Inhibition of multiple trans-sarcolemmal cation flux pathways by dichlorobenzamil in cultured chick heart cells.

Dichlorobenzamil, an analog of amiloride, has been reported to inhibit Na-Ca exchange in sarcolemmal vesicles of guinea pig heart. To examine further the effect of the drug on Na-Ca exchange in intact cardiac cells and the pharmacological specificity of this action, we determined in cultured chick heart cells the effects of dichlorobenzamil on the following: contractile state, Nai-dependent Ca uptake, Ca uptake via slow Ca channels (defined as verapamil-inhibitable Ca uptake), Ca efflux via the sarcolemmal Ca pump, monovalent cation transport, and cellular Ca and Na content. Dichlorobenzamil produced a concentration-dependent decrease in the amplitude of cell motion (EC50 = 5 X 10(-7) M) and abolished the development of ouabain-induced rhythm disturbances and contracture. In normal or Na-loaded cells, dichlorobenzamil inhibited the Ca uptake rate, also in a concentration-dependent manner (EC50 = 6 X 10(-7) M). Dichlorobenzamil (6 X 10(-7) M) also caused a significant inhibition of the isoproterenol-induced elevation of Ca uptake. At 5 X 10(-5) M, dichlorobenzamil blocked completely Ca influx via slow Ca channels. Ca efflux rate was also reduced by dichlorobenzamil (EC50 = 10(-6) M). Replacement of Na with choline in the efflux medium to prevent Ca efflux via Na-Ca exchange did not alter the ED50 of the drug's inhibition of Ca efflux rate. Dichlorobenzamil caused concentration-dependent inhibition of sodium pump activity as judged by ouabain-sensitive 42K uptake (EC50 approximately 2 X 10(-6) M), and, at concentrations above 5 X 10(-7) M), produced an increase in steady state cellular Na content. These results indicate that dichlorobenzamil has several sites of action in intact heart cells and that the negative inotropic action of the drug is due, in part, to inhibition of Ca influx via both Na-Ca exchange and slow Ca channels.