MODULATION OF HORMONE-INDUCED CALCIUM OSCILLATIONS BY INTRACELLULAR PH IN RAT HEPATOCYTES

Single isolated rat hepatocytes were used to investigate the influence of intracellular pH (pH(i)) on hormone-induced cytosolic Ca2+ oscilla tions, using videofluorescence microscopy. Although pH(i) did not vary after alpha-adrenergic stimulation, manipulations of pH(i) induced pr onounced alterations in the frequency of oscillations. Increasing the resting pH(i) with ammonium chloride (5-20 mM), trimethylammonium (2-1 0 mM), or triethylammonium (1.2-8 mM) reduced the frequency of oscilla tions. A change in pH(i) of >0.25 was sufficient to reversibly inhibit oscillations. This effect could be overcome by increasing the agonist concentration or by adding 8-bromoadenosine 3',5'-cyclic monophosphat e, an agent known to potentiate the alpha-adrenergic response. Cellula r acidification, obtained by the ammonium prepulse method as well as b y application of acetate or the ionophore nigericin, in the continuous presence of agonist was accompanied by a modest frequency increase of the oscillations, leading in some cases to an overstimulated state. T his study indicates that pH(1), within a range of values expected to o ccur in vivo (0.1-0.2 pH units), exerts a chronotropic effect on pheny lephrine-induced Ca2+ oscillations. In contrast, oscillations induced by ADP or vasopressin were pH(i) invariant.