Effects of oxytocin, prostaglandin F-2 alpha and prostaglandin E-2 on intracellular free calcium concentrations of longitudinal muscle cells isolated from term pregnant rat myometrium

The effects of oxytocin, PGF(2 alpha) and PGE(2) on [Ca2+](i) of isolated longitudinal muscle cells of term pregnant rats were investigated using Fura-2. Oxytocin, PGF(2 alpha) and PGE(2) induced an initial rapid increase followed by a secondary gradual increase in [Ca2+](i) in the presence of 1.5 mM Ca2+. The initial maximum increases in [Ca2+](i) were obtained at 6 s for oxytocin, 20 s for PGF(2 alpha) and 30 s for PGE(2) after addition of the stimulants. The EC50 values obtained from the curves were 2.0 nM for oxytocin, 250 nM for PGF(2 alpha) and 2,200 nM for PGE(2). On the other hand, the increases in [Ca2+](i) induced by the stimulants were nearly abolished by removal of the extracellular Ca2+. The stimulants induced biphasic increases in [Ca2+](i) which were highly extracellular Ca2+-dependent processes, and the order of potencies for the stimulants was oxytocin much greater than PGF(2 alpha) > PGE(2) in terms of affinity as well as magnitude. These results might indicate differences in the weight of the physiological role of the stimulants for regulating uterine contractility.