Nitric oxide exerts feedback inhibition on EDHF-induced coronary arteriolar dilation in vivo

Abstract

We tested the hypothesis that nitric oxide (NO) inhibits endothelium-derived hyperpolarizing factor (EDHF)-induced vasodilation via a negative feedback pathway in the coronary microcirculation. Coronary microvascular diameters were measured using stroboscopic fluorescence microangiography. Bradykinin (BK)-induced dilation was mediated by EDHF, when NO and prostaglandin syntheses were inhibited, or by NO when EDHF and prostaglandin syntheses were blocked. Specifically, BK (20, 50, and 100 ng · kg−1 · min−1 ic) caused dose-dependent vasodilation similarly before and after administration of N G-monomethyl-l-arginine (l-NMMA) (3 μmol/min ic for 10 min) and indomethacin (Indo, 10 mg/kg iv). The residual dilation to BK withl-NMMA and Indo was completely abolished by suffusion of miconazole or an isosmotic buffer containing high KCl (60 mM), suggesting that this arteriolar vasodilation is mediated by the cytochrome P-450 derivative EDHF. BK-induced dilation was reduced by 39% after inhibition of EDHF and prostaglandin synthesis, and dilation was further inhibited by combined blockade withl-NMMA to a 74% reduction in the response. This suggests an involvement for NO in the vasodilation. After dilation to BK was assessed with l-NMMA and Indo, sodium nitroprusside (SNP, 1–3 μg · kg−1 · min−1ic), an exogenous NO donor, was administered in a dose to increase the diameter to the original control value. Dilation to BK was virtually abolished when administered concomitantly with SNP duringl-NMMA and Indo ( P < 0.01 vs. before SNP), suggesting that NO inhibits EDHF-induced dilation. SNP did not affect adenosine- or papaverine-induced arteriolar dilation in the presence ofl-NMMA and Indo, demonstrating that the effect of SNP was not nonspecific. In conclusion, our data are the first in vivo evidence to suggest that NO inhibits the production and/or action of EDHF in the coronary microcirculation.