NITROVASODILATORS RELAX MESENTERIC MICROVESSELS BY CGMP-INDUCED STIMULATION OF CA-ACTIVATED K-CHANNELS

Nitric oxide (NO) released from endothelial cells or exogenous nitrate s is a potent dilator of arterial smooth muscle; however, the molecula r mechanisms mediating relaxation to NO in the microcirculation have n ot been characterized. The present study investigated the relaxant eff ect of nitrovasodilators on microvessels obtained from the rat mesente ry and also employed whole cell and single-channel patch-clamp techniq ues to identify the molecular target of NO action in myocytes from the se vessels. Bath sodium nitroprusside (SNP) and S-nitroso-N-acetylpeni cillamine (SNAP) relaxed phenylephrine-induced contractions by similar to 80% but were significantly less effective in relaxing contractions induced by 40 mM KCl. Relaxation to SNP was also inhibited by the K+- channel blocker tetraethylammonium or by inhibition of the activity of the guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kin ase (PKG). These results suggest that SNP stimulated K+ efflux by open ing K+ channels via PKG-mediated phosphorylation. Perforated-patch exp eriments revealed that both SNP and SNAP increased outward currents in microvascular myocytes, and single-channel studies identified the hig h-conductance Ca2+- and voltage-activated K+ (BKCa) channel as the tar get of nitrovasodilator action. The effect of nitrovasodilators on BKC a channels were mimicked by cGMP and inhibited by blocking the activit y of PKG. We conclude that stimulation of BKCa-channel activity via cG MP-dependent phosphorylation contributes to the vasodilatory effect of NO on microvessels and that a direct effect of NO on BKCa channels do es not play a major role in this process. We propose that this mechani sm is important for the therapeutic effect of nitrovasodilators on per ipheral resistance and arterial blood pressure.