Effect of porcine relaxin and progesterone on rat, pig and human myometrial activity in vitro.

The in vitro effect of pure porcine relaxin on nonpregnant and pregnant myometrium before and after progesterone treatment was studied in the rat, pig and human. Myometrial strips from 45 rats, 30 pigs and 64 humans were studied. Porcine relaxin inhibited myometrial activity in the nonpregnant and pregnant rat until midpregnancy, when the sensitivity to it gradually decreased. By day 19 of rat pregnancy the myometrium was completely refractory to the inhibitory effect of porcine relaxin. Pretreatment of rat myometrial strips in vitro with progesterone increased their sensitivity to relaxin but did not overcome the refractory period in late pregnancy. In the pig, myometrial contractility was completely inhibited by porcine relaxin both in the nonpregnant state and throughout pregnancy (days 25-109). Progesterone was again synergistic, with relaxin in the pig inhibiting myometrial contractility. However, porcine relaxin had little or no effect on human myometrial contractility either in the nonpregnant state or during the pregnancies tested (28-42 weeks). Pretreatment of human myometrium with progesterone did not induce a response to porcine relaxin.