Mitochondrial myopathy with succinate dehydrogenase and aconitase deficiency. Abnormalities of several iron-sulfur proteins.

Recently, we described a patient with severe exercise intolerance and episodic myoglobinuria, associated with marked impairment of succinate oxidation and deficient activity of succinate dehydrogenase and aconitase in muscle mitochondria (1). We now report additional enzymatic and immunological characterization of mitochondria. In addition to severe deficiency of complex II, manifested by reduction of succinate dehydrogenase and succinate:coenzyme Q oxidoreductase activities to 12 and 22% of normal, respectively, complex III activity was reduced to 37% and rhodanese to 48% of normal. Furthermore, although complex I activity was not measured, immunoblot analysis of complex I showed deficiency of the 39-, 24-, 13-, and 9-kD peptides with lesser reductions of the 51- and 18-kD peptides. Immunoblots of complex III showed markedly reduced levels of the mature Rieske protein in mitochondria and elevated levels of its precursor in the cytosol, suggesting deficient uptake into mitochondria. Immunoreactive aconitase was also low. These data, together with the previous documentation of low amounts of the 30-kD iron-sulfur protein and the 13.5-kD subunit of complex II, compared to near normal levels of the 70-kD protein suggest a more generalized abnormality of the synthesis, import, processing, or assembly of a group of proteins containing iron-sulfur clusters.     

Substrate availability limits human skeletal muscle oxidative ATP regeneration at the onset of ischemic exercise.

We have demonstrated previously that dichloroacetate can attenuate skeletal muscle fatigue by up to 35% in a canine model of peripheral ischemia (Timmons, J.A., S.M. Poucher, D. Constantin-Teodosiu, V. Worrall, I.A. Macdonald, and P.L. Greenhaff. 1996. J. Clin. Invest. 97:879-883). This was thought to be a consequence of dichloroacetate increasing acetyl group availability early during contraction. In this study we characterized the metabolic effects of dichloroacetate in a human model of peripheral muscle ischemia. On two separate occasions (control-saline or dichloroacetate infusion), nine subjects performed 8 min of single-leg knee extension exercise at an intensity aimed at achieving volitional exhaustion in approximately 8 min. During exercise each subject's lower limbs were exposed to 50 mmHg of positive pressure, which reduces blood flow by approximately 20%. Dichloroacetate increased resting muscle pyruvate dehydrogenase complex activation status by threefold and elevated acetylcarnitine concentration by fivefold. After 3 min of exercise, phosphocreatine degradation and lactate accumulation were both reduced by approximately 50% after dichloroacetate pretreatment, when compared with control conditions. However, after 8 min of exercise no differences existed between treatments. Therefore, it would appear that dichloroacetate can delay the accumulation of metabolites which lead to the development of skeletal muscle fatigue during ischemia but does not alter the metabolic profile when a maximal effort is approached.     

Fatty acyl-CoA dehydrogenase enzymes in human skeletal muscle

An electrophoretic and enzyme-substrate staining technique for acyl-CoA dehydrogenase (ACD) enzymes was developed for use with small (less than 100 mg) tissue samples. Based on their electrophoretic mobility and substrate staining specificity, at least two and perhaps three chain-length specific enzymes for dehydrogenation of saturated fatty acids were found in human skeletal muscle. ACD enzymes staining with octanoyl-CoA or palmitoyl-CoA were identified by this technique in human skeletal muscle, heart, and liver, but the ACD enzyme staining with butyryl-CoA was difficult to detect and was definitely visualized only in skeletal muscle.     

Differential investigation of the capacity of succinate oxidation in human skeletal muscle

Procedures are described for the estimation of the succinate:ubiquinone oxidoreductase and succinate:phenazine methosulfate oxidoreductase activities in post-nuclear supernatants of human skeletal muscle homogenates using 2,6-dichlorophenol indophenol as the terminal electron acceptor. The influence of ionic strength and of sucrose upon these assays and upon the succinate:cytochrome c oxidoreductase activity has been investigated. Sucrose markedly interferes with the activation of the succinate dehydrogenase complex. Succinate:cytochrome c oxidoreductase activity and succinate:phenazine methosulfate oxidoreductase activity are inhibited by increasing concentrations of ions and of sucrose. Our results lead us to propose the existence of a single acceptor site for phenazine methosulfate at the succinate dehydrogenase complex, not involved in the physiological electron flux across ubiquinone. Estimation of the enzymatic activities mentioned above allows differential investigation of the functional integrity of a large part of the respiratory chain in patients suspected of suffering from a neuromuscular disorder.     

