The effect of various steroids on adenosine 3',5'-monophosphate in C-6 glial tumor cell cultures was examined. Glucocorticoids increased basal and norepinephrine-stimulated concentrations of cyclic AMP in intact cells 2-fold. Exposure of cells to 2 µM corticosterone for at least 48 hr was required to obtain a maximal effect. The concentration of norepinephrine needed for half-maximal activation and the time course of the cyclic AMP response were unaltered. A 2-fold increase in adenylate cyclase activity of broken cell preparations similarly dependent upon corticosterone concentration and time was observed. The apparent Km for ATP was not changed but the maximum velocity was doubled, suggesting that the steroid induces the synthesis of additional adenylate cyclase. The activity of a second enzyme localized in the surface membrane, (Na+ + K+)-ATPase, was unaltered in glucocorticoid-treated cells. Also, no differences in the activities of cyclic nucleotide phosphodiesterases were detected.
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