ROLE OF NA-PIG ILEUM SMOOTH-MUSCLE( CA++ EXCHANGE IN THE RELAXANT EFFECT OF SODIUM TAUROCHOLATE ON THE GUINEA)

Sodium taurocholate (NaTC), at concentrations below the critical micel lar concentration, caused a transient relaxation of isolated guinea-pi g ileum smooth muscle strips. The relaxation was not inhibited by prev ious incubation with either 10 muM ouabain, 0.4 mM d-tubocurarine or 0 .5 muM apamin, ruling out the participation of hyperpolarization of th e plasma membrane induce by either stimulation of Na+/K+ ATPase or by opening of Ca++-dependent K+ channels. In guinea-pig ileum smooth musc le cultured cells, addition of NaTC (1 mM) stimulated Na+ uptake and C a++ efflux. The relaxation induced by NaTC was inhibited by 3', 4'-dic hlorobenzamil, a blocker of the Na+/Ca++ exchanger. Preincubation with NaTC, or its addition during the early stage of the tonic response, o f the ileum to acetylcholine, enhanced that response, whereas a relaxa tion was observed when NaTC was added at the late stage of the acetylc holine response. In cultured cells, NaTC potentiated the stimulation o f 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.