β-Adrenergic Receptors in Brown Adipose Tissue: Identification by (-)[³H]Dihydroalprenolol Binding

Abstract

The potent β-adrenergic antagonist (-)[³H]dihydroalprenolol (DHA) was used to identify β-adrenergic receptors in brown adipose tissue (BAT) membranes. Specific binding of DHA to unfractionated BAT membranes prepared from newborn rats was rapid, stable, saturable (K_D=150 nM) and entirely reversible by (-)propranolol (K_D=56 nM). β-adrenergic agonists competed for binding sites with an order of potency typical of β₁-adrenergic receptors: Isoprotereǹol (K_D=210 nM) > Norepinephrine (K_D=2.0 μM) ≃ Epinephrine (K_D=4.0 μM). Binding displayed remarkable stereoselectivity towards the (-) isomers of agonists or antagonists. α-adrenergic antagonists did not compete for binding sites. Analysis of the data by Scatchard and Hill plots suggests the existence of negatively cooperative site-site interactions among α-adrenergic receptors and/ or multiple independent binding sites. These findings indicate the presence of α₁-adrenergic receptors in BAT.