Lowering extracellular sodium or pH raises intracellular calcium in gastric cells

The dependence of cytoplasmic free [Ca] (Cai) on [Na] and pH was assessed in individual parietal cells of intact rabbit gastric glands by microfluorimetry of fura-2. Lowering extracellular [Na] (Nao) to 20 mM or below caused a biphasic Cai increase which consisted of both release of intracellular Ca stores and Ca entry across the plasma membrane. The Ca increase was not blocked by antagonists of Ca-mobilizing receptors (atropine or cimetidine) and was independent of the replacement cation. Experiments in Ca-free media and in Na-depleted cells indicated that neither phase was due to reversal of Na/Ca exchange. The steep dependence of the Cai increase on Nao suggested that the response was not due to lowering intracellular [Na] (Nai). The effects of low Nao on Cai were also completely independent of changes in intracellular pH (pHi). Cai was remarkably stable during changes of pHi of up to 2 pH units, indicating that H and Ca do not share a cytoplasmic buffer system. Such large pH excursions required determination of the pH dependence of fura-2. Because fura-2 was found to decrease its affinity for Ca as pH decreased below 6.7, corrections were applied to experiments in which large pHi changes were observed. In contrast to the relative insensitivity of Cai to changes in pHi, decreasing extracellular pH (pHo) to 6.0 or below was found to stimulate release of intracellular Ca stores. Increased Ca entry was not observed in this case. The ability of decreases in Nao and pHo to stimulate release of intracellular Ca stores suggest interactions between Na and H with extracellular receptors.