Effect of age on in vivo rates of mitochondrial protein synthesis in human skeletal muscle

Gene expression profile of aging in human muscle

Studies of gene expression related to aging of skeletal muscle have included few subjects or a limited number of genes. We conducted the present study to produce more comprehensive gene expression profiles. RNA was extracted from vastus lateralis biopsies obtained from healthy young (21-27 yr old, n = 8) and older men (67-75 yr old, n = 8) and was analyzed with high-density oligonucleotide arrays. Of the approximately 44,000 probe sets on the arrays, approximately 18,000 yielded adequate signals for statistical analysis. There were approximately 700 probe sets for which t-tests or rank sum tests indicated a difference (P <or= 0.01) in mean expression between young and old and for which the estimated false discovery rate was <10%. Most of these differences were less than 1.5-fold in magnitude. Genes that encode proteins involved in energy metabolism and mitochondrial protein synthesis were expressed at a lower level in older muscle. Genes encoding metallothioneins, high-mobility-group proteins, heterogeneous nuclear ribonucleoproteins and other RNA binding/processing proteins, and components of the ubiquitin-proteasome proteolytic pathway were expressed at higher levels in older muscle. Expression of numerous genes involved with stress responses, hormone/cytokine/growth factor signaling, control of the cell cycle and apoptosis, and transcriptional regulation appeared to be affected by aging. More transcripts were detected in older muscle, suggesting dedifferentiation, an increased number of splice variants, or increased cellular heterogeneity. We conclude that in human skeletal muscle the expression of many genes tends to increase or decrease between the third and seventh decades. The changes are modest when averaged over all of the cells in the tissue.

Effect of insulin on human skeletal muscle mitochondrial ATP production, protein synthesis, and mRNA transcripts

Mitochondria are the primary site of skeletal muscle fuel metabolism and ATP production. Although insulin is a major regulator of fuel metabolism, its effect on mitochondrial ATP production is not known. Here we report increases in vastus lateralis muscle mitochondrial ATP production capacity (32-42%) in healthy humans (P < 0.01) i.v. infused with insulin (1.5 milliunits/kg of fat-free mass per min) while clamping glucose, amino acids, glucagon, and growth hormone. Increased ATP production occurred in association with increased mRNA levels from both mitochondrial (NADH dehydrogenase subunit IV) and nuclear [cytochrome c oxidase (COX) subunit IV] genes (164-180%) encoding mitochondrial proteins (P < 0.05). In addition, muscle mitochondrial protein synthesis, and COX and citrate synthase enzyme activities were increased by insulin (P < 0.05). Further studies demonstrated no effect of low to high insulin levels on muscle mitochondrial ATP production for people with type 2 diabetes mellitus, whereas matched nondiabetic controls increased 16-26% (P < 0.02) when four different substrate combinations were used. In conclusion, insulin stimulates mitochondrial oxidative phosphorylation in skeletal muscle along with synthesis of gene transcripts and mitochondrial protein in human subjects. Skeletal muscle of type 2 diabetic patients has a reduced capacity to increase ATP production with high insulin levels.

Reduced oxidative power but unchanged antioxidative capacity in skeletal muscle from aged humans

The hypothesis that the aging process is associated with mitochondrial dysfunction and oxidative stress has been investigated in human skeletal muscle. Muscle biopsy samples were taken from seven old male subjects [OS; 75 (range 61-86) years] and eight young male subjects [YS; 25 (22-31) years]. Oxidative function was measured both in permeabilised muscle fibres and isolated mitochondria. Despite matching the degree of physical activity, OS had a lower training status than YS as judged from pulmonary maximal O(2) consumption ( Vdot;O(2)max, -36%) and handgrip strength (-20%). Both maximal respiration and creatine-stimulated respiration were reduced in muscle fibres from OS (-32 and -34%, respectively). In contrast, respiration in isolated mitochondria was similar in OS and YS. The discrepancy might be explained by a biased harvest of "healthy" mitochondria and/or disruption of structural components during the process of isolation. Cytochrome C oxidase was reduced (-40%, P<0.01), whereas UCP3 protein tended to be elevated in OS ( P=0.09). Generation of reactive oxygen species by isolated mitochondria and measures of antioxidative defence (muscle content of glutathione, glutathione redox status, antioxidative enzymes activity) were not significantly different between OS and YS. It is concluded that aging is associated with mitochondrial dysfunction, which appears to be unrelated to reduced physical activity. The hypothesis of increased oxidative stress in aged muscle could not be confirmed in this study.

Reduced amount of mitochondrial DNA in aged human muscle

Muscle concentrations of mRNAs encoded by mitochondrial DNA (mtDNA) decline with aging. To determine whether this can be explained by diminished mtDNA levels, we measured the relative concentrations of mtDNA and a representative mtDNA transcript [encoding cytochrome-c oxidase, subunit 2 (COX-2)] in muscle of young (21-27 yr) and older subjects (65-75 yr). The amount of COX-2 mRNA (relative to 28S rRNA) was 22% lower (P = 0.04) in older muscle, and the amount of mtDNA (relative to nuclear DNA) was 38% lower (P = 0.0002). The average level of mitochondrial transcription factor A (Tfam), a protein essential for mtDNA replication, was similar in younger and older muscle. Tfam mRNA, nuclear respiratory factor-1 mRNA, and several mRNAs encoding proteins required for mtDNA replication were expressed at similar levels in younger and older muscle. The mtDNA concentrations were only weakly related to age-adjusted aerobic fitness (maximal oxygen consumption) and self-reported physical activity levels. We conclude that the lower concentration of mitochondrial mRNAs in older muscle can be explained by a reduced concentration of mtDNA.

Effects of ageing and human whole body and muscle protein turnover

Prevalence of sarcopenia is up to 60% of those individuals over 80 years of age and is associated with increased disability. The causes behind the age-related loss of muscle are difficult to discern. Measurements of protein synthesis/breakdown and net protein balance are important, and further methodological development is warranted. Whole body protein turnover is changed only little - if at all - with ageing, when corrected for fat free mass of the individuals. Discrepancies in reports are often related to inconsistent recordings of energy intake especially protein and variation in subject, gender and physical activity level. Ageing is associated with reduced sensitivity toward amino acids, increased first pass uptake in a splanchnic region and a reduced postprandial stimulation of protein synthesis. Physical activity and amino acids are additive in effect also in elderly individuals, and timing of training and protein intake is crucial, in that early intake of amino acids is advantageous with regards to stimulation of protein synthesis.

Mechanisms of altered protein turnover in chronic diseases: a review of human kinetic studies

PURPOSE OF REVIEW

Changes in hormone secretion, tissue perfusion, oxygen availability, energy-protein intake, free amino acid pattern, hydration state, acid-base balance as well as activation of the systemic inflammatory response may affect protein synthesis and degradation. The overall purpose of this review is to describe how these factors may interact to change protein turnover in the different directions seen in kinetic studies in humans.

RECENT FINDINGS

Evidence indicates that, in vivo, changes of protein synthesis and degradation are strictly related. When protein synthesis is primarily suppressed, protein degradation is found to be unchanged or even slightly decreased. When protein degradation is primarily accelerated, the rate of synthesis is unchanged or even increased. Chronic disease states can, therefore, be characterized either by decreased or accelerated protein turnover. Apparent discrepancies among various studies in chronic uraemia, liver cirrhosis, chronic obstructive pulmonary disease and cancer may stem from the fact that the pathogenesis of protein metabolism abnormalities is multifactorial. When the effects of inflammatory mediators and stress hormones start overwhelming factors that tend to decrease protein synthesis and turnover (decreased protein-energy intake, physical activity, tissue oxygen delivery, leucine levels, etc.), the rate of protein degradation and turnover may increase.

