True versus immunoreactive angiotensin II in human plasma

Abstract

To measure specifically angiotensin-(l-8)octapeptide, peptides were extracted from 2 ml of plasma by reversible adsorption to bonded-phase silica. The angiotensin-(l-8)octapeptide was then isolated by isocratic reversed-phase high-performance liquid chromatography and quantified by radioimmunoassay. The extraction recovery of l25I-angiotensin II added to 2 ml of plasma was 99 ± 2% (mean ± SD). The overall recovery of 5,10, and 20 fmol unlabeled angiotensin II added to 1 ml of plasma was 80 ± 10%. The coefficient of variation for within-assay precision was 0.06 and for between-assay precision 0.13. The detection limit was 0.4 fmol/ml. Buffer and plasma blanks were below the detection limit. Normal subjects on a free diet in supine position averaged 4.2 ± 1.7 fmol/ml angiotensin-(l-8)octapeptide. Furosemide (40 mg p.o.) and standing increased these values to 22 ± 7.6 fmol/ml. In four volunteers, immunoreactive "angiotensin II" (more or less angiotensinlike material) was measured serially before and after converting-enzyme inhibition (Hoe 498) with conventional Dowex extraction. At peak inhibition, plasma immunoreactive "angiotensin II" levels decreased by only 44%. In contrast, angiotensin-(l-8)-octapeptide isolated by high-performance liquid chromatography completely disappeared. In hypertensive patients receiving long-term treatment with enalapril, plasma levels ofangiotensin-(l-8)octapeptide fell from 2.7 ± 0.9 to0.9 ± 0.3 fmol/ml (mean ± SKM) 2 hours after the morning dose, whereas levels of immunoreactive "angiotensin II" were not significantly changed. We found that this sensitive method specifically measured angiotensin-(l-8)octapeptide and demonstrated that true angiotensin II virtually disappears during converting-enzyme inhibition. © 1985 American Heart Association, Inc.