Possible implication of peptidase activity in different potency of angiotensins II and III for displacing [125I]angiotensin II binding in pig aorta.

A single class of [125I]angiotensin II ([125I]AII) binding sites was found in porcine aortic smooth muscle membranes. Des-Asp1-AII (AIII) and des-Asp1-[Ile8]AII were 20 times less potent than AII or [Sar1,Ile8]AII to displace [125I]AII binding. In contrast, AII and AIII equipotently induced an increase in cytosolic free Ca2+ concentration in cultured porcine aortic smooth muscle cells. Des-Asp1-[Ile8]AII and [Sar1,Ile8]AII equipotently inhibited the increase induced by either AII or AIII. In order to explain this discrepancy, we studied [125I]AIII binding. In the presence of amastatin or a potent inhibitor of aminopeptidase M, [125I]AII binding remained stable for 90 min, but [125I]AIII binding decreased gradually after the peak at 30 min. The decrease was completely blocked by the presence of amastatin and phenylmethylsulfonylfluoride. Consequently, AIII was as potent as AII to displace [125I]AII binding in the presence of these two protease inhibitors. These results suggest that the lower potency of AIII to displace [125I]AII binding is related primarily to AIII-specific degradation by proteases.