Development of a novel smart scaffold for human skeletal muscle regeneration

Skeletal muscle defects are notoriously difficult to manage and the current methods used are associated with many limitations. Engineered skeletal muscle tissue has the potential to provide a solution that circumvents these disadvantages. Our previous work has identified a novel three‐dimensionally aligned degradable phosphate glass fibre scaffold that can support myoblast differentiation and maturation. This current study has further developed the scaffold by encasing the fibres within a collagen gel to produce a smart composite scaffold that provides key biomimetic cues and supports the formation of a tissue that may be implanted in vivo. The constructs formed were approximately 30 mm long and microscopic examination confirmed favourable unidirectional cell alignment. There was good cell survival, and gene expression studies demonstrated upregulation of the myogenic regulatory factors and developmental and adult myosin heavy chain isoforms indicating myofibre formation and maturation respectively. Compared with the three‐dimensional glass fibre scaffolds, the composite scaffolds had later gene upregulation, however, the use of collagen gels reinforced with degradable aligned glass fibres offers the opportunity to create a tissue analogue that can be easily manipulated. Furthermore, the glass fibre ends could support tendon/bone formation, and the channels formed as the fibres degrade could allow for vascular ingrowth. Copyright © 2013 John Wiley & Sons, Ltd.     

有氧运动对人全基因组表达的影响

目的:观察有氧运动对骨骼肌全基因组表达的影响.方法:选择6名某部队干休所中很少运动、年龄(66±9)岁的健康老年人集进行为期12周太极拳训练.运动前和运动12周后,所有受试都进行了体质评估.测试指标包括身高、体质量、肺活量、台阶指数、最大摄氧量.在训练前后分别对实验对象进行肌活验,提取总RNA,经处理后与Affymetdx U133A基因芯片进行杂交,分析数据.结果:有氧运动可明显改善老年人心肺功能,同时有一定降低体脂(减肥)功效.有氧运动使老年人骨骼肌全基因组表达发生明显改变,筛选出725条表达有差异的基因.本文对表达差异最显著的20条差异表达基因进行研究(3条基因表达上调,17条基因表达下调).根据基因功能分类对比,差异表达基因分别归属8种细胞组分和生物过程,经KEGG搜索找到4条基因的代谢途径.结论:有氧运动可使三羧酸循环相关酶基因表达上调,肌肉蛋白合成相关基因和神经鞘脂类相关基因表达下调,提示有氧运动有助于保护神经细胞的完整性,对抗衰老有积极作用,同时可加速体内脂类物质有氧代谢.     

Natriuretic peptides enhance the oxidative capacity of human skeletal muscle

Cardiac natriuretic peptides (NP) are major activators of human fat cell lipolysis and have recently been shown to control brown fat thermogenesis. Here, we investigated the physiological role of NP on the oxidative metabolism of human skeletal muscle. NP receptor type A (NPRA) gene expression was positively correlated to mRNA levels of PPARγ coactivator-1α (PGC1A) and several oxidative phosphorylation (OXPHOS) genes in human skeletal muscle. Further, the expression of NPRA, PGC1A, and OXPHOS genes was coordinately upregulated in response to aerobic exercise training in human skeletal muscle. In human myotubes, NP induced PGC-1α and mitochondrial OXPHOS gene expression in a cyclic GMP–dependent manner. NP treatment increased OXPHOS protein expression, fat oxidation, and maximal respiration independent of substantial changes in mitochondrial proliferation and mass. Treatment of myotubes with NP recapitulated the effect of exercise training on muscle fat oxidative capacity in vivo. Collectively, these data show that activation of NP signaling in human skeletal muscle enhances mitochondrial oxidative metabolism and fat oxidation. We propose that NP could contribute to exercise training–induced improvement in skeletal muscle fat oxidative capacity in humans.     

