Effect of porcine relaxin on spontaneous, oxytocin-driven and prostaglandin-driven pig myometrial activity in vitro.

The effect of porcine relaxin on myometrial activity was studied in 18 nonpregnant and 54 pregnant pigs. Relaxin inhibited spontaneous contractility in all luteal phase and pregnant pigs. However, similar doses of relaxin had no effect on oxytocin-, prostaglandin E2- or prostaglandin F2 alpha-induced contractions. Only when the doses were increased fivefold to sixfold, to nonphysiologic levels, did relaxin inhibit the action of these oxytocic agents. The effect of relaxin was the same throughout pregnancy. This finding helps explain the two paradoxical roles of relaxin: it inhibits spontaneous myometrial contractility during pregnancy and thus preterm labor but is able to facilitate labor at term through its cervical ripening action without inhibiting the oxytocin- and prostaglandin-driven contractions of parturition.