Agonist-specific refractoriness induced by isoproterenol. Studies with mutant cells.

The beta-adrenergic catecholamine isoproterenol produces a large, rapid, but often a transient, elevation in cellular content of cyclic AMP. We have used the S49 mouse lymphoma cell line, in which genetic variants with specific defects in the pathway of cyclic AMP generation and function have been isolated, to study the increase and subsequent decrease in cyclic AMP levels (termed refractoriness) following incubation of cells with isoproterenol. In wild type S49 cells, isoproterenol produces a peak response in the cellular content of cyclic AMP within 30 min, but the cyclic AMP level falls rapidly thereafter, approaching basal levels by 6 h. Neither inactivation of the drug nor secretion of a nonspecific inhibitor of adenylate cyclase appears to account for the refractoriness. Because isoproterenol refractory cells can still be stimulated by cholera toxin, refractoriness to isoproterenol does not represent a generalized decrease in cellular cyclic AMP response. Particulate preparations from refractory cells have a selective loss of isoproterenol-responsive adenylate cyclase activity, but their activation constants and stereoselectivity for (-)- and (+)-isoproterenol are unaltered. In addition, refractory cells have decreased specific binding of the beta-adrenergic antagonist 125Iiodohydroxybenzylpindolol. This decrease appears to represent a reduction in the number, but not the affinity, of beta-adrenergic receptor sites. Similar studies in an S49 clone that lacks the enzyme cyclic AMP-dependent protein kinase yield essentially identical findings. Because kinase-deficient cells do not induce the cyclic AMP-degrading enzyme phosphodiesterase after the cellular content of cyclic AMP is increased, induced of phosphodiesterase cannot account for refractoriness to isoproterenol. Cyclic AMP-dependent protein kinase does not appear to be required for either the decrease in beta-adrenergic receptors and isoproterenol-responsive adenylate cyclase, nor does it appear to be required for the development of refractoriness to isoproterenol. In contrast, an S49 clone lacking hormone-responsive adenylate cyclase activity but retaining beta-adrenergic receptors does not appear to lose receptors after being incubated with isoproterenol, either alone or together with dibutyryl cyclic AMP. Therefore, in this clone, receptor occupancy alone or in combination with elevated cyclic AMP levels is insufficient to cause refractoriness. Refractoriness thus appears to require intact adenylate cyclase. This suggests that adenylate cyclase may exert regulatory controls on beta-adrenergic receptors in addition to generation of cyclic AMP.