Specific direct radioimmunoassay of angiotensin II (AT II) in human plasma and the effect of angiotensin converting enzyme (ACE) inhibitor.

Specific direct radioimmunoassay of angiotensin II (AT II) in human plasma was developed to evaluate the dynamics of endogenous AT II in various types of hypertension. Detection limit of this method was less than 2.3 pg/ml, and normal value is less than 25 pg/ml. Cross reactivity of antibody with AT I and III was 0.037% and 21%, respectively. There were good correlation between the value measured by direct method, and that of extraction method (r = 0.96, P < 0.01) and plasma renin activity (r = 0.80, P < 0.01). By oral administration of ACE inhibitor (captopril 50 mg), AT II levels were suppressed to 10 pg/ml or less in most patients with essential hypertension, renal parenchymal hypertension and renovascular hypertension up to 2 h. However, AT II levels in patients treated with ACE inhibitors chronically were not different from the AT II levels in patients without ACE inhibitor. In primary aldosteronism AT II was extremely low levels. AT II markedly increased by the stimulation test using furosemide (1 mg/kg i.v.). These results suggest that this method may be useful to clarify the pathophysiology of hypertension and the escape of the inhibition by ACE inhibitor.