Relations among sodium pump inhibition, Na-Ca and Na-H exchange activities, and Ca-H interaction in cultured chick heart cells

Abstract

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 microM) elevated [Ca]i eightfold and decreased pHi by 0.11 unit (a 30% increase in [H+]). In the presence of 10 microM 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 microM 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 microM) or toxic (100 microM) concentrations of ouabain, initial 24Na uptake rates were significantly greater than in control cells and were significantly reduced in the presence of 10 microM ethylisopropylamiloride. We conclude that ouabain (100 microM) produces intracellular acidification as a result of sodium pump inhibition; calcium accumulation via Na-Ca exchange, and subsequent Ca-H interaction within the cell.(ABSTRACT TRUNCATED AT 250 WORDS)