Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Activation of voltage-dependent Ca2+ channels by high K+ (40 mM) increased the cytosolic Ca2+ level ([Ca2+](i)) (estimated by fura-PE3 fluorescence ratio) and force in myometrium isolated from pregnant (21 days after gestation) and non-pregnant (estrus) rats. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 mM) decreased the high K+-stimulated [Ca2+](i) and force-in a concentration-dependent manner. The inhibitory effect was stronger in the pregnant myometrium than in the non-pregnant myometrium. In the pregnant myometrium, the increase in Ca2+ permeability by ionomycin (1 mu M) greatly increased [Ca2+](i) and force, which were only partially inhibited by verapamil (10 mu M). DPB (1 mu M) inhibited the verapamil-insensitive component of the increases in [Ca2+](i) and muscle tension. Oxytocin (100 nM) and thapsigargin (1 mu M) also induced a verapamil-insensitive increase in [Ca2+](i) and force, and DPB (1 mu M) inhibited these increments. Ca2+ sensitivity of contractile elements, estimated from the relationships between Ca2+ and muscle force in intact and alpha-toxin permeabilized muscle, was not significantly changed by DPB (1 mu M). In summary, DPB inhibits the increase in [Ca2+](i) more strongly in myometrium isolated from pregnant rats than that from non-pregnant rats without any change in the [Ca2+](i)/tension relationship. Since DPB decreased [Ca2+](i)-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+](i).