AGE-RELATED-CHANGES IN ADRENERGIC ALPHA-1-RECEPTOR ALPHA-2-RECEPTOR AND BETA-RECEPTOR OF RAT WHITE FAT-CELL MEMBRANES - AN ANALYSIS USING [H-3] BUNAZOSIN AS A NOVEL LIGAND FOR THE ALPHA-1-ADRENOCEPTOR

Age-related changes in alpha-1-, alpha-2-, and beta-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 alpha-1-adrenoceptor binding experiment, we developed a novel analytical method using the hydrophilic alpha-1-receptor selective antagonist, [H-3]bunazosin. The binding of [H-3]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 alpha-1-adrenoceptors by inhibition experiments using various agonists and antagonists. The number of maximum binding sites (B(max)) of alpha-1-receptor binding was 37.0 +/- 6.5 (young) versus 24.0 +/- 3.2 (aged) fmol/mg protein (P < 0.01). [H-3]Rauwolscine and [H-3]CGP-12177 were used for alpha-2- and beta-receptor binding, respectively. In alpha-2-receptor detection using [H-3]rauwolscine as a ligand, B(max) increased markedly from 19.8 +/- 4.9 to 86.2 +/- 19.5 fmol/mg protein (P < 0.01). In contrast, B(max) for beta-receptor decreased from 69.7 +/- 9.7 to 45.4 +/- 13.9 fmol/mg protein with increasing rat age (P < 0.05). K(d) 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 alpha-2-receptor number and decrease of beta-receptor number. Furthermore, [H-3]bunazosin was demonstrated to be a new suitable radioligand for the detection of the alpha-1-adrenoceptor of rat white fat cell membranes.