Intracellular alkalinization induces Ca2+ influx via non-voltage-operated Ca2+ channels in rat aortic smooth muscle cells

In smooth muscle, the cytosolic Ca2+ concentration (Ca2+](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 Ca2+](i). The application of the NH(4)Cl induced concentration-dependent increases in both pH(i) and Ca2+](i). The extent of Ca2+](i) elevation induced by 20mM NH(4)Cl was approximately 50% of that obtained with 100mM K(+)-depolarization. The NH(4)Cl-induced elevation of Ca2+](i) was completely abolished by the removal of extracellular Ca2+ or the addition of extracellular Ni2+. The 100mM K(+)-induced Ca2+](i) elevation was markedly inhibited by a voltage-operated Ca2+ channel blocker, diltiazem, and partly inhibited by a non-voltage-operated Ca2+ channel blocker, SKF96365. On the other hand, the NH(4)Cl-induced Ca2+](i) elevation was resistant to diltiazem, but was markedly inhibited by SKF96365. It is thus concluded that intracellular alkalinization activates the Ca2+ influx via non-voltage-operated Ca2+ channels and thereby increases Ca2+](i) in the vascular smooth muscle cells. The alkalinization-induced Ca2+ influx may therefore contribute to the enhancement of contraction.