The effects of a progesterone metabolite, 5β-dihydroprogesterone, on oxytocin receptor binding in human myometrial membranes

Objective To determine the effect of the progesterone metabolite 5beta-dihydroprogesterone on human oxytocin receptor binding in myometrial membranes and on whole-cell calcium current in single myometrial cells. Design Receptor binding studies in human myometrial membranes prepared from biopsies taken before or after the onset of labour and in Chinese hamster ovary cells expressing the human oxytocin receptor. Whole cell patch-clamp experiments were undertaken on isolated myometrial cells. Setting University research laboratories and University hospital. Population Patients undergoing caesarean section at term either prior to or following onset of labour. Methods Myometrial biopsies were taken from women undergoing caesarean section. The binding affinities of oxytocin, 5beta-dihydroprogesterone and atosiban were determined in myometrial membranes and Chinese hamster ovary cells expressing the human oxytocin receptor. The effect of 5beta-dihydroprogesterone on inward current was also determined in isolated myometrial cells. Main outcome measures Receptor binding affinity and electrophysiological inward current. Results 5beta-Dihydroprogesterone did not reduce oxytocin receptor binding in myometrial membranes or Chinese hamster ovary cells expressing the human oxytocin receptor. Nor did it influence calcium current under whole-cell patch conditions in single myometrial cells. In contrast, atosiban inhibited binding in myometrial membranes prepared from samples taken either prior to or following labour (K-i = 112 and 108 nM, respectively). The affinity of atosiban for the oxytocin receptor was much lower than oxytocin (K-i = 5 and 6 nM in samples taken before or after labour, respectively) in myometrial membranes and in Chinese hamster ovary cells expressing the human oxytocin receptor (K-i = 63 M and 1 nM for atosiban and oxytocin, respectively). Conclusions We conclude that 5beta-dihydroprogesterone is unlikely to regulate myometrial activity as a result of a direct effect on oxytocin receptor binding or inward calcium current.