Endothelial release of nitric oxide contributes to the vasodilator effect of adenosine in humans

Background: The endogenous nucleoside adenosine plays an important role in the regulation of vascular tone, especially during ischemia. Experimental data derived from animal models suggest that nitric oxide (NO) contributes to the vasodilator effect of adenosine. The primary purpose of this investigation was to determine whether the endothelial release of NO contributes to adenosine-induced vasodilation in humans. Methods and Results: Venous occlusion plethysmography was used to assess the forearm blood flow (FBF) responses to graded intra-arterial infusions of adenosine (1.5 to 500 μg/min). Dose-response curves were constructed before and during intra- arterial infusion of the NO synthase inhibitor N(G)-monomethyl-L-arginine (L- NMMA) (2 mg/min, n=6) or vehicle (n=6). Before infusion of L-NMMA, adenosine caused a dose-dependent increase in FBF from 2.3 to 15.9 mL · min^-1 · dL^-1. During concurrent infusion of L-NMMA, adenosine increased FBF from 1.7 to 10.0 mL · min^-1 · dL^-1, and this change from baseline was significantly reduced compared with that before L-NMMA (P<.05). L-NMMA also attenuated the FBF response to adenosine when the basal constrictor effect of L-NMMA was prevented by coinfusion of the NO donor sodium nitroprusside (n=6, P<.01). In contrast, L-NMMA did not affect the FBF response to intra-arterial infusion of the endothelium-independent vasodilator verapamil (from 2.0 to 13.9 mL · min^-1 · dL^-1 before L-NMMA and from 1.3 to 13.6 mL · min^{- 1} · dL^-1 during L-NMMA; n=6, P=NS). The second objective of this study was to determine whether the adenosine-induced release of NO is mediated by activation of endothelial potassium channels, putatively coupled to adenosine receptors. Thus, the FBF response to adenosine was measured before and during infusion of the ATP-dependent potassium channel blocker tolbutamide (1 mg/min, n=6), or the potassium channel blocker quinidine (0.5 mg/min, n=6). The adenosine-mediated increments in FBF were not attenuated by either potassium channel blocker. Conclusions: Adenosine-induced vasodilation in humans is mediated, at least in part, by endothelial release of NO. The transducing mechanism of this phenomenon is not known, but it does not appear to involve the activation of either ATP-dependent or quinidine-sensitive potassium channels.