Oxytocin-induced Ca-free contraction of rat uterine smooth muscle: effects of divalent cations and drugs.

When rat myometrium that had been depleted of Ca by incubation with 3 mM EGTA for 50 min was challenged with 10(-2) unit/ml oxytocin, it showed sustained contraction in a medium with no added Ca (Ca-free contraction). It also showed Ca-free contraction of similar magnitude in the presence of 1 mM EGTA. The effects on this contraction of divalent cations (Co2+, Ni2+ and Mn2+) singly and in combination with D-600 (3 X 10(-6) M) were investigated. Co2+ and Ni2+ potentiated Ca-free contraction concentration-dependently, and their effects were greater in the presence of D-600. In contrast, Mn2+ evoked a triphasic response; first transient potentiation, second relaxation, and third persistent increase in tension. D-600 did not block the first or second, but blocked the third, resulting in persistence of the second phase of relaxation. The relaxing action of papaverine on Ca-free contraction was not affected by D-600. Isoproterenol and dibutyryl cyclic AMP also relaxed the contraction.