Role of AT(1) receptors in area postrema on baroreceptor reflex in spontaneously hypertensive rats

Intravenous injection of angiotensin II type I (AT(1)) receptor antagonist improves the baroreceptor reflex gain in spontaneously hypertensive rats (S HRs). To investigate the role of area postrema in the modulation of the bar oreflex control by AT(1) receptor, the effects of intravenous injection of CV-11974 (AT(1) receptor antagonist) on the baroreflex control of renal sym pathetic nerve activity (RSNA) and heart rate (HR) were examined in sham an d area postrema-lesioned SHRs. The baseline mean arterial pressure was simi lar in both groups. However, baseline heart rate was significantly lower (p < 0.01) in area postrema-lesioned SHR than in sham-lesioned SHR, 307 +/- 1 1 and 365 +/- 10 beats/min (bpm), respectively. Intravenous CV-11974 (0.05 mg/kg) significantly decreased mean arterial pressure; however, it did not change HR and RSNA in either group. Reflex changes in RSNA and HR were elic ited by intravenous infusion of either phenylephrine or sodium nitroprussid e before and after intravenous injection of CV-11974. Intravenous CV-11974 increased baroreflex control of RSNA (G(max); -1.57 +/- 0.08vs. -1.42 +/- 0 .12%/mmHg, p < 0.05) and HR (G(max); -0.54 +/- 0.12 vs. 1.25 +/- 0.24 bpm/m mHg, p < 0.05) in sham-lesioned SHRs. However, intravenous CV-11974 failed to alter the baroreflex sensitivities in area postrema-lesioned SHRs. These results suggest that the area postrema does not play a crucial role in mai ntenance of high blood pressure in adult SHRs, and rat the improvement of b aroreflex control of RSNA and HR by intravenous CV-11974 is mediated via th e area postrema in SHRs. (C) 1999 Elsevier Science B.V. All rights reserved .