Evidence for heterogeneous distribution of alpha 1, alpha 2- and beta-adrenergic binding sites on rat-liver cell...

Fractionation of preparations of rat-liver membranes on linear sucrose gradients revealed different profiles for the binding of alpha 1-, alpha 2- and beta-adrenergic radioligands. The peaks of binding activities of [3H]prazosin and [3H]epinephrine were clearly separated from those of [3H]yohimbine and [125I]iodocyanopindolol which appeared at lower sucrose densities. Enzyme marker activities in the sucrose subfractions indicated the presence of plasma membranes in all of the subfractions. Furthermore, the binding peaks of the various adrenergic radioligands cannot be correlated with the presence of membranes derived from microsomes, lysosomes or Golgi apparatus. Pretreatment of rat livers with concanavalin A, in order to prevent the fragmentation of the plasma membranes during isolation, resulted in the shift of the binding of [3H]yohimbine and [125I]iodocyanopindolol to sucrose-gradient subfractions of higher densities, clearly separate from fractions containing microsomes and Golgi apparatus. There was no distinct separation of the binding peaks of prazosin, yohimbine, and cyanopindolol in sucrose-gradient subfractions from concanavalin A-pretreated livers. These results are consistent with the hypothesis that alpha 1-, alpha 2-, and beta-adrenergic binding sites are associated with plasma membranes, and are heterogeneously distributed on the rat-liver cell surface.