SUMMARY

Low-protein turnover conditions are usually associated with the adequate sparing of body proteins, whereas in high-protein turnover conditions protein loss may proceed at a faster rate. Nonetheless, impaired recovery from acute complications and the reduced renewal of damaged and toxic proteins are potential undesired consequences of low-protein turnover.

Human protein metabolism: its measurement and regulation

The body's protein mass not only provides architectural support for cells but also serves vital roles in maintaining their function and survival. The whole body protein pool, as well as that of individual tissues, is determined by the balance between the processes of protein synthesis and degradation. These in turn are regulated by interactions among hormonal, nutritional, neural, inflammatory, and other influences. Prolonged changes in either the synthetic or degradative processes (or both) that cause protein wasting increase morbidity and mortality. The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of both protein synthesis and breakdown in vivo. These insights, including those from studies of insulin, insulin-like growth factor I, growth hormone, and amino acid-mediated regulation of muscle and whole body protein turnover, provide opportunities to develop and test therapeutic approaches with promise to minimize or prevent these adverse health consequences.

Sarcopenia, weight loss, and nutritional frailty in the elderly

The progression of the aging process leads to a decreased margin of homeostatic reserve and a reduced ability to accommodate metabolic challenges, including nutritional stress. Nutritional frailty refers to the disability that occurs in old age owing to rapid, unintentional loss of body weight and loss of lean body mass (sarcopenia). Sarcopenia, a loss of muscle mass and strength, contributes to functional impairment. Weight loss is commonly due to a reduction in food intake; its possible etiology includes a host of physiological and nonphysiological causes. The release of cytokines during chronic disease may also be an important determinant of frailty. In addition to being anorectic, cytokines also contribute to lipolysis, muscle protein breakdown, and nitrogen loss. Whereas the multiple causes of nutritional frailty are not completely understood, clinical interventions for weight loss, sarcopenia, and cytokine alterations have been used with modest success.

Performance and physiologic adaptations to resistance training

Weight lifting, or resistance training, is a potent stimulus to the neuromuscular system. Depending on the specific program design, resistance training can enhance strength, power, or local muscular endurance. These improvements in performance are directly related to the physiologic adaptations elicited through prolonged resistance training. Optimal resistance training programs are individualized to meet specific training goals. When trained properly (i.e., similar intensity and volume), these functional and physiologic adaptations are similarly impressive among women and the aged as they are among young men. Yet, in contrast to relative measurements, sex and age differences exist in the absolute magnitude of adaptation. Of equal importance, perhaps most notably among the elderly, are the important health benefits that may also be derived from resistance training. For example, bone density, insulin sensitivity, and co-morbidities associated with obesity can be effectively managed with resistance exercise when it is conducted on a regular basis. The extent of the functional and health benefits to be accrued from resistance training depend on factors such as initial performance and health status, along with the specification of program design variables such as frequency, duration, intensity, volume, and rest intervals.

Resistance-training-induced adaptations in skeletal muscle protein turnover in the fed state

Resistance training changes the balance of muscle protein turnover, leading to gains in muscle mass. A longitudinal design was employed to assess the effect that resistance training had on muscle protein turnover in the fed state. A secondary goal was investigation of the potential interactive effects of creatine (Cr) monohydrate supplementation on resistance-training-induced adaptations. Young (N = 19, 23.7 +/- 3.2 year), untrained (UT), healthy male subjects completed an 8-week resistance-training program (6 d/week). Supplementation with Cr had no impact on any of the variables studied; hence, all subsequent data were pooled. In the UT and trained (T) state, subjects performed an acute bout of resistance exercise with a single leg (exercised, EX), while their contralateral leg acted as a nonexercised (NE) control. Following exercise, subjects were fed while receiving a primed constant infusion of [d5]- and [15N]-phenylalanine to determine the fractional synthetic and breakdown rates (FSR and FBR), respectively, of skeletal muscle proteins. Acute exercise increased FSR (UT-NE, 0.065 +/- 0.025 %/h; UT-EX, 0.088 +/- 0.032 %/h; P < 0.01) and FBR (UT-NE, 0.047 +/- 0.023 %/h; UT-EX, 0.058 +/- 0.026 %/h; P < 0.05). Net balance (BAL = FSR - FBR) was positive in both legs (P < 0.05) but was significantly greater (+65%) in the EX versus the NE leg (P < 0.05). Muscle protein FSR and FBR were greater at rest following T (FSR for T-NE vs. UT-NE, +46%, P < 0.01; FBR for T-NE vs. UT-NE, +81%, P < 0.05). Resistance training attenuated the acute exercise-induced rise in FSR (T-NE vs. T-EX, +20%, P = 0.65). The present results demonstrate that resistance training resulted in an elevated resting muscle protein turnover but an attenuation of the acute response of muscle protein turnover to a single bout of resistance exercise.

Antioxidant pathways in human aged skeletal muscle: relationship with the distribution of type II fibers

Type II fiber loss and reactive oxygen species (ROS)-induced damage are hallmarks of muscle aging. The aim of this study was to analyze whether there exists a relationship between age-dependent changes in cellular antioxidant capacity and type II fiber loss in aged human skeletal muscles. Forty-five male and female subjects ranging in age from 65 to 90 year-old were divided into +40 and -40% type II fiber groups. We measured both total and Mn superoxide dismutase (total and MnSOD), glutathione peroxidase (GSHPx) and catalase (CAT) activities. We also measured the reduced and oxidized forms of glutathione (GSH and GSSG) and lipid peroxide (LPO) levels. Total SOD activity was lower in the -40% type II fiber group than in the +40% group; MnSOD tended to be lower but data are not statistically consistent. Both GSHPx and CAT activities remained unchanged; as did GSH, GSSG and GSH/GSSG ratio. Finally, muscle samples with -40% type II fibers had a significantly higher LPO content compared to those with +40% type II fibers. In summary, a relationship between human skeletal muscle aging, type II fiber loss and ROS reactions seems to exist.

Activities of cytochrome c oxidase and citrate synthase in lymphocytes of obese and normal-weight subjects

BACKGROUND

Obesity represents a heterogeneous group of disorders associated with broad spectrum of metabolic and endocrine abnormalities. The metabolic changes in obesity may also concern the efficacy of mitochondrial system of energy provision. The aim of our study was to analyse activities of mitochondrial enzymes cytochrome c oxidase (COX) and citrate synthase (CS) in isolated lymphocytes of obese and normal-weight subjects.

RESULTS

In the group of 304 non-obese controls, differences between men and women were found neither in the COX and CS activities nor in the COX/CS ratio in isolated lymphocytes. The activity of COX did not change even with age, whereas the activity of CS decreased significantly resulting in age-dependent increase of the COX/CS ratio (P<0.01). In the group of 60 obese patients aged 17-75 y, the COX activity was 1.2-fold higher (P<0.01) and the CS activity was 1.3-fold lower (P<0.01) compared to 151 non-obese healthy age-matched controls. Consequently, the COX/CS ratio became 1.7-fold higher (P<0.01) in the obese patients compared to the non-obese population, which indicates that both the absolute and relative oxidative capacity are increased.

