大鼠膈肌水溶性蛋白组的研究

本研究应用蛋白组的核心技术——高分辨率二维电泳对同属骨骼肌的膈肌和趾长伸肌水溶性蛋白组的二维电泳图谱进行了比较,并对两者在老化过程与年龄相关的水溶性蛋白进行了比较。我们发现膈肌和趾长伸肌水溶性蛋白的二维图谱极为相似,大部分蛋白都能较好地重叠。我们未能找到持续而稳定且特异于膈肌的水溶性蛋白。同时我们发现了三个与年龄相关蛋白（S1,S2和S3）,随增龄,蛋白S1只在24月龄趾长伸肌存在,而在8月龄和24月龄的膈肌中却不能被检测到。S2在8月龄的膈肌和趾长伸肌存在,而在24月龄的膈肌和趾长伸肌中不能被检测到。S3在8月龄的膈肌和趾长伸肌中不能被检测,而在24月龄的膈肌和趾长伸肌中能被检测到。本研究提示膈肌和趾长伸肌水溶性蛋白在其成年时表达基本一致。老化而造成水溶性蛋白表达的变化在膈肌和趾长伸肌也较为相似。     

烧伤后大鼠不同类型骨骼肌蛋白降解率的变化规律及其与糖皮质激素间的关系

为了解烧伤后不同类型骨骼肌蛋白降解率和血浆中糖皮质激素含量的变化 ,比较不同类型骨骼肌对烧伤刺激的反应 ,分析烧伤后骨骼肌萎缩的调节因子 ,应用 30 %Ⅲ度烫伤大鼠模型 ,借助骨骼肌充分氧供离体孵育系统 ,采用氨基酸全谱分析仪测定大鼠伤后不同时间点伸趾长肌和比目鱼肌蛋白降解率 ,用放射免疫分析测定大鼠血浆中糖皮质激素含量。结果发现 :烧伤后大鼠伸趾长肌总蛋白降解率和肌纤维蛋白降解率均明显增加 ,以烧伤后 12h和 2 4h为著 ,其中肌纤维蛋白降解率增幅明显 ;比目鱼肌总蛋白降解率和肌纤维蛋白降解率烧伤后均无显著变化 ;烧伤后不同时间点血浆糖皮质激素含量较正常对照组均显著增加 (P <0 0 1) ;伸趾长肌蛋白降解率与血浆糖皮质激素增加密切相关。说明烧伤后骨骼肌萎缩主要是由于肌纤维蛋白大量降解所致 ,快白肌比慢红肌对烧伤刺激敏感 ;糖皮质激素可能是导致骨骼肌蛋白降解增强的重要调节因子。     

8周爬梯负重训练诱导大鼠比目鱼肌和趾长伸肌PGC1α/FNDC5/MSTN差异表达

目的:观察8周爬梯负重训练对SD大鼠比目鱼肌和趾长伸肌PGC1α/FNDC5/MSTN表达及血清Irisin浓度的影响。方法:20只SD大鼠随机平均分为对照组(Con组,n=10)和爬梯训练组(Lad组,n=10),Lad组采用改进的爬梯负重抗阻力量训练模型,3天训练1轮,共进行8周。训练结束后48小时取比目鱼肌和趾长伸肌,用定量PCR和Western Blot法检测肌肉组织中PGC1α/FNDC5/MSTN的基因和蛋白表达,用Elisa法检测血清Irisin浓度。结果:与Con组相比,Lad组比目鱼肌PGC1α表达显著降低(P<0.05)、FNDC5表达有降低趋势,MSTN表达显著增加(P<0.05);而趾长伸肌PGC1α表达显著增加(P<0.05)、FNDC5表达有增加趋势,MSTN表达显著降低(P<0.05);血清Irisin浓度无明显差异(P>0.05)。结论:8周爬梯负重训练诱导SD大鼠比目鱼肌和趾长伸肌PGC1α/FNDC5/MSTN差异表达但不影响血清Irisin浓度,提示长期抗阻力量训练后机体可能存在复杂的平衡拮抗机制。     

