GLUCOSE UPTAKE RATE AND GLYCOGEN SYNTHASE ACTIVITY BOTH LIMIT SKELETAL MUSCLE GLYCOGEN ACCUMULATION

Abstract

The glycogen synthase kinase 3β inhibitors GF109203x (GF) and lithium were used in combination with insulin and the insulin-mimetic bpV(phen) [bpV] to evaluate the contributions of glucose uptake rate and glycogen synthase (GS) activity to glycogen accumulation. Rat epitrochlearis muscles were incubated with insulin, 36 mM glucose, and 10 μM, GF, 2 mM, LiCl, or 0.1 mM bpV. Glucose uptake was assayed with 8 mM 3H-3-O-methylglucose. Both GF and LiCl induced greater glucose-6-phosphate independent GS activity (GS%, 37 ± 2% and 24 ± 2%, respectively) than 60 μU/mL insulin alone (18 ± 2%) after 3 hrs of incubation (GF > LiCl > insulin, p < 0.05). However, GF did not increase glycogen accumulation or glucose uptake relative to 60 μU/mL insulin alone, while LiCl increased both glucose transport and glycogen synthesis. After 5 hrs of glycogen accumulation (final 50 ± 3 μmol glucosyl units/gram from basal of 9 ± 1 μmol/g) stimulated by 2 mU/mL insulin, GS% was reduced by ∼70% from the basal level (p < 0.05). Inclusion of GF prevented the decline in GS% and increased glycogen accumulation by ∼20% (p < 0.05). Addition of bpV for the final two hours also increased glycogen accumulation by ∼20% vs. insulin alone (p < 0.05), and the bpV and GF effects were additive (p < 0.05). BpV (no insulin) reduced GS% to ∼40% of basal level. While bpV increased glucose uptake by ∼50% (p < 0.05) compared to maximal insulin, there was minimal glycogen accumulation. GF added to bpV increased GS% by ∼60% (p < 0.05) and doubled glycogen accumulation vs. bpV alone (p < 0.05). We conclude that 1) the rate of glucose uptake limits glycogen accumulation when GS activity is at least at basal, fasted level; 2) raising GS activity above the fasting baseline level does not result in an increase in glycogen accumulation; and 3) when GS activity is reduced markedly below basal level, increasing glucose transport or GS activity both increase the rate of glycogen synthesis. Supported by HL10212 and DK18986.