SYNOPSIS A semi-micro method for determination of plasma renin activity (PRA) by radioimmunoassay of generated angiotensin I (Al) is described. The method permits measurement of PRA on 250 pl of plasma without loss of specificity, sensitivity, accuracy or precision.The small sample size has considerable application in terms of investigation of infants and young children.A reference range was established for healthy children on free diets. There was a 15-fold decline in PRA with age from a mean value of 1404 pgAI/ml h-1 (ngAI/l. h-1) in infancy to a mean of 85 pgAI/ml h-' (ngAI/l. h-1) in adult life.Several methods utilizing radioimmunoassay techniques have been reported for the measurement of plasma renin activity (PRA) based on the generation rate of angiotensin I (Al) in plasma (Boyd etal, 1969;Haber et al, 1969;Vallotton, 1969;Hollemans et al, 1969;Cohen et al, 1971;Kurtz, 1971;Rossler et al, 1971;Menard and Catt, 1972;Katz and Smith, 1972;Sealey et al, 1972;Malvano et al, 1972;Brech and Franz, 1972;Viol et al, 1972).Many of these techniques required comparatively large quantities of blood and this has hampered the investigation of the renin angiotensin system in young children. This study describes a semi-micro method for measuring PRA and a preliminary reference range which has been determined in children.
PrincipleSince it is not possible to measure the level of plasma renin directly, indirect methods have been developed. Plasma renin activity measures the rate of generation of Al as a result of the action of renin on its natural substrate. This reaction takes place under controlled conditions of incubation and in the presence of enzymatic inhibitors. which prevent Al degradation. The generated Al is measured by radioimmunoassay.