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SUMMARY Vascular renln-llke activity was studied in the aortas and the cerebral microvessels of Sprague-Dawley rats and in the aortas of spontaneously hypertensive rats. Methods were employed to maximize detection of tissue renin and to simultaneously minimize contamination of that activity by either plasma renin or nonspecific proteases capable of angiotensin I generation. To this end, renin activity was measured near its pH optimum; plasma renin was eliminated by nephrectomy; and nonspecific proteases such as cathepsin D were either inhibited by proteolytic blockers or removed by chromatography over immobilized bovine hemoglobin. Aortic vascular renin-like activity was detected in rats not subjected to nephrectomy and could be inhibited by preincubation of samples with antimouse renin antibody shown to cross-react and inhibit rat plasma renin activity. extrarenal vessels may be a source of renin;'" 16 but measurement of vascular renin (i.e., renin arising in vessel wall) can be complicated by the presence in vessel wall of other enzymatic activities capable of generating angiotensin I (AI) in the renin assay. 1718 In the present study, methods were used to maximize detection of vascular renin and to simultaneously minimize contamination of that activity by either nonspecific proteases or plasma renin. To maximize detection of vascular renin, activity was assayed near the pH optimum of renin. "• 19~22 Since nonspecific acid proteases can generate AI from angiotensinogen at near the optimal pH for renin activity, 17 ' 18 ' a all samples were incubated with proteolytic inhibitors and any residual activity was removed by chromatography over immobilized bovine hemoglobin. Activity of nonspecific acid proteases was monitored by radiochemical assay. 24 Despite the removal of nonspecificFrom the Hypertension-Endocrine Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland.Current address for Dr. Fordis: Laboratory of Chemical Biology, N1ADDK, Bldg. 10, Room 9N-321, National Institute of Health, Bethesda, Maryland 20205.Address for reprints: Dr. Harry R. Keiser, National Heart, Lung, and Blood Institute, Building 10, Room 8C-1O3, Bethesda, Maryland 20205.Received March 2, 1982; revision accepted April 18, 1983. acid proteases, a renin-like activity could be detected in rat aorta. To eliminate the possibility that the reninlike activity in rat aorta represented plasma renin contamination, rats were nephrectomized; and plasma renin activity and vascular renin-like activity were measured at 2, 6, and 24 hours after nephrectomy. After the disappearance of plasma renin activity, no vascular renin-like activity could be detected in aortas collected from either Sprague-Dawley or spontaneously hypertensive rats (SHR). Vascular renin-like activity was also absent from microvessels collected from the rat cerebral cortex. Materials and MethodsExperiments were designed to examine vascular renin-like activity in both large vessels (aortas) and small resistance vessels (cerebral micr...