Age-related changes in adrenergic CY,, CY*, and ,6 receptors of rat white fat cell membranes: an analysis using (3H)bunazosin as a novel ligand for

Age-related changes in al-, a?-, and 0-catecholamine receptors on membranes of rat epididymal fat cells were in- vestigated. Both young (6 weeks old, weight about 190 g) and aged (20 weeks old, weight about 490 g) Sprague-Dawley male rats were used. For the a)-adrenoceptor binding experiment, we developed a novel analytical method using the hydrophilic a)- receptor selective antagonist, (3H)bunazosin. The binding of (3H)bunazosin to its binding sites was rapid, reversible, saturable, and stereospecific. Scatchard binding analysis showed a single class of binding site. The sites were characterized as a)- adrenoceptors by inhibition experiments using various agonists and antagonists. The number of maximum binding sites (Bmm) of al-receptor binding was 37.0 f 6.5 (young) versus 24.0 f 3.2 (aged) fmol/mg protein (P < 0.01). (3H)Rauwol- scine and (3H)CGP-12177 were used for a2- and j3-receptor binding, respectively. In a2-receptor detection using (3H)rau- wolscine as a ligand, B,, increased markedly from 19.8 + 4.9 to 86.2 + 19.5 fmol/mg protein (P < 0.01). In contrast, B,,, for @-receptor decreased from 69.7 + 9.7 to 45.4 + 13.9 fmol/mg protein with increasing rat age (P < 0.05). Kd showed no change in each of the binding experiments between young and aged rats. The cell volume increased from 0.07 & 0.02 to 0.15 * 0.06 nl. It is implied that anti-lipolytic activity strengthened on the whole mainly with the marked increase of a2-receptor number and decrease of j3-receptor number. Fur- thermore, (3H)bunazosin was demonstrated to be a new suitable radioligand for the detection of the al-adrenoceptor of rat white fat cell membranes.-Kobatake, T., Y. Watanabe, Y. Mat- suzawa, K. Tokunaga, S. Fujioka, T. Kawamoto, Y. Keno, S. Tarui, and H. Yoshida. Age-related changes in adrenergic CY), a?, and j3 receptors of rat white fat cell membranes: an analysis using (3H)bunazosin as a novel ligand for the a, adrenoceptor. J Lipid Res. 1991. 32: 191-196.