Carbachol partially inhibits the plasma-membrane Ca2+-pump in microsomes from pig stomach smooth muscle

A plasmalemmal enriched membrane fraction, prepared from pig stomach smooth-muscle, contains a calmodulin-stimulated (Ca2+ + Mg2+)-ATPase and presents an ATP-dependent 45Ca-uptake. If these smooth-muscle strips are preincubated with 10(-3) M-carbachol, this Ca2+ + Mg2+)-ATPase and the 45Ca-uptake are reduced by 21.4% and 13.5%, respectively, as compared to controls. This inhibitory effect of carbachol can be completely blocked by atropine. Carbachol does neither affect the passive permeability of the microsomes to 45Ca, nor the passive 45Ca-binding to the vesicles. Neither does it exert an effect on the proportion of closed inside-out plasma-membrane vesicles. Likewise, preincubation of rat myometrium with 90 nM-oxytocin induces a 20.4% inhibition of the ATP-dependent 45Ca-uptake, without having an effect on the passive 45Ca-binding, the permeability to 45Ca or the sideness of the vesicles. From these results, it is concluded that some agonists as carbachol and oxytocin induce a decrease in the activity of the plasmalemmal Ca2+-pump.