Characterization of the human lymphocyte beta-adrenergic receptor by photoaffinity labeling. Alterations with desensitization.

Desensitization of the leukocyte beta-receptor system has been associated with a functional uncoupling of the components of the beta-receptor complex. In order to determine whether desensitization and uncoupling of the leukocyte beta-receptor is associated with any structural alterations in the beta-receptor, we studied labeling of lymphocytes using the photoactive beta-adrenergic antagonist p-azido-m-[125I]iodobenzylcarazolol. Labeled peptides were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and detected using autoradiographic techniques. In broken cell preparations, specific labeling was demonstrated in two major peptide bands: mol wt approximately equal to 68,000 and mol wt approximately equal to 55,000. Inhibition of photolabeling was stereospecific and demonstrated an order of potency for agonists consistent with labeling of a beta 2-receptor. Preincubation of cells with the beta-agonist, isoproterenol, resulted in a reduction in beta-adrenergic-mediated adenylate cyclase activity to 60% of control, but no change in total binding sites as determined by [125I]iodocyanopindolol binding. In photolabeling studies, desensitization was associated with a reduction in proportional labeling of the 55,000 mol wt band as compared to the 68,000 mol wt band to 58 +/- 3% of control and a reduction in mobility of the upper band. These studies suggest that structural alterations in the human lymphocyte beta-receptors occur with desensitization, analogous to changes in several other beta-receptor model systems. Also, since the techniques described can identify alterations in human beta-receptor structure, these methods may be exploited to determine whether structural alterations in lymphocyte beta-receptors may occur in human disease states.