Intracellular alkalinization induces Ca 2+ influx via non-voltage-operated Ca 2+ channels in rat aortic smooth muscle cells

Abstract

In smooth muscle, the cytosolic Ca 2+ concentration ([Ca 2+] i) is the primary determinant of contraction, and the intracellular pH (pH i) modulates contractility. Using fura-2 and 2′,7′-biscarboxyethyl-5(6) carboxyfluorescein (BCECF) fluorometry and rat aortic smooth muscle cells in primary culture, we investigated the effect of the increase in pH i on [Ca 2+] i. The application of the NH 4Cl induced concentration-dependent increases in both pH i and [Ca 2+] i. The extent of [Ca 2+] i elevation induced by 20 mM NH 4Cl was approximately 50% of that obtained with 100 mM K +-depolarization. The NH 4Cl-induced elevation of [Ca 2+] i was completely abolished by the removal of extracellular Ca 2+ or the addition of extracellular Ni 2+. The 100 mM K +-induced [Ca 2+] i elevation was markedly inhibited by a voltage-operated Ca 2+ channel blocker, diltiazem, and partly inhibited by a non-voltage-operated Ca 2+ channel blocker, SKF96365. On the other hand, the NH 4Cl-induced [Ca 2+] i elevation was resistant to diltiazem, but was markedly inhibited by SKF96365. It is thus concluded that intracellular alkalinization activates the Ca 2+ influx via non-voltage-operated Ca 2+ channels and thereby increases [Ca 2+] i in the vascular smooth muscle cells. The alkalinization-induced Ca 2+ influx may therefore contribute to the enhancement of contraction.