OXYTOCIN-INDUCED CA-FREE CONTRACTION OF RAT UTERINE SMOOTH-MUSCLE - EFFECTS OF DIVALENT-CATIONS AND DRUGS

Abstract

When rat myometrium that were depleted of Ca by incubation with 3 mM EGTA [ethylene glycol bis[.beta.-amino ethylether) N,N,N-tetraacetic acid] for 50 min was challenged with 10-2 U/ml oxytocin, it showed sustained contraction in a medium with no added Ca (Ca-free contraction). It 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 [methoxy verapamil] (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. Mn2+ evoked a triphasic response; 1st transient potentiation, 2nd relaxation and 3rd persistent increase in tension. D 600 did not block the 1st or 2nd, but blocked the 3rd, resulting in persistence of the 2nd phase of relaxation. The relaxing action of papaverine on Ca-free contraction was not affected by D 600. Isoproterenol and dibutyryl cAMP relaxed the contraction.