1. To examine the role of angiotensin II in the maintenance of blood pressure and control of aldosterone secretion, eight normal human subjects were studied on a tilt table in sodium-replete and sodium-depleted states, before and after the administration of an angiotensin converting-enzyme inhibitor (CEI).
2. Administration of CEI was followed by a marked fall in blood pressure on tilting in sodium-depleted, but not in sodium-replete, subjects. CEI administration also resulted in a rise in plasma renin activity in the supine position, in the absence of haemodynamic change. The rise in plasma aldosterone observed both in response to tilting and sodium depletion did not occur after CEI, even though plasma renin activities were higher.
3. These results indicate that: (a) angiotensin II is essential for blood pressure control in the sodium-depleted individual; (b) angiotensin II exerts direct feedback control on renin secretion; (c) angiotensin II is the primary stimulus to aldosterone secretion in response to both sodium depletion and posture.