Angiotensin antagonists and the adrenal cortex and medulla

Several analogs of angiotensin in which the phenylalanine in position 8 of the peptide chain was replaced by an aliphatic amino acid residue are specific antagonists of angiotensin in aorta, the adrenal medulla, and adrenal zona glomerulosa. In the adrenal cortex and medulla, all actapeptide analogs have more agonist activity than in aortic strips. In studies with N-terminally substituted analogs, it appears that adrenal degradation of the angiotensin molecule by aminopeptidase(s) does not occur or is not retarded by N-terminal mocifications such as sarcosine substitution. The decapeptide analog [Ile8]-angiotensin I and heptapeptide analog [des-Asp1, Ile8]-angiotensin II were excellent antagonists in the adrenal medulla and each peptide was devoid of intrinsic activity. These substituted homologs of angiotensin may offer a novel approach for the development of selective antagonists of angiotensin receptors. In the adrenal cotex, [des-Asp1, Ile8]-heptapeptide possessed greater receptor affinity than any of the angiotensin octapeptides studied. This C-terminally substituted heptapeptide does have significant intrinsic activity in the adrenal cortex which would limit the use of this compound as an antagonist of vascular responses to angiotensin II. In studies with [Ile8]-angiotensin II, [Sar1, Ile8]-angiotensin II, and [des-Asp1, Ile8]-angiotensin II, the pA2 values calculated indicate that the N-terminal residue is not important for receptor binding in the adrenal cortex but may be of significance in binding to adrenal medullary and aortic smooth muscle receptors. At the present time it appears unlikely that any single animal model or assay system can reliably predict the agoinst/antagonist activities of angiotensin analogs for all the various end organs which respond to the angiotensins.