In vivo location and mechanism of EDHF-mediated vasodilation in canine coronary microcirculation

Responses of epicardial coronary arteriolos to ACh were measured using stro boscopic fluorescence microangiography in dogs (n = 38). ACh (0.1 and 0.5 p g mu g.kg(-1).min(-1) ic) dilated small (<100 mu m, 11 +/- 2 and 19 +/- 2%, respectively) and large (>100 mu m, 6 +/- 3 and 13 +/- 3%, respectively) a rterioles at baseline. Combined administration of N-omega-monomethyl-L-argi nine (L-NMMA; 1.0 mu mol/min icf and indomethacin (10 mg/kg iv) eliminated ACh-induced dilation in large coronary arterioles but only partially attenu ated that in small arterioles. Suffusion of a buffer containing 60 mM KCI t high KCI) completely abolished cromakalim-induced dilation in arterioles an d in combination with L-NMMA plus indomethacin completely blocked ACh-induc ed dilation in small arterioles, This indicated that the vasodilation to AC h that persists in small arterioles after administration of L-NMMA and indo methacin is mediated via a hyperpolarizing factor. The ACh-induced vasodila tion remaining after L-NMMA and indomethacin was completely blocked by the large-conductance potassium-channel antagonist iberiotoxin or by epicardial suffusion of miconazole or metyrapone, inhibitors of cytochrome P-450 enzy mes. These observations are consistent with the view that endothelium-deriv ed hyperpolarizing factor (EDHF) is a product of cytochrome P-450 enzymes a nd produces vasodilation by the opening of large-conductance potassium chan nels. We conclude that ACh-induced dilation in large coronary arterioles is mediated mainly by nitric oxide (NO), whereas, in small arterioles both NO and EDHF mediate dilation to ACh. These data provide the first direct evid ence for an in vivo role of EDHF in small coronary arterioles.