Department of Physical Performance, Norwegian School of Sport Sciences, P. O. Box 4014 Ullevål Stadion, N-0806 Oslo, Norway.
Am J Physiol Cell Physiol. 2012 May 15;302(10):C1539-47. doi: 10.1152/ajpcell.00430.2011. Epub 2012 Mar 7.
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 ~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 Ser(641), 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 ~0.8, ~0.4, and ~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 ~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.
在本研究中，研究了胰岛素和收缩对瘦素和肥胖 Zucker 大鼠的后肢屈肌中糖原合酶 (GS) 动力学特性和磷酸化的影响。与瘦鼠相比，肥胖鼠后肢屈肌中的总 GS 活性和蛋白表达水平降低了约 15%。与肥胖鼠相比，胰岛素刺激的 GS 分数活性和对 UDP-葡萄糖的亲和力降低（Km 值较高）。在肥胖鼠胰岛素刺激的肌肉中，GS Ser(641)和 Ser(645、649、653、657)磷酸化水平更高，这与 GS 激活水平降低一致。在肥胖 Zucker 大鼠的肌肉中，收缩介导的 GS Ser(641)、Ser(641+645)、Ser(645、649、653、657)和 Ser(7+10)去磷酸化正常，并且 GS 分数活性在瘦鼠和肥胖鼠的后肢屈肌中增加到相似水平。在含有 0、0.17 和 12 mM 葡萄糖 6-磷酸的测定缓冲液中，GS 对 UDP 葡萄糖的亲和力（Km 值）分别约为 0.8、0.4 和 0.1 mM。在含有 0.17 mM 葡萄糖 6-磷酸的生理浓度下，收缩增加了对 UDP-葡萄糖的亲和力（Km 值降低），在瘦鼠和肥胖鼠的肌肉中均降低至约 0.2 mM。有趣的是，在测定缓冲液中没有葡萄糖 6-磷酸的情况下，收缩（和胰岛素）不会影响 GS 对 UDP-葡萄糖的亲和力，表明亲和力受葡萄糖 6-磷酸的敏感性调节。总之，在肥胖 Zucker 大鼠的肌肉中，收缩介导的 GS 激活和去磷酸化正常，而胰岛素介导的 GS 激活和去磷酸化受损。