CONCLUSION

Isolated lymphocytes from peripheral blood contribute very little to the overall metabolic turnover, but they may serve as easily available marker cells for studying the changes of mitochondrial energy converting systems in obesity.

Synthesis rate of muscle proteins, muscle functions, and amino acid kinetics in type 2 diabetes

Improvement of glycemic status by insulin is associated with profound changes in amino acid metabolism in type 1 diabetes. In contrast, a dissociation of insulin effect on glucose and amino acid metabolism has been reported in type 2 diabetes. Type 2 diabetic patients are reported to have reduced muscle oxidative enzymes and VO(2max). We investigated the effect of 11 days of intensive insulin treatment (T(2)D+) on whole-body amino acid kinetics, muscle protein synthesis rates, and muscle functions in eight type 2 diabetic subjects after withdrawing all treatments for 2 weeks (T(2)D-) and compared the results with those of weight-matched lean control subjects using stable isotopes of the amino acids. Whole-body leucine, phenylalanine and tyrosine fluxes, leucine oxidation, and plasma amino acid levels were similar in all groups, although plasma glucose levels were significantly higher in T(2)D-. Insulin treatment reduced leucine nitrogen flux and transamination rates in subjects with type 2 diabetes. Synthesis rates of muscle mitochondrial, sarcoplasmic, and mixed muscle proteins were not affected by glycemic status or insulin treatment in subjects with type 2 diabetes. Muscle strength was also unaffected by diabetes or glycemic status. In contrast, the diabetic patients showed increased tendency for muscle fatigability. Insulin treatment also failed to stimulate muscle cytochrome C oxidase activity in the diabetic patients, although it modestly elevated citrate synthase. In conclusion, improvement of glycemic status by insulin treatment did not alter whole-body amino acid turnover in type 2 diabetic subjects, but leucine nitrogen flux, transamination rates, and plasma ketoisocaproate level were decreased. Insulin treatments in subjects with type 2 diabetes had no effect on muscle mitochondrial protein synthesis and cytochrome C oxidase, a key enzyme for ATP production.

Age is independently related to muscle metabolic capacity in premenopausal women

The purpose of this study was to determine whether muscle metabolic capacity was inversely related to age after adjusting for physical activity in sedentary premenopausal women. Eighty-three women (ages 23-47 yr) had their free-living, activity-related energy expenditure evaluated with doubly labeled water procedures, and room calorimeter determined sleeping energy expenditure. Maximum O(2) uptake and strength were evaluated in all subjects, whereas 31P-magnetic resonance spectroscopy determined metabolic economy during maximal exercise, and muscle biopsy maximal enzyme activity was evaluated in subsets of the sample (48 and 18 subjects, respectively). Age was significantly related to whole body treadmill endurance time (r = -0.32), plantar flexion strength (r = -0.29), maximum O(2) uptake (r = -0.27), (31)P-magnetic resonance spectroscopy ADP recovery rate (r = -0.44), and anaerobic glycolytic capacity (r = -0.37), and muscle biopsy citrate synthase activity (r = -0.48), glyceraldehyde-3-phosphate dehydrogenase (r = -0.54), phosphofructokinase (r = -0.62), and phosphorylase (r = -0.58) activity even after adjusting for activity-related energy expenditure. These data suggest that, in sedentary premenopausal women, both oxidative and glycolytic muscle capacity decrease with age even when physical activity is taken into account.

Effects of aging on human skeletal muscle myosin heavy-chain mRNA content and protein isoform expression

The purpose of this investigation was to determine the role played by pretranslational events in the decreased rate of myosin heavy-chain (MyHC) protein synthesis in old age. It was hypothesized that the decreased rate of MyHC protein synthesis reported in the elderly population is, at least in part, related to lower MyHC messenger RNA (mRNA) in old age. MyHC protein expression and mRNA levels for the three MyHC isoforms expressed in human muscle, that is, types I, IIa, and IIx/d, were measured in percutaneous vastus lateralis muscle biopsies from 16 young and 16 old healthy men. The MyHC isoform mRNA content was determined by quantitative, real-time reverse transcriptase polymerase chain reaction, and it was normalized to 18S ribosomal RNA; the relative MyHC protein isoform content was measured on silver-stained 7% sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. The old men demonstrated signs of sarcopenia, such as loss of muscle force, a trend toward a loss in lean body mass, and an increased percentage of body fat. Statistically significant correlations were observed between the isoform expression of different MyHCs at the protein and mRNA levels. However, the expression of the different MyHC isoforms at the mRNA and protein levels did not differ between the young and old men. Thus, the present results do not support the hypothesis that pretranslational events in MyHC protein synthesis are playing a significant role in the development of sarcopenia.

Behavioral dysfunction, brain oxidative stress, and impaired mitochondrial electron transfer in aging mice

Behavioral tests, tightrope success, and exploratory activity in a T maze were conducted with male and female mice for 65 wk. Four groups were defined: the lower performance slow males and slow females and the higher performance fast males and fast females. Fast females showed the longest life span and the highest performance, and slow males showed the lowest performance and the shortest life span. Oxidative stress and mitochondrial electron transfer activities were determined in brain of young (28 wk), adult (52 wk), and old (72 wk) mice in a cross-sectional study. Brain thiobarbituric acid reactive substances (TBARS) were increased by 50% in old mice and were approximately 15% higher in males than in females and in slow than in fast mice. Brain Cu,Zn-superoxide dismutase (SOD) activity was increased by 52% and Mn-SOD by 108% in old mice. The activities of mitochondrial enzymes NADH-cytochrome c reductase, cytochrome oxidase, and citrate synthase were decreased by 14-58% in old animals. The cumulative toxic effects of oxyradicals are considered the molecular mechanism of the behavioral deficits observed on aging.

HO2*: the forgotten radical

HO2*, usually termed either hydroperoxyl radical or perhydroxyl radical, is the protonated form of superoxide; the protonation/deprotonation equilibrium exhibits a pK(a) of around 4.8. Consequently, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form. This ratio is rather accurately reflected by the published literature on the two species, as identified by a PubMed search; at the time of writing only 28 articles mention "HO2," "hydroperoxyl" or "perhydroxyl" in their titles, as against 9228 mentioning superoxide. Here it is argued that this correlation is not justifiable: that HO2*'s biological and biomedical importance far exceeds the attention it has received. Several key observations of recent years are reviewed that can be explained much more economically when the participation of HO2* is postulated. It is suggested that a more widespread appreciation of the possible role of HO2* in biological systems would be of considerable benefit to biomedical research.

