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Adrenergic Receptors in Human Liver Plasma Membranes: Predominance of β₂- and ₁-Receptor Subtypes^*

Departments of Biochemistry and Pathology Nashville, Tennessee 37208
Pathology, Meharry Medical College Nashville, Tennessee 37208
Clinical Laboratories of the Veterans Administration Medical Center Murfreesboro, Tennessee 37130

Address all correspondence and requests for reprints to: Dr. Ifeanyi J. Arinze, Department of Biochemistry, Meharry Medical College, Nashville, Tennessee 37208.

Adrenergic receptors in human liver plasma membranes were characterized by radioligand binding assays. The binding of [³H] dihydroalprenolol ([³H]DHA) to partially purified membranes was rapid, of high affinity, saturable, and stereospecific. The binding of [¹²⁵I]iodocyanopindolol to the same membranes was also saturable and stereospecific, but extremely slow, and at 37 C required about 6 h for equilibration. The maximum number of binding sites from six livers determined with these two β-receptor ligands was 36–83 fmol/mg protein. Catecholaminergic agonists competed for these binding sites in the order typical for β₂-adrenergic receptors. IPS 339 [(tertiarybutylamino-3-ol-2-propyl)oximino-9-fluorene hydrochloride], a β₂-selective antagonist, was at least 3 orders of magnitude more potent in inhibiting the binding of [³H]DHA than the β₁-antagonist, atenolol. Computer-aided analysis of the competition curves as well as Hofstee transformations of the binding data indicated the predominance of the β₂subtype. The GTP analog guanyl-5 ' -yl-imidodiphosphate, decreased the binding affinity of the agonist, l-isoproterenol, indicating the modulation of agonist-promoted coupling of the receptors to guanine nucleotide regulatory proteins. The maximum number of binding sites for the binding of [³H]prazosin and [³H]dihydroergocryptine were the same (60–70 fmol/mg protein), indicating that the majority of the -receptors are of the ₁-subtype. Competition experiments with prazosin and yohimbine confirmed the predominance of the ₁-receptor subtype, although the presence of ₂-receptors cannot be completely ruled out. These results indicate that adrenergic receptors in human liver plasma membranes are predominantly of the β₂- and ₁-subtypes

^* This work was supported by NIH Grant HD-08792 and by Grant 1-598 from the March of Dimes-Birth Defects Foundation.

Received January 24, 1985.

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