INHIBITION OF OXYTOCIN- OR PROSTAGLANDIN F2α-DRIVEN MYOMETRIAL ACTIVITY BY RELAXIN IN THE RAT IS OESTROGEN-DEPENDENT

Abstract

Relaxin in doses of 5 μg i.v. completely but reversibly abolishes 'spontaneous' myometrial activity in anaesthetized ovariectomized rats. Similar levels of myometrial activity, evoked in oestrogen-treated rats (which normally have quiescent uteri) by infusions of oxytocin or prostaglandin F2α (PGF2α), were also reduced to complete quiescence by relaxin in small doses. However when spontaneous myometrial activity in untreated ovariectomized rats was slightly stimulated by oxytocin the uterus became completely refractory to the inhibitory effects of relaxin even at doses of 50 μg. Relaxin was also ineffective in reducing myometrial activity in similar rats during intra-arterial infusion of PGF2α. It is suggested that the ability of relaxin to inhibit uterine smooth muscle during exogenous stimulation is oestrogen-dependent.