Tissue-specific regulation of mitochondrial and cytoplasmic protein synthesis rates by insulin

In vivo studies have reported conflicting effects of insulin on mixed tissue protein synthesis rates. To test the hypothesis that insulin has differential effects on synthesis rates of various protein fractions in different organs, we infused miniature swine (n = 8 per group) with saline, insulin alone (at 0.7 mU/kg(-1). min(-1)), or insulin plus an amino acid mixture for 8 h. Fractional synthesis rate (FSR) of mitochondrial and cytoplasmic proteins in liver, heart, and skeletal muscle, as well as myosin heavy chain (MHC) in muscle, were measured using L-[1-(13)C]leucine as a tracer. The FSR of mitochondrial and cytoplasmic proteins were highest in liver, followed by heart and then muscle. Mitochondrial FSR in muscle was higher during insulin and insulin plus amino acid infusions than during saline. Insulin had no significant effect on FSR of MHC in muscle. In contrast, FSR of both mitochondrial and cytoplasmic proteins were not stimulated by insulin in liver. Insulin also did not increase FSR of mitochondrial in heart, whereas insulin and amino acid stimulated FSR of cytoplasmic protein. In conclusion, insulin stimulates the synthesis of muscle mitochondrial proteins, with no significant stimulatory effect on synthesis of sarcoplasmic and MHC. These results demonstrate that insulin has different effects on synthesis rates of specific protein fractions in the liver, heart, and skeletal muscle.

Garbage catastrophe theory of aging: imperfect removal of oxidative damage?

Increasing evidence suggests an important role of oxidant-induced damage in the progress of senescent changes, providing support for the free radical theory of aging proposed by Harman in 1956. However, considering that biological organisms continuously renew their structures, it is not clear why oxidative damage should accumulate with age. No strong evidence has been provided in favor of the concept of aging as an accumulation of synthetic errors (e.g. Orgel's 'error-catastrophe' theory and the somatic mutation theory). Rather, we believe that the process of aging may derive from imperfect clearance of oxidatively damaged, relatively indigestible material, the accumulation of which further hinders cellular catabolic and anabolic functions. From this perspective, it might be predicted that: (i) suppression of oxidative damage would enhance longevity; (ii) accumulation of incompletely digested material (e.g. lipofuscin pigment) would interfere with cellular functions and increase probability of death; (iii) rejuvenation during reproduction is mainly provided by dilution of undigested material associated with intensive growth of the developing organism; and (iv) age-related damage starts to accumulate substantially when development is complete, and mainly affects postmitotic, cells and extracellular matrix, not proliferating cells. There is abundant support for all these predictions.

Does aging adversely affect muscle mitochondrial function?

Muscle oxidative function appears to decline with aging, and evidence suggests that this is related to reduced synthesis of mitochondrial and other muscle proteins. Causes for these events may include mtDNA damage or reduced mtDNA copy numbers, reduced oxidative enzyme activities and ATP production, and increased proton leak.

Age-related mitochondrial genotypic and phenotypic alterations in human skeletal muscle

To have a clearer picture of how mitochondrial damages are associated to aging, a comprehensive study of phenotypic and genotypic alterations was carried out, analyzing with histochemical and molecular biology techniques the same skeletal muscle specimens of a large number of healthy subjects from 13 to 92 years old. Histochemical data showed that ragged red fibers (RRF) appear at about 40 years of age and are mostly cytochrome c oxidase (COX)-positive, whereas they are almost all COX-negative thereafter. Molecular analyses showed that the 4977 bp deletion of mitochondrial DNA (mtDNA(4977)) and the 7436 bp deletion of mtDNA (mtDNA(7436)) are already present in individuals younger than 40 years of age, but their occurrence does not change with age. After 40 years of age the number of mtDNA deleted species, as revealed by Long Extension PCR (LX-PCR), increases, the 10422 bp deletion of mtDNA (mtDNA(10422)) appears, although with a very low frequency of occurrence, and mtDNA content is more than doubled. Furthermore, mtDNA(4977) level directly correlates with that of COX-negative fibers in the same analyzed subjects. These data clearly show that, after 40 years of age, the phenotypic and genotypic mitochondrial alterations here studied appear in human skeletal muscle and that they are closely related.

Insulin's effect on synthesis rates of liver proteins. A swine model comparing various precursors of protein synthesis

Insulin's effect on the synthesis of liver proteins remains to be fully defined. Previous studies using various surrogate measures of amino acyl-tRNA have reported variable results of insulin's effect on liver protein synthesis. We determined the effect of insulin with or without amino acid supplementation on the synthesis rates of liver proteins (tissue, albumin, and fibrinogen) using L-[1-13C]Leu as a tracer in 24 male miniature swine. In addition, we compared the isotopic enrichment of different precursors of liver proteins with that of amino acyl-tRNA using L-[1-13C]Leu and L-[15N]Phe as tracers. Although liver tissue fluid enrichment of [13C]Leu and [15N]Phe and that of plasma [13C]ketoisocaproatic acid (KIC) were very similar to that of tRNA, plasma isotopic enrichment of both Leu and Phe were substantially higher (P < 0.01) and VLDL apolipoprotein-B100 enrichment was lower (P < 0.01) than the respective amino acyl-tRNA enrichment. Plasma KIC enrichment most accurately predicted leucyl-tRNA enrichment, whereas plasma Leu enrichment was best correlated with that of tRNA. Neither insulin alone nor insulin plus amino acid infusion had an effect on liver tissue protein synthesis. In contrast, insulin alone decreased the albumin synthesis rate, and insulin with amino acids maintained the albumin synthesis rate. Insulin with or without amino acids inhibited the fibrinogen synthesis rate. These results, based on synthetic rates using amino acyl-tRNA, were consistent with those obtained using KIC or tissue fluid Leu or Phe as precursor pools. These studies demonstrated that plasma KIC enrichment is a convenient and reliable surrogate measure of leucyl-tRNA in liver. We also concluded that insulin has differential effects on the synthesis rates of liver proteins. Whereas insulin with or without amino acid supplement has no acute effect on the synthesis of liver tissue protein, insulin has a substantial inhibitory effect on fibrinogen synthesis. In contrast, insulin administration along with amino supplement is necessary to maintain albumin synthesis rate.

Sarcopenia

Sarcopenia is a term utilized to define the loss of muscle mass and strength that occurs with aging. Sarcopenia is believed to play a major role in the pathogenesis of frailty and functional impairment that occurs with old age. Progressive muscle wasting occurs with aging. The prevalence of clinically significant sarcopenia is estimated to range from 8.8% in young old women to 17.5% in old old men. Persons who are obese and sarcopenic (the "fat frail") have worse outcomes than those who are sarcopenic and non-obese. There is a disproportionate atrophy of type IIa muscle fibers with aging. There is also evidence of an age-related decrease in the synthesis rate of myosin heavy chain proteins, the major anabolic protein. Motor units innervating muscle decline with aging, and there is increased irregularity of muscle unit firing. There are indications that cytokines-especially interleukin-1beta, tumor necrosis factor-alpha, and interleukin-6-play a role in the pathogenesis of sarcopenia. Similarly, the decline in anabolic hormones-namely, testosterone, dehydroepiandrosterone growth hormone, and insulin-like growth factor-I-is also implicated in the sarcopenic process. The role of the physiologic anorexia of aging remains to be determined. Decreased physical activity with aging appears to be the key factor involved in producing sarcopenia. An increased research emphasis on the factors involved in the pathogenesis of sarcopenia is needed.

Senescence-related changes in gene expression in muscle: similarities and differences between mice and men

A microarray study of the effect of senescence in mice on gene expression in muscle has been published recently. The present analysis was done to evaluate the extent to which the age-related differences in gene expression in murine muscle are also evident in human muscle. RNA extracted from muscle of young (21-24 yr) and old men (66-77 yr) was studied both by serial analysis of gene expression (SAGE) and by oligonucleotide microarrays. SAGE tags were detected for 61 genes homologous to genes reported to be differentially expressed in young and old murine muscle. The microarray had probe sets for 70 homologous genes. For 17 genes, there was evidence for a similar age-related change in expression in muscles of mice and men. For 32 other genes, there was evidence that the effect of age on the level of expression is not the same in mice and men. There was no evidence that older human muscle has increased expression of the stress response genes that are increased in old murine muscle.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.

