Characterization of a binding site for angiotensin IV on bovine aortic endothelial cells

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Abstract

We have characterized a specific binding site for angiotensin IV on bovine aortic endothelial cell membranes. Pseudo-equilibrium studies at 37°C for 2 h have shown that this binding site recognizes angiotensin IV with a high affinity (Kd=0.71; average of two experiments that yielded values of 0.71 and 0.72 nM). The binding site is saturable and relatively abundant with a maximal binding capacity of 0.59 pmol/mg protein (average of two experiments that yielded values of 0.39 and 0.78 pmol/mg of protein). Non-equilibrium kinetic analyses at 37°C revealed a calculated Kd of 59 pM (average of two experiments that yielded values of 67 and 50 pM). The binding site is pharmacologically distinct from the classic angiontensin receptors AT1 or AT2. An analysis of specificity showed that the binding site displays a high affinity for angiotensin IV, low affinities for angiotensin II, [Sar1, Val5, Ala8]angiotensin II and does not recognize L-158,809 (5,7-dimethyl-2-ethyl-3-[(2′-(H-tetrazol-5-yl)[1,1′-biphenyl]-4-yl)methyl]-3H-imidazo [4,5-β]pyridine H2O) and PD 123319 (1-[4-(dimethylamino)3-methylphenyl]methyl-5-(diphenylacety)4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid). A few unrelated hormones (bradykinin, [Arg8]vasopressin, endothelin-1, atrial natriuretic factor, isoproterenol and adrenocorticotropic hormone) were unable to inhibit any 125I-angiotensin IV binding. The affinities of different structural analogues of angiotensin IV revealed that the N-terminal position is critical for receptor recognition and the C-terminal proline is also important. GTPγS and polyvinyl sulfate did not affect the binding, suggesting that the receptor is not coupled to a G-protein. The divalent cations Mg2+ and Ca2+ were shown to diminish the binding of 125I-angiotensin IV. Cross-linking of 125I-angiotensin IV to bovine aortic endothelial cell membranes in the presence of disuccinimidyl suberate, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed a major band of 186±12 kDa. The presence in high concentration of this angiotensin binding site an aortic endothelial cells suggests the existence of a novel mechanism involved in the control of vascular tone or vascular permeability.

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