Identification and subclassification of rat adipocyte β-adrenoceptors using (+-)-[125I]cyanopindolol

The ligand binding characteristics of β-adrenoceptors on membranes derived from rat whole fat pads and isolated adipocytes have been compared to those present on lung membranes using the specific ligand (+-)-[125I]cyanopindolol ([125I]CYP). The equilibrium dissociation constant (Ksub(D)) of [125I]CYP was similar in all three preparations, whilst the degree of stereospecificity displayed by the isomers of propranolol varied between lung and isolated adipocyte membranes. The rank order of potency in displacing [125I]CYP binding was isoprenaline > adrenaline > noradrenaline in lung and whole fat pads, suggesting an overall β2-receptor subtype, and isoprenaline > noradrenaline > adrenaline in isolated adipocytes, suggesting a β1-receptor. In view of the high proportion of β2-receptors on whole fat pad membranes, the overall amount of β2-receptors on isolated adipocytes may reflect contamination from other cell types. The nature of the β1-receptors present on fat cells is discussed in relation to the apparently atypical β-receptor involved in mediating functional lipolytic response. (Auth.)