Changes in uncoupling protein-2 and -3 expression in aging rat skeletal muscle, liver, and heart

Uncoupling protein (UCP)-2 and -3 mediate mitochondrial (mt) proton leak in vitro and are potential regulators of energy expenditure and ATP production. Aging is associated with alteration of tissue functions, suggesting impaired mtATP production. To determine whether age-related changes in UCP expression occur, we measured the transcript levels of UCP-2 and -3 in skeletal muscle, liver, and heart in 6- and 27-mo-old rats. UCP-2 transcripts were higher in old animals in the white (+100%) and red (+70%, both P < 0.04) gastrocnemius muscle and in the liver (+300%, P < 0.03), whereas they were comparable in the heart in both age groups. UCP-2 transcript levels correlated positively with mitochondrial-encoded cytochrome c oxidase transcripts normalized for mtDNA (P < 0.01) and negatively with mtDNA copy number (P < 0.001). UCP-3 transcripts were lower in the less oxidative white (-50%, P < 0.04) and unchanged in the more oxidative red (-15%, P = 0.41) gastrocnemius muscle in old animals. Similar changes at protein level were confirmed by UCP-2 protein in aging liver (+300%, P < 0.01) and UCP-2 (+85%, P < 0.05) and UCP-3 (-30%, P = 0.4) protein in aging mixed gastrocnemius muscle. Aging is thus associated with tissue-specific changes of UCP-2 and -3 gene expression. Increased UCP-2 expression may limit ATP production and is related to mitochondrial gene expression in aging muscles and liver. Different age-related changes may reflect differential regulation of UCP-2 and -3 in skeletal muscle. The current data suggest a potential role of uncoupling proteins to alter energy production in aging tissues.

Age-related decreases in basal limb blood flow in humans: time course, determinants and habitual exercise effects

Basal whole-limb blood flow is lower in older than in young healthy sedentary men due to a lower limb vascular conductance. In Study 1, we determined whether age-associated reductions in basal whole-leg (femoral artery) blood flow and vascular conductance are modulated by habitual physical activity by studying 89 healthy men aged 20-35 or 55-75 years (26 sedentary, 31 physically active and 32 endurance exercise trained). Femoral blood flow (duplex Doppler) and vascular conductance were approximately 20-30 % lower (P < 0.01) in the older men in all three physical activity groups. In Study 2, to determine the temporal pattern and relation to local metabolism and lean tissue mass of the age-associated reductions in femoral blood flow, we studied 142 healthy men aged 18-79 years. Femoral blood flow (r = -0.40) and vascular conductance (r = -0.51) were linearly and inversely related to age (both P < 0.001). Leg fat-free mass (r = -0.48) and estimated leg oxygen consumption (r = -0.49) declined with advancing age (both P < 0.001), and were strongly and positively related (r = 0.75; P < 0.001). The age-associated decline in femoral blood flow correlated with the corresponding reductions in leg fat-free mass and estimated leg oxygen consumption (both r = 0.47; P < 0.001). We concluded that: (1) basal whole-limb blood flow and vascular conductance decrease progressively with advancing age in healthy men; (2) reductions in both limb fat-free mass and oxygen consumption are related to the decline in whole-limb blood flow with age; and (3) habitual aerobic exercise does not appear to modulate the age-related reductions in basal limb blood flow and vascular conductance.

Resistance exercise decreases skeletal muscle tumor necrosis factor alpha in frail elderly humans

Skeletal muscle protein and function decline with advancing age but the underlying pathophysiology is poorly understood. To test the hypothesis that the catabolic cytokine tumor necrosis factor alpha (TNF-alpha) contributes to this process, we studied the effects of aging and resistance exercise on TNF-alpha expression in human muscle. Using in situ hybridization, TNF-alpha message was localized to myocytes in sections of skeletal muscle from elderly humans. Both TNF-alpha mRNA and protein levels were elevated in skeletal muscle from frail elderly (81+/-1 year) as compared to healthy young (23+/-1 year) men and women. To determine whether resistance exercise affects TNF-alpha expression, frail elderly men and women were randomly assigned to a training group or to a nonexercising control group. Muscle biopsies were performed before and after 3 months. Muscle TNF-alpha mRNA and protein levels decreased in the exercise group but did not change in the control group. Muscle protein synthesis rate in the exercise group was inversely related to levels of TNF-alpha protein. These data suggest that TNF-alpha contributes to age-associated muscle wasting and that resistance exercise may attenuate this process by suppressing skeletal muscle TNF-alpha expression.

Age effect on transcript levels and synthesis rate of muscle MHC and response to resistance exercise

Experimental evidence indicates that a lower synthesis rate of muscle contractile protein myosin heavy chain (MHC) occurs in age-related muscle wasting and weakness. To determine the molecular mechanism of this lower synthesis of MHC, we measured transcript levels of isoforms of MHC (MHCI, MHCIIa, and MHCIIx) in muscle biopsy samples of 7 young (20-27 yr), 12 middle-aged (47-60 yr), and 14 older (>65 yr) people. We further determined the effect of 3 mo of resistance exercise training (exercise) vs. nonintervention (control) on transcript levels of MHC isoforms on these subjects and the fractional synthesis rate (FSR) of MHC in 39 people aged 46-79 yr. MHCI mRNA levels did not significantly change with age, but MHCIIa decreased 38% (P < 0.05) from young to middle age and further decreased 50% (P < 0.05) from middle to old age. MHCIIx decreased 84% (P < 0.05) from young to middle age and 48% from middle to old age (P < 0.05). Exercise increased FSR of MHC by 47% (P < 0.01) and mixed muscle protein by 56% (P < 0.05). Exercise training results in an increase (85%) in transcript levels of MHCI and a decrease in the transcript levels of MHCIIa and MHCIIx. In conclusion, an age-related lowering of the transcript levels of MHCIIa and MHCIIx is not reversed by exercise, whereas exercise results in a higher synthesis rate of MHC in association with an increase in MHCI isoform transcript levels.

Metabolism 2000: the emperor needs new clothes

Metabolism is one of the corner stones of nutritional science. As biology enters the post-genomic era and with functional genomics beginning to takeoff, we anticipate that the study of metabolism will play an increasingly important role in helping to link advances made via the reductionist paradigm, that has been so successful in molecular and cellular biology, with those emerging from observational studies in animals and human subjects. A reconstructive metabolically-focused approach offers a timely paradigm for enhancing the elegance of nutritional science. Here we give particular attention to the use of tracers as phenotyping tools and discuss the application of our metaprobe concepts with respect to some novel features of metabolism, including 'underground metabolism', 'metabolic hijacking', 'catalytic promiscuity' and 'moonlighting proteins'. The opportunities for enhancing the study of metabolism by new and emerging technologies, and the importance of the interdisciplinary research enterprise are also touched upon. We conclude that: (1) the metaprobe concepts and approach, discussed herein, potentially yield a quantitative physiological (metabolic) phenotype against which to elaborate partial or focused genotypes; (2) physiological (metabolic) phenotypes which have a whole-body or kinetically-discernible inter-organ tissue-directed metabolic signature are an ideal target for this directed tracer-based definition of the 'functional' genotype; (3) metabolism, probed with tracer tool kits suitable for measuring rates of turnover, change and conversion, becomes in the current sociology of the 'Net', like AOL, Yahoo. Alta Vista, Lycos or Ask Jeeves, the portal for an exploration of the metabolic characteristics of the 'Genomics Internet'.

