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Am J Physiol Regul Integr Comp Physiol 291: R803-R809, 2006. First published March 23, 2006; doi:10.1152/ajpregu.00808.2005
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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Nitric oxide and prostaglandins influence local skeletal muscle blood flow during exercise in humans: coupling between local substrate uptake and blood flow

Kari K. Kalliokoski,1 Henning Langberg,1 Ann Kathrine Ryberg,1 Celena Scheede-Bergdahl,2 Simon Doessing,1 Andreas Kjær,3,4 Michael Kjær,1 and Robert Boushel1,5

1Copenhagen Muscle Research Centre, Institute of Sports Medicine, Bispebjerg Hospital, 2Department of Medical Physiology, Copenhagen Muscle Research Centre, Panum Institute, 3Department of Clinical Physiology, Nuclear Medicine, and Positron Emission Tomography, Rigshospitalet, and 4Cluster for Molecular Imaging, University of Copenhagen, Copenhagen, Denmark; and 5Department of Exercise Science, Concordia University, Montreal, Canada

Submitted 16 November 2005 ; accepted in final form 18 March 2006


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Synergic action of nitric oxide (NO) and prostaglandins (PG) in the regulation of muscle blood flow during exercise has been demonstrated. In the present study, we investigated whether these vasodilators also regulate local blood flow, flow heterogeneity, and glucose uptake within the exercising skeletal muscle. Skeletal muscle blood flow was measured in seven healthy young men using near-infrared spectroscopy and indocyanine green and muscle glucose uptake using positron emission tomography and 2-fluoro-2-deoxy-D-[18F]glucose without and with local blockade of NO and PG at rest and during one-legged dynamic knee-extension exercise. Local blockade was produced by infusing nitro-L-arginine methyl ester and indomethacin directly in the muscle via a microdialysis catheter. Blood flow and glucose uptake were measured in the region of blockade and in two additional regions of vastus lateralis muscle 1 and 4 cm away from the infusion of blockers. Local blockade during exercise at 25 and 40 watts significantly decreased blood flow in the infusion region and in the region 1 cm away from the site of infusion but not in the region 4 cm away. During exercise, muscle glucose uptake did not show any regional differences in response to blockade. These results show that NO and PG synergistically contribute to the local regulation of blood flow in skeletal muscle independently of muscle glucose uptake in healthy young men. Thus these vasodilators can play a role in regulating microvascular blood flow in localized regions of vastus lateralis muscle but do not influence regional glucose uptake. The findings suggest that local substrate uptake in skeletal muscle can be regulated independently of regional changes in blood flow.

perfusion; distribution; metabolic vasodilators

SUFFICIENT SUPPLY OF OXYGEN through increased blood flow and adequate substrate delivery to skeletal muscle is important to meet metabolic demands both at rest and during muscular contractions. It is well known that blood flow distributes heterogeneously within skeletal muscle (41, 44) like within a variety of other tissues (5, 18, 23, 52). Evidence also exists that heterogeneous blood flow distribution may have a significant impact on oxygen and nutrient delivery in different tissues (14, 15, 23, 29, 31, 42, 43, 60). However, the mechanisms underlying this blood flow heterogeneity and the degree of coupling between metabolism and circulation during muscular activity in humans are still not clearly understood. Is the magnitude of blood flow governing the substrate uptake and utilization, or is substrate uptake closely coupled to metabolic needs within the muscle? The present study was undertaken to investigate this coupling.

A prevailing explanation for blood flow heterogeneity within different tissues is the vascular tree structure (4, 7, 51, 56, 58) whereby microvascular units are not precisely matched to motor units within skeletal muscle. During the last decade, evidence has emerged that a key factor influencing flow distribution, at least in myocardium, is the heterogeneous metabolism within the tissue for which blood flow is matched (1, 12, 19, 32, 57). The link between local metabolism and perfusion likely involves signals that alter the levels of locally produced vasodilators. Previously, Iversen and coworkers studied the role of various physiological stimuli on skeletal muscle perfusion heterogeneity in different animals (24–26, 28). One hypothesis tested was that nitric oxide (NO)-mediated vasodilation could play a role in the regulation of blood flow heterogeneity, but blocking of NO formation with NG-monomethyl-L-arginine (L-NMMA) had no influence (24). Blockade of NO formation alone has a variable, if any, effect on whole muscle blood flow during exercise in humans (17, 46, 50, 53), but, with combined inhibition of NO and PGs (8, 50) or endothelium-derived hyperpolarizing factor (EDHF; see Ref. 21), blood flow is reduced significantly. We therefore hypothesized that NO and PGs could have synergistic roles also in the regulation of local muscle microvascular blood flow during exercise.

