Glycogen content regulates insulin- but not contraction-mediated glycogen synthase activation in the rat slow-twitch soleus muscles

Aim: The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined. Methods: Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin. Results: Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser⁶⁴¹ (R = -0.93, P < 0.001) and Ser^{645,649,653,657} (R = -0.85, P < 0.001) phosphorylation, but not with Ser⁷ phosphorylation. Conclusion: Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser⁶⁴¹ and Ser^{645,649,653,657} seems to regulate GS activity in soleus.