Oxytocin contracts the human uterus at term by inhibiting the myometrial Ca2+-extrusion pump.

The sarcolemma of smooth-muscle cells from human pregnant myometrium possesses a high-affinity Ca2+-ATPase, which has the characteristics of an active Ca2+-extrusion pump. This pump enzyme, either membrane-bound or solubilized, was strongly inhibited by oxytocin (half-maximal inhibition at about 4 microU/ml or 10 pM). However, under similar conditions, oxytocin did not inhibit Ca2+-extrusion ATPase of the erythrocyte membrane. The inhibitory concentrations of oxytocin correspond to hormone plasma levels which initiate spontaneous labor.