急性跑台运动对骨骼肌氧化应激及Fas/FasL、Bcl-2/Bax基因表达的影响

摘要 目的：通过检测急性跑台运动中骨骼肌氧化应激情况以及Fas/FasL、Bcl-2/Bax等凋亡调控基因表达的动态变化,探究急性跑台运动诱导骨骼肌细胞凋亡的分子机制。 方法：以小鼠急性递增负荷跑台运动为实验模型,连续观察与分别测定安静组（Con）、急性跑台运动45min组（E45）、90min组（E90）、120min组（E120）和150min组（E150）的腓肠肌丙二醛（MDA）、过氧化氢（H2O2）、一氧化氮（NO）含量和超氧化物歧化酶（SOD）活性变化以及Fas/FasL、Bcl-2/Bax基因表达水平。 结果：①骨骼肌MDA和H2O2含量在急性跑台运动第45分钟时显著升高（P＜0.05）,维持一段时间后又稳步下降,直到运动结束;骨骼肌SOD活性却呈现与之基本相反的变化态势;骨骼肌NO含量在急性跑台运动第45分钟时显著升高（P＜0.01）,并始终维持较高水平,直到运动结束。②骨骼肌Fas/FasL mRNA表达水平在急性跑台运动第90—120分钟时显著上升（P＜0.01）,随后有稳步下降态势;骨骼肌Bcl-2 mRNA表达水平在急性跑台运动第90—120分钟时显著降低（P＜0.05）,随后有稳步上升态势;骨骼肌Bax mRNA表达水平在急性跑台运动第45分钟时显著上升（P＜0.05）,在第90分钟时达到峰值（P＜0.01）,维持到第120分钟（P＜0.05）后呈现下降态势;Bcl-2/Bax比率在急性跑台运动第45分钟时显著降低（P＜0.05）,在90min时达最低水平（P＜0.01）,维持到第120分钟（P＜0.05）后呈现上升态势。 结论：急性跑台运动可使骨骼肌MDA、H2O2、NO含量显著上升、SOD活性下降,致使自由基过度生成与聚集,氧化应激水平提高。急性跑台运动可显著增强骨骼肌Fas/FasL和Bax基因表达、降低Bcl-2基因表达和Bcl-2/Bax比率,使骨骼肌细胞发生凋亡,这可能与自由基过度聚集,氧化应激水平提高有关。     

Effects of insulin infusion on human skeletal muscle pyruvate dehydrogenase, phosphofructokinase, and glycogen synthase. Evidence for their role in oxidative and nonoxidative glucose metabolism.

To determine whether activation by insulin of glycogen synthase (GS), phosphofructokinase (PFK), or pyruvate dehydrogenase (PDH) in skeletal muscle regulates intracellular glucose metabolism, subjects were studied basally and during euglycemic insulin infusions of 12, 30, and 240 mU/m2 X min. Glucose disposal, oxidative and nonoxidative glucose metabolism were determined. GS, PFK, and PDH were assayed in skeletal muscle under each condition. Glucose disposal rates were 2.37 +/- 0.11, 3.15 +/- 0.19, 6.71 +/- 0.44, and 11.7 +/- 1.73 mg/kg X min; glucose oxidation rates were 1.96 +/- 0.18, 2.81 +/- 0.28, 4.43 +/- 0.32, and 5.22 +/- 0.52. Nonoxidative glucose metabolism was 0.39 +/- 0.13, 0.34 +/- 0.26, 2.28 +/- 0.40, and 6.52 +/- 1.21 mg/kg X min. Both the proportion of active GS and the proportion of active PDH were increased by hyperinsulinemia. PFK activity was unaffected. Activation of GS was correlated with nonoxidative glucose metabolism, while activation of PDH was correlated with glucose oxidation. Sensitivity to insulin of GS was similar to that of nonoxidative glucose metabolism, while the sensitivity to insulin of PDH was similar to that of glucose oxidation. Therefore, the activation of these enzymes in muscle may regulate nonoxidative and oxidative glucose metabolism.     

Effect of atorvastatin on energy expenditure and skeletal muscle oxidative metabolism at rest and during exercise

Statins are associated with adverse effects in skeletal muscle. This study tested the hypothesis that atorvastatin would increase the respiratory exchange ratio (RER) at rest and during exercise. Twenty-eight healthy subjects (mean age 52 years) were enrolled in a double-blind, placebo-controlled, randomized study of the effects of atorvastatin (40 mg/day) on whole body energetics over 8 weeks. Ventilatory gas exchange measurements, at rest and during bicycle ergometry, were used to assess muscle oxidative metabolism. Thirteen subjects from each treatment arm completed the study. Eight weeks of atorvastatin lowered plasma low-density lipoprotein cholesterol concentration but had no effect on resting or submaximal energy expenditure, RER, or calculated fatty acid oxidation rates. Atorvastatin did not affect maximal exercise oxygen consumption or the anaerobic threshold. Eight weeks of atorvastatin therapy was not associated with alterations in substrate oxidation, or muscle oxidative function at rest, or during exercise in healthy adults.     

