Carboxyeosin decreases the rate of decay of the [Ca²⁺](i) transient in uterine smooth muscle cells isolated from pregnant rats

In myometrial smooth muscle cells the rate of decline of intracellular calcium ([Ca²⁺](i)) is determined by Ca²⁺ extrusion from the cell and uptake into intracellular stores. The relative quantitative contribution of these processes however, has not been established. We therefore examined the effect of the sarcolemmal Ca²⁺ pump inhibitor, carboxyeosin, on the rate of the [Ca²⁺](i) transient decline in myocytes isolated from pregnant rat uterus. Indo-1 was used in conjunction with the whole-cell patch-clamp technique to measure [Ca²⁺](i) simultaneously with transmembrane calcium current (I(Ca)). [Ca²⁺](i) transients were elicited by repetitive membrane depolarization to simulate the natural pattern of uterine electrical activity. The rate of [Ca²⁺](i) removal was calculated from the falling phase of the [Ca²⁺](i) transient. Pretreatment of the cells with 2 μM carboxyeosin led to a marked decrease in the rate of [Ca²⁺](i) transient decay, suggesting that the sarcolemmal Ca²⁺ pump is involved in the calcium extrusion process. Removal of the extracellular Na also decreased the rate: of [Ca²⁺](i) decay, indicating an important role for the Na⁺/Ca²⁺ exchange. When both the sarcolemmal Ca²⁺ pump and Na⁺/Ca²⁺ exchange were inhibited the cell failed to restore [Ca²⁺i after the stimulation. Comparison of the rate constants of [Ca²⁺](i) decay in control conditions and after carboxyeosin treatment shows that approximately 30% of [Ca²⁺](i) decay is due to the sarcolemmal calcium pump activity. The remaining 70% can be attributed to the activity of Na⁺/Ca²⁺ exchanger and the intracellular calcium stores.