Specific measurement of angiotensin metabolites and in vitro generated angiotensin II in plasma.
Combining high-performance liquid chromatography with radioimmunoassay enabled the precise measurement of different angiotensins and their metabolites in plasma. Peptides were extracted from 2 ml of plasma by reversible adsorption to phenylsilyl-silica, separated by isocratic high-performance liquid chromatography, and quantitated by radioimmunoassay using a sensitive but suitably cross-reacting angiotensin II antiserum. For the C-terminal angiotensin II metabolites (2-8)heptapeptide, (3-8)hexapeptide, and (4-8)pentapeptide, overall recoveries of 10 fmol peptide added to 1 ml of plasma were (mean +/- SD), 74 +/- 6, 68 +/- 8, and 67 +/- 11%, respectively. The detection limit for these peptides in plasma was 0.2 fmol/ml. Blanks were below the detection limits. In eight seated normal subjects treated for 4 days with enalapril, 20 mg p.o., q.d., angiotensin II metabolites tended to decrease during the 4 postdrug hours. However, their cumulated concentration in relation to octapeptide increased from 54 to 163% on Day 1 and from 62 to 103% on Day 4. After 4 hours of converting enzyme inhibition with enalapril there was still a close correlation between plasma renin activity and angiotensin-(1-8)octapeptide level (r = 0.83, p less than 0.05) and between blood angiotensin I and angiotensin-(1-8)octapeptide levels (r = 0.86, p less than 0.01). Adding angiotensin I in vitro raised the angiotensin-(1-8)octapeptide levels after incubation at 4 degrees C for 4 hours. Thus, immunoreactive "angiotensin II" does not disappear after converting enzyme inhibition largely because of the cumulated contribution of cross-reacting metabolites and partly because of in vitro generation of true angiotensin II.
D000328:Adult; D000803:Angiotensin I / Q000097:blood; D000804:Angiotensin II / Q000097:blood*; D000805:Angiotensin III / Q000097:blood; D000806:Angiotensin-Converting Enzyme Inhibitors; D000809:Angiotensins / Q000097:blood* / Q000276:immunology; D002851:Chromatography, High Pressure Liquid; D003429:Cross Reactions; D004656:Enalapril / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D011863:Radioimmunoassay
D000806:Angiotensin-Converting Enzyme Inhibitors; D000809:Angiotensins; D000804:Angiotensin II; D000805:Angiotensin III; D004656:Enalapril; D000803:Angiotensin I