In vivo production of plasma angiotensin I: is plasma renin sufficient?

Production and metabolism of angiotensin I (Ang I) were studied during a constant intravenous infusion of mono-iodinated 125I-Ang I in seven patients with essential hypertension taking 50 mg captopril twice a day. The elimination of arterially delivered 125I-Ang I reached 80% in the kidneys, 45% in the forearm, 58% in the leg and 95% in the hepatomesenteric vascular bed. Despite the high level of elimination of Ang I, plasma levels of Ang I in the leg, forearm and renal vein were not different from that of arterial plasma. In the hepatic vein the plasma level of Ang I was 50% lower than in the aorta. Thus, 50-90% of endogenous Ang I in the veins is derived from new regional production. The blood transit time through the kidneys, limbs and hepatomesenteric region is too short for plasma renin activity to account for the measured new Ang I production. Our results indicate that a high percentage of plasma Ang I may be produced locally in the tissues, and not in circulating plasma.