Abstract

We have used two experimental approaches--receptor inactivation with an irreversible antagonist and changes in receptor expression during passage of cells through the cell cycle--to explore the relationship between beta-adrenergic receptor number and response in intact S49 lymphoma cells. beta-Receptors in asynchronous cultures of S49 cells were blocked to varying degrees with the irreversible antagonist bromoacetylalprenololmenthane (BAAM). Blockade by BAAM was noncompetitive and did not alter the affinity of receptors for the agonist isoproterenol. Intracellular accumulation of cAMP in response to 1 microM isoproterenol was proportional to receptor number both at times of initial and maximal accumulation. In contrast, when intracellular accumulation of cAMP in response to isoproterenol was measured in synchronized cultures of S49 cells (obtained by centrifugal elutriation), a notably different relationship was observed. Cells were least responsive, that is, receptors appeared "uncoupled," during S phase of the cell cycle. This attenuation of response was not due to alterations of receptor number, receptor affinity for agonist, or expression of the catalytic unit of adenylate cyclase. Use of the antibiotic mycophenolic acid, a selective inhibitor of the synthesis of GTP, elicited response patterns in asynchronous cells similar to those seen in synchronized cells. These results confirm that wild-type S49 cells do not possess spare receptors. In addition to the importance of total receptor number in determining maximal response to isoproterenol, receptors may show differential efficacy in promoting cAMP accumulation as cells traverse the cell cycle. Changes in cellular levels or utilization of GTP during the cell cycle may serve to regulate the coupling of receptors to the stimulation of adenylate cyclase.