Metabolism of tetradecapeptide, angiotensinogen and angiotensin I and II by isolated perfused rat hindlimbs.

1. We investigated the mechanism of tetradecapeptide-induced vasoconstriction by studying the metabolism of tetradecapeptide (TDP), angiotensinogen, and angiotensin I (AI) and angiotensin II (AII) by isolated perfused rat hindlimbs. 2. Using HPLC and specific RIAs we have quantified six angiotensin peptides: pentapeptide(4-8), hexapeptide(3-8), heptapeptide(2-8), octapeptide(1-8), nonapeptide(2-10) and decapeptide(1-10) in hindlimb effluent. 3. TDP-induced vasoconstriction was associated with generation of predominantly AI and AII, with smaller amounts of the other peptides measured. 4. Captopril prevented vasoconstriction and inhibited AII production by 80%, indicating a dominant role for AI generation in the vascular response to TDP. 5. Evidence that renin is not the enzyme responsible for AI generation from TDP includes: first, the failure of angiotensinogen to cause vasoconstriction or angiotensin peptide generation despite very high amounts of AI and AII generation from TDP; second, the resistance of the TDP-induced vasoconstriction and angiotensin peptide generation to inhibition by pepstatin; and third, the failure of bilateral nephrectomy 24 h before the experiment to influence the vascular and angiotensin peptide response to TDP. 6. AII was cleared with 41% efficiency, with generation of penta-, hexa-, and heptapeptides. 7. AI was cleared with 59% efficiency; this was reduced to 24% by captopril, indicating a conversion of at least 35% of AI to AII by ACE. 8. These studies have identified vascular metabolism of AI and AII to be an efficient process, with both ACE and aminopeptidases playing an important role, and indicate that those peptidases which cleave TDP to generate AI are unlikely to play any role in AI generation in vivo.