肌肉运动与骨骼肌细胞的凋亡

背景:不少医学研究表明,细胞凋亡能导致大量自由基增多、Ca~(2+)浓度升高、线粒体膜电位下降引起运动能力的下降.因此,研究细胞凋亡与运动训练的关系意义重要.目的:总结与探索关于肌肉运动与骨骼肌细胞凋亡的相关问题.方法:计算机检索中国期刊全文数据(网址http://dlib.cnki.net/kns50/index.aspx)及PubMed数据库(网址http://www.ncbi.nlm.nih.gov/pubmed/)1990-01/2009-06期间的相关文章,检索词为"肌肉运动,骨骼肌细胞凋亡,muscle exercise,apoptosis in the skeletal muscle".纳入与肌肉运动与骨骼肌细胞的凋亡研究现状与发展密切相关.①有关骨骼肌细胞凋亡的研究.②运动与骨骼肌细胞凋亡研究.③运动诱发骨骼肌细胞凋亡的基因调控研究.④骨骼肌细胞凋亡的分子机制研究.⑤同一领域选择近期发表或在权威杂志上发表的文章.排除重复性研究.结果与结论:运动后,正常肌肉中或是病理状态下的肌肉中骨骼肌细胞都会出现凋亡,凋亡的形态学表现与普通凋亡细胞相似,即核固缩、质膜发泡、细胞器紧缩,凋亡小体形成,其凋亡过程大致可分为3个阶段,即启始阶段、效应阶段和降解阶段.骨骼肌细胞凋亡的增加是导致运动性疲劳的重要原因.目前国内外对骨骼肌细胞凋亡的基因调控研究主要是从凋亡调控因子Bcl-2蛋白、肿瘤坏死因子α及死亡蛋白酶半胱氨酸天冬氨酸酶着手.bcl-2基因蛋白的抗凋亡作用主要是通过阻止线粒体通透性转换孔的开放,阻止线粒体释放促凋亡蛋白、防止线粒体膜脂质过氧化以及线粒体基质Ca2+释放实现的.肿瘤坏死因子家族在启动死亡因子及其受体途径中起重要作用,此途径的启动依赖于死亡配体与死亡受体相结合,激活半胱氨酸天冬氨酸酶,导致细胞凋亡.通过研究探索运动强度与骨骼肌细胞凋亡及坏死的界限关系,有利于在运动中认识运动性疲劳产生的机制及有效消除疲劳.     

ATP breakdown products in human skeletal muscle during prolonged exercise to exhaustion

Summary. To study changes in muscle energy state during prolonged exercise, especially in relation to fatigue, muscle biopsies were obtained from seven healthy males working until exhaustion on a cycle ergometer at 68% (63–74%) of their maximal oxygen uptake. Biopsies were taken at rest, after 15 and 45 min of exercise and at exhaustion, and analysed for ATP, ADP, AMP, inosine monophosphate (IMP) and hypoxanthine content by high performance liquid chromatography (HPLC), and for creatine phosphate (CP), lactate and glycogen by enzymatic fluorometric techniques. Glycogen content at exhaustion was approximately 30% of the pre-exercise level. The CP content decreased steeply during the first 15 min of exercise (P<0·01) and continued to decrease during the rest of the exercise period (P<0·05). Pronounced increases in contents of IMP (64%P<0·001) and hypoxanthine (69%, P<0·05) were found when exhaustion was approaching. Furthermore, energy charge [EC; (ATP+0·5 ADP)/(ATP+ADP+AMP)] was decreased at exhaustion (P<0·05). The increases in IMP and hypoxanthine which occurred when exhaustion was approaching during prolonged submaximal exercise together with the decrease in EC during this phase of exercise suggest a failure of the exercising skeletal muscle to regenerate ATP at exhaustion.     

