Angiotensin-Induced Changes in the Apparent Size of Rat Liver Angiotensin Receptors

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Abstract
Angiotensin receptors from rat liver were labeled using four different ligands : (Sar1-(3H)Tyr4)-Angiotensin II ((3H)SarAII); (Sar1-(3H)Tyr4-IIe8) -Angiotensin II ((3H)SarIIeAII); (Sar1-(125I)Tyr4-(4′-N3)Phe8)-Anglotensin II (IN3AII); (Sar1-(125I)Tyr4-(4′N3D-Phe)8)-Angiotensin II (IN3DPheAII) (3H) SarAII and IN3AII behaved like agonists and (3H) SarlleAII and IN3DPheAII like antagonists. All four ligands labeled the same population of sites. The azido derivatives allowed covalent labeling of receptors with a high yield (about 40%). Membranes were solubilized by Triton X-100 under experimental conditions which ensured complete solubilization of the liganded receptors in a stable form (less than 40% dissociation after 20 h). The apparent size of liganded angiotensin receptors was determined by gel filtration on Ultrogel ACA-34 columns and by SDS gel electrophoresis (in the case of covalent labeling). The apparent Stokes radius of solubilized angiotensin receptors was different wether the receptor was labeled with an agonist (Stokes radius = 6.2 ± 0. 1 nm (6) after labeling with (3H) SarAII) or with an antagonist (Stokes radii of 5. 5 ± 0. 1 (7), and 5.6 ± 0.1 nm (4) after labeling with (3H) SarIIeAII and IN3DPheAII respectively). After covalent labeling with IN3All anglotensin receptors were eluted as a mixture of light and heavy forms. SDS gel electrophoresis revealed only one molecular entity of Mr 64,000. It is concluded that binding of an agonist to liver angiotensin receptors triggers or stabilizes an interaction with another membrane component Involved in the coupling of the receptor to its primary effector.