Role of Na+/Ca++ exchange in the relaxant effect of sodium taurocholate on the guinea-pig ileum smooth muscle.

Sodium taurocholate (NaTC), at concentrations below the critical micellar concentration, caused a transient relaxation of isolated guinea-pig ileum smooth muscle strips. The relaxation was not inhibited by previous incubation with either 10 microM ouabain, 0.4 mM d-tubocurarine or 0.5 microM apamin, ruling out the participation of hyperpolarization of the plasma membrane induced by either stimulation of Na+/K+ ATPase or by opening of Ca(++)-dependent K+ channels. In guinea-pig ileum smooth muscle cultured cells, addition of NaTC (1 mM) stimulated Na+ uptake and Ca++ efflux. The relaxation induced by NaTC was inhibited by 3',4'-dichlorobenzamil, a blocker of the Na+/Ca++ exchanger. Preincubation with NaTC, or its addition during the early stage of the tonic response of the ileum to acetylcholine, enhanced that response, whereas a relaxation was observed when NaTC was added at the late stage of the acetylcholine response. In cultured cells, NaTC potentiated the stimulation of Ca2+ influx by acetylcholine. Our results suggest that NaTC acts on the smooth muscle cell membrane causing a stimulation of the Na+/Ca++ exchange mechanism.