Cytosolic acidification leads to Ca mobilization from intracellular stores in single and populational parietal cells and platelets

Regulatory relationship and gain control between cytosolic free Ca²⁺ concentration (Ca_i) and cytosolic pH (pH_i) were evaluated by two different cell types, gastric parietal cells, and blood platelets. Studies were carried out in both single cells and populations of cells, using Ca²⁺-indicative probe fura-2 (1-(2-(5′-carboxyoxazol-2′-yl)-6-aminobenzofuran-5-oxy)-2-(2′-amino-5′-methylphenoxy) ethane-N,N,N′,N′-tetraacetic acid) and pH-indicative probe BCECF (2′,7′-bis(carboxyethyl) carboxyfluorescein). Stimulation of single and populational parietal cells and platelets with gastrin and thrombin, respectively, resulted in an increase in Ca_i. In both populational cell types, an initial change in pH_i during agonist stimulation occurred almost simultaneously with the mobilization of Ca²⁺; an initial transient decrease in pH_i was followed by a slower increase in pH_i above the prestimulation level. When populational platelets were preloaded with the Ca²⁺ chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′tetraacetic acid), the thrombin-induced initial large increase in Ca_i was apparently inhibited, whereas the pH_i decrease induced by thrombin was not altered. This suggests that the initial Ca_i change is not a prerequisite for the pH_i change. The effect of pH_i on Ca_i was examined next. In both single and populational cell types, application of the K⁺-H⁺ ionophore nigericin, which induced a transient decrease in pH_i, led to the release of Ca²⁺ from intracellular stores. In single parietal cells double-labeled with fura-2 and BCECF, a temporal decrease in pH_i preceded the rise in Ca_i after stimulation with nigericin. A decrease in pH_i, and an increase in Ca_i occurred at 1.5 and 4 s, respectively. In single parietal cells, replacement of medium Na⁺ with N-methyl- -glucamine (NMG⁺), which also induced a decrease in pH_i, resulted in repetitive Ca²⁺ spike oscillations. The source of Ca²⁺ utilized for the Ca²⁺ oscillation that was induced by NMG⁺ originated from the agonist-sensitive pool. Thus, several maneuvers, which were capable of decreasing pH_i, led to an increase in Ca_i. Cytosolic acidification may be a part of the trigger for Ca²⁺ mobilization from intracellular stores in both parietal cells and platelets.