In vivo evidence against a role for adenosine in the exercise pressor reflex in humans.

The pressor response to exercise is of great importance in both physiology and pathophysiology. Whether endogenous adenosine is a trigger for this reflex remains controversial. Muscle interstitial adenosine concentration can be determined by microdialysis. However, there are indications that local muscle cell damage by the microdialysis probe confounds these measurements in exercising muscle. Therefore, we used the nucleoside uptake inhibitor dipyridamole as pharmacological tool to bypass this confounding. We used microdialysis probes to measure endogenous adenosine in forearm skeletal muscle of healthy volunteers during two cycles of 15 min of intermittent isometric handgripping. During the second contraction, dipyridamole (12 microg.min(-1).dl forearm(-1)) was administered into the brachial artery. Dipyridamole potentiated the exercise-induced increase in dialysate adenosine from 0.30 +/- 0.08 to 0.48 +/- 0.10 micromol/l (n = 9, P < 0.05), but it did not potentiate the exercise-induced increase in blood pressure. A time-control study without dipyridamole revealed no difference in exercise-induced increase in adenosine between both contractions (n = 8). To exclude the possibility that the dipyridamole-induced increase in dialysate adenosine originates from extravasation of increased circulating adenosine, we simultaneously measured adenosine with microdialysis probes in forearm muscle and antecubital vein. In a separate group of nine volunteers, simultaneous intrabrachial infusion of 100 microg.min(-1).dl(-1) dipyridamole and 5 microg.min(-1).dl(-1) adenosine increased dialysate adenosine from the intravenous but not the interstitial probe, indicating preserved endothelial barrier function for adenosine. We conclude that dipyridamole significantly inhibits uptake of interstitial adenosine without affecting the pressor response to exercise, suggesting that interstitial adenosine is not involved in the pressor response to rhythmic isometric exercise.

Glyburide inhibits dipyridamole-induced forearm vasodilation but not adenosine-induced forearm vasodilation.

BACKGROUND: The mechanism of the vasodilator response to adenosine has not been elucidated in humans. Stimulation of adenosine receptors on endothelial and vascular smooth muscle cells with subsequent endothelial release of nitric oxide and opening of adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels has been suggested. AIM: The aim of this study was to investigate the involvement of K(ATP) channels in the vasodilator response to adenosine and the nucleoside transport inhibitor dipyridamole. Methods and results In healthy male volunteers, adenosine (0.6, 1.9, 5.6, 19, 57, and 190 nmol. min(-1). dL(-1)) was infused into the brachial artery, and forearm blood flow (FBF) was measured by use of strain-gauge plethysmography. Adenosine increased the FBF ratio (FBF in experimental arm/FBF in control arm) from 1.3 +/- 0.2 to 1.2 +/- 0.2, 1.5 +/- 0.2, 2.8 +/- 0.4, 7.3 +/- 2.3, 11.1 +/- 4.1, and 12.9 +/- 3.7 for the six increasing adenosine doses, respectively. Simultaneous infusion of glyburide (INN, glibenclamide), a blocker of K(ATP) channels, did not affect this response (from 1.7 +/- 0.4 to 1.5 +/- 0.2, 2.2 +/- 0.3, 4.0 +/- 1.0, 9.3 +/- 4.0, 13.5 +/- 6.4, and 15.9 +/- 5.3 for the 6 increasing doses of adenosine, respectively; P =.439, n = 6). The increase in FBF ratio during infusion of the nucleoside transport inhibitor dipyridamole (20, 60, and 200 nmol. min(-1). dL(-1)) was significantly reduced by glyburide, as follows: from 1.2 +/- 0.1 to 1.7 +/- 0.2, 2.4 +/- 0.5, and 2.9 +/- 0.4, respectively, during saline solution and from 1.6 +/- 0.2 to 1.8 +/- 0.2, 2.1 +/- 0.3, and 2.2 +/- 0.4, respectively, during glyburide (P =.010 for effect of glyburide on response from baseline, ANOVA for repeated measures; n = 8). The vasodilator response to dipyridamole was significantly inhibited by the adenosine receptor antagonist theophylline. CONCLUSION: Opening of vascular K(ATP) channels is involved in the forearm vasodilator response to dipyridamole but not to adenosine. Differences in stimulated cell type (endothelium for adenosine versus smooth muscle cells for dipyridamole) may underlie this divergent pharmacologic profile.

The influence of diazepam and midazolam on adenosine-induced forearm vasodilation in humans.

