Effects of Oxygen Tension on Flow-Induced Vasodilation in Porcine Coronary Resistance Arterioles

Abstract

Vascular tone has been shown to be importantly influenced by flow-induced release of endothelium-derived vasodilators. The purpose of the present study was to test the hypothesis that in porcine coronary resistance-size arterioles, flow-induced vasodilation is sensitive to oxygen tension. Arterioles (55–150 μm) were studiedin vitrounder conditions of constant intraluminal pressure to dynamically measure arteriolar diameter in response to changes in flow or, alternatively, in response to bradykinin under three conditions: hyperoxia (pO2400 mm Hg), normoxia (pO2160 mm Hg), and hypoxia (pO240 mm Hg). Under conditions of constant pressure and no flow, hypoxia alone resulted in vasodilation that was blocked by the nitric oxide synthase inhibitor Ω-nitro-l-arginine methyl ester (L-NAME). Hypoxia did not alter the vasodilator response to bradykinin when compared to the vasodilator response to bradykinin during normoxia. During hyperoxia, flow-induced vasodilation was significantly reduced by either indomethacin, or L-NAME. Indomethacin and L-NAME combined completely abolished flow-induced vasodilation under conditions of hyperoxia. Under conditions of normoxia and hypoxia, indomethacin or L-NAME alone only partially blocked flow-induced vasodilation. No further inhibition was observed when indomethacin and L-NAME were combined. Glybenclamide failed to alter flow-induced vasodilation either alone or in combination with indomethacin and L-NAME. The results suggest that the mechanisms responsible for flow-induced vasodilation in coronary arterioles are complex and are different depending upon the oxygen tension. During hyperoxia, vasodilation is due to the combined actions of prostanoids and nitric oxide, while under conditions of normoxia and hypoxia, flow-induced vasodilation is the result of not only prostanoids and nitric oxide, but of another as of yet unidentified oxygen-sensitive endogenous vasodilator.