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The direct measurement of active renin with monoclonal antibodies (IRMA) in plasma from hypertensive patients was compared with the traditional PRA method. Two monoclonal antibodies were used: 3E8 and 4G1. The first was coupled to magnetizable beads and was used to trap both active and inactive renin from plasma. The second antibody, 4G1, was iodinated and used to detect active renin trapped by 3E8. The correlation coefficient between the two methods was very high (r = 0.98, p less than 0.001) in plasma samples whose PRA values were higher than 2 ng/ml/h; in low renin samples (PRA lower than 2 ng/ml/h) no significant correlation was found (r = 0.12 n.s.). When PRA and IRMA were performed before and after trypsin activation of inactive renin, the percentage of inactive over total renin was 86.8% and 84% as calculated with PRA and IRMA respectively. The direct monoclonal antibodies method for measuring active renin can be usefully adopted, in conjunction with the traditional PRA procedure, in studying both clinical and pathophysiological aspects of the renin-angiotensin system.

The experimental conditions of trypsin activation of inactive renin in rat plasma have been studied. Our results showed that the pH optimum of trypsin-activated renin is 6.2, the same as that of rat plasma active renin. Trypsin concentration and incubation time might also interfere with the activation process. Trypsin concentration below 2 mg/ml cannot activate inactive renin. Trypsin at a concentration of 6 mg/ml for 1 min. can cleave dialyzable fragments from renin substrate which can generate Angiotensin I at 37 degrees C with a pH optimum of 5.3 but which do not affect Angiotensin generation at pH 6.2. Longer incubations (more than 2 min.), on the contrary, can produce a cleavage at 4 degrees C of Angiotensin I containing fragments directly from renin substrate strongly interfering with measurements of renin activity at pH 6.2. Trypsin concentrations lower than endogenous rat plasma anti-trypsin activity do not activate inactive renin. Much higher concentrations of trypsin, however, are needed to obtain optimum activation of inactive renin. In average, 40% of the total circulating renin in rat plasma can be activated by trypsin.

Arterial and renal venous active and inactive renin were studied in 5 patients with long established moderate hypertension following unilateral acute reductions of renal perfusion pressure (15% and 70% of control) by inflating a balloon catheter introduced into the right renal artery. This procedure failed to induce the expected release of active renin; total and inactive renin levels were also unchanged. On the contrary in a single normotensive patient smaller reductions of the renal perfusion pressure (-15% and -30%) were able to acutely increase the release of active renin with a concurrent conversion of inactive renin but without inducing blood pressure changes. These findings show that the renin pattern typical of unilateral renovascular hypertension, including the intrarenal activation of inactive renin, could be reproduced acutely in a normotensive subject. Moreover, a complete reversal of the above mentioned active and inactive renin pattern was observed in a recent onset renovascular hypertensive patient within 30 min from successful percutaneous transluminal dilation. The negative results observed in our hypertensive patients suggest that structural changes induced by the long duration of hypertension might have reduced the sensitivity of the baroceptors involved in renin release.

The role of the kidney as a possible source or as activator of inactive renin was studied in 22 patients with Essential Hypertension (EH) and in 20 patients with Unilateral Renal Artery Stenosis (RAS). Active and inactive renin (trypsin activation) were measured in blood samples taken simultaneously from both renal veins and from a peripheral artery during acute diuretic stimulation induced by furosemide 40 mg i.v. In EH pts active and trypsin-activated renin were significantly higher in both renal veins than in arterial blood (P less than 0.001 and P less than 0.02 respectively) whereas no difference was seen as far as inactive renin is concerned. In unilateral RAS trypsin-activated and active renin from the ischemic kidney were significantly higher (P less than 0.01 and P less than 0.005 respectively) while inactive renin was significantly lower (P less than 0.005) than in arterial blood. No significant difference was seen between arterial and renal venous blood from the contralateral kidney as far as active and inactive renin are concerned. When comparing the V-A differences for active renin to the corresponding V-A differences for inactive renin from the ischemic kidney a significant negative correlation appeared (r = -0.49 p less than 0.05) whereas no correlation was found from the contralateral kidney (r = -0.26 n.s.). These data demonstrate that the ischemic kidney, in addition to its ability to release active renin, can also activate circulating inactive renin.