Submitted on February 19, 2007
Accepted on September 11, 2007
Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles
Yu-Chiang Lai1, Jorid Thrane Stuenes2, Chia-Hua Kuo3, and Jorgen Jensen4*
1 Department of Physiology, National Institute of Occupational Health, Oslo, Norway; The Norwegian University of Sport and Physical Education, Norway
2 Department of Physiology, National Institute of Occupational Health, Oslo, Norway
3 Exercise Biochemistry, Taipei Physical Education College, Taipei, Taiwan - Republic of China
4 Department of Physiology, National Institute of Occupational Health, Oslo, Norway; Oslo, Norway
* To whom correspondence should be addressed. E-mail: jorgen.jensen{at}stami.no.
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 dw-1), low (LG; 108.1±4.5 mmol · kg dw-1) and high glycogen (HG; 482.7±42.1 mmol · kg dw-1) were contracted or rested prior to 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). Additionally, 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; these effects were observed independently of glycogen content. In rested muscles GS Ser641 and Ser7 phosphorylations were decreased in LG and increased in HG compared to NG. GSK-3
Ser9 and AMPK
Thr172 phosphorylation were 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 UDP-glucose independently of glycogen content and creates a unique phosphorylation pattern.