Nonthermal ultrasound and exercise in skeletal muscle regeneration

OBJECTIVE: To determine whether continuous nonthermal therapeutic ultrasound (US) and low-intensity exercise (Ex) influence skeletal muscle regeneration after a standardized contusion injury in an animal model. DESIGN: Randomized controlled trial with blinded comparisons in a 2 x 2 factorial (US by Ex) design. SETTING: Animal care facility and exercise physiology biochemistry laboratory. ANIMALS: Twenty male Wistar rats (age, 8 mo) received a reproducible bilateral contusion injury to the gastrocnemius muscles. Ten gastrocnemius muscles from 5 noninjured, nontreated rats provided baseline control data. INTERVENTIONS: US (continuous duty cycle, 3 MHz; intensity, 0.1 W/cm2 ; transducer, 1cm2 ; duration, 5 min/d; duty cycle, 100%) and exercise (20 min/d of low-intensity treadmill walking at 14 m/min). Gastrocnemius muscles from injured rats received exercise treatment alone (Ex + NoUS), exercise and US treatment (Ex + US), US treatment alone (NoEx + US), and no treatment (NoEx + NoUS). MAIN OUTCOME MEASURES: Ninety-six-hour postinjury muscle mass, contractile protein concentration, fiber cross-sectional area, number of nuclei per fiber, and myonuclear density. RESULTS: Myonuclei per fiber were statistically greater in injured than in noninjured gastrocnemius muscle (P < .05). There were no statistical differences (P > .01) among the 4 injured treatment groups for any of the outcome measures chosen as biomarkers of skeletal muscle regeneration. CONCLUSIONS: There is no evidence that the specific continuous US and Ex protocols investigated enhanced skeletal muscle regeneration after contusion injury.     

An improved spectrophotometric assay of pyruvate dehydrogenase in lactate dehydrogenase contaminated mitochondrial preparations from human skeletal muscle

In mitochondria-enriched preparations of human skeletal muscle, the measurement of pyruvate dehydrogenase activity, as determined by conventional spectrophotometric assay of NADH accumulation, is underestimated due to the oxidizing activity of the contaminating lactate dehydrogenase. Using a model reaction system consisting of varying mixtures of purified lactate and pyruvate dehydrogenases, we found that the presence of oxamate, a competitive inhibitor of the lactate dehydrogenase, allowed the measurement of a linear rate of pyruvate dehydrogenase activity without interference from lactate dehydrogenase. In the presence of 25 mM oxamate, this holds true up to a ratio of 30:1 for lactate to pyruvate dehydrogenases, respectively. A similar result was obtained when using human skeletal muscle mitochondria contaminated by lactate dehydrogenase. Rates of pyruvate dehydrogenase activity ranging from 50 to 120 nmol/min/mg protein could be routinely measured in such mitochondrial fractions. We concluded that the use of oxamate allows a spectrophotometric assay for pyruvate dehydrogenase activity to be utilized when screening for pyruvate dehydrogenase deficiency in mitochondria-enriched preparations of human skeletal muscle.     

Maximum rate of oxygen consumption related to succinate dehydrogenase activity in skeletal muscle fibres of chronic heart failure patients and controls

Previous studies indicate that the low maximum rate of oxygen consumption (VO2max) of chronic heart failure (CHF) patients is not because of impaired pump function of the heart. We hypothesize that VO2 during maximum exercise is determined by the total oxidative capacity of skeletal muscle. VO2max of six controls and 14 CHF patients, New York Heart Association class I-III, was determined using an incremental bicycle ergometer test. Cryostat sections of a biopsy from the quadriceps femoris muscle were incubated for succinate dehydrogenase (SDH) using quantitative histochemistry. VO2max (range: 29 ml O2 kg muscle(-1) min(-1) in a class III patient to 118 ml O2 kg muscle(-1) min(-1) in a control subject) correlates with the mean SDH activity of skeletal muscle fibres (r=0.79 or r=0.81, including or excluding oxygen uptake at rest, respectively; P<0.001). The relationship between VO2max and SDH activity is similar to that determined previously using isolated single muscle fibres and myocardial trabeculae under hyperoxic conditions. From the product of SDH activity and the cross-sectional area of the fibre (i.e. spatially integrated SDH activity), it is possible to calculate the maximum oxygen uptake rate per unit muscle fibre length. This uptake rate is linearly related to the number of capillaries per fibre (r=0.76, P<0.001) in all subjects, suggesting that oxidative capacity of skeletal muscle fibres in CHF patients decreases in proportion to the oxygen supply capacity of the microcirculation.     

