Fomin, Victor P., Blair E. Cox, and R. Ann Word. Effect of progesterone on intracellular Ca21 homeostasis in human myometrial smooth muscle cells. Am. J. Physiol. 276 (Cell Physiol. 45): C379–C385, 1999.—Although it is well known that progesterone alters uterine contractility and plays an important role in maintenance of pregnancy, the biochemical mechanisms by which progesterone alters uterine contractility in human gestation are less clear. In this investigation we sought to identify progesterone-induced adaptations in human myometrial smooth muscle cells that may alter Ca21 signaling in response to contractile agents. Cells were treated with vehicle or the progesterone analog medroxyprogesterone acetate (MPA) for 5 days, and intracellular free Ca21 concentration ([Ca]i) was quantified after treatment with oxytocin (OX) or endothelin (ET)-1. OXand ET-1-induced increases in [Ca]i were significantly attenuated in cells pretreated with MPA in a dose-dependent manner. Progesterone receptor antagonists prevented the attenuated Ca21 transients induced by MPA. ETA and ETB receptor subtypes were expressed in myometrial cells, and treatment with MPA resulted in significant downregulation of ETA and ETB receptor binding. MPA did not alter ionomycin-stimulated increases in [Ca]i and had no effect on inositol trisphosphate-dependent or -independent release of Ca21 from internal Ca21 stores. We conclude that adaptations of Ca21 homeostasis in myometrial cells during pregnancy may include progesterone-induced modification of receptor-mediated increases in [Ca]i.