Effects of the specific cAMP antagonist, (Rp)-adenosine cyclic 3',5'-phosphorothioate, on the cAMP-dependent protein kinase-induced activity of hepatic glycogen phosphorylase and glycogen synthase.
The cAMP-dependent protein kinase-induced effects on phosphorylase and glycogen synthase activities and glucose production were studied in hepatocytes isolated from fed rats in the presence of the diastereomers of adenosine cyclic 3',5'-phosphorothioate, (Sp)-cAMPS and (Rp)-cAMPS. Incubation of hepatocytes with (Sp)-cAMPS or glucagon, both of which lead to cAMP-dependent protein kinase activation, resulted in a concentration-dependent increase in glycogen phosphorylase activity and a decrease in glycogen synthase activity. Incubation of hepatocytes with the cAMP-dependent protein kinase antagonist, (Rp)-cAMPS, in the absence of an agonist, had no significant effect on phosphorylase or glycogen synthase activities. Incubation of hepatocytes with a half-maximally inhibitory concentration of (Rp)-cAMPS shifted the agonist-induced activation curves for phosphorylase and the agonist-induced inhibition curves for glycogen synthase to 5-fold higher concentrations for both (Sp)-cAMPS and glucagon. Phosphorylase activity was very sensitive to the rapid, concentration-dependent inhibition by (Rp)-cAMPS of agonist-induced activation of cAMP-dependent protein kinase. The effects on phosphorylase activity were observable in 30 s and were concentration-dependent with half-maximal inhibition at 10 microM, similar to that observed for cAMP-dependent protein kinase. In contrast, glycogen synthase activity was less sensitive to (Rp)-cAMPS inhibition of agonist-induced activation of cAMP-dependent protein kinase. The effects on glycogen synthase activity lagged behind those on phosphorylase activity and the concentration dependence did not parallel the cAMP-dependent protein kinase effect, but was shifted to higher concentrations of (Rp)-cAMPS with half-maximal inhibition at 60 microM. Glucose (10 to 40 mM) increased the sensitivity of glycogen synthase to (Rp)-cAMPS inhibition of cAMP-dependent protein kinase over a narrow range of agonist concentration, but had no significant effect throughout most of the agonist-induced activation range. Thus, the diastereomers, (Sp)- and (Rp)-cAMPS, influence glycogen metabolism and the glycogenolytic enzymes through their modulation of cAMP-dependent protein kinase levels.
D000818:Animals; D000242:Cyclic AMP / Q000031:analogs & derivatives* / Q000494:pharmacology; D004789:Enzyme Activation; D005934:Glucagon / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006006:Glycogen Synthase / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000187:drug effects; D009132:Muscles / Q000201:enzymology*; D006005:Phosphorylases / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D013873:Thionucleotides / Q000494:pharmacology*
D013873:Thionucleotides; C016957:adenosine-3',5'-cyclic phosphorothioate; D005934:Glucagon; D000242:Cyclic AMP; D006005:Phosphorylases; D006006:Glycogen Synthase; D011494:Protein Kinases; D005947:Glucose