Peroxynitrite Inhibits Ca 2+ -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 (O 2 ·− ) and nitric oxide (NO), inhibits calcium-activated K + (K Ca ) 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 (K Ca 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 (NP o ) of K Ca channel by 50±12%. O 2 ·− generated by XA+XO had no effect on BK-induced dilation and NP o of K Ca channels. These results suggest that ONOO − , but not O 2 ·− , inhibits K Ca 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 O 2 ·− .