Effect of insulin and contraction on glycogen synthase phosphorylation and kinetic properties in epitrochlearis muscles from lean and obese Zucker rats.

Lin FC, Bolling A, Stuenaes JT, Cumming KT, Ingvaldsen A, Lai YC, Ivy JL, Jensen J. Effect of insulin and contraction on glycogen synthase phosphorylation and kinetic properties in epitrochlearis muscles from lean and obese Zucker rats. Am J Physiol Cell Physiol 302: C1539-C1547, 2012. First published March 7, 2012; doi:10.1152/ajpcell.00430.2011.-In the present study, the effects of insulin and contraction on glycogen synthase (GS) kinetic properties and phosphorylation were investigated in epitrochlearis muscles from lean and obese Zucker rats. Total GS activity and protein expression were similar to 15% lower in epitrochlearis from obese rats compared with lean rats. Insulin-stimulated GS fractional activity and affinity for UDP-glucose were lower (higher K-m) in muscles from obese rats. GS Ser(641) and Ser(645,649,653,657) phosphorylation was higher in insulin-stimulated muscles from obese rats, which agreed with lower GS activation. Contraction-mediated GS dephosphorylation of Ser641, Ser(641+645), Ser(645,649,653,657), and Ser(7 + 10) was normal in muscles from obese Zucker rats, and GS fractional activity increased to similar levels in epitrochlearis muscles from lean and obese rats. GS affinity for UDP glucose was similar to 0.8, similar to 0.4, and similar to 0.1 mM with assay buffers containing 0, 0.17, and 12 mM glucose 6-phosphate, respectively. Contraction increased affinity for UDP-glucose (reduced K-m) at a physiological concentration of glucose 6-phosphate (0.17 mM) to similar to 0.2 mM in muscles from both lean and obese rats. Interestingly, in the absence of glucose 6-phosphate in the assay buffer, contraction (and insulin) did not influence GS affinity for UDP-glucose, indicating that affinity is regulated by sensitivity for glucose 6-phosphate. In conclusion, contraction-mediated activation and dephosphorylation of GS were normal in muscles from obese Zucker rats, whereas insulin-mediated GS activation and dephosphorylation were impaired.