EDRF AND NOREPINEPHRINE-INDUCED VASODILATION IN THE CANINE CORONARY CIRCULATION

Experiments were performed to characterize the role of endothelium-derived relaxing factor (EDRF) in coronary vasodilation caused by norepinephrine. The circumflex or left anterior descending coronary artery was cannulated and pump perfused with constant pressure or constant flow in closed-chest anesthetized dogs. Prostaglandin synthesis was blocked with ibuprofen. During constant-pressure perfusion, EDRF inhibition with intracoronary N-omega-nitro-L-arginine (L-NNA) did not affect the vasodilation due to nitroglycerin (an endothelium-independent process). However, L-NNA did significantly inhibit (P < 0.001) the vasodilation due to acetylcholine (an endothelium-dependent process). In response to bolus injections of norepinephrine, EDRF inhibition with L-NNA significantly reduced coronary vasodilation (P < 0.001). This inhibition was partially reversed with L-arginine infusion. To determine whether an increase in shear stress due to an increase in flow was the stimulus for EDRF release, experiments were performed during constant-flow conditions. Vasodilation caused by nitroglycerin was not inhibited by L-NNA, but EDRF inhibition did blunt acetylcholine-induced vasodilation significantly (P < 0.001) during constant-flow perfusion. During EDRF inhibition with L-NNA, vasodilation due to norepinephrine was not significantly altered when coronary flow was held constant (P = 0.19). In conclusion, EDRF plays a role in norepinephrine-induced coronary vasodilation that is largely flow dependent.