Cardiovascular and neurohormonal changes following central sympathetic blockade with clonidine in human unilateral renal artery stenosis

Abstract

The role of central pressor mechanisms in the maintenance of blood pressure in six hypertensive patients with angiographically proven unilateral renal artery stenosis was investigated by studying the haemodynamic and hormonal responses before and after central sympathetic blockade with clonidine. To assess the dependency of blood pressure on the direct effects of angiotensin II, the same patients were studied on a separate occasion after administration of the angiotensin converting enzyme (ACE) inhibitor, captopril. There was a substantial fall in blood pressure after administration of clonidine with a smaller fall after captopril. Clonidine-induced hypotension was accompanied by a fall in cardiac output, through a reduction in the stroke volume and heart rate. Forearm vascular resistance was unchanged. There was a selective decrease in digital skin vascular resistance and plasma noradrenaline, indicating reduced sympathetic activity. Plasma renin activity and aldosterone levels did not fall. After administration of captopril, there was a fall in cardiac output due to a fall in the stroke volume but not in the heart rate. Forearm vascular resistance, digital skin vascular resistance and plasma noradrenaline were unchanged. Plasma renin activity rose and plasma aldosterone fell. We conclude that in our hypertensive patients with renal artery stenosis, clonidine lowered blood pressure by a reduction in central sympathetic activity independently of renin suppression. In these patients captopril had minimal hypotensive effects, indicating a smaller role for the direct vascular effects of angiotensin II.