Adrenergic receptors in human liver plasma membranes: predominance of beta 2- and alpha 1-receptor subtypes

Abstract

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 beta-receptor ligands was 36-83 fmol/mg protein. Catecholaminergic agonists competed for these binding sites in the order typical for beta 2-adrenergic receptors. IPS 339 [(tertiarybutylamino-3-ol-2-propyl)oximino-9-fluorene hydrochloride], a beta 2-selective antagonist, was at least 3 orders of magnitude more potent in inhibiting the binding of [3H]DHA than the beta 1-antagonist, atenolol. Computer-aided analysis of the competition curves as well as Hofstee transformations of the binding data indicated the predominance of the beta 2-subtype. 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 alpha-receptors are of the alpha 1-subtype. Competition experiments with prazosin and yohimbine confirmed the predominance of the alpha 1-receptor subtype, although the presence of alpha 2-receptors cannot be completely ruled out. These results indicate that adrenergic receptors in human liver plasma membranes are predominantly of the beta 2- and alpha 1-subtypes.