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 stares. 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 mu M 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.