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

We examined the hypothesis that ONOO-, a product of the interaction between superoxide (O-2(.-)) and nitric oxide (NO), inhibits calcium-activated K+ (Ka(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 rnicrovessels was determined by immunochistochemistry 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 (NPo) of K-Ca channel by 50 +/- 12%. O-2(.-) generated by XA+XO had no effect on BK-induced dilation and NPo 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(.-).