Hepatic alpha-adrenoreceptor: specific and irreversible labeling by [3H]-phenoxybenzamine.

The binding of [³H]-phenoxybenzamine, a potent irreversible alpha-adrenergic antagonist, to alpha-adrenergic binding sites in rat liver plasma membranes was a saturable process with a number of binding sites of 1600 fmol/mg of membrane protein. No significant dissociation of this ligand occurred after dilution for 2 hours, or repeated washing of the labeled membranes. The specificity of the binding site was characterized by an order of potency of various drugs typical for an alpha-adrenergic receptor. Binding showed strict stereospecificity since R(+)phenoxybenzamine exhibited a two order of magnitude higher affinity than its S(?)isomer. A SH group was involved in the binding of both [³H]-phenoxybenzamine and [³H]-dihydroergocryptine to alpha-receptor. It appears therefore that [³H]-phenoxybenzamine binds irreversibly and specifically to the alpha-adrenoreceptor and that it might prove adequate in the solubilization and purification of the alpha-adrenoreceptor.