SUMMARY Investigations were performed on components of the renin-angiotensin system (RAS) in homogenate extracts of vascular tissue and aortic smooth muscle cells cultivated in vitro. Determinations of isoelectric points and pH optima indicated the existence in aortic homogenate extracts of two local angiotensin I (AI)-forming enzymes (AIFE) that were different from those of plasma, renal cortex, veins, and aortic smooth muscle cells. The pH optima for Al-converting enzyme (ACE) from vascular tissues, aortic smooth muscle cells, and plasma were in the same range (pH 8.0-8.5), and in agreement with those measured previously in other tissues. In contrast, in vitro studies with the ACE inhibitors MK-421 and MK-422 and measurement of isoelectric points suggested that aortic ACE was different from the plasma enzyme. AIFE and ACE activities were found to be elevated in spontaneously hypertensive rats (SHR). The biochemical characteristics of the enzymes investigated in the vascular tissue of SHR were not different from those of the normotensive controls. AI-and All-degrading enzymes were found both in aortic tissue and in aortic smooth muscle cells. One potent Al-degrading enzyme different from ACE was observed in aortic tissue. A high ratio of AI/AII immunoreactivities in arterial walls suggests the availability of renin substrate, and that Al-degrading enzymes are the rate-limiting enzymes for AH formation. were the first to demonstrate the existence of arterial and also venous wall renin by using systematically performed biochemical methods. In the last few years, there has been renewed interest in the vascular reninangiotensin system (RAS). Now, as before, it is thought to play a role in the regulation of blood pressure and in the development and maintenance of hypertension. The present investigation continues our previous studies in this field 4 ' 3 and was undertaken to recognize, quantify, and partly characterize those components of

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