Angiotensin II as a modulator of baroreceptor reflexes in the brainstem of conscious rats.

The effect of microinjection into the nucleus tractus solitarii (NTS) of angiotensin II (Ang II) on baroreceptor control of heart rate (HR) in conscious, freely moving rats was evaluated with a new method of long-term cannulation of the dorsal brainstem areas. Reflex changes in HR were produced by intravenous bolus injections of either phenylephrine or sodium nitroprusside (0.2-25.6 micrograms/kg) both after saline and after unilateral microinjection of Ang II into the NTS (24 ng, 0.2 microliter) and compared with those produced after administration of Ang II into the fourth ventricle (24 ng, 0.2 microliter) or intravenously (1-2 ng/kg/min). Baseline levels of mean arterial pressure (MAP) and HR were not affected by the route of Ang II application but reflex bradycardia during MAP increase was significantly attenuated after injections of Ang II into the NTS. Both the slope and the intercept of the regression line function between delta HR and delta MAP were reduced by 43% from the control value of -1.55 +/- 0.13 beats/min/mm Hg (p less than 0.01) and -14 +/- 5 beats/min (p less than 0.05), respectively. Similar reductions were observed after Ang II administration into the fourth ventricle or intravenously, although microinjections into the cerebellum produced no effect. Endogenous blockade of Ang II by saralasin (22 ng) in the NTS facilitated the bradycardic response (-2.29 +/- 0.91 beats/min/mm Hg). Nitroprusside-induced tachycardia was not altered by Saralasin microinjection into the NTS or by Ang II application to the NTS, fourth ventricle, or intravenously.(ABSTRACT TRUNCATED AT 250 WORDS)