Sarcopenia: current concepts

Sarcopenia, the loss of muscle mass and strength with age, is becoming recognized as a major cause of disability and morbidity in the elderly population. Sarcopenia is part of normal aging and does not require a disease to occur, although muscle wasting is accelerated by chronic diseases. Sarcopenia is thought to have multiple causes, although the relative importance of each is not clear. Neurological, metabolic, hormonal, nutritional, and physical-activity-related changes with age are likely to contribute to the loss of muscle mass. In this review, we discuss current concepts of the pathogenesis, treatment, and prevention of sarcopenia.

The utility of resistance exercise training and amino acid supplementation for reversing age-associated decrements in muscle protein mass and function

Advancing age is associated with reduced skeletal muscle protein synthesis, altered expression of and chemical modifications to muscle proteins, reduced muscle strength, muscle strength per unit muscle mass and muscle power (sarcopenia). These age-associated impairments in the quantity and quality of contractile protein contribute to physical disability and frailty, a loss of independent function, the risk of falling and fractures, and escalating health-care costs. Progressive resistance exercise training is a potent, non-pharmacologic, efficacious therapy for the impairment in muscle quantity and quality in middle age and physically frail adults. Evidence is accumulating that dietary amino acid supplementation may also improve muscle protein balance in the elderly. Several potential cellular mechanisms for the loss of muscle protein and resistance exercise-induced improvements in muscle quantity and quality in elderly adults are reviewed.

High-abundance mRNAs in human muscle: comparison between young and old

To gain a better understanding of the potential role of altered gene expression in the diminished muscle function in old age, we performed a broad search for transcripts expressed at quantitatively different levels in younger (21-24 yr) and older (66-77 yr) human vastus lateralis muscle by serial analysis of gene expression (SAGE). Because SAGE was based on RNA pooled from muscle of several different subjects, relative concentrations of selected mRNAs also were determined in individual muscle samples by quantitative RT-PCR. There were 702 SAGE tags detected at least 10 times in one or both mRNA pools, and the detection frequency was different (at P < 0.01) between young and older muscle for 89 of these. The ratio of myosin heavy chain 2a mRNA to myosin heavy chain 1 mRNA was reduced in older muscle. The mRNAs encoding several mitochondrial proteins involved in electron transport (including several subunits of cytochrome-c oxidase and NADH dehydrogenase) and subunits of ATP synthase were approximately 30% less abundant in older muscle. Several mRNAs encoding enzymes involved in glucose metabolism also were less abundant in older muscle. Analysis of individual samples revealed that the differences suggested by SAGE were not artifacts of atypical gene expression in one or a few individuals. These data suggest that some of the phenotypic changes in senescent muscle may be related to altered gene transcription.

Oxidative capacity and ageing in human muscle

This study determined the decline in oxidative capacity per volume of human vastus lateralis muscle between nine adult (mean age 38.8 years) and 40 elderly (mean age 68.8 years) human subjects (age range 25-80 years). We based our oxidative capacity estimates on the kinetics of changes in creatine phosphate content ([PCr]) during recovery from exercise as measured by (31)P magnetic resonance (MR) spectroscopy. A matched muscle biopsy sample permitted determination of mitochondrial volume density and the contribution of the loss of mitochondrial content to the decline in oxidative capacity with age. The maximal oxidative phosphorylation rate or oxidative capacity was estimated from the PCr recovery rate constant (k(PCr)) and the [PCr] in accordance with a simple electrical circuit model of mitochondrial respiratory control. Oxidative capacity was 50 % lower in the elderly vs. the adult group (0.61 +/- 0.04 vs. 1.16 +/- 0.147 mM ATP s(-1)). Mitochondrial volume density was significantly lower in elderly compared with adult muscle (2.9 +/- 0.15 vs. 3.6 +/- 0.11 %). In addition, the oxidative capacity per mitochondrial volume (0.22 +/- 0.042 vs. 0.32 +/- 0.015 mM ATP (s %)(-1)) was reduced in elderly vs. adult subjects. This study showed that elderly subjects had nearly 50 % lower oxidative capacity per volume of muscle than adult subjects. The cellular basis of this drop was a reduction in mitochondrial content, as well as a lower oxidative capacity of the mitochondria with age.

Effect of contractile activity on protein turnover in skeletal muscle mitochondrial subfractions

To determine the role of intramitochondrial protein synthesis (PS) and degradation (PD) in contractile activity-induced mitochondrial biogenesis, we evaluated rates of [(35)S]methionine incorporation into protein in isolated rat muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria. Rates of PS ranged from 47 to 125% greater (P < 0.05) in IMF compared with SS mitochondria. Intense, acute in situ contractile activity (10 Hz, 5 min) of fast-twitch gastrocnemius muscle resulted in a 50% decrease in PS (P < 0.05) in SS but not IMF mitochondria. Recovery, or continued contractile activity (55 min), reestablished PS in SS mitochondria. In contrast, PS was not affected in either SS or IMF mitochondria after prolonged (60-min) contractile activity in the presence or absence of a recovery period. PD was not influenced by 5 min of contractile activity in the presence or absence of recovery but was reduced after 60 min of contractions followed by recovery. Chronic stimulation (10 Hz, 3 h/day, 14 days) increased muscle cytochrome-c oxidase activity by 2.2-fold but reduced PS in IMF mitochondria by 29% (P < 0.05; n = 4). PS in SS mitochondria and PD in both subfractions were not changed by chronic stimulation. Thus acute contractile activity exerts differential effects on protein turnover in IMF and SS mitochondria, and it appears that intramitochondrial PS does not limit the extent of chronic contractile activity-induced mitochondrial biogenesis.

Resistance exercise acutely increases MHC and mixed muscle protein synthesis rates in 78-84 and 23-32 yr olds

We determined whether short-term weight-lifting exercise increases the synthesis rate of the major contractile proteins, myosin heavy chain (MHC), actin, and mixed muscle proteins in nonfrail elders and younger women and men. Fractional synthesis rates of mixed, MHC, and actin proteins were determined in seven healthy sedentary 23- to 32-yr-old and seven healthy 78- to 84-yr-old participants in paired studies done before and at the end of a 2-wk weight-lifting program. The in vivo rate of incorporation of 1-[(13)C]leucine into vastus lateralis MHC, actin, and mixed proteins was determined using a 14-h constant intravenous infusion of 1-[(13)C]leucine. Before exercise, the mixed and MHC fractional synthetic rates were lower in the older than in the younger participants (P < or = 0.04). Baseline actin protein synthesis rates were similar in the two groups (P = not significant). Over a 2-wk period, participants completed ten 1- to 1. 5-h weight-lifting exercise sessions: 2-3 sets per day of 9 exercises, 8-12 repetitions per set, at 60-90% of maximum voluntary muscle strength. At the end of exercise, MHC and mixed protein synthetic rates increased in the younger (88 and 121%) and older participants (105 and 182%; P < 0.001 vs. baseline). These findings indicate that MHC and mixed protein synthesis rates are reduced more than actin in advanced age. Similar to that of 23-32 yr olds, the vastus lateralis muscle in 78-84 yr olds retains the capacity to increase MHC and mixed protein synthesis rates in response to short-term resistance exercise.

