Pharmacological and biochemical characterization of alpha1 adrenoceptor in rat liver

Hepatic alpha-adrenoceptors have been characterized pharmacologically and biochemically. In isolated rat liver cells, activation of glycogen phosphorylase by adrenergic agonists and its inhibition by subsite selective antagonists are mediated by alpha(,1)-receptors that do not display the phenomenon of receptor reserve. When tested in dilute cell suspensions to minimize uptake and metabolism of ligands, these receptors have properties indistinguishable from those of alpha(,1)-receptors in other tissues. In purified liver plasma membranes, the reversible radioligand {('3)H}prazosin labels a single population of high affinity binding sites with properties similar to the alpha(,1)-receptors mediating the glycogenolytic response of intact cells. Affinity labeling of these sites was achieved by using {('3)H}phenoxybenzamine ({('3)H}POB), synthesized in our laboratory. In intact liver cells {('3)H}POB binds irreversibly to stereoselective sites with properties expected from alpha-receptors. Half-maximal blockade of the glycogenolytic response to adrenaline and half-maximal saturation of binding sites occur at the same, low nanomolar concentrations of POB, at which no labeling of serotonin, histamine or muscarinic receptors can be detected. Further analysis of {('3)H}POB binding in purified plasma membranes indicates selective labeling of the alpha(,1)-receptor subtype. Molecular characterization of detergent-solubilized, {('3)H}POB-labeled alpha(,1)-receptors indicates that it is a protein with a Stokes radius of 6 nm, and a subunit molecular weight of 80,000