OXYTOCIN DECREASES BLOOD-PRESSURE IN MALE BUT NOT IN FEMALE SPONTANEOUSLY HYPERTENSIVE RATS

The aim of the present study was to investigate the effects of repeate d injections of oxytocin on blood pressure and heart rate in spontaneo usly hypertensive rats (SHR). For this purpose subcutaneous (s.c.) inj ections of oxytocin 1 mg/kg or saline were given for 5 days to male an d female SHR. Blood pressure and heart rate were measured daily before , during and after the oxytocin treatment period. In male rats, a sign ificant decrease in blood pressure (systolic; p < 0.01, diastolic; p < 0.05), but no effect on heart rate, was seen the day after the first injection of oxytocin, when compared to saline-treated controls. Blood pressure decreased further in response to each injection and a maxima l difference of 21 mmHg (systolic) (p < 0.01), compared to controls, w as reached after the last injection. The significant effect was gone 3 days after the last injection, although a tendency to a lower blood p ressure in the oxytocin-treated rats persisted, On day 10, the oxytoci n-treated SHR males again had a significantly lower systolic blood pre ssure(p < 0.05). In female SI-IR, the same treatment with oxytocin aff ected neither blood pressure nor heart rate. These results show that o xytocin may cause a sustained decrease in blood pressure, without affe cting heart rate, in male but not in female SHR. (C) 1997 Elsevier Sci ence B.V.