Pressor effect of posterior hypothalamic stimulation is enhanced in pregnant rats

In contrast to the attenuated pressor response to intravenous angiotensin II (ANG II) in rat pregnancy, the response to intracerebroventricular ANG II is not blunted. To test whether this difference was due to enhanced effects of sympathetic nervous system activation, blood pressure, heart rate, and renal and mesenteric blood flow were measured in conscious term-pregnant and age-matched virgin rats during electrical stimulation of the posterior hypothalamus and during intravenous infusion of norepinephrine (NE). The pressor responses to endogenous NE release by hypothalamic stimulation were significantly greater (P less than 0.05) in pregnant rats at 10, 15 and 20 Hz. In contrast, the pressor response to exogenous NE was significantly blunted in gravid animals. These differences were not associated with significant differences in renal or mesenteric vascular resistance changes. We conclude that the lack of an attenuated response to intracerebroventricular ANG II in the pregnant rat can be explained in part by an enhanced pressor effect of sympathetic nervous system activation.