Estimation of Plasma Renin Activity by Radioimmunoassay

(I) Factors Influencing the Plasma Renin Activity in Various conditions

Abstract

The renin-angiotensin system had been received a great deal of attention in references with the development of arterial hypertension. Although the measurement of the plasma renin activity (PRA) is very important for evaluating the patients with arterial hypertension, it may be difficult to decide whether the PRA measured is normal or abnormal since the rate of renin secretion from the kidney is said to be greatly determined by a variety of factors even under normotensive states. The recent introduction of radioimmunoassay has promoted studies in the dynamics of renin-angiotensin system.
The purpose of this study is to evaluate the immunoassay method, to det ermine the normal dynamic ranges of PRA levels in the various conditions and to search for the factors influencing PRA. Radioimmunoassay of PRA was performed using the kits commercially distributed by Schwarz-Mann (SM), CEA-CEN-SORIN (CCS) and CEA-IRESORIN (CIS). For all of these kits PRA was given as an amount of angiotensin I generated in ng/ml/hr at 37°C and the range of the measurement was directed as from 0.05 to 0.80 ng.
1) PRA levels in the sample tubes increased lineally with the incubation time being elapsed from 0.5 to 6 hours.
2) The normal PRA levels under various conditions were measured in normotensive patients(patient controls). Under normal diet, PRA in mean at rest was 0.61±0.35(S. D. )ng/ml/hr by SM & CCS and 2.02±0.56ng/ml/hr (CIS), and after 4 hour-standing it increased to 1.67±0.55ng/ml/hr (SM & CCS) and 3.54 ±1.76ng/ml/hr (CIS). Under Na restricted diet (30mEq/day), PRA at rest was 1.46 ±O.62ng/ml/hr (SM & CCS),5.19±0.72 ng/ml/hr (CIS), and after 4 hour-standing under furosemide administration it was 6.96 ±2.19ng/ml/hr (SM & CCS),10.8 1±4.13ng/ml/hr (CIS).
3) Postural changes in PRA was investigated to prove that it was required one and a half, and two hours for PRA level to stabilize after taking recumbent, and standing position, respectively.
4) Diurnal change in PRA was studied in three healthy subjects and its level was found to be higher from the midnight to the morning and remained in the lower level in the afternoon.
5) The menstrual cycle was also found to affect PRA level in 4 normal females that it was higher in the luteinizing phase than in the follicular one.
It was discussed that the measurement of PRA and its interpretation would be valid only when the factors and conditions of the patients such as diet, posture and so forth be clearly defined.