Measurement of Plasma Renin Substrate by Radioimmunoassay of Angiotensin I: Concentration in Syndromes Associated with Steroid Excess

Abstract

Plasma renin substrate (PRS) and plasma renin activity (PRA) were measured by radioimmunoassay of generated angiotensin I in normal subjects and patients with steroid excess syndromes. Significant elevations of plasma renin substrate were observed in subjects with Cushing's syndrome, glucocorticosteroid therapy and oral contraceptive agents or estrogens. Plasma renin activity was significantly suppressed only in subjects with primary aldosteronism. The significance of elevations of PRS was assessed by determining the Km of the plasma renin-substrate reaction at physiologic pH. The apparent Km for the reaction was 1,000 ng Angiotensin I in PRS units. Correction of measured PRA to a standardized PRA (PRAJ was achieved by calculation of the reaction rate that would occur at the mean normal PRS concentration of 1100 ng/ml. The results indicate that elevations of PRS which occur in Cushing's syndrome or with administration of glucocorticoids, oral contraceptive agents, or estrogens are sufficient to significantly elevate the renin substrate reaction in plasma. In contrast to primary aldosteronism, glucocorticoid excess appears not to suppress plasma renin activity even when reaction rates are corrected for increased PRS. (J Clin Endocrinol Metab37: 110, 1973)