the absence of effects of these treatments on basal MLDF and MAP. Regardless, we have previously shown that neither changes in vascular tone per se (21), nor changes in renal perfusion pressure (22), significantly alter responses of MLDF to RNS in anesthetized rabbits. As we have found previously (16, 17, 21, 34, 35), responses to RNS were highly reproducible across the course of the experiment in control rabbits (group 1). By analyzing our results mainly in a within-animal fashion, we were able to detect relatively modest effects of ANG II and PD123319 on responses to RNS, even though responses to RNS can vary somewhat in different rabbits. A limitation of our study is that we are unable to determine the sites at which exogenous and endogenous ANG II act to modulate neural control of regional kidney perfusion. ANG II can 1) act on renal vascular smooth muscle to directly mediate vasoconstriction and enhance responses to norepinephrine (2), 2) release multiple paracrine factors from the vascular endothelium and tubular epithelium (2, 8, 12, 23, 31, 36, 40), and 3) enhance norepinephrine release from sympathetic nerves (6, 15, 33, 39). We believe our current data are consistent with the hypothesis that exogenous ANG II activates paracrine systems to blunt renal neurovascular function in the medullary circulation. In contrast, the ability of endogenous ANG II to enhance renal neurovascular function in both the cortical and medullary circulations is likely mediated through prejunctional and/or postjunctional effects within vascular smooth muscle and its associated sympathetic nerves. This hypothesis, however, remains to be formally tested.

The dose of PD123319 that we used has previously been shown to effectively and selectively antagonize AT2 receptors in the renal circulation (13, 14). In anesthetized rabbits, this dose of PD123319 (1 mg/kg plus 1 mg·kg–1·h–1) enhanced ANG II-induced reductions in RBF and CLDF (13). The AT1-receptor antagonist candesartan had the opposite effect (13). This PD123319 dose regimen, but not candesartan, also uncovered ANG II-induced increases in MLDF in anesthetized rabbits (13) and rats (14). PD123319, but not candesartan, also increased basal MLDF in rats with renovascular hypertension, reflecting blockade of the effects of endogenous ANG II mediated by AT2 receptors (14). Thus we can be confident that the PD123319 dose regimen we used provides effective antagonism of the actions of both exogenous and endogenous ANG II mediated via AT2 receptors, without appreciable antagonism of AT1 receptors. We administered PD123319 intravenously, so AT2 receptors outside the kidney were likely also blocked. However, this is unlikely to have greatly confounded our observations, since the stimuli we studied were confined to the kidney (RNS with the nerves sectioned cranially and renal arterial infusion of ANG II), and basal MAP remained stable across the course of the experiment.

Interestingly, PD123319 blunted responses of CLDF to RNS when given alone (group 4), but not when given after the renal arterial infusion of ANG II had commenced (group 3). These apparently contradictory observations might reflect complex interactions between the actions of exogenous and endogenous ANG II mediated by AT1 and AT2 receptors but could alternatively be explained if the efficacy of PD123319 in vivo depends on whether it is administered before or after ANG II. We are not aware of any studies that have directly addressed this latter possibility under in vivo conditions. Nevertheless, our major conclusions are robust, since PD123319 both prevented (group 4) and reversed (group 3) the effect of exogenous ANG II to blunt responses of MLDF to RNS, providing unequivocal evidence that this effect of exogenous ANG II is mediated by AT2 receptors. Furthermore, the observation that PD123319 alone (group 4) blunted responses of CLDF and MLDF to RNS provides evidence for roles of endogenous ANG II and AT2 receptors in modulating neurally mediated vasoconstriction within the kidney.

In conclusion, taken together with the results of our previous studies (21, 35), the present observations suggest that ANG II has multiple effects on neural control of the renal circulation, which are expressed in a regionally specific manner. Thus, although the net effect of activation of AT1 and AT2 receptors by endogenous ANG II appears to be facilitation of renal neurovascular function within both the cortex and medulla, ANG II also appears able to inhibit renal neurovascular function, specifically within the medullary circulation. This latter effect, seen when ANG II is infused into the renal artery at doses that reduce RBF (but not MLDF) by ≥ 30%, appears to be mediated by AT2 receptors and may be dependent on NO. Because this effect of ANG II is not expressed by the endogenous peptide under physiological conditions, but induced by exogenous ANG II, it is tempting to deem it a pharmacological curiosity rather than a physiological phenomenon. However, it may be engaged under pathological conditions associated with activation of the endocrine renin-angiotensin system, such as renovascular hypertension. Experiments are currently under way in our laboratory to test this hypothesis.


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 ABSTRACT
 METHODS
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 DISCUSSION
 GRANTS
REFERENCES
 
This work was supported by grants from the National Health and Medical Research Council of Australia (143603, 143785, 384101, 384299), the National Heart Foundation of Australia (G04M1550, PF04M1758), the Ramaciotti Foundations, and Monash University. N. W. Rajapakse was a recipient of a Commonwealth Postgraduate Scholarship (Australia).


   ACKNOWLEDGMENTS
 
We thank Dr. John Ludbrook (Biomedical Statistical Consulting Service, Melbourne, Australia) for advice regarding the statistical analysis of our data.

Present address for N. W. Rajapakse: Department of Physiology, Medical College of Wisconsin, 8701, Watertown Plank Rd., Milwaukee WI 53213 USA.


   FOOTNOTES
 

Address for reprint requests and other correspondence: Roger G. Evans, Dept. of Physiology, PO Box 13F, Monash Univ., Melbourne 3800, Australia (E-mail: roger.evans{at}med.monash.edu.au)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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