To investigate this, we conducted experiments in which blockers of nitric oxide synthase [NOS; nitro-L-arginine methyl ester (L-NAME)] and PGs (indomethacin) were infused directly in the vastus lateralis muscle using the microdialysis method. Regional microvascular blood flow was measured without and with local combined blockade in the infusion region and two other adjacent regions within the same muscle using near-infrared spectroscopy (NIRS) and indocyanine green (ICG) dye as a tracer. Furthermore, to investigate the association between the changes in blood flow and metabolism, glucose uptake was also measured in the same regions as flow with and without combined blockade using positron emission tomography (PET) and fluoro-deoxy-D-[18F]glucose ([18F]FDG).


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Subjects. Seven healthy men participated in this study (age 24 ± 5 yr, height 183 ± 6 cm, weight 76.3 ± 6.9 kg, and body mass index 22.9 ± 2.5 kg/m2). Written informed consent was obtained after the purpose, nature, and potential risks were explained to the subjects. The study protocol was reviewed and approved by the Ethical Committee of Copenhagen and Frederiksberg communities (11–140/03), and the studies were performed in accordance with the Declaration of Helsinki.

Study protocol. The study protocol is shown in Fig. 1. The measurements were performed on three separate days. Blood flow measurements were performed at least 4 h after a light breakfast, whereas glucose uptake measurements were performed in the fasting state. On day 1, muscle blood flow was measured at rest and during dynamic one-leg knee extension with and without NOS and prostaglandin (PG) blockade with L-NAME and indomethacin, respectively. With the use of the Seldinger technique, a catheter was placed in the femoral vein of the right leg for infusion of ICG. Four microdialysis catheters (CMA 60; 20 kDa molecular cut off, 0.5 mm outer diameter; length 30 mm; CMA/Microdialysis) were inserted parallel to the muscle fibers in the vastus lateralis muscle. The microdialysis fibers were positioned at 1-cm depth in the muscle. Three of the fibers were inserted in the right leg (exercising leg) and one in the left leg (resting leg). The fibers in the exercising (right) leg were placed so that the infusion catheter was in the middle and the other two were positioned 1 and 4 cm away from the middle one to the distal and proximal directions, respectively. After placement of the microdialysis probes, the subjects rested for 2 h. During this period, NIRS probes for blood flow measurements were positioned on the surface of the leg directly over the microdialysis catheters (one in the middle and the others 1 and 4 cm away), and the subject was moved to the one-legged knee-extension ergometer (2).


Figure 1
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  		Fig. 1. Study design. Blood flow measurements were performed during day 1 and glucose uptake measurements during days 2 and 3. [18F]FDG, 2-fluoro-2-deoxy-D-[18F]glucose; PET, positron emission tomography; L-NAME, nitro-L-arginine methyl ester.

 
The measurements started with resting blood flow measurements (two times). Thereafter, the subjects began one-legged kicking exercise for 15 min at 25 watts and subsequently at 40 watts. Blood flow was measured during both intensities at 7–8 and 11–13 min after the start of the exercise. After the exercise periods, the subjects recovered for 30 min. After the recovery, infusion of L-NAME and indomethacin in one of the microdialysis catheters (the middle one in the exercise leg) commenced and continued until the end of the study. After 1 h of infusion, blood flow was measured in the resting state, followed by the measurements during the same exercise loads as without inhibitors. The control exercise bout was performed first followed by the blockade trial. This was because of the half-life of the inhibitors, which would have prohibited completion of both trials in the same day if the design had been randomized.

