Detection of alpha-1 adrenergic receptors in rat fat cells by ^^3 H-bunazosin HCl

The role of alpha-1 adrenoceptors in rat fat cells in Ca^2+ movement or PI turnover were reported so far. But the presence of alpha-1 adrenergic receptors in rat fat cells have not been demonstrated through binding experiments, so we studied the direct binding to alpha-1 adrenergic receptors using a new radiolabeled hydrophilic alpha-1 antagonist, ^^3 H-bunazosin (^^3 H-BZ). The fat cell membrane was prepared by the method of Kaahiwagi, et al. (J Biol Chem 258; 13685-13692, 1983) The binding sites of ^^3 H-Bz were rapid, reversible, saturable and stereospesific. Scatchard analysis showed a single class of binding sites with Kd of about 0.4 nM and Bmax of about 2.8 fmole/mg prot. The sites were characterized as alpha-1 receptors with inhibition experiments using various agonists and antagonists. Thus ^^3 H-bunazosin, hydrophilic alpha-1 antagonist, is useful radioligand for ditection of alpha-1 adrenoceptors of rat fat cells.