Renin-angiotensin modulation of sympathetic reflex function in essential hypertension and in the elderly

The renin-angiotensin system and the sympathetic nervous system are important control mechanisms in blood pressure regulation. A large body of in vitro and in vivo animal data indicate that angiotensin II regulates noradrenaline release and modifies sympathetic reflexes, although the physiological relevance of these to normal man remains unclear. It is established, however, that increasing age and hypertension independently modify autonomic responses in man. The hypothesis was therefore examined that angiotensin II exerts a pre-junctional effect on noradrenaline release and sympathetic cardiovascular function, in the elderly and in patients with essential hypertension. Inhibition of angiotensin-converting enzyme with enalapril and cilazapril, after short-term administration at doses sufficient to reduce circulating angiotensin II concentrations, did not alter sympathetically mediated orthostatic test, Valsalva's manoeuvre and cold pressor test, in young or elderly normotensives. Similarly, but in a separate study, the pressor, chronotropic and sympathoadrenal response to submaximal dynamic exercise were unchanged, following short-term converting enzyme inhibition in normal persons and essential hypertensive patients. These findings do not support a role for angiotensin II in presynaptic facilitation or post-junctional potentiation of noradrenergic transmission. It is proposed that the renin-angiotensin system exerts an independent, rather than a regulatory influence on sympathetic function in blood pressure control, in normal and hypertensive man.