Fatty acid and amino acid modulation of glucose cycling in isolated rat hepatocytes

We studied the influence of glucose/glucose 6-phosphate cycling on glycogen deposition from glucose in fasted-rat hepatocytes using S4048 and CP320626 , specific inhibitors of glucose-6-phosphate translocase and glycogen phosp horylase respectively. The effect of amino acids and oleate was also examin ed. The following observations were made: (1) with glucose alone, net glyco gen production was low. Inhibition of glucose-6-phosphate translocase incre ased intracellular glucose 6-phosphate (3-fold), glycogen accumulation (5-f old) without change in active (dephosphorylated) glycogen synthase (GSa) ac tivity, and lactate production (4-fold). With both glucose 6-phosphate tran slocase and glycogen phosphorylase inhibited, glycogen deposition increased 8-fold and approached reported in vivo rates of glycogen deposition during the fasted --> fed transition. Addition of a physiological mixture of amin o acids in the presence of glucose increased glycogen accumulation (4-fold) through activation of GS and inhibition of glucose-6-phosphatase flux. Add ition of oleate with glucose present decreased glycolytic flux and increase d the flux through glucose 6-phosphatase with no change in glycogen deposit ion. With glucose 6-phosphate translocase inhibited by S4048, oleate increa sed intracellular glucose 6-phosphate (3-fold) and net glycogen production (1.5-fold), without a major change in GSa activity. It is concluded that gl ucose cycling in hepatocytes prevents the net accumulation of glycogen from glucose. Amino acids activate GS and inhibit flux through glucose-6-phosph atase, while oleate inhibits glycolysis and stimulates glucose-6-phosphatas e flux. Variation in glucose 6-phosphate does not always result in activity changes of GSa. Activation of glucose 6-phosphatase flux by fatty acids ma y contribute to the increased hepatic glucose production as seen in Type 2 diabetes.