1. To study which factors determine the balance between the antagonistic and agonistic effects of the angiotensin II analogue [Sar1,Ala8]-angiotensin II (saralasin) in man, saralasin was infused in subjects on a ‘normal’ sodium intake (group 1) during sodium restriction with appropriately elevated plasma angiotensin II levels (group 2) and in sodium-restricted subjects in whom plasma angiotensin II was suppressed by converting enzyme inhibition with captopril (group 3).
2. The action of saralasin was agonistic in group 3, antagonistic in group 2 and variable in group 1.
3. For groups 1 and 2 together the saralasin-induced changes of arterial pressure, of plasma aldosterone and of plasma renin were significantly related to control plasma angiotensin II but also to the 24 h urinary sodium excretion. When group 3 was included the changes remained significantly related to plasma angiotensin II but not to the urinary sodium excretion.
4. The results indicate that angiotensin II and not sodium status determines the agonistic/antagonistic balance of saralasin's actions.