BRAIN ANGIOTENSIN-II PARTIALLY MEDIATES THE EFFECTS OF RELAXIN ON VASOPRESSIN AND OXYTOCIN RELEASE IN ANESTHETIZED RATS

Experiments were conducted in anesthetized rats to assess the contribu tion of the brain angiotensin-II system in the relaxin-induced secreti on of vasopressin and oxytocin. Intravenous injection of porcine relax in (5 mu g) caused a significant (P < 0.05, by analysis of variance) i ncrease in plasma concentrations of both hormones. Peak concentrations of both vasopressin (75.2 +/- 2.9 pmol/liter) and oxytocin (38.4 +/- 1.2 pmol/liter) were observed 1-2.5 min after relaxin injection. There after, concentrations fell significantly (P < 0.05) but remained eleva ted for a further 25 minutes. Continuous infusion of a specific angiot ensin-II receptor antagonist into the lateral cerebral ventricle did n ot affect baseline levels of either vasopressin or oxytocin, but did s ignificantly reduce (P < 0.05) the relaxin-induced release of both pep tides. A significant (P < 0.05) short term increase in both plasma vas opressin and oxytocin occurred 1 min after injection of 5 mu g relaxin , iv, in angiotensin-II-antagonized rats, but the concentrations of bo th neuropeptides were significantly (P < 0.05) lower than those observ ed in the angiotensin-intact relaxin-treated controls. These data sugg est that relaxin may act through the central angiotensin-II system to induce the release of vasopressin and oxytocin.