RELATIONS AMONG SODIUM-PUMP INHIBITION, NA-CA AND NA-H EXCHANGE ACTIVITIES, AND CA-H INTERACTION IN CULTURED CHICK HEART-CELLS

Sodium pump inhibition in cardiac muscle cells is associated with changes in intracellular sodium, calcium, and hydrogen concentrations as well as in membrane ion transport activity. We examined further the functional relations among these entities using cultured chick ventricular cells. [Ca]i and pHi were determined from fluorescence signals obtained from cells loaded with fura-2 or BCECF, respectively. Ouabain (100 .mu.M) elevated [Ca]i eightfold and decreased pHi by 0.11 unit (a 30% increase in [H+]). In the presence of 10 .mu.M ethylisopropylamiloride, a potent inhibitor of Na-H exchange, ouabain elevated [Ca]i 3.5-fold and reduced pHi by 0.16 unit (a 48% increase in [H]). Exposure to sodium-free (sodium replaced with potassium) medium produced a twelvefold increase in [Ca]i and a 0.12 pH unit decrease in pHi. In cells treated with 100 .mu.M ouabain, exposure to sodium-free (lithium) medium resulted in a 22-fold sustained increase in [Ca]i and a rapid intracellular acidification (pH 7.15 to 6.60). The effect of ouabain or sodium-free medium on pHi was abolished in calcium-free medium; addition of 1 mM Ca rapidly increased [Ca]i and decreased pHi. In cells treated with subtoxic (3 .mu.m) or toxic (100 .mu.M) concentrations of ouabain, initial 24Na uptake rates were significantly greater than in control cells and were significantly reduced in the presence of 10 .mu.M ethylisopropylamiloride. We conclude that ouabain (100 .mu.M) produces 1) intracellular acidification as a result of sodium pump inhibition; 2) calcium accumulation via Na-Ca exchange, and 3) subsequent Ca-H interaction within the cell. The decrease in pHi stimulates H+ efflux and Na+ influx via Na-H exchange to limit further decline in pHi and to further augment [Na]i, which tends to elevate [Ca]i as a result of increased calcium accumulation via Na-Ca exchange. Thus, Na-H exchange may play an important role in the positive inotropic and also the toxic effects of cardiac glycosides.