悬吊大鼠后肢骨骼肌等长收缩功能变化的动态特征

目的观测模拟失重1、2和4周大鼠比目鱼肌与趾长伸肌收缩性能的变化,并分析萎缩比目鱼肌收缩性能变化的可能机制.方法采用离体骨骼肌肌条灌流技术,观测比目鱼肌与趾长伸肌的等长收缩功能.结果悬吊1周大鼠比目鱼肌即出现萎缩,悬吊2周与4周进一步萎缩,在各时间点两组的差别均具有显著性或非常显著性意义.比目鱼肌等长收缩的最大张力在悬吊第1周仅呈降低趋势[对照组(1.76±0.18)g/mm2,悬吊组(1.46±0.26)g/mm2,P＞0.05],第2周的降低具有显著性意义[(1.25±0.09)g/mm2,P＜0.05],第4周则进一步降低[(0.90±0.06)g/mm2,P＜0.01].悬吊1周组大鼠趾长伸肌的最大张力未改变,2周与4周组的降低具有非常显著性意义(P＜0.01).悬吊1、2和4周组大鼠比目鱼肌等长收缩达到最大张力一半的时间与同步对照组相比,两组间的差别均无显著性意义(P＞0.05),但是,悬吊1与2周组达到最大张力的时间与从张力峰值到舒张75%的时间缩短(P＜0.05),悬吊4周组则进一步缩短(P＜0.01).与同步对照组相比,悬吊组趾长伸肌的时间参数均未发生改变(P＞0.05).结论由于比目鱼肌最大张力的降低与其萎缩在发生时间上不同步,而等长收缩的时间参数却较早发生改变,提示模拟失重萎缩比目鱼肌中调节性收缩蛋白可能较早发生改变,进而影响收缩性能.     

硝苯地平对去负荷比目鱼肌肌球蛋白重链(MHC)异构体转化的影响

目的观测L型钙离子通道抑制剂硝苯地平对尾部悬吊大鼠比目鱼肌重量,以及肌球蛋白重链(MHC)表达水平的影响。方法经饮水每天给予大鼠10mg／kg体重的硝苯地平1与2周。称量比目鱼肌(SOL)与趾长伸肌(EDL)的湿重,用低温SDS-聚丙烯酰胺凝胶电泳观测MHC异构体蛋白表达,并用RT—PCR方法观测MHCmRNA的表达。结果与正常对照组相比,给予硝苯地平处理1与2周的同步对照组大鼠SOL的相对重量未见明显改变;而尾部悬吊1周与2周大鼠SOL的相对重量则分别降低了39．5％和51．7％。经硝苯地平处理的1与2周悬吊大鼠的SOL相对重量较其对照组分别降低了36．6％与52．0％,但与同步悬吊组相比,无明显差异。各组EDL相对重量均未见明显改变。正常对照组与经硝苯地平处理的对照组可检测到MHCⅠ、Ⅱa mRNA与蛋白的表达,悬吊组与经硝苯地平处理的悬吊组可检测到MHCⅠ、Ⅱa、Ⅱb与Ⅱx mRNA以及MHCⅠ与Ⅱa蛋白的表达。在对照组与悬吊组,硝苯地平对Ⅱ型MHC mRNA表达均产生抑制作用,并降低MHC Ⅱa蛋白表达。结论硝苯地平不能防止去负荷引起的SOL萎缩,但在转录水平抑制萎缩SOL的MHC异构体由慢型向快型转化。     

后肢去负荷对小鼠运动耐力的影响及其机制

目的研究不同持续时间后肢去负荷对小鼠运动耐力的影响并探讨其可能的机制。方法小鼠去负荷7、14、28 d,采用跑台记录小鼠运动至耗竭的距离以评价运动耐力,并利用免疫荧光染色、Western blot等方法检测快慢肌纤维蛋白的表达和肌纤维含量。结果在去负荷7、14、28 d肌萎缩形成过程中,小鼠运动耐力分别比对照组下降30.2%、65.5%、71.7%(P0.001)。运动耐力进行性下降伴随小鼠后肢比目鱼肌肌肉重量的持续丢失,分别下降16.3%(P0.05)、27.2%(P0.001)、29.4%(P0.001)。Western blot结果表明,比目鱼肌中慢肌纤维特异蛋白Troponin I-SS蛋白表达呈进行性下降,而快肌纤维特异蛋白Troponin I-FS的表达上升。免疫组化分析提示,去负荷7、14和28 d后比目鱼肌中慢肌纤维含量分别比对照组下降2.76%、7.75%(P0.05)、14.97%(P0.05)。可见伴随后肢去负荷时间的延长,运动耐力持续下降,且伴有显著慢肌萎缩及慢肌纤维含量减少。结论后肢去负荷可引起运动耐力的渐进性下降,且这种规律性变化与慢肌萎缩和慢肌纤维的选择性丢失有关。     