Adenosine is an endogenous purine with vasodilating and cardioprotective properties. Animal experiments have shown that some benzodiazepine-induced effects can be explained by potentiation of adenosine effects, via inhibition of the nucleoside transport system. The objective of this study was to determine whether the frequently used benzodiazepines diazepam and midazolam increase adenosine-induced vasodilation in the human forearm vascular bed, measured by venous occlusion plethysmography. Adenosine (0.6, 6, 20, and 60 nmol/min/dl ForeArm Volume) was infused into the brachial artery with and without concomitant separate infusion of diazepam (21 nmol/min/dl, n = 9) and midazolam (23 nmol/min/dl, n = 8). Plasma concentrations of diazepam resp. midazolam at the end of the infusion protocol averaged 0.5 +/- 0.2 microg/ml plasma (1.6 microM) for diazepam versus 1.2 +/- 0.4 microg/ml plasma (3 microM) for midazolam. Intra-arterial infusion of the benzodiazepines did not alter baseline vascular tone, and had no significant influence on the forearm vasodilator response to adenosine. The adenosine-induced relative change in Forearm Vascular Resistance (FVR) was -3 +/- 7, -48 +/- 8, -75 +/- 6, and -85 +/- 3% in the absence and 3.5 +/- 11, -54 +/- 5, -74 +/- 5, and -82 +/- 3% resp. in the presence of diazepam (P > 0.1, repeated measures ANOVA, n = 9). Likewise, in the absence resp. presence of midazolam, FVR fell by 1 +/- 6, 55 +/- 5, 74 +/- 3, and 84 +/- 2% resp. 11 +/- 11, 59 +/- 2, 80 +/- 3, and 87 +/- 2% (P > 0.1, n = 7). Intra-brachial infusion of diazepam and midazolam resulting in forearm concentrations in the high therapeutic range does not augment adenosine-induced forearm vasodilation. A possible interaction at supra-therapeutic levels of the benzodiazepines can not be excluded from the present study, but lacks clinical significance.

Diadenosine pentaphosphate vasodilates the forearm vascular bed: inhibition by theophylline and augmentation by dipyridamole.

BACKGROUND: In rats, diadenosine pentaphosphate (AP(5)A) has been implicated in the pathogenesis of essential hypertension. This study describes for the first time the vasomotor action of AP(5)A in humans by means of the "perfused forearm technique." RESULTS: AP(5)A evoked a dose-dependent forearm vasodilator response equal to that of adenosine but less than that of adenosine triphosphate (ATP) at equimolar doses. The P(1)-purinoceptor antagonist theophylline (0.28 micromol/min per deciliter) reduced the percentage decrease in forearm vascular resistance (FVR) to AP(5)A (0.6, 6, and 20 nmol/min/dL): -8% +/- 6%, -50% +/- 6%, and -68% +/- 4% during saline solution versus -7% +/- 4%, -33% +/- 5%, and -45% +/- 6% during theophylline (mean +/- standard error [SE]; ANOVA for repeated measures; P <.05 for the interaction between purine dose and theophylline; n = 10). An inhibitor of equilibrative nucleoside transport, dipyridamole (7.4 nmol/min per deciliter), augmented the AP(5)A (0.6 and 6 nmol/min per deciliter)-induced reduction in FVR as follows: -34% +/- 6% and -67% +/- 5% during saline versus -49% +/- 5% and -80% +/- 3% during dipyridamole (P <.05 for the effect of dipyridamole; n = 6). The bivalent cation chelator ethylenediaminetetra-acetic acid (EDTA) inhibited the rapid degradation of AP(5)A in vitro. In vivo, the highest tolerated intra-arterial EDTA dose was not sufficient to inhibit AP(5)A metabolism. CONCLUSION: Intra-arterial AP(5)A caused a dose-dependent reduction in FVR. This is, at least in part, mediated by its degradation product adenosine. The data do not support an in vivo vasoconstrictor action of AP(5)A, and as such AP(5)A does not seem likely to contribute to the pathogenesis of primary hypertension in humans.

No vasodilator response to low-dose heparin in the forearm skeletal-muscle vascular bed.

OBJECTIVE: Recent reports show that heparin induces vasodilation. This heparin-induced vasodilation might interfere with the results of vascular studies using the perfused-forearm technique because the arterial catheter is often flushed with a heparin solution in those experiments. Therefore, we investigated the vascular effects of flushing an arterial catheter with a heparin solution compared with a placebo solution (NaCl 0.9%) in six healthy volunteers. METHODS: During the first half of each experiment, the arterial catheter was flushed several times with the placebo solution. In the second half, identical flushes were performed with a solution of heparin diluted in NaCl 0.9% (4U x ml(-1)). RESULTS: Forearm blood flow (venous occlusion plethysmography) response was similar with heparin and placebo after all flushes. CONCLUSION: We conclude that low-dose heparin in the solution to flush an arterial catheter has no short-term vasodilator effect in the human forearm vascular bed. Therefore, the results of studies using the perfused-forearm technique are not confounded by flushes with a low-dose heparin solution.