Effects of amiloride and ouabain on contractile state, Ca and Na fluxes, and Na content in cultured chick heart cells.

Amiloride has been reported to reduce the positive inotropic and toxic actions of cardiac glycosides in patients as well as in experimental animals. To investigate the mechanism by which amiloride interacts with glycosides at the cellular level, we examined the effect of amiloride and ouabain on cellular Na content and uptake, Ca flux via Na-Ca exchange, and contractile state. Amiloride (1 mM) reduced cellular Na content by 16% (p less than 0.05) under normal conditions and by 45% in the presence of 1 mM ouabain compared to respective control values observed in the absence of amiloride. Amiloride (1 mM) reduced the initial rate of 45Ca uptake by 40% in ouabain (1 mM)-treated cells. This reduction of 45Ca uptake could be mimicked by lowering cellular Na content by 42%. Amiloride (1 mM) did not alter significantly the initial rate of 24Na uptake under normal conditions but reduced it by 32% in the presence of 3 microM ouabain. Amiloride (1 mM) produced a transient increase followed by a gradual decrease in the amplitude of cell motion over 60 min to 10% of control level. At other concentrations between 0.1 and 3 mM, amiloride produced negative inotropic effects only. Amiloride increased the concentration of ouabain needed to produce rhythm disturbances and contracture, and reduced Na-free contracture amplitude by 18%. These results are consistent with the view that amiloride antagonizes the arrhythmogenic effects of ouabain by inhibiting the glycoside-induced elevation in cellular Na content and, consequently, the increases in [Ca]i that occur via Na-dependent pathways. The reduced cellular Na content appears to be due to decreased Na influx via Na-H exchange.