Angiotensin II receptor blockade with losartan does not increase baroreflex gain in conscious, pregnant rabbits

Pregnancy is associated with decreases in the slope or gain of baroreflex relationships between arterial blood pressure (BP) and heart rate (HR) or sympathetic activity. Plasma angiotensin II (AII) levels increase during pregnancy, and in some circumstances AII has been shown to decrease baroreflex gain. Therefore, the hypothesis that the reduced gain of pregnancy is mediated by increased AII was tested by determining if blockade of AII type 1 receptors with losartan (LOS; 5 mg/kg bolus followed by 40 μg/min infusion) increases gain in pregnant (P), but not nonpregnant (NP), conscious rabbits (n=7). Baroreflex curves were generated by changing BP with nitroprusside and phenylephrine and recording changes in HR. Before LOS, P rabbits exhibited reduced BP (68±1 vs. 59±3 mmHg; p<0.01), reduced maximum reflex gain (38±14 vs. 11±1 bpm/mmHg; p<0.05) and elevated minimum reflex HR (107±3 vs. 140±10 bpm; p<0.01). LOS decreased BP only in P rabbits (to 42±4 mmHg; p<0.05), but did not significantly alter gain in either group. LOS also decreased maximum reflex HR in P (from 284±9 to 250±5 bpm; p<0.01) but not NP (273±8 to 268±8 bpm) rabbits. Moreover, LOS shifted the HR/BP curve to a lower BP level in P (56±3 to 44±3 mmHg; p<0.05) but not NP (67±2 to 62±2 mmHg) rabbits. Minimum HR was not affected by LOS in either group. In conclusion, AII does not mediate the reduced baroreflex gain observed in P rabbits. However, during pregnancy, endogenous AII contributes to maintenance of BP, reflex maximum HR and the position of the HR baroreflex curve.