Molecular Pharmacology, Vol 13, 1159-1169, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Beta Adrenergic Regulation of Glycogen Phosphorylase Activity and Adenosine Cyclic 3',5'-Monophosphate Accumulation in Control and Desensitized C-6 Astrocytoma Cells

¹ Friedrich Miescher-Institut, CH-4002 Basel, Switzerland

(-)-Isoproterenol (0.01-10 nM) caused the rapid, complete conversion of glycogen phosphorylase from the b to the a form in cultured C-6 astrocytoma cells. This was associated with a 60-fold elevation in cellular cyclic 3',5'-AMP content. Both effects were blocked stereoselectively by (-)-propranolol. The beta adrenoceptor partial agonists (±)-salbutamol (1 nM-10 µM) and (±)-hydroxybenzylpindolol (0.1 nM-10 µM) also caused full conversion of phosphorylase b to a, although their maximal effects on cellular cyclic AMP content were only 80% and 17.5%, respectively, of that obtainable with (-)-isoproterenol. Prostaglandin E₁ (PGE₁), which caused only a 1.6-fold elevation in cyclic AMP content, gave rise to 54% conversion of phosphorylase b to the a form. Prior incubation of cells with 3-isobutyl-1-methylxanthine caused a 1.4-fold elevation of cyclic AMP and converted 36% of phosphorylase b to the a form. Under those conditions dibutyryl cyclic AMP fully activated phosphorylase b to a , and the effects of beta adrenergic agonists on both cyclic AMP levels and phosphorylase b to a conversion were enhanced. These results suggest that beta adrenoceptor agonists and PGE₁ convent phosphorylase b to a by a mechanism involving cyclic AMP. However, conversion of phosphorylase b to a was essentially complete when the cellular cyclic AMP content had risen by only 6% of the maximum amount attainable by beta adrenoceptor stimulation. Because of this relationship between cell cyclic AMP content and phosphorylase conversion, cells that had been desensitized to beta adrenoceptor stimulation by prior incubation with low concentrations of (-)-isoproterenol still responded to a subsequent addition of (-)-isoproterenol with full phosphorylase b to a conversion. However, in these cells complete conversion required 5-15-fold higher concentrations of (-)-isoproterenol than control cells. The divalent cation ionophore A23187, which did not produce a measurable rise in cell cyclic AMP levels, activated phosphorylase by 50%. This effect, unlike the action of beta adrenergic agonists, was inhibited by lowering the extracellular divalent cation concentration and may therefore occur by a different, cyclic AMP-independent mechanism.

Note:
ACKNOWLEDGMENTS We are indebted to Ms. M. Kubler and Ms. E. Müller for their excellent technical assistance.