缺氧和收缩状态下不同肌纤维类型骨骼肌葡萄糖转运速率与肌糖原含量的关系

目的:观察大鼠不同类型骨骼肌肌纤维在缺氧、收缩以及两者联合刺激下,其葡萄糖跨膜转运速率(GTR)与肌糖原含量的关系。方法:取不同肌纤维占优势的3种大鼠骨骼肌(比目鱼肌、趾长伸肌和肱骨内上髁肌),离体条件下进行不同的刺激处理(基础对照、缺氧、收缩、缺氧 收缩),测定GTR和肌糖原含量并分析它们之间的关系。结果:骨骼肌肌纤维类型不同,其GTR对缺氧、收缩、缺氧 收缩刺激的反应有所不同,其肌糖原的基础量、不同刺激后的残留量及消耗量也不尽相同。3种肌纤维类型骨骼肌的GTR均与肌糖原的残留量呈负相关倾向,而与消耗量呈正相关倾向。氧化型(Ⅱa型和Ⅰ型)肌纤维(趾长伸肌和比目鱼肌)比酵解型(Ⅱb型)肌纤维(肱骨内上髁肌)在肌糖原消耗量较少的情况下可引起GTR较高的增加。这一结果从肌纤维类型的角度揭示了有氧运动(可更多地动员氧化型肌纤维)在预防和改善胰岛素抵抗中能够发挥重要作用的机理。     

睫状神经营养因子对失重性肌萎缩及其纤维表型转换的干预作用

目的 探讨CNTF对失重性肌萎缩及其纤维表型转换的干预作用.方法 用尾吊方法制备失重性肌萎缩模型,通过采用RT-PCR和Western blot分析方法检测MHC-l/llb和p130或Myf5表达的变化,以揭示CNTF对失重性肌萎缩及其纤维表型转换的影响.结果 与对照组相比,体内注射CNTF显著逆转失重诱导的慢肌比目鱼肌重量的丢失.此外,尾吊过程中的动态分析结果也提示,CNTF处理可明显削弱失重诱导的慢肌比目鱼肌重量的丢失及其慢肌向快肌纤维表型的转换.课题组进一步发现CNTF的这种干预效应伴随着肌卫星细胞特异标志p130和Myf5蛋白的上调,这就提示了CNTF处理的慢肌比目鱼肌中肌卫星细胞库增加.结论 本研究首次揭示了CNTF可通过增加慢肌的肌卫星细胞库干预失重性肌萎缩及其纤维表型的转换.     

钙调神经磷酸酶在耐力运动大鼠骨骼肌纤维类型和大小转变中的作用

目的:探讨钙调神经磷酸酶(CaN)在耐力运动时骨骼肌纤维类型和大小转变中的作用。方法:雄性SD大鼠(150~200g)随机分成对照组和运动组。运动组大鼠进行6周无坡度跑台训练(20m/min~28m/min),并在第3周末进一步随机分成2组:运动 环孢素组,皮下注射环孢素(CSA,15mg/kg体重/day)3周;单纯运动组,注射等量生理盐水。各组大鼠第6周末取比目鱼肌和趾长伸肌,分别用SDS-PAGE电泳法和肌纤维ATP酶染色法测定肌球蛋白重链(MHC)组成比例和肌纤维横断面积,同时用比色法测CaN活性,并用Westernblotting法测CaN和活化T细胞核因子2(NFAT2)蛋白含量。结果:单纯运动组大鼠比目鱼肌MHCⅠ比例(93.4±6.0%)显著高于对照组(82.7±6.6%),而运动 环孢素组大鼠MHCⅠ比例(79.3±8.4%)无增加,但其Ⅰ型肌纤维横断面积显著小于对照组和单纯运动组(P<0.05)。单纯运动组大鼠趾长伸肌胞浆CaN活性及胞核NFAT2蛋白含量显著高于对照组(P<0.05),但纤维比例无显著改变(P>0.05);运动 环孢素组大鼠趾长伸肌肌原纤维蛋白浓度显著高于单纯运动组(P<0.05),且该组各型肌纤维横断面积均显著小于对照组和单纯运动组(P<0.05)。结论:CaN参与了耐力运动骨骼肌纤维类型和大小的调控,且具有肌肉特异性。     

