Peroxynitrite inhibits Ca2+-activated K+ channel activity in smooth muscle of human coronary arterioles

Abstract

We examined the hypothesis that ONOO-, a product of the interaction between superoxide (O2.-) and nitric oxide (NO), inhibits calcium-activated K+ (KCa) channel activity in vascular smooth muscle cells (VSMCs) of human coronary arterioles (HCAs), thereby reducing hyperpolarization-mediated vasodilation. HCAs were dissected from right atrial appendages. The interaction of ONOO- with microvessels was determined by immunohistochemistry using a nitrotyrosine antibody. Strong staining was observed in arteries exposed to authentic ONOO- or to sodium nitroprusside (SNP)+xanthine (XA)+xanthine oxidase (XO). Dilation to 10-8 mol/L bradykinin (BK) was abolished in vessels exposed to ONOO- (-2.5±8%; P<0.05) but not DC-ONOO- (65±8%). Reduced dilation to BK was also observed after application of XO and SNP. Dilation to NS1619 (KCa channel opener) was reduced in endothelial denuded arterioles treated with ONOO-. In isolated VSMCs, whole-cell peak K+ current density was reduced by ONOO- (control 65±15 pA/pF; ONOO- 42±9 pA/pF; P<0.05). Iberiotoxin had no further effect on whole-cell K+ current. In inside-out patches, ONOO- but not DC-ONOO- decreased open state probability (NPo) of KCa channel by 50±12%. O2.- generated by XA+XO had no effect on BK-induced dilation and NPo of KCa channels. These results suggest that ONOO-, but not O2.-, inhibits KCa channel activity in VSMCs possibly by a direct effect. This mechanism may contribute to impaired EDHF-mediated dilation in conditions such as ischemia/reperfusion where increased activity of NO synthase occurs in the presence of excess of O2.-.