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 NH₄Cl induced concentration-dependent increases in both pH_i and [Ca²⁺]_i. The extent of [Ca²⁺]_i elevation induced by 20 mM NH₄Cl was approximately 50% of that obtained with 100 mM K⁺-depolarization. The NH₄Cl-induced elevation of [Ca²⁺]_i was completely abolished by the removal of extracellular Ca²⁺ or the addition of extracellular Ni²⁺. The 100 mM 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 NH₄Cl-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.