Protein oxidation and degradation during proliferative senescence of human MRC-5 fibroblasts

One of the highlights of age-related changes of cellular metabolism is the accumulation of oxidized proteins. The aging process on a cellular level can be treated either as the ongoing proliferation until a certain number of cell divisions is reached (the Hayflick limit) or as the aging of nondividing cells, that is, the age-related changes in cells without proliferation. The present investigation was undertaken to reveal the changes in protein turnover, proteasome activity, and protein oxidation status during proliferative senescence. We were able to demonstrate that the activity of the cytosolic proteasomal system declines dramatically during the proliferative senescence of human MRC-5 fibroblasts. Regardless of the loss in activity, it could be demonstrated that there are no changes in the transcription and translation of proteasomal subunits. This decline in proteasome activity was accompanied by an increased concentration of oxidized proteins. Cells at higher proliferation stages were no longer able to respond with increased degradation of endogenous [(35)S]-Met-radiolabeled proteins after hydrogen peroxide- or quinone-induced oxidative stress. It could be demonstrated that oxidized proteins in senescent human MRC-5 fibroblasts are not as quickly removed as they are in young cells. Therefore, our study demonstrates that the accumulation of oxidized proteins and decline in protein turnover and activity of the proteasomal system are not only a process of postmitotic aging but also occur during proliferative senescence and result in an increased half-life of oxidized proteins.

Effects of aging on mitochondrial DNA copy number and cytochrome c oxidase gene expression in rat skeletal muscle, liver, and heart

Mitochondrial DNA (mtDNA) deletions and mutations have been reported to occur with aging in various tissues. To determine the functional impact of these changes, we measured mtDNA copy number, mitochondria-encoded cytochrome c oxidase (COX) subunit I and III transcript levels, and COX enzyme activity in skeletal muscles (medial and lateral gastrocnemius and soleus), liver, and heart in 6- and 27-month-old rats. Substantial age-related reductions of mtDNA copy number occurred in skeletal muscle groups (-23-40%, p < 0.03) and liver (-50%, p < 0.01) but not in the heart. The decline in mtDNA was not associated with reduced COX transcript levels in tissues with high oxidative capacities such as red soleus muscle or liver, while transcript levels were reduced with aging in the less oxidative mixed fiber gastrocnemius muscle (-17-22%, p < 0.05). Consistent with transcript levels, COX activity also remained unchanged in aging liver and heart but declined with age in the lateral gastrocnemius (-32%, p < 0.05). Thus, the effects of aging on mitochondrial gene expression are tissue-specific. A substantial age-related decline in mtDNA copy number proportional to tissue oxidative capacities is demonstrated in skeletal muscle and liver. mtDNA levels are in contrast preserved in the aging heart muscle, presumably due to its incessant aerobic activity. Reduced mtDNA copy number has no major effects on mitochondrial encoded transcript levels and enzyme activities in various tissues under these base-line study conditions. In contrast, maintenance of mitochondrial transcript levels that may be linked to oxidative metabolism and energy demand appears to be the main determinant of mitochondrial oxidative capacity in aging tissues.

Reduced synthesis of muscle proteins in chronic renal failure

Muscle wasting and weakness occur frequently in patients with chronic renal failure. The mechanism(s) by which these abnormalities occur is unclear. We hypothesized that such findings were due to defective muscle protein synthesis. We measured synthetic rates of mixed muscle proteins, myosin heavy chain, and mitochondrial proteins in serial muscle biopsy samples during a continuous infusion of L[1-(13)C]leucine from 12 patients with chronic renal failure and 10 healthy control subjects under identical study conditions. Patients with chronic renal failure have significantly lower synthetic rates of mixed muscle proteins and myosin heavy chain (27 and 37% reductions, respectively, P < 0.05 and P < 0.02). Significant declines in the synthetic rates of muscle mitochondrial protein (27%) (P < 0.05), muscle cytochrome c-oxidase activity (42%) (P < 0.007), and citrate synthase (27%) (P < 0.007) were also observed in patients with chronic renal failure. The synthetic rates of muscle proteins and activity of mitochondrial enzymes were negatively correlated to the severity of renal failure. These results indicate that in chronic renal failure there is a decrease in the synthesis of muscle contractile and mitochondrial proteins and a decrease in muscle mitochondrial oxidative enzymes. Reduced synthetic rate of several muscle proteins is the likely biochemical basis of muscle loss and muscle weakness in people with chronic renal failure.

Changes in protein, carbohydrate, and fat metabolism with aging: possible role of insulin

Age is associated with modifications of body composition, i.e., an increase in body fat mass and a decrease in protein mass. Because insulin controls substrate disposal and production, these changes could theoretically be related to changes in either insulin action or secretion on the various substrates. On the basis of available evidence, insulin action on whole-body amino acid and protein metabolism seems not to be impaired in the aged. Decreased synthesis of contractile and mitochondrial proteins in muscle, associated with decreased gene expression, was described in humans. Decreased physical activity apparently represents an important factor responsible for decreased muscle protein synthesis and mass in the elderly. Exercise in the elderly may acutely revert these changes, although its chronic effects are still uncertain. In addition, the possible interaction between insulin and exercise in the maintenance of muscle mass needs to be specifically investigated in aged people. Higher free fatty acid (FFA) absolute flux and oxidation rates were observed in healthy elderly subjects in both the fasting state and following hyperinsulinemia, but not when normalized over fat mass. This suggests that FFA kinetics reflect the established changes in fat mass. Insulin sensitivity on glucose metabolism is usually normal in the aged, despite subtle impairments in insulin secretion, hepatic uptake, and onset of action. Finally, data support the operation of the Randle cycle (i.e., inverse relationships between fat and glucose oxidation) in the elderly.

The effect of age on protein metabolism

The mechanisms of senescence remain to be fully defined. This review focuses on recent advances in our understanding of body protein turnover, which is essential for the remodeling of tissues and production of specific proteins in time of need. Recent advances in technology make it possible to measure the synthesis rate of muscle myosin heavy chain, mitochondrial proteins and sarcoplasmic proteins, providing insight into the mechanisms of the sarcopenia of aging. A reduced synthesis rate of myosin heavy chain and mitochondrial protein may explain muscle weakness and fatiguability that occurs with aging. Aging also seems to affect selected liver proteins such as fibrinogen. The potential roles of exercise and hormone replacement in slowing the age-related decline in protein turnover is discussed.

Muscle mass and function in thyrotoxic patients before and during medical treatment

OBJECTIVE

Development of muscle weakness and atrophy are well known complications of thyrotoxicosis, although little is known about its clinical course. The present longitudinal study was therefore undertaken to monitor muscle mass and strength before and during treatment of hyperthyroidism.

DESIGN AND PATIENTS

Five patients (2 male, 3 female; Age 41 +/- 6 years; BMI 22.2 +/- 1.1 kg/m2) with newly diagnosed hyperthyroidism were studied with respect to muscle area, muscle strength, body composition and substrate metabolism at baseline and after 1, 3, 6, 9 and 12 months of treatment.

