Molecular mode of inhibition of glycogenolysis in rat liver by the dihydropyridine derivative, BAY R3401: inhibition and inactivation of glycogen phosphorylase by an activated metabolite.
The racemic prodrug BAY R3401 suppresses hepatic glycogenolysis. BAY W1807, the active metabolite of BAY R3401, inhibits muscle glycogen phosphorylase a and b. We investigated whether BAY R3401 reduces hepatic glycogenolysis by allosteric inhibition or by phosphatase-catalyzed inactivation of phosphorylase. In gel-filtered liver extracts, racemic BAY U6751 (containing active BAY W1807) was tested for inhibition of phosphorylase in the glycogenolytic (in which only phosphorylase a is active) and glycogen-synthetic (for the evaluation of a:b ratios) directions. Phosphorylase inactivation by endogenous phosphatase was also studied. In liver extracts, BAY U6751 (0.9-36 micromol/l) inhibited glycogen synthesis by phosphorylase b (notwithstanding the inclusion of AMP), but not by phosphorylase a. Inhibition of phosphorylase-a-catalyzed glycogenolysis was partially relieved by AMP (500 micromol/l). BAY U6751 facilitated phosphorylase-a dephosphorylation. Isolated hepatocytes and perfused livers were tested for BAY R3401-induced changes in phosphorylase-a:b ratios and glycogenolytic output. Though ineffective in extracts, BAY R3401 (0.25 micromol/l-0.5 mmol/l) promoted phosphorylase-a dephosphorylation in hepatocytes. In perfused livers exposed to dibutyryl cAMP (100 micromol/l) for maximal activation of phosphorylase, BAY R3401 (125 micromol/l) inactivated phosphorylase by 63% but glucose output dropped by 83%. Inhibition of glycogenolysis suppressed glucose-6-phosphate (G6P) levels. Activation of glycogen synthase after phosphorylase inactivation depended on the maintenance of G6P levels by supplementing glucose (50 mmol/l). We conclude that the metabolites of BAY R3401 suppress hepatic glycogenolysis by allosteric inhibition and by the dephosphorylation of phosphorylase a.
Bergans
N
N
Department of Radiology, Faculty of Medicine, Katholieke Universiteit Leuven, Belgium.
Stalmans
W
W
Goldmann
S
S
Vanstapel
F
F
D000249:Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D004095:Dihydropyridines / Q000494:pharmacology*; D004789:Enzyme Activation; D005663:Furans / Q000494:pharmacology*; D019298:Glucose-6-Phosphate / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008112:Liver Glycogen / Q000378:metabolism*; D008297:Male; D010477:Perfusion; D010762:Phosphorylase a / Q000378:metabolism; D010763:Phosphorylase b / Q000378:metabolism; D006005:Phosphorylases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D017378:Quinolinic Acid / Q000494:pharmacology; D011805:Quinolinic Acids; D051381:Rats; D017208:Rats, Wistar
C413462:BAY R3401; C110474:Bay W1807; D002121:Calcium Channel Blockers; D004095:Dihydropyridines; D005663:Furans; D008112:Liver Glycogen; D011805:Quinolinic Acids; D000249:Adenosine Monophosphate; D019298:Glucose-6-Phosphate; D003994:Bucladesine; D010762:Phosphorylase a; D010763:Phosphorylase b; D006005:Phosphorylases; D017378:Quinolinic Acid