On days 2 and 3, subjects repeated the exercise protocol for measurements of glucose uptake by PET during either control or blockade conditions as on day 1. Two subjects withdrew from the study before the PET studies, and thus five subjects completed this portion of the study. Four microdialysis catheters were again placed in the vastus lateralis muscles exactly as in experimental day 1. Thereafter, the subjects were allowed to rest for 2 h during which two venous catheters were inserted, one in an antecubital forearm vein for injection of [18F]FDG and the other in the opposite antecubital vein for blood sampling of arterialized (heated arm) venous blood. Thereafter, the subjects were moved to the one-legged kicking ergometer, and the infusion of either normal perfusate (day 2) was continued or infusion of both L-NAME and indomethacin (day 3) was started in one of the microdialysis catheters (the middle one in the exercise leg) and continued until the end of the study. After 1 h, the subject began exercise at the intensity of 25 watts. After 10 min of exercise, 385 ± 23 MBq of [18F]FDG in 5 ml of saline were infused, and blood sampling commenced and continued until the end of the study. A total of 23 venous blood samples were taken for determination of blood radioactivity. After the injection of the tracer, the subject continued kicking for another 25 min. Immediately after exercise, the subject was moved into the PET scanner, and the PET scanning was performed.

Local pharmacological inhibition of NO and PGs. Cyclooxygenase inhibition was accomplished by indomethacin (Confortid, Alpharma, Denmark) and NOS inhibition by L-NAME (Clinalfa, Laufelfingen, Switzerland). Indomethacin and L-NAME were mixed with saline so that the mixture contained 100 mg/ml indomethacin and 10 mg/ml L-NAME. This solution was infused (as explained in Study protocol) in the vastus lateralis muscle via the middle microdialysis catheter at the rate of 2 µl/min. The doses used for the infusion were based on previous experiments with whole body (systemic) infusion of the same substances (8) and pilot studies that ensured local blockade of PG release by 100% in the infusion fiber, 90% in the fiber 1 cm away, and 50% in the fiber 4 cm away from the infusion probe (mean values for n = 3).

Blood flow measurements using NIRS. Muscle blood flow was measured using NIRS and ICG as a tracer. The method has been previously validated for blood flow measurements during exercise in humans (9). A 5-mg bolus of ICG was injected rapidly in the femoral vein followed by a 10-ml flush of saline. ICG accumulation in muscle was recorded using 2 NIRO 300 (Hamamatsu Photonics) spectrophotometers with dual-channel laser diodes comprising emitting and receiving optodes placed on the exercising muscle and on the resting contralateral resting leg, and interfaced with an online data acquisition system. The optode separation distance for both muscles was 40 mm, corresponding to a penetration depth of ~20 mm. Light attenuation in muscle was measured at 6 Hz immediately after venous bolus injection of ICG. Changes in tissue ICG concentration were determined by measuring light attenuation at 775-, 813-, 850-, and 913-nm wavelengths, analyzed with an algorithm incorporating the Modified Beer-Lambert Law (9).

The ICG contribution to the absorption signal was isolated using a matrix operation (MATLAB) incorporating path length-specific extinction coefficients for each of the light-absorbing chromophores (Hb/MbO2 and ICG) at each wavelength employed by the NIRS device (Hamamatsu Photonics). Blood flow was calculated from the ratio of muscle ICG accumulation over time (µg/s) measured by NIRS according to the Sapirstein (49) bolus principle. The slope calculated over the initial 10–90% increase in muscle ICG represents the interval over which one-half (50%) of the bolus is received in tissue on the arterial side of the circulation upon which basis this tracer method for calculating blood flow index closely correlates with absolute values for flow requiring an arterial input function over the same time interval. This method has been validated in animals (34) with a correlation of 0.81 against the microsphere technique, and a similar value of r2 = 0.89 has been obtained in humans comparing absolute flow values with blood flow index (Boushel R, Gemmer C, and Kjaer M, unpublished data).

Glucose uptake measurements using PET. For PET scanning, a GE Advance scanner (General Electric Medical Systems, Milwaukee, WI) was used. The scanner has 15 crystal rings forming 35 two-dimensional imaging planes spaced by 4.25 mm. In the emission scans, both legs distal to the midthigh (starting at 2 cm to the proximal direction from the most proximal microdialysis catheter) were scanned in 3 x 4-min time frames from four different scanner positions. After the emission scans, transmission scans for attenuation correction were performed for the four different regions of the leg using germanium-68 pin sources. All data sets were corrected for dead time and random coincidences. The axial and in-plane resolution of the reconstructed images were ~5 mm full width at half-maximum. The data sets were reconstructed using the filtered back-projection method with a Hanning filter.

