Role of angiotensin II in altered baroreflex function of conscious rabbits during late pregnancy

OBJECTIVE: Pregnancy alters baroreflex control of heart rate in conscious rabbits, but the mechanism for this action is unknown. This study tested the hypothesis that endogenous angiotensin II is the mediator. STUDY DESIGN: To test this hypothesis the baroreflex relationship between arterial pressure and heart rate in conscious rabbits was determined before and after administration of the angiotensin II AT₁ receptor antagonist losartan (n = 7) before pregnancy and at the end of gestation. RESULTS: Pregnancy decreased mean arterial pressure, increased heart rate, and modified the reflex by shifting the mean arterial pressure-heart rate relationship to a lower pressure level, by increasing minimum heart rate, and by decreasing baroraflex gain (P < .05). Before pregnancy, losartan decreased baroreflex gain but had no other effect on reflex function. In contrast, during late gestation losartan further decreased mean arterial pressure, further decreased reflex gain, decreased maximum heart rate, and shifted the curve to a lower mean arterial pressure level (P < .05). CONCLUSION: These results suggest that in conscious rabbits during pregnancy endogenous angiotensin II contributes to hypotension-induced tachycardia but does not decrease reflex gain or elevate minimum heart rate.