跑台运动对肥胖大鼠比目鱼肌毛细血管生成及apelin表达的影响

目的:观察跑台运动对高脂饮食诱导的肥胖大鼠骨骼肌毛细血管生成及apelin/APJ表达的影响。方法:5周龄健康雄性SD大鼠随机分成对照组和高脂喂养组,分别进行普通饲料和高脂饲料喂养,12周后筛选肥胖大鼠16只,随机分为高脂安静组和高脂运动组。跑台训练持续10周,每周5次,每次60分钟,跑速26米/分钟。测定大鼠体重、体脂重及血脂水平,免疫组化法观察比目鱼肌毛细血管密度和apelin蛋白表达水平,放免法测定比目鱼肌apelin含量,real-time PCR方法测定比目鱼肌apelin和APJ的m RNA表达。结果:与对照组相比,高脂饮食喂养大鼠体重和体脂显著增加(P<0.01),总甘油三酯、总胆固醇和低密度脂蛋白胆固醇水平显著升高,高密度脂蛋白胆固醇水平显著降低(P<0.05)。与对照组比,肥胖大鼠比目鱼肌毛细血管生成无显著变化,apelin和APJ m RNA表达无显著变化(P>0.05)。与单纯高脂饮食组大鼠相比,10周跑台运动大鼠体重显著降低(P<0.01),血脂改善;此外,比目鱼肌毛细血管密度显著升高,apelin和APJ m RNA表达显著上升(P<0.05),apelin蛋白表达水平显著升高(P<0.05)。结论:跑台运动能显著增加肥胖大鼠比目鱼肌毛细血管生成,其作用可能与运动诱导比目鱼肌apelin表达相关。     

In vitro 1H-NMR spectroscopic analysis of metabolites in fast- and slow-twitch muscles of young rats.

The lactate (LAC), creatine (CRN), taurine (TAU), anserine (ANS) and carnosine (CAR) content of the masseter muscles (MM), long extensor muscles of digits (EDL) and soleus muscles (SOL) of young rats were determined using in vitro 1H-NMR spectroscopy to assess the significance of CRN, TAU, ANS and CAR in these muscles. The muscles of Wistar rats at the ages of 6, 12 and 18 weeks were dissected after decapitation and used for the metabolite analyses. The LAC and CAR content of all muscle groups showed no age dependence. The CRN content was increased age-dependently in MM but not in EDL or SOL. The LAC and CRN content was higher in MM and EDL (fast-twitch) than in SOL (slow-twitch) (P<0.01-0.001 at 18 weeks). A significant positive correlation existed between the LAC and CRN content (P<0.00001, r=0.80), suggesting that the CRN content reflects the capacity of the anaerobic glycolysis of the individual muscles. The TAU content was higher in SOL and MM than in EDL (P<0.05) and showed an approximately 1.5-fold increase with age in all three muscle groups. The ANS content was higher in EDL than in SOL and MM (P<0.001), and showed an approximately threefold increase with age in all three muscle groups. The ANS content positively correlated with the LAC content (P<0.001, r=0.41), and the chemical shift of the imidazole proton in ANS showed a correlation with the LAC content (P<0.0001, r>0.76), indicating that ANS would buffer the pH change produced by LAC. These results suggest that 1H-NMR spectroscopy would provide an adjunct method of assessing the muscle types and their development.     

Vasomodulation of skeletal muscle BOLD signal

PURPOSE: To evaluate whether the BOLD signal from skeletal muscle can be modulated by exercise and ingestion of vasoactive substances. MATERIALS AND METHODS: The right calf muscles of healthy adult volunteers were imaged using a GE 1.5-Tesla scanner and a gradient-echo sequence with spiral readout. Time-varying changes in the BOLD signal were induced through cyclic phases of normoxia (90 seconds of 20.8% O2) and hyperoxia (45 seconds of 100% O2 at 22 L/minute). Superimposed on this paradigm were pre- and post-exercise regimes, with and without ingestion of caffeine (100 mg) or antihistamine (4 mg chlorpheniramine). The numbers of voxels within slow-twitch (soleus) and fast-twitch (gastrocnemius) muscles that significantly responded to the paradigms were scored and compared using the AFNI software (NIMH). RESULTS: Cycling-inspired O2 produced a corresponding BOLD modulation that increased in magnitude with exercise. Chlorpheniramine significantly (P<0.01) prevented the overall increase in exercise-induced soleus muscle BOLD signal, while caffeine accentuated the increase (P<0.05) in the gastrocnemius relative to control (no vasomodulator) conditions. CONCLUSION: BOLD signal changes with exercise can be modulated by standard doses of chlorpheniramine (antihistamine) and caffeine. We suggest that chlorpheniramine may act detrimentally on slow-twitch muscle contractility, while caffeine appears to improve fast-twitch muscle function.     

