In obesity, accumulation of lipid droplets in skeletal muscle fibers and a shift towards fast muscle fiber types can both contribute to insulin resistance. However, it is not yet clear how intramyocellular lipid accumulation and fiber type changes are associated. Therefore, we investigated to what extent the lipids accumulated in a fiber type-specific manner in the functionally similar fast-, intermediate- and slow-twitch gastrocnemius, plantaris, and soleus muscles, respectively, in high-fat diet-induced obese 54-week-old female C57BL/6JOlaHsd mice (n = 9) compared to control standard-diet-treated lean mice (n = 9). A high-fat diet was administered for 26 weeks. Fiber-type specific intramyocellular lipid content analysis and muscle fiber typing were performed using histochemical analysis of lipids with Sudan Black and immunohistochemical analysis of myosin heavy chains on serial sections of skeletal muscles. Compared to the lean mice, the lipid accumulation was most prominent in types 2a and 2x/d fibers (p < 0.05) of fast-twitch gastrocnemius and intermediate plantaris muscles in the obese mice, while in slow-twitch soleus muscle, there was no significant lipid accumulation in the obese animals. Furthermore, the slow-twitch soleus muscle of the obese mice with no significant change in muscle fiber diameters exhibited the most pronounced shift towards fast-type myosin heavy chain isoform expression (p < 0.05). In contrast, the fast-twitch and intermediate-twitch gastrocnemius and plantaris muscles, respectively, in which the muscle fiber diameters increased (p < 0.05), were more resistant toward myosin heavy chain expression changes. In conclusion, we demonstrated both muscle- and fiber-type specificity in intramyocellular lipid accumulation in obese mice, suggesting that in obesity, similar muscle fiber types in different muscles accumulate lipids differentially.

中文翻译：

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肥胖小鼠骨骼肌和纤维类型特异性肌细胞内脂质积累。

在肥胖症中，骨骼肌纤维中脂滴的积累和向快速肌纤维类型的转变都可能导致胰岛素抵抗。然而，目前尚不清楚肌细胞内脂质积累和纤维类型变化之间的关系。因此，我们研究了在高脂饮食诱导的肥胖中，分别在功能相似的快肌、中肌和慢肌腓肠肌、跖肌和比目鱼肌中，脂质以纤维类型特异性方式积累的程度 54-与对照标准饮食治疗的瘦小鼠 (n = 9) 相比，一周大的雌性 C57BL/6JOlaHsd 小鼠 (n = 9)。给予高脂肪饮食 26 周。纤维类型特异性肌细胞内脂质含量分析和肌纤维分型使用苏丹黑脂质的组织化学分析和骨骼肌连续切片上肌球蛋白重链的免疫组织化学分析进行。与瘦小鼠相比，肥胖小鼠的快肌腓肠肌和跖中肌的2a型和2x/d型纤维的脂质积累最为显着（p < 0.05），而慢肌比目鱼肌则没有肥胖动物体内有显着的脂质积累。此外，肌肉纤维直径没有显着变化的肥胖小鼠的慢肌比目鱼肌表现出向快速型肌球蛋白重链异构体表达的最明显转变（p < 0.05）。相比之下，快速抽动和中速抽动的腓肠肌和跖肌，分别，其中肌纤维直径增加（p < 0.05），对肌球蛋白重链表达变化更具抵抗力。总之，我们在肥胖小鼠的肌细胞内脂质积累中证明了肌肉和纤维类型的特异性，这表明在肥胖症中，不同肌肉中相似的肌纤维类型不同地积累脂质。