Oxytocin-stimulated capacitative calcium entry in human myometrial cells

Abstract

Objective: Our purpose was to investigate the relative contribution of extracellular calcium recruitment and release of calcium from intracellular stores in an immortalized myometrial cell line derived from a pregnant woman (PHM1-41) and to determine the importance of capacitative calcium entry in the oxytocin-stimulated rise in intracellular free calcium. Study Design: The PHM1-41 immortalized myometrial cell line, which retains smooth muscle phenotype, estrogen, and oxytocin receptors and responds to oxytocin with an increase in intracellular free calcium, was used for this study. Intracellular free calcium was measured directly in cells loaded with Fura 2-AM. Results: The oxytocin-stimulated rise in intracellular free calcium decreased in the absence of extracellular calcium or in the presence of phospholipase C inhibitors, suggesting mobilization of calcium from both extracellular and intracellular sources to increase intracellular free calcium. Phospholipase C inhibitors resulted in greater inhibition of the oxytocin-stimulated rise in intracellular free calcium than expected on the basis of experiments performed in the absence of extracellular calcium. This implies interdependence of the intracellular and extracellular pathways for elevation of intracellular free calcium and suggests some capacitative entry of calcium as a consequence of depletion of intracellular stores. The oxytocin-stimulated intracellular free calcium increase resulting from calcium entry was blocked by store depletion by thapsigargin or cyclopiazonic acid, consistent with a capacitative calcium entry mechanism. Conclusion: Oxytocin stimulates both capacitative and noncapacitative calcium entry in a pregnant human myometrium cell line. (Am J Obstet Gynecol 1999;181:424-9.)