Effect of porcine relaxin on the human umbilical artery.

Relaxin decreases human myometrial contractions in vitro; that effect is synergized by progesterone. We examined the effects of porcine relaxin on the contractility of isolated perfused human umbilical arterial strips in vitro. One experimental group received relaxin (1.5 micrograms/mL), the second received progesterone (500 ng/mL) plus relaxin, and the control strips received neither. Both resting and agonist-stimulated (KCl or serotonin) isometric tension were compared with profile analysis for all groups. Relaxin had no effect on either resting or agonist-stimulated tension either with or without progesterone. Neither higher concentrations of relaxin nor longer exposures altered contractility. Therefore, the human umbilical artery, unlike the cervix and myometrium, is not sensitive to porcine relaxin. Porcine relaxin could be used as a tocolytic or cervical ripening agent without adversely affecting fetal placental circulation.