NA+/K+ PUMP INHIBITION INDUCES CELL SHRINKAGE IN CULTURED CHICK CARDIAC MYOCYTES

Myocardial cell swelling occurs in ischemia and in reperfusion injury before the onset of irreversible injury. Swelling has been attributed to failure of the Na+/K+ pump and the accumulation of intracellular Na+. To evaluate the role of the pump-leak model of cell volume maintenance, short term changes in cell volume in response to Na+/K+ pump inhibition were studied in aggregates of cultured embryonic chick cardiac myocytes using optical and biochemical methods. Exposure to 100 muM ouabain over 20 min induced cell shrinkage of approximately 10 %. Cell water was also decreased by Na+/K+ pump inhibition; incubation for 1 hr either in the presence of 100 muM ouabain or in K+-free solution reduced cell water by 18.4 % and 28.4 % respectively. When exposed to ouabain in the absence of extracellular Ca2+, the aggregates swelled by approximately 15 %, indicating that extracellular Ca2+ was required for the ouabain-induced shrinkage to occur. Ouabain still caused shrinkage, however, in the presence of the Ca2+ channel blockers verapamil (10 muM) and nifedipine (10 muM), suggesting that Na+/Ca2+ exchange, rather than Ca2+ channels, is the route for Ca2+ influx during Na+/K+ pump inhibition. Efflux of amino acids (taurine, aspartate, glutamate, glycine and alanine) from confluent monolayers of chick heart cells exposed to ouabain for 20 min was nearly double that observed in control solution. These results suggest that, during Na+/K+ pump inhibition, chick heart cells can limit accumulation of intracellular sodium by means of Na+/Ca2+ exchange, and that a rise in intracellular [Ca2+], also mediated by Na+/Ca2+ exchange, promotes the loss of amino acids and ions to cause cell shrinkage. Therefore, swelling during ischemic injury may not result from Na+/K+ pump failure alone, but may reflect the exhaustion of alternative volume regulatory transport mechanisms.