MEASUREMENTS

Midthigh muscle areas were assessed by computed tomography (CT), while bioelectrical impedance analysis (BIA) was used for assessment of body composition. The isometric strength of the biceps brachialis and quadriceps muscles was assessed by means of a dynamometer and the maximal static ins- and ex-piratory mouth pressures were measured with a respiratory pressure module.

RESULTS

Prior to treatment thyrotoxic patients all displayed elevated levels of total and free T3 and T4 together with suppressed TSH. BMI, fat mass and lean body mass increased significantly during the treatment period, while energy expenditure (EE) decreased. Thigh muscle areas increased by 24% (101.5 +/- 11.5 vs. 125.3 +/- 13.1 cm2, P < 0.05) from entry to peak. Peak time was 9 +/- 0.9 months. During treatment a significant (P < 0.01) increase in muscle strength was observed; arm capacity increased by 48%, while leg capacity increased by 51%. Peak time (months) was: Right arm: 8 +/- 3, left arm: 7 +/- 2, right leg: 5 +/- 3, left leg: 9 +/- 2. Respiratory muscle strength, expressed as maximal ins- or ex-piratory mouth pressure, was significantly impaired among patients at entry. A significant increase in inspiratory and expiratory strength was found from entry to peak (P < 0.05), as inspiratory strength increased by 35% and expiratory by 19%. Inspiratory strength peaked after 7 +/- 1 months, expiratory muscle strength after 6 +/- 1 months.

CONCLUSIONS

In conclusion we find that in patients with thyrotoxicosis muscle mass is reduced by approximately 20% and muscle strength by approximately 40% and that between 5 and 9 months elapse before normal muscle mass and function are reestablished.

Protein and physical performance

Important advances have been made in the understanding of the regulation of protein metabolism, which are of relevance to those interested in maximizing muscle performance in sport and aging. The use of 24 h amino acid infusion studies, the development of methods to measure skeletal muscle fractional breakdown rate and the fractional synthetic rate of specific proteins have and will continue to enhance our understanding of protein metabolism in exercise. Recent studies have demonstrated potentially beneficial effects upon protein metabolism by altering the composition and timing of nutrient delivery. Exercise and nutritional interventions may positively influence the negative effects of aging upon protein metabolism.

Tracers to investigate protein and amino acid metabolism in human subjects

Three tracer methods have been used to measure protein synthesis, protein breakdown and protein oxidation at whole-body level. The method using L-[1-(13)C]leucine is considered the method of reference. These methods have contributed greatly to the existing knowledge on whole-body protein turnover and its regulation by feeding, fasting, hormones and disease. How exercise and ingestion of mixed protein-containing meals affect whole-body protein metabolism is still open to debate, as there are discrepancies in results obtained with different tracers. The contribution of whole-body methods to the future gain of knowledge is expected to be limited due to the fact that most physiological disturbances have been investigated extensively, and due to the lack of information on the relative contribution of various tissues and proteins to whole-body changes. Tracer amino acid-incorporation methods are most suited to investigate these latter aspects of protein metabolism. These methods have shown that some tissues (liver and gut) have much higher turnover rates and deposit much more protein than others (muscle). Massive differences also exist between the fractional synthesis rates of individual proteins. The incorporation methods have been properly validated, although minor disagreements remain on the identity of the true precursor pool (the enrichment of which should be used in the calculations). Arterio-venous organ balance studies have shown that little protein is deposited in skeletal muscle following a protein-containing meal, while much more protein is deposited in liver and gut. The amount deposited in the feeding period in each of these tissues is released again during overnight fasting. The addition of tracers to organ balance studies allows the simultaneous estimation of protein synthesis and protein breakdown, and provides information on whether changes in net protein balance are caused primarily by a change in protein synthesis or in protein breakdown. In the case of a small arterio-venous difference in a tissue with a high blood flow, estimates of protein synthesis and breakdown become very uncertain, limiting the value of using the tracer. An additional measurement of the intracellular free amino acid pool enrichment allows a correction for amino acid recycling and quantification of the inward and outward transmembrane transport. However, in order to obtain reliable estimates of the intramuscular amino acid enrichment and, therefore, of muscle protein synthesis and breakdown in this so-called three-pool model, the muscle should be freeze-dried and the resulting fibres should be freed from connective tissue and small blood clots under a dissection microscope. Even when optimal precautions are taken, the calculations in these tracer balance methods use multiple variables and, therefore, are bound to lead to more variability in estimates of protein synthesis than the tracer amino acid incorporation methods. In the future, most studies should focus on the measurement of protein synthesis and breakdown in specific proteins in order to understand the mechanisms behind tissue adaptation in response to various stimuli (feeding, fasting, exercise, trauma, sepsis, disuse and disease). The tracer laboratories, therefore, should improve the methodology to allow the measurement of low tracer amino acid enrichments in small amounts of protein.

Lower recovery of muscle protein lost during starvation in old rats despite a stimulation of protein synthesis

Sarcopenia could result from the inability of an older individual to recover muscle lost during catabolic periods. To test this hypothesis, we compared the capacity of 5-day-refed 12- and 24-mo-old rats to recover muscle mass lost after 10 days without food. We measured gastrocnemius and liver protein synthesis with the flooding-dose method and also measured nitrogen balance, 3-methylhistidine excretion, and the gene expression of components of proteolytic pathways in muscle comparing fed, starved, and refed rats at each age. We show that 24-mo-old rats had an altered capacity to recover muscle proteins. Muscle protein synthesis, inhibited during starvation, returned to control values during refeeding in both age groups. The lower recovery in 24-mo-old rats was related to a lack of inhibition of muscle proteolysis during refeeding. The level of gene expression of components of the proteolytic pathways did not account for the variations in muscle proteolysis at both ages. In conclusion, this study highlights the role of muscle proteolysis in the lower recovery of muscle protein mass lost during catabolic periods.

Resistance exercise training increases mixed muscle protein synthesis rate in frail women and men >/=76 yr old

Muscle atrophy (sarcopenia) in the elderly is associated with a reduced rate of muscle protein synthesis. The purpose of this study was to determine if weight-lifting exercise increases the rate of muscle protein synthesis in physically frail 76- to 92-yr-old women and men. Eight women and 4 men with mild to moderate physical frailty were enrolled in a 3-mo physical therapy program that was followed by 3 mo of supervised weight-lifting exercise. Supervised weight-lifting exercise was performed 3 days/wk at 65-100% of initial 1-repetition maximum on five upper and three lower body exercises. Compared with before resistance training, the in vivo incorporation rate of [(13)C]leucine into vastus lateralis muscle protein was increased after resistance training in women and men (P < 0.01), although it was unchanged in five 82 +/- 2-yr-old control subjects studied two times in 3 mo. Maximum voluntary knee extensor muscle torque production increased in the supervised resistance exercise group. These findings suggest that muscle contractile protein synthetic pathways in physically frail 76- to 92-yr-old women and men respond and adapt to the increased contractile activity associated with progressive resistance exercise training.

Aging and Muscle Loss

Aging is associated with a decrease in fat-free mass, an increase in fat mass, and progressive impairment of muscle function and performance. Diminishing anabolic hormone levels and progressive declines in muscle protein turnover contribute to the multifactorial pathophysiology of age-associated sarcopenia. The potential effects of anabolic hormone replacement on body composition and functional capacity are only beginning to be studied.