Direct radioimmunoassay of human renin: comparison with renin activity in plasma and amniotic fluid.

Human plasma and amniotic liquid were activated by dialysis at pH 3.3. Then, renin before and after acidification was determined by two methods: enzymatic activity measurement, and direct radioimmunoassay. The identity between nonactivated and activated renin in plasma and amniotic fluid on the one hand, and pure renin on the other, was demonstrated by the dilution curves in radioimmunoassay. After acidification, mean plasma renin activity in 17 patients with high renin activity rose from 26.8 +/- 11.7 pmoles A I ml-1 h-1 to 67.9 +/- 29.3 pmoles A I ml-1 h-1, whereas the mean renin concentration tested by direct radioimmunoassay remained constant at 13.8 +/- 10.5 and 14.8 +/- 11.2 fmol/ml before and after acidification respectively. In amniotic fluid, renin activity increased from 9.7 to 227 pmoles angiotensin I/ml/h, but the renin concentration did not change. Direct radioimmunoassay of renin may therefore be considered as measuring total renin, regardless of its enzymatic activity. In 12 hypertensive patients undergoing bilateral renal-vein catheterization, the direct measurement of renin was very significantly correlated to the non-activated (r = 0.883) and activated renin values (r = 0.963).