T he demonstration that endothelin (ET) induces rat uterine contraction, coupled with the observation that ET is present in human amniotic fluid, suggests that the myometrium may be an important target organ for this hormone. We show that in quiescent human myometrial cells ET produced a dose-dependent increase in cytosolic free Ca2+ (Cai2+), which was markedly attenuated when the cells were studied in Ca2(+)-free media. Preincubation with nicardipine, diltiazem, or verapamil reduced the ET-evoked Cai2+ transient by 30, 40, and 65%, respectively. The presence of voltage sensitive Ca2+ channels was demonstrated by Mn2+ quenching of fura-2. Activation of the Na+/H+ antiport could not be demonstrated with ET stimulation. In nonquiescent cells, the ET-evoked Cai2+ transient was significantly reduced, while the response to oxytocin was retained. This is at least partially explained by a reduction in Bmax (maximal binding capacity) for ET (mean +/- SEM) from 3,506 +/- 268 binding sites/cell in quiescent cells to 2,411 +/- 300 binding sites/cell, as well as 72% increase in Kd (equilibrium dissociation constant), in the nonquiescent cells. We conclude that, in human myometrial cells, ET and oxytocin modulate Cai2+ through independent receptors and propose that ET, like oxytocin, is an important endogenous modulator of uterine contractility.