Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles

Lai Y-C, Stuenaes JT, Kuo C-H, Jensen J. Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles. Am J Physiol Endocrinol Metab 293: E1622-E1629, 2007. First published September 18, 2007; doi: 10.1152/ajpendo.00113.2007. - Glycogen content and contraction strongly regulate glycogen synthase (GS) activity, and the aim of the present study was to explore their effects and interaction on GS phosphorylation and kinetic properties. Glycogen content in rat epitrochlearis muscles was manipulated in vivo. After manipulation, incubated muscles with normal glycogen [NG; 210.9 +/- 7.1 mmol/kg dry weight (dw)], low glycogen (LG; 108.1 +/- 4.5 mmol/kg dw), and high glycogen (HG; 482.7 +/- 42.1 mmol/kg dw) were contracted or rested before the studies of GS kinetic properties and GS phosphorylation ( using phospho-specific antibodies). LG decreased and HG increased GS Km for UDP-glucose ( LG: 0.27 +/- 0.02 < NG: 0.71 +/- 0.06 < HG: 1.11 +/- 0.12 mM; P < 0.001). In addition, GS fractional activity inversely correlated with glycogen content ( R = -0.70; P < 0.001; n = 44). Contraction decreased Km for UDP-glucose ( LG: 0.14 +/- 0.01 = NG: 0.16 +/- 0.01 < HG: 0.33 +/- 0.03 mM; P < 0.001) and increased GS fractional activity, and these effects were observed independently of glycogen content. In rested muscles, GS Ser(641) and Ser(7) phosphorylation was decreased in LG and increased in HG compared with NG. GSK-3 beta Ser(9) and AMPK alpha Thr(172) phosphorylation was not modulated by glycogen content in rested muscles. Contraction decreased phosphorylation of GS Ser641 at all glycogen contents. However, contraction increased GS Ser7 phosphorylation even though GS was strongly activated. In conclusion, glycogen content regulates GS affinity for UDP-glucose and low affinity for UDP-glucose in muscles with high glycogen content may reduce glycogen accumulation. Contraction increases GS affinity for UDPglucose independently of glycogen content and creates a unique phosphorylation pattern.