Effect of acidosis on Ca2+-activated K+ channels in cultured porcine coronary artery smooth muscle cells

Although acidosis induces vasodilation, the vascular responses mediated by large-conductance Ca2+-activated K+ (K-Ca) channels have not been investigated in coronary artery smooth muscle cells. We therefore investigated the response of porcine coronary arteries and smooth muscle cells to acidosis, as well as the role of K-Ca channels in the regulation of muscular tone. Acidosis (pH 7.3-6.8), produced by adding HCl to the extravascular solution, elicited concentration-dependent relaxation of precontracted, endothelium-denuded arterial rings. Glibenclamide (20 mu M) significantly inhibited the vasodilatory response to acidosis (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 low-pH solution, K-Ca channel activity in cell-attached patches increased. However, pretreatment of these cells with 10 or 30 mu M O, O'-bis(2-aminophenyl)ethyleneglycol-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl)ester (BAPTA-AM), a Ca2+ chelator for which the cell membrane is permeable, 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 ethyleneglycolbis(beta-aminoethylester)-N,N,N',N'-tetraacetic acid (EGTA) at pH 7.3], K-Ca channels were activated by H+ concentration dependently. However, when these patches were exposed to a [Ca2+] of 10(-6) M adjusted with BAPTA at pH 7.3, H+ inhibited K-Ca channel 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 acidosis. These results indicate that the modulation of K-Ca channel kinetics by acidosis plays an important role in the determination of membrane potential and, hence, coronary arterial tone.