The reliability of the measurement of plasma renin activity by radioimmunoassay.

A radioimmunoassay of angiotensin I has been applied to the measurement of plasma renin activity. Angiotensin I was generated in plasma samples by 3 h incubation at 37 degrees C and pH 5.6 after addition of EDTA and Dowex. The generated amount of angiotensin I was measured by radioimmunoassay in the eluate of the Dowex column. With this method a negligible amount of angiotensin I was measured after incubation at 4 degrees C (0.8 ng/ml per 3 h). Eluate of blank plasma had no measurable effect on the standard curve. The mean recovery of angiotensin I was 87%. The limit of detection of the assay was 0.5 ng/ml per 3 h. The results obtained using different antisera were equal. A marked variation was found in immunological properties of different standard preparations of angiotensin I tested. The mean value of angiotensin I generation per Goldblatt Unit (G.U.) renin was 3.9 with 10-4 ng/h. In normotensive control subjects, the plasma renin concentration, whileon unrestricted diet and after 3 h of ambulation, was on average 0.39 with 10-minus 4 G.U./ml, range 0.12 with 10-minus 4-0.91 with 10-minus 4. With the use of the same plasma extracts for radioimmunoassay and bioassay, a perfect correlation was found between the plasma renin activities measured with both assays. The differences found between the results of both assays could be fully explained by the different biological activities of the standards used (Angiotensin I, Schwarz Mann, and Angiotensin II, Ciba-Geigy). With a direct radioimmunoassay, angiotensin I was generated in plasma by 3 h incubation at 37 degrees C and pH 5.6 after addition of phenylmethanesulfonyl fluoride, 8-hydroxyquinoline and 2,3-dimercaptopropanol (dimercaprol). The generated amount of angiotensin I was measured by the above mentioned radioimmunoassay. A fairish correlation was found between the generated amounts of angiotensin I measured in the Dowex eluate and those found in the incubated plasma. Especially in the lowest range, lower values were obtained by the latter assay. However, the generated amounts of angiotensin I measured in non-incubated plasma samples (3 h at 4 degrees C) was on average 6.4 ng/ml per 3 h and accounted for 748% of the amounts found after incubation at 37 degrees C.