川芎嗪和黄芪对尾部悬吊大鼠比目鱼肌肌球蛋白ATP酶活性及肌萎缩的影响

目的 探讨川芎嗪和黄芪对吊尾大鼠比目鱼肌（SOL）肌球蛋白ATP酶（mATPase）活性及肌萎缩的影响。方法 尾部悬吊法建立模拟失重模型,Ca^2＋-ATPase法测定SOL的mATPase活性。结果 1）与吊尾组（TS）相比,在腹腔注射川芎嗪组和黄芪组,SOL中Ⅱ型肌纤维比例均显著降低,且黄芪组Ⅰ、Ⅱ型肌纤维的比例与同步饲养组相比亦无显著差异;2）川芎嗪组SOL中Ⅰ型肌纤维横截面积（CSA）显著增大,与同步饲养组相比无显著性差异,黄芪组Ⅰ、Ⅱ型肌纤维CSA及平均CSA均显著地增大,且与同步饲养组相比亦无显著性差异;3）川芎嗪组和黄芪组梭内肌各纤维的mATPase染色均与同步饲养组相近。结论 川芎嗪和黄芪对大鼠SOL的失重性肌萎缩及慢肌向快肌的转化均有明显的对抗作用,且均可显著抑制模拟失重导致的梭内肌各纤维mATPase活性的升高。     

Composition and turnover of contractile proteins in volume-overtrained skeletal muscle

The purpose of this study was to find the composition shift of myosin heavy chain (MyHC) isoforms in overtraining in fast- and slow-twitch skeletal muscles and different changes in MyHC isofom composition, synthesis and turnover rate between 4-week and 6-week overtraining. Male Wistar rats were randomly assigned to 4-week and 6-week endurance training, 4-week and 6-week overtraining groups. Plantaris (Pla), extensor digitorum longus (EDL), and soleus (Sol) muscles were studied. Daily excretion of 3-methylhistidine (3-MeHis) pool as an indicator for protein degradation increased in the 4-week and 6-week overtraining group to 4.04 +/- 0.21 and 4.32 +/- 0.23 %/day subsequently in comparison with the control group (2.16 +/- 14 %/day, p < 0.001). In Pla muscle MyHC I isoform synthesis rate was 33 200 +/- 2150 (after 6-week overtraining 27 100 +/- 1800, p < 0.05), IIa 32 600 +/- 2100; IId 27 300 +/- 1890 and IIb isoform 20 100 +/- 1600 (after 6-week overtraining 15 500 +/- 1400, p < 0.05) dpm/M leucine/min. Actin synthesis rate increased in fast-twitch muscles during 4- and 6-week overtraining, and in soleus muscle during 6-week overtraining. In EDL and Sol muscle MyHC isoform composition during 6-week overtraining did not change significantly. During the 6-week overtraining the relative content of MyHC I and IIb isoforms decreased and IIa and IId isoforms increased in Pla muscle. The initial increase of MyHC IIb isoform after 4-week overtraining shows the higher stability of this isoform in comparison with MyHC I isoform in fast-twitch muscles during high volume exercise.     