PET data analysis. Small regions of interest were drawn on the same regions where blood flow was measured on the first day. That is, over the infusion region and 2 and 5 cm away from it. Muscle glucose uptake index was calculated by dividing the tissue radioactivity by blood radioactivity (61). Yokoyama and colleagues (61) recently compared this method with traditional graphical analysis (39) with an excellent linear relationship between absolute glucose uptake from graphical analysis and glucose uptake index from the simplified method (r = 0.968–0.984). The first time frame of the scanning (the first 4 min from the thigh region) was used for the analysis, as was the blood sample from the respective time point. Because the PET imaging was performed immediately after the exercise, the results reflect very well the uptake of [18F]FDG during the exercise period (33). Because the tracer content in the blood is very low (<5% of the injected amount) 25 min after the tracer injection and exercise, the period between the end of exercise and the start of the scan has only a minor influence on the measured values of skeletal muscle glucose uptake.

Statistical analysis. SAS statistical package version 8.2 was used for the statistical analysis. Normal distribution of the blood flow and glucose uptake values was tested using the Shapiro-Wilk test. Blood flow values were normally distributed but glucose uptake values were not; therefore, glucose uptake values were log-transformed before proceeding with further analysis. In the analysis, the effect of exercise intensity on blood flow and glucose uptake was first tested using ANOVA for repeated measures with three factors (region, blockade, and intensity). Thereafter, the effect of two other factors (region and blockade) was tested separately for each intensity (rest, 25 watts, and 40 watts in the blood flow measurements; rest and 25 watts in the glucose uptake measurements). The results are expressed as means ± SD. A P value < 0.05 was considered statistically significant.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Muscle blood flow. Blood flow index increased significantly from rest to exercise at 25 watts (P < 0.001) and further with exercise at 40 watts (P < 0.001), both without and with local inhibition of NO and PGs (Fig. 2). Blood flow index was similar between the three muscle regions without blockade, but inhibition of NO and PGs decreased blood flow in the infusion region, to a lesser extent in the region 1 cm from the infusion site, and not in the region 4 cm from the infusion site compared with control exercise (Fig. 2).


Figure 2
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  		Fig. 2. Skeletal muscle blood flow at rest and during dynamic one-legged knee-extension exercise with 25 and 40 watts without (A) and with (B) combined local blockade of nitric oxide and prostaglandins in 7 healthy men. Local blockade was produced by infusion of L-NAME and indomethacin directly in the muscle by the microdialysis method. White bars, blood flow in the infusion region; gray bars, blood flow in the region of 1 cm away from the infusion site; black bars, blood flow in the region of 4 cm away from the infusion site. *P < 0.05, **P < 0.01, and ***P < 0.001 for the difference between with and without blockade in the same region and same intensity. {dagger}{dagger}P < 0.01 and {dagger}{dagger}{dagger}P < 0.001 for the difference from the region of 4 cm away from the infusion site during the same exercise intensity.

 
Muscle glucose uptake. Muscle glucose uptake was measured on two separate days: first without and then with local inhibition of NO and PGs. Glucose uptake was not significantly different between the regions or study days (Fig. 3). Blockade-induced changes in glucose uptake did not correlate with blockade-induced changes in blood flow in any regions during exercise at 25 watts (r2 = 0.00–0.22).


Figure 3
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  		Fig. 3. Skeletal muscle glucose uptake at rest and during dynamic one-legged knee-extension exercise with 25 watts without (A) and with (B) combined local blockade of nitric oxide and prostaglandins in 5 healthy men.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The main finding in the present study was that combined local inhibition of NO and PG administered directly in the vastus lateralis muscle elicited very localized reductions in microvascular blood flow within the same muscle during exercise, but without affect on muscle glucose uptake, which remained homogenous between the regions. This suggests that the combined effects of NO and PG play a role in the local regulation of muscle microvascular blood flow and raise the possibility that these substances may have a potential influence on blood flow heterogeneity in contracting muscle. However, this localized blunting of blood flow did not exert any primary control over local glucose uptake in human skeletal muscle in healthy young men.

