title: Effects of graded intravenous doses of fructose on glycogen synthase in the liver of fasted rats.
abstract: We have examined in fasted rats the effects of graded doses of intravenous fructose (50 to 500 mg/kg) in order to determine potential mechanisms by which different concentrations of fructose reaching the liver may modify the activity of glycogen synthase (and phosphorylase). With increasing fructose doses the % synthase I increased threefold to a maximum at a dose of 125 mg/kg and then decreased progressively after higher fructose doses were given. The % phosphorylase a decreased by 30% to a minimum at a dose of 125 mg/kg but increased with higher doses to 370% of the control values. Both the % synthase I and the % phosphorylase a were elevated above the control values at fructose doses of 175 to 225 mg/kg. The increase in % synthase I after low doses of fructose occurred with a significant increase in glucose-6-P but no significant change in hepatic fructose, glucose, UDPglucose, ATP/Mg++, Pi, cAMP, plasma insulin, or glucagon concentrations. The reciprocal decrease in % synthase I and increase in % phosphorylase a occurred despite increases in glucose and glucose-6-P, at fructose doses resulting in no change in ATP/Mg++, Pi or cAMP, and only a small increase (0.39 mmol/L) in the fructose-1-P concentration. We propose that activation of synthase phosphatase by a rise in the glucose-6-P concentration is responsible for the increase in % synthase I after low doses of fructose. The mechanism by which higher fructose doses overcome the expected activation of synthase phosphatase by glucose and glucose-6-P and a decreased ATP/Mg++ ratio is uncertain.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D005215:Fasting; D005632:Fructose / Q000008:administration & dosage*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000378:metabolism; D006006:Glycogen Synthase / Q000378:metabolism*; D007275:Injections, Intravenous; D008099:Liver / Q000201:enzymology* / Q000378:metabolism; D008297:Male; D010762:Phosphorylase a / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D005958:Glucosephosphates; D005632:Fructose; D019298:Glucose-6-Phosphate; D010762:Phosphorylase a; D006006:Glycogen Synthase