Oxytocin-stimulated capacitative calcium entry in human myometrial cells

OBJECTIVE: Our purpose was to investigate the relative contribution of extr acellular calcium recruitment and release of calcium from intracellular sto res in an immortalized myometrial cell line derived from a pregnant woman ( PHM1-41) and to determine the importance of capacitative calcium entry in t he 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 o xytocin with an increase in intracellular free calcium, was used for this s tudy. Intracellular free calcium was measured directly in cells loaded with Fura 2-AM. RESULTS: The oxytocin-stimulated rise in intracellular free calcium decreas ed in the absence of extracellular calcium or in the presence of phospholip ase C inhibitors, suggesting mobilization of calcium from both extracellula r and intracellular sources to increase intracellular free calcium. Phospho lipase C inhibitors resulted in greater inhibition of the oxytocin-stimulat ed rise in intracellular free calcium than expected on the basis of experim ents performed in the absence of extracellular calcium. This implies interd ependence 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-stimula ted intracellular free calcium increase resulting from calcium entry was bl ocked by store depletion by thapsigargin or cyclopiazonic acid, consistent with a capacitative calcium entry mechanism. CONCLUSION: Oxytocin stimulates both capacitative and noncapacitative calci um entry in a pregnant human myometrium cell line.