Abstract

We have examined the metabolism of beta-adrenergic receptors in intact S49 lymphoma cells. Centrifugal elutriation was used to prepare synchronized cells enriched in particular phases of the cell division cycle. In these synchronized cells, the rate of appearance of beta-adrenergic receptors (i.e., [125I]iodocyanopindolol binding sites) was continuous, approximately 75 sites/cell/hr, and receptor number per cell increased in proportion to the increase in cell size. Thus, receptor number, normalized for cell size, remains constant throughout the cell cycle. Receptors on cells in G1, S, and G2/M phases of the cell cycle displayed similar affinities for the radiolabeled antagonist [125I]iodocyanopindolol and apparent affinities for the agonist isoproterenol. We examined rates of receptor turnover in asynchronous cells by following receptor recovery after inactivation of beta-adrenergic receptors with bromoacetylalprenololmenthane (BAAM), an irreversible beta-adrenergic antagonist. The beta-adrenergic receptors on S49 cells demonstrated an average "half-life" of 28-30 hr. Since the population doubling time for S49 cells is 16-17 hr, this would indicate that receptor protein is conserved through successive cell generations. Moreover, receptor reappearance after blockade by BAAM was a function of newly appearing receptors during the S49 cell cycle and not loss of BAAM from receptors. The rate of receptor metabolism indicates that, under basal conditions (i.e., in the absence of agonist), beta-adrenergic receptors on S49 cells are metabolized more slowly than are other classes of receptors that bind peptides and cholinergic agonists in several other cell types.