Application of a Radioimmunoassay for Angiotensin I to the Physiologic Measurements of Plasma Renin Activity in Normal Human Subjects1

A radioimmunoassay for angiotensin I and its application to the determination of renin activity is described. The assay employs antibodies raised to copolymers of angiotensin I and succinylated poly-l-lysine. Angiotensin labeled with 125I and purified by high voltage paper electrophoresis is employed as a tracer. Incubation is carried out in plasma in the presence of 3 metal binding reagents which serve to inhibit effectively proteolytic attack on angiotensin I. Immunoassay of generated angiotensin I is carried out directly on plasma diluted 1:20. Fifteen normal volunteers were studied on a metabolic ward at 2 levels of Na intake, during recumbency and upright posture, and following the administration of furosemide. Renin activity increased with Na restriction, was significantly higher on upright activity during both normal and restricted Na intake, and was most markedly elevated following the diuretic. Renin values obtained by immunoassay of angiotensin I correspond closely to those observed by bioassay in similar metabolic studies. The immunoassay provides the advantages of relative simplicity, specificity and reproducibility.