Angiotensin-(1#x2013;8)octapeptide (angiotensin II) is the active principle of the reninangiotensin system. Crossreaction of angiotensin II-antisera with inactive precursors and metabolic fragments prevented the specific quantitation of this hormone in biological fluids. Peptide-extraction on bonded-phase silica followed by peptide-separation using isocratic reverse-phase high performance liquid chromatography and subsequent radioimmunoassay rendered possible the octapeptide-specific measurement of angiotensin II in 2 ml plasma with a detection limit of 0.4fmol/ml. The coefficient of variation for intra-assay precision was 0.06 and for inter-assay precision 0.13.125Iangiotensin II was recovered from plasma by solid-phase extraction to 99#xB1;2 (mean #xB1; S.D.). The overall recovery of 5, 10 and 20 fmol unlabeled angiotensin II added to plasma was 80#xB1;10. Plasma concentrations in supine normal humans averaged 4.1 #xB1; 1.6 fmol/ml and were suppressed below the detection limit by angiotensin I converting enzyme inhibition.
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