The cardiovascular effects of porcine relaxin in Brattleboro rats

The effects of porcine relaxin on arterial blood pressure, heart rate, and the release of vasopressin and oxytocin were investigated in homozygous diabetes insipidus (di/di) Brattleboro and Long-Evans rats. Acute iv injection of relaxin (5 microg) caused a significant increase in mean arterial, systolic and diastolic blood pressures in Long-Evans rats compared with control injections of saline, but had no pressor effect in Brattleboro rats. Circulating concentrations of vasopressin were also significantly elevated above baseline in the Long-Evans rats 1 min after relaxin treatment, but remained undetectable in the relaxin-treated Brattleboro rats. Relaxin increased heart rate in both groups of animals 4 min after injection. The chronotropic effect of relaxin was, however, attenuated in the Brattleboro rats. Intravenous relaxin injection also caused a significant increase in plasma oxytocin concentrations 5 min posttreatment in both the Long-Evans and Brattleboro rats. The change in plasma oxytocin above basal concentrations was significantly greater in Brattleboro rats compared with Long-Evans controls. The data in this study demonstrate that iv relaxin increases heart rate, but not arterial blood pressure in Brattleboro rats. Furthermore, the relaxin-induced release of oxytocin in Brattleboro rats does not result in an acute pressor response.