Baroreceptor reflex modulation by circulating angiotensin II is mediated by AT1 receptors in the nucleus tractus solitarii

Circulating angiotensin II (ANG II) modulates the baroreceptor reflex control of heart rate, at least partly via activation of ANG II type 1 (AT1) receptors on neurons in the area postrema. In this study we tested the hypothesis that the effects of circulating ANG II on the baroreflex also depend on AT1 receptors within the nucleus tractus solitarii (NTS). In confirmation of previous studies in other species, increases in arterial pressure induced by intravenous infusion of ANG II had little effect on heart rate in urethane-anesthetized rats, in contrast to the marked bradycardia evoked by equipressor infusion of phenylephrine. In the presence of a continuous background infusion of ANG II, the baroreflex control of heart rate was shifted to higher levels of heart rate, but had little effect on the baroreflex control of renal sympathetic activity. The modulatory effects of circulating ANG II on the cardiac baroreflex were significantly reduced by microinjection of candesartan, an AT1 receptor antagonist, into the area postrema, and were virtually abolished by microinjections of candesartan into the medial NTS. After acute ablation of the area postrema, a background infusion of ANG II still caused an upward shift of the cardiac baroreflex curve, which was reversed by subsequent microinjection of candesartan into the medial NTS. The results indicate that AT1 receptors in the medial NTS play a critical role in the modulation of the cardiac baroreflex by circulating ANG II, via mechanisms which are at least partly independent of AT1 receptors in the area postrema.