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 repeated injections of oxytocin on blood pressure and heart rate in spontaneously hypertensive rats (SHR). For this purpose subcutaneous (s.c.) injections of oxytocin 1 mg/kg or saline were given for 5 days to male and female SHR. Blood pressure and heart rate were measured daily before, during and after the oxytocin treatment period. In male rats, a significant 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 maximal difference of 21 mmHg (systolic) (p < 0.01), compared to controls, was reached after the last injection. The significant effect was gone 3 days after the last injection, although a tendency to a lower blood pressure in the oxytocin-treated rats persisted. On day 10, the oxytocin-treated SHR males again had a significantly lower systolic blood pressure (p < 0.05). In female SHR, the same treatment with oxytocin affected neither blood pressure nor heart rate. These results show that oxytocin may cause a sustained decrease in blood pressure, without affecting heart rate, in male but not in female SHR.