4周悬吊大鼠比目鱼肌强直收缩力降低及其机理分析

目的：观测尾部悬吊4周大鼠比目鱼肌（SOL）强直收缩力降低的动态特征，并探讨其可能机理。方法：采用离体SOL与趾长伸肌（EDL）灌流方法。测量其单次与强直收缩张力。结果：尾部悬吊模拟失重4周，大鼠SOL明显萎缩，但EDL重量保持不变。悬吊大鼠SOL欠收缩的最大张力虽然显著降低，但未见明显的刺激电压依赖性，时间参数亦未发生改变，悬吊大鼠SOL强直收缩的最大张力显著降低，且发生迅速的衰退，在强直收缩的第33秒时，其强直收缩的张力已降低了73％，悬吊大鼠SOL强直收缩张力的动态曲线与EDL的相似，EDL的强直收缩最大张力也显著降低，但衰减速率未改变。结论：悬吊4周大鼠SOL强直收缩最大张力降低且易衰退，前者可能与单位面积内收缩装置减少以及每一横桥产力减少相关，后者则可能是由于收缩与调节蛋白异构体发生转化，以及肌肉电兴奋性改变所导致。     

Effects of aging on muscle T2 relaxation time: difference between fast- and slow-twitch muscles.

RATIONALE AND OBJECTIVES: To determine whether the T2 relaxation time of skeletal muscle is affected by aging and to compare the effects of aging between fast- and slow-twitch muscles in a human study. To investigate the mechanisms of age-related changes in T2 relaxation time in an animal (mouse) study. METHODS: T2 relaxation times of the soleus (slow-twitch, rich in type I fiber) and gastrocnemius (fast-twitch, rich in type II fiber) muscles were examined in 59 healthy human subjects, 22 to 76 years of age, by clinical magnetic resonance imaging. In mice, T2 relaxation times, fat ratios, and extracellular space ratios (extracellular space/intracellular plus extracellular space) of the spinalis (fast-twitch, rich in type II fiber) muscles were also examined (group of 7 old mice, 24-26 months; group of 7 young mice, 8-10 weeks). RESULTS: In the human study, the T2 relaxation time of the gastrocnemius muscle increased significantly with aging (r = 0.53, P < 0.01) while that of the soleus muscle did not. In the animal study, the T2 relaxation time of the spinalis muscle was significantly longer (P < 0.05) and the extracellular space ratio of the spinalis muscle significantly wider (P < 0.01) in old than in young mice. No significant difference in fat ratio was observed between old and young mice. A significant, positive correlation was seen between the extracellular space ratio and T2 relaxation time (r = 0.84, P < 0.01). CONCLUSIONS: The T2 relaxation time of fast-twitch muscle increases with aging, due mainly to increased extracellular space, reflecting age-related type II fiber atrophy.     

The effect of gadolinium DTPA on tissue water compartments in slow- and fast-twitch rabbit muscles

Proton T2 relaxation and its biexponential components have been determined in rabbit skeletal muscle in the presence and absence of GdDTPA. The effect of GdDTPA, which distributes only in the extracellular space, was greatest in the longer-relaxing T2 component (T22). A 27% reduction in T22 was measured for slow-twitch (red) muscle and 17% for fast-twitch (white) muscle, consistent with the larger extracellular space of the former. Magnetic resonance images demonstrated apparent contrast between red and white rabbit muscles. This contrast was instantaneously enhanced by administration of GdDTPA and returned to near normal levels after approximately 30 min. These functional changes in tissue contrast are consistent with differences in blood perfusion and biological water compartmentation between fast- and slow-twitch skeletal muscles.     

Steroid receptors in two types of rabbit skeletal muscle

Radiolabeled synthetic steroid hormones and a charcoal adsorption assay were used to identify cytosolic androgen, glucocorticoid, and estrogen receptors in skeletal muscle from rabbits. The presence of the receptors was verified by saturation analysis showing low-capacity, high-affinity binding for the steroid-receptor complexes, specific for each class of steroids. The concentration of androgen and estrogen receptors were of the same magnitude, whereas the corresponding value for the glucocorticoid receptor was about tenfold higher. Comparisons of fast-twitch (the gastrocnemius/plantaris complex) and slow-twitch (soleus) muscles revealed that the latter contained higher concentrations (expressed per g of tissue wet weight) of glucocorticoid and estrogen receptors, but not of androgen receptor. Expressed per mg of soluble protein, the slow-twitch muscle contained higher concentrations of all three receptors, but when related to the concentrations of all three receptors, but when related to the concentration of DNA, only the concentration of estrogen receptor was higher in the slow-twitch muscle. Different response of the two fiber types to direct action of steroid hormones can hence be expected. The fast-twitch muscle contained a higher concentration of soluble protein, whereas the slow-twitch muscle contained higher concentration of DNA, resulting in lower protein/DNA ratio, i.e., smaller "cell units," in the latter muscle.