Abstract
1)[U-14C] glucose incorporations into hepatocytes, glycogen and nucleotide fractions were stimulated by insulin (200ng/ml): this effect seemed apparent as early as at two to four hours and most prominent at 24 hours after insulin addition.
2) At four hours, [U-14C] glucose incorporations into hepatocytes (100%), glycogen (8% of [U-14C] glucose incorporation into hepatocytes), lipid (9%), nucleotide fractions (29%) and medium lactate (340%) were all stimulated with insulin (200 ng/ml) by 28-54% over control. At 24 hours, [U-14C] glucose incorporations into hepatocytes (100%), glycogen (15% of [U-14C] glucose incorporation into hepatocytes), lipid (11%), nucleotide fractions (39%) and medium lactate (530%) increased in proportion to the glucose concentration up to 800 mg/dl and were further significantly stimulated by 21-112% over control with insulin supplementation.
3) Addition of cycloheximide (1μg/ml) totally abolished the effect of insulin on [U-14C] glucose incorporation into hepatocytes at 24 hours, while no significant suppressive effect was observed at four hours.
4) As for key enzyme activities, at four hours after the addition of insulin, activation of glycogen synthase (increase of % I activity) and pyruvate kinase (increased at 0.2 mM phosphoenol pyruvate) was noted, while at 24 housr, these two enzymes increased in the total activity. Regarding glucokinase and glucose 6-phosphate dehydrogenase, both enzyme activities increased by 30-35% and 65-93% at four and 24 hours, respectively.
5) Glucose utilization was increased in a dose-response manner in the presence of insulin (5-200ng/ml).
It is suggested that insulin stimulates several pathways of glucose metabolism in hepatocytes mainly through activation of glycogen synthase and glycolytic key enzymes as an early effect and probably through induction of these enzyme levels as a delayed effect.
