Evidence of classic beta 3-adrenergic receptors in porcine adipocytes

Adipocytes from several mammalian species have predominant beta 3-adrenergic receptors (beta 3-AR). Attempts to classify porcine adipocyte beta-adrenergic receptors (beta-AR) into subtypes have not been successful. Selectivity of agonists and antagonists for stimulation of lipolysis and for ligand binding is considerably more restrictive than for the classic rat and guinea pig beta-AR subtypes. The unique pattern for activity of agonists and antagonists in porcine beta-AR precludes analogy to classic receptors and consequently there is no conclusive evidence regarding porcine beta-AR subtypes. Porcine adipocyte membranes were used in ligand binding experiments designed specifically to demonstrate beta 3-AR. Equilibrium saturation curves with dihydroalprenolol, CGP 12,177, or iodocyanopindolol indicated saturation at low concentrations with a single binding site. Equilibrium competitive ligand binding with iodocyanopindolol as radioligand and isoproterenol or propranolol as competitive ligands indicated both ligands totally inhibited radioligand binding; propranolol was more potent than isoproterenol. Nonradioactive CGP 12,177 also competed with iodocyanopindolol. Ligand binding experiments provided no evidence of a low-affinity beta-AR (binding at high concentrations of ligand), the beta 3-AR. Positive evidences of a beta 3-AR were that CGP 12,177, a beta 1- and beta 2-adrenergic receptor antagonist but a beta 3-AR agonist, partially stimulated porcine adipocyte lipolysis. Furthermore, transcripts for a beta 3-AR, as well as a beta 1- and beta 2-AR, have been demonstrated in RNA from porcine adipocytes in other studies. The beta-AR subtypes expressed and functional in porcine adipocytes remain unknown. The multiple ligand binding sites cannot be attributed to classic beta-AR subtypes. The porcine beta-AR may be a single unique receptor to impart atypical binding properties, or more likely, multiple subtypes, each different enough from classic subtypes to impart the unique properties observed.