Human relaxin. In vitro response of human and pig myometrium

Human myometrial strips were excised at hysterectomy and cesarean section and allowed to contract spontaneously in a water bath. Porcine myometrial strips from midpregnancy were also collected. Recombinant human relaxin completely inhibited spontaneous myometrial activity in the pig at a concentration of 0.6 micrograms/mL, but human relaxin had much less of an effect or no effect on human myometrium at concentrations up to 7.5 micrograms/mL. Any effect of human relaxin on human myometrium was seen only in estrogen-primed human tissues; that effect was never greater than a 5% reduction in amplitude and 50% reduction in frequency. When contractions were stimulated with oxytocin or prostaglandin F2 alpha, relaxin had no inhibitory effect on either porcine or human myometrium at doses up to eight times the concentration of relaxin that had attenuated spontaneous contractions. Pretreatment with progesterone did not enhance the action of relaxin on human myometrium. The limited effect of human relaxin on human myometrium as compared to the marked inhibitory action of both porcine and human relaxin on porcine myometrial activity suggests that the species specificity does not lie with the relaxins but with the target tissues. Human relaxin H2 might not play a major role in the control of myometrial activity in the human.