Direct analysis of β-adrenergic receptor subtypes on intact adult ventricular myocytes of the rat

Abstract

β1- and β2-Adrenergic receptors co-exist in the adult rat ventricle. We have employed radioligand binding and cell purification techniques to determine the cellular origin of these receptors. The β-adrenergic antagonist ligand (±)-[125I]iodocyanopindolol binds to 2 x 105 receptors per purified adult rat cardiomyocyte, with a dissociation constant of 70 pM. The subtype-selective antagonists betaxolol (β1), and zinterol (β2) compete for [125I]iodocyanopindolol-binding sites on intact myocytes in monophasic manners with dissociation constants of 46, 845, and 923 nM, respectively. [125I]iodocyanopindolol binding to membranes prepared from nonmyocyte elements of rat ventricle occurs with a dissociation constant of 43 pM and a capacity of 88 fmol/mg membrane protein. Computer analysis of competition of [125I]iodocyanopindolol binding by betaxolol, practolol, and zinterol in nonmyocyte membranes demonstrates biphasic curves that comprise binding to both β1- and β2-receptors. These data demonstrate that purified adult ventricular myocytes possess only β1-receptors, and that the β2-receptors found in rat ventricle are located on nonmyocyte cell types.