Cyclic Phosphodiesterase Inhibitor Research Articles

Stimulation of Guanosine 3',5'-Monophosphate-Phosphodiesterase Activity by Adrenocorticotropic Hormone-Activated Increase of Guanosine 3',5'-Monophosphate in Isolated Adrenocortical Carcinoma Cells*

The decline of the ACTH-stimulated cGMP to the basal level in isolated adrenocortical carcinoma cells was inhibited by cyclic phosphodiesterase inhibitors. Time-course experiments showed that the ACTH-induced level of cGMP preceded the activation of phosphodiesterase. Cells incubated with increasing concentrations of exogenous cGMP responded with a corresponding increase in the cGMP-phosphodiesterase activity. cAMP was 100-fold less effective than cGMP in the activation of cGMP-phosphodiesterase, indicating the nucleotide specificity of enzyme activation. The activation of cGMP-phosphodiesterase by ACTH-induced cGMP or exogenous cGMP was rapid. In contrast to these observations, there was no activation of cAMP-phosphodiesterase by exogenous cAMP or cGMP. These results indicate that one mechanism by which isolated adrenocortical carcinoma cells regulate the ACTH-induced increase in cGMP is the cyclic nucleotide control of the activity of its phosphodiesterase, thereby regulating cGMP degradation.

Formation of adenosine 3′,5′-monophosphate from adenosine in mouse thymocytes

The effect of adenosine on the mouse thymocyte adenylate cyclase-adenosine 3′:5′-monophosphate (cyclic AMP) system was examined. Adenosine, like prostaglandin E 1, can cause 5-fold or greater increases in thymocyte cyclic AMP content in the presence but not in the absence of certain cyclic phosphodiesterase inhibitors. Two non-methylxanthine inhibitors potentiated the prostaglandin E 1 and adenosine responses, while methylxanthines selectively inhibited the adenosine response. Adenosine increased cyclic AMP content significantly wihtin 1 min and was maximal by 10 to 20 min with approx. 2 and 10 μM adenosine being minimal and half-maximal effective doses, respectively. Combinations of prostaglandin E 1, isoproterenol and adenosine were near additive and not synergistic. Of the adenosine analogues tested, only 2-chloro- and 2-fluoroadenosine significantly increased cyclic AMP. Thymocytes prelabeled with [ 14C] adenine exhibited dramatic increases in cyclic [ 14C]AMP 10 min after addition of adenosine or prostaglandin E 1 which corresponded to simultaneously determined increases in total cyclic AMP. Using [ 14C]adenosine, the percent of total cyclic AMP increase due to adenosine was only 16%. Adenosine was also shown to elicit a 40% increase in particulate thymocyte adenylate cyclase activity. Therefore, the increased content of cyclic AMP seen in mouse thymocytes after incubation with adenosine was due primarily to stimulation of adenylate cyclase and only partially to conversion of adenosine to cyclic AMP. The increased cellular content of cyclic AMP may be, in part, responsible for various immunosuppressive effects of adenosine.

Mechanism of puromycin activation of tadpole liver glycogen synthetase

Tadpole liver glycogen synthetase (EC 2.4.1.11), which is completely glucose 6-phosphate dependent, is activated by insulin. Puromycin mimics the hormone and activates the enzyme to a similar degree. The antibiotic initiates a series of events which appears to result in the release of insulin. Inhibition of cyclic phosphodiesterase by puromycin raises hepatic cyclic AMP to a level which stimulates phosphorylase b kinase. The increased glycogenolysis, depletion of liver glycogen and hyperglycemia which result would lead to the release of insulin. By using alloxan-treated and pancreatectomized tadpoles we have shown that the puromycin activation of glycogen synthetase is contigent on functional β-cells in the pancrease to produce insulin.