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

Age-related changes in al-, a?-, and 0-catecholamine receptors on membranes of rat epididymal fat cells were investigated. 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]Rauwolscine and [3H]CGP-12177 were used for a2and j3-receptor binding, respectively. In a2-receptor detection using [3H]rauwolscine 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. Furthermore, [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. Matsuzawa, 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. Supplementary key words adipocytes [3H]bunazosin [3H]rauwolscine [3H]CGP-12177 rat white adipose tissue aging 9 cell size cyclase. On the other hand, catecholamines also perform an anti-lipolytic role by stimulating az-adrenoceptors and inhibiting adenylate cyclase (1). The existence of at-adrenoceptors, which could not be shown owing to the impossibility of direct binding of [3H]prazosin (2) on rat white fat cell membranes, has been suggested indirectly by the demonstration of responses to al-adrenoceptor agonists such as phosphatidylinositol turnover or Caz + mobilization (3), although their physiological roles in white fat cells have not been