The role of the renin-angiotensin system in the hormonal and renal responses to tilt in normal man.

Head-up tilt is associated with rapid increases in total peripheral and renal resistance and falls in the urinary excretion of water and sodium. The influence of the acute oral administration of the converting enzyme inhibitor captopril (50 mg) on the consequences 60 degrees head-up tilt were assessed in 6 normal subjects on unrestricted sodium intake. Captopril did not modify the responses of arterial pressure and heart rate to tilt, although a significant decrease in supine mean arterial pressure was induced by captopril. In addition, the agent blunted by 50-60% all the changes in renal hemodynamics and urinary excretion of water and electrolytes except for that of the fractional excretion of sodium. Urinary kallikrein activity which markedly fell and plasma aldosterone concentration which increased during tilt before captopril were not affected after administration of the drug. These studies indicate that angiotensin might mediate at least in part the renal hemodynamic and functional responses to tilt in normal man and is the primary determinant of the aldosterone stimulating effect of tilt.