Estimation of plasma renin activity by radioimmunoassay of angiotensin I.

The renin-angiotensin-aldosterone hor­ monal system has been shown to be involved in the pathogenesis of certain hypertensive disorders, notably primary aldosteronism, malignant hypertension, renovascular hy­ pertension and that induced by oral con­ traceptives. 1 " 7 Plasma renin assay has be­ come an important diagnostic tool for in­ vestigating these disorders. Previously, plasma renin activity has been measured by a tedious, complicated bioassay. The availability of synthetic 1 2 5Iangiotensin I and its antibody has made possible the measurement of renin activity in plasma by radioimmunoassay, 8" 11 by utilizing the principle of saturation analy­ sis. Normal human plasma contains angiotensinogen (a substrate) and the enzymes renin and angiotensinase. During the in­ cubation of plasma, the decapeptide angio­ tensin I is generated by the action of renin on the substrate. The further conversion of angiotensin I to angiotensin II by the action of angiotensinase can be prevented by the addition of EDTA, dimercaprol and 8 -hydroxyquinoline to the plasma sample. The angiotensin I generated per ml of plasma per hour (ng/m l/hr) at37°C (plasma renin activity or PRA) is determined by radio­ immunoassay. Collection o f Blood Specim ens The blood specimen should be drawn between 8:00 and 11:00 AM after the pa­ tient has been in the upright position for one hour. It should be taken into a chilled Vacutainer tube containing Na2EDTA (not the potassium salt), which is then im­ mersed immediately in ice-water. The blood is centrifuged in the cold (2° to 4°C ), the plasma separated without delay and kept frozen ( —20°C) until analyzed.