title: Reciprocal modulation of the binding of angiotensin agonists and antagonists to angiotensin receptors in smooth muscle.
abstract: 1. Direct ligand binding studies have shown that the agonist 125I-[Sar1]Ang II and the antagonist 125I-[Sar1Ile8]Ang II bind to bovine uterus smooth muscle membranes in a time-dependent, reversible and saturable manner; both ligands had the same number of high affinity sites. 2. [Sar1Ile8]Ang II inhibited the binding of 125I-[Sar1]Ang II in a non-competitive manner by decreasing the number of high affinity sites without changing the binding affinity of the radioligand. 3. [Sar1]Ang II also inhibited the binding of 125I-[Sar1Ile8]Ang II in a non-competitive manner. 4. Dissociation of both radioligands from their receptor sites was fast enough that pseudo irreversible occupancy of the binding sites could not account for the observed non-competitive inhibition. 5. Displacement studies using 125I-[Sar1Ile8]Ang II as the radioligand provided evidence for the existence of two binding sites when the displacing ligand was [Sar1]Ang II but not when the displacing ligand was [Sar1Ile8]Ang II. 6. GTPS gamma S had no discernible effect on the binding of either 125I-[Sar1]Ang II or 125I-[Sar1Ile8]Ang II to bovine uterine membranes. 7. The present findings are consistent with an allosteric mechanism of antagonism for [Sar1Ile8]Ang II. The data are also consistent with a mechanism wherein agonist and antagonist ligands occupy different binding modes at the same receptor site and induce long-term conformational changes in the receptor which are idiosyncratic with respect to the nature of the ligand. An emerging relationship between the actions of angiotensin peptides and non-peptide mimetics of angiotensin is presented.
authors.affiliation: Department of Medical Biochemistry, University of Calgary, Alberta, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Koziarz
authors.forename: J
authors.initials: J
authors.lastname: Beck
authors.forename: G J
authors.initials: GJ
authors.lastname: Moore
mesh_terms: D015059:1-Sarcosine-8-Isoleucine Angiotensin II / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology; D000804:Angiotensin II / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology / Q000502:physiology*; D057911:Angiotensin Receptor Antagonists; D000818:Animals; D002417:Cattle; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D008968:Molecular Conformation; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D011869:Radioligand Assay; D011945:Receptors, Angiotensin / Q000187:drug effects / Q000378:metabolism*; D014599:Uterus / Q000187:drug effects / Q000378:metabolism
chemical_list: D057911:Angiotensin Receptor Antagonists; D011945:Receptors, Angiotensin; D000804:Angiotensin II; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C074793:angiotensin II, Sar(1)-; D015059:1-Sarcosine-8-Isoleucine Angiotensin II