Characterization of a new, highly specific, beta 3-adrenergic receptor radioligand, [3H]SB 206606.

The RR-enantiomer of the beta 3-adrenergic receptor agonist BRL 37344 was tritiated to yield a high specific activity compound, [3H]SB 206606. This new, potentially specific, beta 3-adrenergic receptor ligand was characterized by binding studies using membranes from both Chinese hamster ovary K1 cells transfected with the rat beta 3-adrenergic receptor and rat interscapular brown adipose tissue, where beta 1-, beta 2-, and beta 3-adrenergic receptor subtypes are known to coexist. [3H]SB 206606 was found to bind to a single population of binding sites in both preparations. The Kd values for [3H]SB 206606 binding to membranes from Chinese hamster ovary K1 cells and brown adipose tissue were quite comparable (58 and 38 nM, respectively). At 37 degrees, the time courses of association and dissociation of [3H]SB 206606 with membranes of brown adipose tissue were quite short. At 4 degrees, the T1/2 were found to be 13 and 40 min, respectively. The Ki values for various beta-adrenergic agonists and antagonists in brown adipose tissue membranes were similar to those obtained in Chinese hamster ovary K1 cell membranes with both [3H]SB 206606 and [125I]iodocyanopindolol. The order of binding affinity was BRL 37344 >> (-)-isoproterenol = (-)-norepinephrine > (-)-epinephrine = (+)-isoproterenol. The similarity of the Kd values and of the Ki values for various beta-adrenergic agonists and antagonists in both systems tested indicates that, in a complex membrane system, [3H]SB 206606 binds selectively to the beta 3-adrenergic receptor. The affinity of [3H]SB 206606 is 76 times higher for the beta 3-adrenergic receptor than for the beta 1/beta 2-adrenergic receptors, thus allowing, under controlled conditions, measurement of interactions only with the beta 3-adrenergic receptor in complex membrane systems.