Increased Inhibitory Effect of Phorbol Ester on Cytosolic Ca²⁺ Level and Contraction in Rat Myometrium after Gestation

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Activation of voltage-dependent Ca²⁺ channels by high K⁺ (40 mM) increased the cyiosolic 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 μM) greatly increased [Ca²⁺]i and force, which were only partially inhibited by verapamil (10 μM). DPB (1 μM) inhibited the verapamil-insensitive component of the increases in [Ca²⁺]i and muscle tension. Oxyiocin (100 nM) and thapsigargin (1 μM) also induced a verapamil-insensitive increase in [Ca²⁺]i and force, and DPB (1 μM) inhibited these increments. Ca²⁺ sensitivity of contractile elements, estimated from the relationships between Ca²⁺ and muscle force in intact and α-toxin permeabilized muscle, was not significantly changed by DPB (1 μM). 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 [Ca^{2+]i-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+]i.}