[Effects of steroid hormones on change in [Ca2+]i following oxytocin stimulation in cultured human myometrial cells, and on myometrial oxytocin receptor]

Abstract

It is considered that steroid hormones, the concentrations of which vary during pregnancy, play an important role in the initiation of delivery. In the present study, we measured intracellular Ca2+ ([Ca2+]i) change following oxytocin stimulation in cultured human myometrial cells with fura-2, a fluorescent Ca2+ indicator. We also studied the effect of the steroid hormones on changes in [Ca2+]i due to oxytocin stimulation. When the [Ca2+]i change due to 10(-6) M oxytocin reached 100%, the addition of 10(-6) M DHAS (dehydroepiandrosterone sulfate) just before the stimulation raised the [Ca2+]i level to 207%, whereas that of 10(-6) M progesterone dropped to 64%. Moreover DHAS accelerated the speed of increase until the maximum response of [Ca2+]i, while progesterone decelerated it. In another study, human myometrial plasma membrane was solubilized in 7.5mM CHAPSO solution and applied to a binding study. Oxytocin receptors of two different molecular weights, namely 350kD(OTR-1) and 39kD(OTR-2), were extracted from solubilized plasma membrane by employing a gel filtration column. Binding assays were performed for OTR-1 and OTR-2 in the presence of DHAS and progesterone (10(-5) M). The results showed that DHAS enhanced the binding affinity of the receptors, whereas progesterone reduced the maximum binding capacity. It is therefore considered that the steroid hormones added just before oxytocin stimulation might act on receptor levels and modify intracellular Ca2+ response over short periods.