Roy D, Jóhannsson E, Bonen A, Marette A
Department of Physiology, Laval University Hospital Research Center, Ste-Foy, Québec, Canada.
Am J Physiol. 1997 Oct;273(4):E688-94. doi: 10.1152/ajpendo.1997.273.4.E688.
Insulin and contraction independently stimulate glucose transport in skeletal muscle. Whereas insulin activates glucose transport more in muscles composed of type I and IIa fibers, electrical stimulation increases glucose transport at least as much in type IIb fiber-enriched muscles despite the fact that the latter fiber type contains less GLUT-4 glucose transporters. The aim of the present study was to test the hypothesis that a greater GLUT-4 translocation to the cell surface may underlie the higher contraction-stimulated glucose transport in type IIb myofibers. Leg muscles from rats were stimulated in situ at 100 Hz (200 ms) each 2 s via the sciatic nerve over a period of 20 min while the contralateral leg was kept at rest. Muscle 2-[3H]deoxy-D-glucose uptake (2-DG) was measured in separated red gastrocnemius (RG, type I and IIa fibers) and white gastrocnemius (WG, type IIb fibers) muscles. Resting 2-DG uptake was greater in RG than WG. Electrical stimulation increased 2-DG uptake over resting values similarly in WG and RG. Fractions enriched with either plasma membranes, transverse (T) tubules, triads, or GLUT-4-enriched intracellular membranes were isolated from RG and WG using a recently developed subcellular fractionation procedure. Electrical stimulation similarly increased GLUT-4 protein content in plasma membranes of RG and WG, whereas it stimulated GLUT-4 translocation more (approximately 50%) in T tubules of WG than in RG. GLUT-4 content was not changed in triads of both muscle types. The increments in cell surface GLUT-4 protein levels were paralleled by significant reductions in the amount of the transporter in the intracellular membrane fractions of both muscle types (by 60% in RG and 56% in WG). It is concluded that electrically induced contraction stimulates GLUT-4 translocation more in T tubules of WG than RG. The physiological implications of this finding for glucose uptake by contracting RG and WG muscles is discussed.
胰岛素和收缩可独立刺激骨骼肌中的葡萄糖转运。胰岛素在由I型和IIa型纤维组成的肌肉中对葡萄糖转运的激活作用更强，而电刺激在富含IIb型纤维的肌肉中至少能同等程度地增加葡萄糖转运，尽管后一种纤维类型所含的GLUT - 4葡萄糖转运蛋白较少。本研究的目的是检验以下假设：在IIb型肌纤维中，更多的GLUT - 4向细胞表面转位可能是收缩刺激的葡萄糖转运更高的基础。通过坐骨神经以每秒1次的频率在20分钟内对大鼠腿部肌肉进行100 Hz（200毫秒）的原位刺激，同时对侧腿保持静止。在分离的红色腓肠肌（RG，I型和IIa型纤维）和白色腓肠肌（WG，IIb型纤维）中测量肌肉2 - [³H]脱氧 - D - 葡萄糖摄取（2 - DG）。静息状态下，RG的2 - DG摄取量高于WG。电刺激使WG和RG的2 - DG摄取量相对于静息值同样增加。使用最近开发的亚细胞分级分离程序，从RG和WG中分离出富含质膜、横管（T管）、三联体或富含GLUT - 4的细胞内膜的组分。电刺激同样增加了RG和WG质膜中GLUT - 4蛋白含量，而在WG的T管中，它刺激GLUT - 4转位的程度比RG中更大（约50%）。两种肌肉类型的三联体中GLUT - 4含量均未改变。细胞表面GLUT - 4蛋白水平的增加与两种肌肉类型细胞内膜组分中转运蛋白量的显著减少同时出现（RG中减少60%，WG中减少56%）。得出的结论是，电诱导收缩在WG的T管中比在RG中更能刺激GLUT - 4转位。讨论了这一发现对收缩的RG和WG肌肉摄取葡萄糖的生理意义。
