EFFECT OF ACIDOSIS ON CA2-ACTIVATED K+ CHANNELS IN CULTURED PORCINE CORONARY-ARTERY SMOOTH-MUSCLE CELLS()

Although acidosis induces vasodilation, the vascular responses mediate d by large-conductance Ca2+-activated K+ (K-Ca) channels have not been investigated in coronary artery smooth muscle cells. We therefore inv estigated the response of porcine coronary arteries and smooth muscle cells to acidosis, as well as the role of K-Ca channels in the regulat ion of muscular tone. Acidosis (pH 7.3-6.8), produced by adding HCl to the extravascular solution, elicited concentration-dependent relaxati on of precontracted, endothelium-denuded arterial rings. Glibenclamide (20 mu M) significantly inhibited the vasodilatory response to acidos is (pH 7.3-6.8). Charybdotoxin (100 nM) was effective only at pH 6.9-6 .8. When we exposed porcine coronary artery smooth muscle cells to a l ow-pH solution, K-Ca channel activity in cell-attached patches increas ed. However, pretreatment of these cells with 10 or 30 mu M O, 2-amino phenyl)ethyleneglycol-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethy l)ester (BAPTA-AM), a Ca2+ chelator for which the cell membrane is per meable, abolished the H+-mediated activation of K-Ca channels in cell- attached patches. Under these circumstances H+ actually inhibited K-Ca channel activity. When inside-out patches were exposed to a [Ca2+] of 10(-6) M [adjusted with colbis(beta-aminoethylester)-N,N,N',N'-tetraa cetic acid (EGTA) at pH 7.3], K-Ca channels were activated by H+ conce ntration dependently. However, when these patches were exposed to a [C a2+] of 10(-6) M adjusted with BAPTA at pH 7.3, H+ inhibited K-Ca chan nel activity. Extracellular acidosis had no significant direct effect on K-Ca channels, suggesting that extracellular H+ exerts its effects after transport into the cell, and that K-Ca channels are regulated by intracellular H+ and by cytosolic free Ca2+ modulated by acute acidos is. These results indicate that the modulation of K-Ca channel kinetic s by acidosis plays an important role in the determination of membrane potential and, hence, coronary arterial tone.