The role of NO in the regulation of local blood flow and blood flow distribution has been studied previously in animals in skeletal (24) and cardiac (13) muscle. Iversen and colleagues (24) studied blood flow distribution in cat skeletal muscle at rest and during exercise both without and with NO inhibition induced by L-NMMA and during reversal by L-arginine. No changes were found in blood flow distribution after blocking NOS or restoration of NO by L-arginine. Correspondingly, Deussen and coworkers (13) showed no effect on myocardial blood flow heterogeneity by infusion of L-NAME in dogs. The present results are consistent with a reduction in muscle blood flow with systemic NOS and PG blockade (8, 50) and with recent findings of an attenuated limb flow with NO and PG inhibitors infused in the forearm. Importantly, this study shows that these similar flow reductions can be seen with PG and NOS inhibition directly within localized regions of a single muscle. Recent human studies show that systemic blockade of NO production has only minimal effect on muscle blood flow during exercise (17, 46, 50, 53), but, with the combined systemic blockade of NO and PGs, muscle blood flow is reduced markedly (8, 50). In one study examining forearm blood flow responses to exercise, NOS and PG blockade had independent and combined effects (50). The present findings suggest that similar redundant mechanisms are active in the regulation of muscle blood flow and its distribution at the regional microvascular level.

Because blood flow was reduced only by ~30% with the blockade, a significant part of the exercise-induced muscle blood flow was still maintained. Thus other factors enabled blood flow to increase during exercise. One potential factor is EDHF, which is considered to play a role in muscle vasodilatation (22). Recently, it has been shown that combined blockade of NO and cytochrome P-450 (needed for formation of EDHF) also significantly reduces muscle blood flow during exercise (21). Furthermore, the finding that production of EDHF is attenuated by increased NO (6) may imply that EDHF is elevated under NOS inhibition. Another potential vasodilator is adenosine, which has been shown to account alone for a significant part (~20%) of the exercise-induced increase in muscle blood flow (45). However, although the vasodilatory response to adenosine is attenuated with NOS blockade (54), no compensatory increases in interstitial adenosine have been shown during exercise with L-NAME (16). Another potential vasodilator is bradykinin, since it has recently been demonstrated that bradykinin release in the muscle interstitium increases during exercise and is correlated with exercise-induced increases in muscle blood flow (35). The potential role of these and other vasodilators, such as ATP in the local regulation of skeletal muscle blood flow and flow heterogeneity, remain to be investigated further.

The second main finding in the present study was that blocking of NO and PGs did not have any effect on muscle glucose uptake distribution between the three regions. There is no clear consensus on whether or not NO has a role in the regulation of skeletal muscle glucose uptake during exercise. Although insulin-stimulated glucose uptake seems to be NO dependent (48, 59), studies regarding exercise- or muscle contraction-stimulated glucose uptake are controversial, showing it to be both NO independent (20) and NO dependent (10, 55). The studies on the effects of PGs on skeletal muscle glucose uptake have also shown partly diverging results. As with NO, most of the studies suggest that PGs have an influence on insulin-stimulated glucose uptake (36–38), but the effects of PGs on exercise-stimulated glucose uptake are unknown. Some experimental evidence, although not directly measured during exercise, suggests that PGs would not be important for exercise-stimulated glucose uptake (62). Thus, although PGs and NO may play some role in muscle glucose uptake regulation in the whole muscle in humans, especially during insulin stimulation, they appear not to be involved in the local regulation of muscle glucose uptake during acute exercise. However, no other data exist concerning the effects of combined inhibition on muscle glucose uptake. It cannot be excluded that an alternative explanation to our findings is that the demonstrated reduction in muscle blood flow with blockade led to a reduction in tissue oxygen content, and this in itself caused a compensatory increase in glucose uptake in the area of reduced flow. We do not have data to fully explore that question, but if it is the case, it still indicates that the magnitude of glucose uptake at a certain work output is maintained despite manipulation of blood flow.

The measurements of muscle blood flow and glucose uptake were performed on separate days and under different dietary conditions. This may limit the conclusions of the present study. However, the study days were so long that it was not reasonable to perform all the measurements during the same day. Also, because of long radioactive decay {[18F]FDG (half-life 109.2 min)}, glucose uptake measurements without and with blockade could not be performed within 1 day. On the other hand, the metabolic conditions would have perhaps differed at least as much, if not even more, if all the measurements would have been performed within 1 day. In any case, we do not believe that these differences had any influence on the main findings of the present study (lack of local effects on glucose uptake with blockade of NO and PGs).

