Distinction between Two Subpopulations of β₁‐Adrenergic Receptors in Human Adipose Cells

The beta-adrenergic receptors in human adipose membranes were identified by the specific and saturable binding of the beta-adrenergic antagonist (--)-[3H]dihydroalprenolol. The total number of sites in control membranes was 0.32 +/- 0.03 pmol/mg protein and the equilibrium dissociation constant for binding (Kd) was 2.6 nM and 2.5 nM as determined by Scatchard analysis of experiments on equilibrium binding and kinetics, respectively. The beta 1-adrenergic nature of the receptors was derived from the order of potencies of beta-adrenergic agonists (isoproterenol greater than norepinephrine greater than epinephrine) to complete with (--)-[3H]dihydroalprenolol for binding. Studies of saturation binding, kinetics and competition binding revealed the presence of a single class of beta 1-adrenergic receptors. Prolonged incubation of human adipose cells in the presence of (--)-norepinephrine decreases the lipolytic response to beta-adrenergic agonists, and reduces by 50% the concentration of beta-adrenergic receptors. The Kd values for (--)-[3H]dihydroalprenolol and the beta-adrenergic agonists remain unchanged. Catecholamines also produce a rapid conformational change of approximatively 50% of the receptors in control membranes as revealed by their increased sensitivity towards inactivation by the alkylating agent N-ethylmaleimide. This inactivation process is not observed in desensitized membranes, which indicates that desensitization and inactivation by agonists plus N-ethylmaleimide affect the same receptor population. The beta 1-adrenergic receptors in human adipocytes can thus be divided into two subpopulations on the basis of the different consequences of their interaction wtih agonist molecules.