Sodium pump inhibition, enhanced calcium influx via sodium-calcium exchange, and positive inotropic response in cultured heart cells.

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Abstract
The effects of Na pump inhibition produced by exposure to the cardiac glycosides, ouabain or dihydroouabain or by reduction in extracellular K to 1.0 mM, on contractile state and Na-Ca exchange were studied in primary monolayer cultures of chick embryo ventricular cells. Ouabain, 10-6 M, dihydroouabain, 5 .times. 10-5 M and extracellular K of 1.0 mM all induced similar and prominent positive inotropic effects. These effects were accompanied, in each case, by 40-50% inhibition of the rate of active uptake of 42K and by similar increases in steady state Na content. Stimulation of the rate of 45Ca uptake on exposure to zero extracellular Na occurred in response to extracellular K (1.0 mM) or to glycoside concentrations that induced a positive inotropic effect and Na-K pump inhibition. Reactivation of the Na pump after return from 1.0 to 4.0 mM extracellular K was rapid and was associated with membrane hyperpolarization and slowing of spontaneous beating rate. With pump reactivation under these circumstances, the time course of disappearance of stimulation of Na-Ca exchange on exposure to O extracellular Na was similar to the time course of loss of the positive inotropic effect. Under physiological conditions (4.0 mM extracellular K), exposure to positively inotropic but nontoxic concentrations of ouabain or dihydroouabain caused a small but consistent increase in unidirectional Ca influx, but had no discernible effect on Ca efflux. Since similar inotropic effects were produced for comparable degrees of glycoside or low extracellular K-induced Na pump inhibition and increases in cellular Na content, Na pump inhibition rather than a glycoside-specific change in Ca binding appears to underlie the inotropic response. These findings are further consistent with the view that the primary mechanism of the positive inotropic effects of digitalis and low extracellular K in this experimental preparation is Na pump inhibition resulting in increased intracellular Na. Increased Ca influx via Na-Ca exchange is the principal mechanism whereby increased intracellular Na results in enhanced Ca availability to the myofibrils, but an additional effect on Ca efflux is not excluded.

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