An important question in exercise physiology is how well muscle metabolism and blood flow are matched. It has recently been suggested that regional heterogeneous metabolism could explain heterogeneous blood flow (1, 12, 19, 32, 57). In the present study, we found evidence against this hypothesis in skeletal muscle, since local blood flow changed independently of local glucose uptake. Iversen and Nicolaysen (27) studied the relationship between local blood flow and glucose uptake in rabbit skeletal muscle and found a poor correlation between local blood flow and glucose uptake at rest and only a moderate relation during exercise. Similarly, Peltoniemi and colleagues (40) showed a poor correlation between local blood flow and glucose uptake in the resting human skeletal muscle and somewhat better correlation (r = ~0.5) during exercise. Thus, although both flow and glucose uptake in skeletal muscle increase almost linearly with increased exercise workload, the coupling is not precise at the microcirculatory level, which is consistent with the concept that motor units and microvascular units are not precisely matched. However, glucose uptake from blood is not the only source of energy during exercise, and this can explain the relatively poor association between glucose uptake and blood flow in skeletal muscle, even during exercise. Oxygen, instead, is the most important substance for muscle metabolic demand during dynamic exercise. The supply of oxygen is highly dependent on blood flow; thus, oxygen uptake and blood flow might to some extent be expected to correlate with each other. Surprisingly, the results from several studies using different methods in humans have shown that there is a large variation in matching local blood flow to oxygen uptake within a skeletal muscle (3, 11, 30, 47). Furthermore, when muscle blood flow is manipulated, for example, by systemic blockade of NO and PG during exercise (blood flow decreases; see Ref. 8) or by infusion of adenosine at rest (blood flow increases; see Ref. 45) oxygen uptake is preserved by adjusting the arteriovenous oxygen extraction. Thus the coupling between metabolism and blood flow is a controversial issue. In part, the degree of matching of flow and metabolism depends on whether the relationship is examined at the level of the whole limb or at the regional microunit level of tissue. Whereas most studies have examined the first question, very few studies have focused on the latter level of organization that remains unresolved.

In the present study, glucose uptake was calculated using the ratio between tissue and blood activity (glucose uptake index). It has been shown that this index has an almost perfect linear relationship to the quantitative glucose uptake values obtained using graphical analysis, which is the golden standard for analyzing glucose uptake from [18F]FDG-PET images (61). Thus, although it is semiquantitative in nature, the glucose uptake index reflects linearly the absolute changes in tissue glucose uptake. Glucose uptake was only measured with the lowest exercise intensity for two reasons. First, because a radioactive tracer was used, we limited the total dose to below the recommended levels of radioactivity safety. Second, the exercise period needed to be long enough that most of the tracer was taken up during exercise while on the other hand at a low intensity so that the subjects did not exhaust during the exercise. For these reasons, measurements of blood flow and glucose uptake were performed at slightly different time points. Although blood flow was measured at 7 and 11 min after the start of the exercise, glucose uptake was measured during the 10–35 min of exercise. However, because the exercise was performed only locally, this difference likely did not have any significant effect on the results. The independent effects of NO and PGs were not studied because that would also have meant more injections of the radioactive tracer.

In conclusion, the results of the present study provide evidence that synergic action of NO and PGs contributes to the regulation of local blood flow and may account for an effect of flow heterogeneity within the exercising skeletal muscle in healthy young men. Furthermore, the results support the findings that the magnitude of local blood flow can be altered without any effect on the regional magnitude of muscle substrate uptake during exercise.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
Academy of Finland (Grants 206970 and 204240), Ministry of Education in Finland (Grants 143/722/2002, 51/722/2003, and 40/627/2005), Juho Vainio Foundation (Finland), the Danish National Research Foundation (Denmark), Danish Medical Research Council, Lundbeck Foundation (Denmark), Fonds de la recherche en Sante Quebec, and the Natural Science and Engineering Research Council of Canada have given financial support for this work.


   ACKNOWLEDGMENTS
 
We thank the personnel at the Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, and at the Department of Clinical Physiology, Nuclear Medicine, and Positron Emission Tomography, Rigshospitalet, Copenhagen, Denmark, for help during the study.


   FOOTNOTES
 

Address for reprint requests and other correspondence: K. Kalliokoski, Turku PET Centre, Univ. of Turku, Kiinamyllynkatu 4–8, FIN-20520 Turku, Finland (e-mail: kari.kalliokoski{at}tyks.fi)

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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