Relaxin acts centrally to inhibit oxytocin release during parturition: an effect that is reversed by naloxone.
Experiments were carried out to establish whether infusion of relaxin prolongs gestation and labour in the rat by suppressing release of oxytocin, and whether the effects of relaxin on birth could be reversed by the opioid antagonist naloxone. Female rats were implanted with subcutaneous osmotic minipumps for the infusion of purified porcine relaxin into the jugular vein for 84 h from either day 19 or day 20 of gestation. Infusion of relaxin delayed the onset of labour and in those animals which delivered during relaxin infusion, delivery was longer by approximately 45 min. Plasma oxytocin levels 40 min after delivery of the first fetus were 45.25 +/- 3.6 pmol/l (mean +/- S.D.) in unoperated controls and significantly (P less than 0.01) depressed (23.89 +/- 3.9) in rats that delivered during infusion of relaxin. Rats that delivered after the infusion of relaxin had finished, gave birth significantly (P less than 0.05) faster than controls and plasma oxytocin levels were significantly (P less than 0.01) raised (77.87 +/- 15.9 pmol/l). Naloxone treatment (1 mg/kg; i.m.) given immediately after the delivery of the first fetus reversed the inhibitory effect of relaxin and the interval between successive deliveries was slightly faster than that of controls. Plasma oxytocin levels in relaxin-infused naloxone-treated rats were significantly (P less than 0.01) higher than values in unoperated control rats. The results confirm that relaxin suppresses oxytocin release possibly through an opioid system and this may be important in the control of the timing of birth.
D000818:Animals; D005260:Female; D007743:Labor, Obstetric / Q000187:drug effects / Q000502:physiology*; D009270:Naloxone / Q000494:pharmacology*; D010121:Oxytocin / Q000097:blood / Q000378:metabolism*; D011247:Pregnancy; D051381:Rats; D011919:Rats, Inbred Strains; D012065:Relaxin / Q000502:physiology*; D013997:Time Factors
D009270:Naloxone; D010121:Oxytocin; D012065:Relaxin