Simultaneous radioimmunoassay of six angiotensin peptides in arterial and venous plasma of man

Using antibodies raised against angiotensin I and II, and high-performance liquid chromatography (HPLC) of plasma extracts, we have quantified six angiotensin peptides in venous (cubital vein) and arterial (brachial) plasma of normal male subjects. The concentrations of venous plasma (fmol/ml, mean +/- s.d., n = 29) for these six peptides were: pentapeptide-(4-8): 1.5 +/- 1.1; hexapeptide-(3-8): 1.0 +/- 0.8; heptapeptide-(2-8): 2.4 +/- 2.6; octapeptide-(1-8): 10.7 +/- 6.6; nonapeptide-(2-10): 3.7 +/- 2.1; and decapeptide-(1-10): 18.7 +/- 10.7. No significant differences in the levels of each peptide were found for arterial and venous plasma; thus the rate of production of these peptides in the forearm is equivalent to their rate of clearance by the forearm. Both angiotensin I and II were significantly correlated with plasma renin, and there was a highly significant correlation between angiotensin I and II. With respect to the mechanism of local production of angiotensin, the close correlations between renin and angiotensin I and II in venous blood provide evidence that, for the forearm, plasma renin is a major determinant of local production of angiotensin. The ratio of angiotensin I:II, 1.83: 1, was similar for arterial and venous plasma, and indicates that despite the ubiquitous distribution of the angiotensin converting enzyme, the conversion of angiotensin I to angiotensin II is a significant rate-limiting step in angiotensin II formation.