Endogenous angiotensin I concentration in human plasma.

A reliable radioimmunoassay of endogenous angiotensin I concentration in plasma, extracted on a Dowex-resin at pH 7.4, is described. The concentration of angiotensin I in the pH 7.4-extracted plasma samples is significantly correlated with the angiotensin I concentration in the samples extracted at pH 5.5, the pH optimum of the in vitro reaction of human renin with human renin substrate. The presence of immunoreactive angiotensin I-like material in plasma and the relatively high cross-reaction of renin substrate with the anti-angiotensin I antiserum necessitates a chromatographic purification on a Dowex resin. The normal range found for angiotensin I in venous blood of apparently normal, male subjects was 25 to 143 pg/ml. A significant correlation (r = 0.78; p less than 0.001) between PA-I and PRA was found in normal subjects. No differences (p greater than 0.10) between arterial and venous angiotensin I concentration were observed in hypertensive patients.