Short-term increase and long-term decrease of blood pressure in response to oxytocin-potentiating effect of female steroidal hormones

To investigate how the effects of oxytocin on blood pressure an influenced by female sex hormones, oxytocin (1 mg/kg, s.c.) was given to intact cyclin g and ovariectomized (OVX) female rats. Oxytocin caused a transient increas e in blood pressure, most pronounced during proestrus (p < 0.01) and estrus (p < 0.01). This increase was partially antagonized by an oxytocin antagon ist. When oxytocin was given for 5 days, blood pressure decreased (intact r ats: 123 +/- 1.5 vs. 130 +/- 1.3 mm Hg; p < 0.001, OVX rats: 120 +/- 3.0 vs . 129 +/- 1.1 mm Hg; p < 0.001). This decrease, not abolished by the oxytoc in antagonist, persisted for 3 weeks in intact rats and for 8 days in OVX r ats. If oxytocin treatment of OVX rats continued, a nadir of 12 mm Hg (118 +/- 1.7 mm Hg; p < 0.001) was reached after 8 days. Thereafter heart rate d ecreased significantly (p < 0.05). One daily oxytocin injection for 12 days to OVX rats decreased blood pressure for 3 weeks, as in intact rats. These results show that acute and chronic oxytocin treatment cause opposite effe cts on blood pressure, and that these effects are modified by female sex ho rmones.