Radioimmunological determination of plasma renin activity during the menstrual cycle and during acute progesterone administration.

Abstract

ABSTRACT Antibodies to ileu5angiotensin I were produced in rabbits with the poly-l-lysine succinyl complex of angiotensin I. Iodine-125 was used for the labelling of angiotensin I. The tracer was purified on a G-15 Sephadex column. For the determination of plasma renin activity plasma samples containing EDTA and BAL were incubated for two hours at 37°C. while a parallel sample was kept at 4°C to serve as a plasma blank. The angiotensin I content in 50 μl of plasma diluted 1:20 was quantitated directly, without prior extraction. For the immunoassay the samples and standards were incubated with 125I-angiotensin I (about 2500 cpm) and antiserum at a final dilution of 1:50 000–1:60 000 for 16–18 hours at 4°C. Dextran-coated charcoal was used for the separation of free and antibody-bound angiotensin I. The antiserum did not bind angiotensin II. The lower limit of detection was 20 pg. An average recovery of added angiotensin I of 100.5 % ± 11 (sd) was obtained. In duplicate determinations a coefficient of variation of 6.9 % was found. The method was used to measure renin activity during the menstrual cycle. In the follicular phase a range of 0.8–1.5, mean 1.14 ng ang.I/ml/h, was found, whereas the values in the luteal phase were 2.1–4.0, mean 3.06 ng ang.I/ml/h, respectively. When 500 mg progesterone, divided into 5 doses over a period of 24 hours was given to menopausal women, an increase in renin activity was found. No increase in aldosterone excretion or in the K/Na ratio in the urine was observed during this period.