Inactive renin--a renin proenzyme?

Asynchronous changes in prorenin and renin secretion after captopril in patients with renal artery stenosis

An assay of plasma prorenin was developed in which the conversion to renin occurred under apparently optimal conditions. Some characteristics of the assay were 1) prorenin was activated by Sepharose-bound trypsin at 4 degrees C; 2) the concentration of activator was not critical provided that incubation was prolonged until renin activity had reached a plateau; and 3) this plateau was stable and had the same height as after maximal activation with acid, pepsin, plasmin or urokinase. Maximal activity with Sepharose-bound trypsin at 4 degrees C was higher than with cryoactivation, and optimal conditions were more readily reproduced than with trypsin at 37 degrees C or with acid-activation. The assay was used for measurements in peripheral and renal vein plasma after captopril in hypertensive patients with unilateral renal artery stenosis. Peripheral renin rose within 30 minutes after a first dose of captopril, 50 mg orally, and it remained high with chronic treatment. In contrast, peripheral prorenin fell initially and rose after 4 hours. These changes in peripheral plasma were related to changes in the secretion rates of the two forms of renin from the affected kidney. Thus chronic, but not acute, stimulation of renin release was associated with an increased secretion rate of prorenin. The late rise in prorenin is probably an indication of enhanced synthesis in the kidney, so that more prorenin is available for conversion. The data suggest that prorenin is indeed a biosynthetic precursor of renin and that, at least under certain circumstances, a major proportion of circulating prorenin originates from the kidney.

Dynamic responses of active and inactive renin in patients with essential and renovascular hypertension

We studied the dynamic responses of inactive renin and the form of renin released by the kidney in the hypertensive patients. Significant increase of active renin concentration (p less than 0.01) and decrease of the percentage of inactive renin concentration (p less than 0.01) after sodium depletion was observed in 15 essential hypertensive subjects with normal plasma renin activity. In eight of 15 patients, significant increase of inactive renin concentration (p less than 0.01) was observed after sodium depletion. In the remaining seven patients, no significant change of inactive renin concentration was demonstrated. A small increase of active and inactive renin concentration was observed following sodium depletion in six essential hypertensive subjects with low plasma renin activity (PRA). In the unilateral renal hypertension after upright tilting, active renin concentration in the renal vein of the affected kidney was significantly (p less than 0.02) higher than that in the renal vein of the non-affected kidney and the inferior vena cava. Inactive renin concentration in the renal vein of the affected kidney was significantly (p less than 0.02) lower than that in the renal vein of the nonaffected kidney and the inferior vena cava. In four of five cases, the inactive renin concentration in the femoral artery was less than that in the inferior vena cava. Therefore, we might conclude that only active renin was released from the affected kidney, and active renin became inactive by unknown mechanisms; the ischemic kidney might also activate inactive renin.

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).

Renin-activation by kallikrein: evidence for alkaline-activation of plasma renin

Porcine pancreatic kallikrein leads to a significant activation of human plasma renin when incubated at pH 8.2. 'Endogenous' activation of plasma renin can be seen by incubation of plasmas at the same pH. Both 'endogenous' alkaline- and kallikrein-activation of plasma renin can be inhibited by aprotinin. Both, kallikrein- and 'endogenous' alkaline activation of plasma renin are significantly correlated to cryo- and acid-activation. We suggest that kallikrein-and 'endogenous' alkaline-activiation come close to the in vivo activation process.

Acid- and cryoactivation of renin in human plasma

Inactive renin in normal human plasma was activated in vitro either by cryoactivation (incubation at 0 degrees C/-5 degrees C up to 3 months) or acid-activation (dialysis to pH 3.0 for 48 h followed by diaylsis to pH 7.5). Plasma-renin-concentration was similar after either activation procedure (96 +/- 50 microU/ml vs 100 +/- 43 microU/ml). There was no further significant acid-activable renin after cryoactivation. These data suggest that conversion of inactive to active renin by optimized procedures is complete. The term "total renin" for activation results under these conditions seems to be valid.