Deficiency of the pyruvate dehydrogenase component in pyruvate dehydrogenase complex-deficient human fibroblasts. Immunological identification.

A previously reported deficiency of "total" pyruvate dehydrogenase complex activity is further characterized. Dihydrolipoyl transacetylase (E2) and lipoamide dehydrogenase (E3) activities in the patient's fibroblasts were normal. Pyruvate dehydrogenase activity (E1) was 33% of that in fibroblasts from an age-matched control. The amounts of each of the components of pyruvate dehydrogenase complex were analyzed using an immunoblot technique and specific antibodies. Levels of components E2 and E3 were the same in fibroblasts from the patient and control, confirming the activity measurements. However, the levels of E1 alpha and E1 beta were reduced markedly in fibroblasts from the patient. Thus, impairment in the pyruvate dehydrogenase complex activity was due to a reduction in the amount of the E1 component of the complex.     

骨骼肌的运动适应机制

背景:骨骼肌运动适应机制的研究对提高运动成绩,预防和治疗一些代谢紊乱性疾病具有重要意义.目的:探讨骨骼肌运动适应的机制.方法:应用计算机检索PubMed 数据库和中文期刊全文数据库2011-03 前发表的相关文章,检索词分别为"skeletal muscle,exercise,adaptation,cytoskeleton,dystrophin"和"骨骼肌,运动,适应,骨架蛋白,肌营养不良蛋白",共检索到56 篇文献,纳入所述内容与骨骼肌运动适应机制相关的文献,排除重复性研究,保留31 篇进行综述.结果与结论:激烈的运动使肌肉结构和细胞代谢产生应激反应,包括肌肉损伤和氧化应激反应.高强度的离心运动可造成肌肉超微结构损伤,但运动性肌损伤后存在肌肉再重建反应.运动训练可促进健康的个体对一氧化氮系统产生各种各样的适应,通过各种机制增强骨骼肌的生物学有效性,这些适应性变化可有效增加运动能力,对心血管系统具有保护作用.目前,大多数人类骨骼肌运动适应机制还没有被发现.     

骨骼肌的运动适应机制

背景：骨骼肌运动适应机制的研究对提高运动成绩,预防和治疗一些代谢紊乱性疾病具有重要意义。目的：探讨骨骼肌运动适应的机制。方法：应用计算机检索PubMed数据库和中文期刊全文数据库2011-03前发表的相关文章,检索词分别为＂skeletal muscle,exercise,adaptation,cytoskeleton,dystrophin＂和＂骨骼肌,运动,适应,骨架蛋白,肌营养不良蛋白＂,共检索到56篇文献,纳入所述内容与骨骼肌运动适应机制相关的文献,排除重复性研究,保留31篇进行综述。结果与结论：激烈的运动使肌肉结构和细胞代谢产生应激反应,包括肌肉损伤和氧化应激反应。高强度的离心运动可造成肌肉超微结构损伤,但运动性肌损伤后存在肌肉再重建反应。运动训练可促进健康的个体对一氧化氮系统产生各种各样的适应,通过各种机制增强骨骼肌的生物学有效性,这些适应性变化可有效增加运动能力,对心血管系统具有保护作用。目前,大多数人类骨骼肌运动适应机制还没有被发现。     

Vitamin D Deficiency, hypocalcemia, and increased skeletal muscle degradation in rats.

The myopathy associated with vitamin D deficiency was examined in vitamin D-deficient and vitamin D-supplemented rats. When compared with either vitamin D-supplemented ad lib. or pair-fed rats, weight gain and muscle mass were decreased in vitamin D-deficient hypocalcemic animals. With the exception of a modest decrease in muscle creatine phosphate levels, muscle composition was unchanged by vitamin D deficiency. Muscle protein turnover rates were determined in both in vivo and in vitro studies and demonstrated that myofibrillar protein degradation was increased in vitamin D deficiency. Normal growth rates could be maintained be feeding the rats vitamin D-deficient diets containing 1.6% calcium, which maintained plasma calcium within the normal range. In addition to its role in maintaining plasma calcium, vitamin D-supplemented rats had significantly higher levels of the anabolic hormone insulin. Vitamin D supplementation may affect muscle protein turnover by preventing hypocalcemia, as well as directly stimulating insulin secretion, rather than by a direct effect within skeletal muscle.