Role of AT1 receptors in the central control of sympathetic vasomotor function

1. In a number of species, high concentrations of angiotensin II (AngII) receptors have been found in the rostral ventrolateral medulla (RVLM) in the hindbrain, which is an important region involved in the modulation of sympathetic vasomotor tone. The present review describes studies in which the contribution of angiotensin receptors in the brainstem to cardiovascular regulation, in particular sympathetic vasomotor reflexes, has been examined in conscious and anaesthetized rabbits. 2. In conscious rabbits, fourth ventricular infusions of AngII produced dose-dependent pressor responses as does 400 times less than equipressor intravenous doses. Chronic baroreceptor denervation increased the sensitivity to AngII by 1000-fold. Administration of prazosin i.v. blocked the pressor response, suggesting that the mechanism involved sympathetic vasoconstriction. 3. The pattern of haemodynamic changes in response to AngII injected into the fourth ventricle (4V) involved decreased total peripheral conductance and mesenteric conductance, but a rise in hindlimb conductance. Sinoaortic denervation changed the hindlimb fall in conductance to an increase, suggesting that muscle vasomotor pathways were particularly inhibited by baroreceptor feedback mechanisms. 4. In anaesthetized rabbits, infusion of AngII into the RVLM increased blood pressure and transiently increased resting renal sympathetic nerve activity. The renal sympathetic baroreflex curves were shifted to the right and the upper plateau of the sympathetic reflex increase was markedly increased. 5. The pressor actions of 4V AngII were blocked by administration of a peptide antagonist injected into the RVLM or by the angiotensin AT1 antagonist losartan injected into the 4V. These results suggest that mainly AT1 receptors are involved and that the RVLM is a likely candidate site for the modulation of the renal sympathetic baroreflex. 6. Losartan administration into the 4V in conscious rabbits increased resting renal sympathetic tone and enhanced renal sympathetic baroreflex and chemoreflexes. 7. Our studies suggest that there are sympathoexcitatory AT1 receptors in the RVLM accessible to AngII from the cerebrospinal fluid. In addition, an AT1 receptor pathway normally inhibits the sympathoexcitation produced by baroreceptor unloading or chemoreceptor activation. The effect of losartan suggests that there is greater tonic activity within the sympathoinhibitory pathways. These two actions suggest that angiotensin receptors in the brainstem modulate sympathetic responses to specific afferent inputs, thus forming part of a potentially important mechanism for the integration of characteristic autonomic response patterns.