Skeletal Muscle Of Aged Rats Research Articles

Point: a call for proper usage of “gender” and “sex” in biomedical publications

the term “gender” became popular among sociologists, psychologists, and sex researchers in the 1970s as a means to differentiate biological differences between men and women (sex differences) and differences due to socialization experiences (gender differences), or the act of having sex (e.g., [

Proteomic DIGE analysis of the mitochondria‐enriched fraction from aged rat skeletal muscle

Skeletal muscle aging is associated with a loss in tissue mass and contractile strength, as well as fiber type shifting and bioenergetic adaptation processes. Since mitochondria represent the primary site for energy generation via oxidative phosphorylation, we investigated potential changes in the expression pattern of the mitochondrial proteome using the highly sensitive DIGE approach. The comparative analysis of the mitochondria-enriched fraction from young adult versus aged muscle revealed an age-related change in abundance for 39 protein species. MS technology identified the majority of altered proteins as constituents of muscle mitochondria. An age-dependent increase was observed for NADH dehydrogenase, the mitochondrial inner membrane protein mitofilin, peroxiredoxin isoform PRX-III, ATPase synthase, succinate dehydrogenase, mitochondrial fission protein Fis1, succinate-coenzyme A ligase, acyl-coenzyme A dehydrogenase, porin isoform VDAC2, ubiquinol-cytochrome c reductase core I protein and prohibitin. Immunoblotting, enzyme testing and confocal microscopy were used to validate proteomic findings. The DIGE-identified increase in key mitochondrial elements during aging agrees with the concept that sarcopenia is associated with a shift to a slower contractile phenotype and more pronounced aerobic-oxidative metabolism. This suggests that mitochondrial markers are reliable candidates that should be included in the future establishment of a biomarker signature of skeletal muscle aging.

Exercise alters SIRT1, SIRT6, NAD and NAMPT levels in skeletal muscle of aged rats

Silent information regulators are potent NAD +-dependent protein deacetylases, which have been shown to regulate gene silencing, muscle differentiation and DNA damage repair. Here, changes in the level and activity of sirtuin 1 (SIRT1) in response to exercise in groups of young and old rats were studied. There was an age-related increase in SIRT1 level, while exercise training significantly increased the relative activity of SIRT1. A strong inverse correlation was found between the nuclear activity of SIRT1 and the level of acetylated proteins. Exercise training induced SIRT1 activity due to the positive effect of exercise on the activity of nicotinamide phosphoribosyltransferase (NAMPT) and thereby the production of sirtuin-fueling NAD +. Exercise training normalized the age-associated shift in redox balance, since exercised animals had significantly lower levels of carbonylated proteins, expression of hypoxia-inducible factor-1alpha and vascular endothelial growth factor. The age-associated increase in the level of SIRT6 was attenuated by exercise training. On the other hand, aging did not significantly increase the level of DNA damage, which was in line with the activity of 8-oxoguanine DNA glycosylase, while exercise training increased the level of this enzyme. Regular exercise decelerates the deleterious effects of the aging process via SIRT1-dependent pathways through the stimulation of NAD + biosynthesis by NAMPT.

Setting the “tone” for aging in the skeletal muscle microcirculation

the ability to regulate vasomotor tone in resistance arteries and arterioles plays an integral role in mediating the circulatory adjustments that occur during exercise. During exercise, skeletal muscle blood flow must increase to actively contracting skeletal muscle to supply oxygen and nutrients

Evidence for differential regulation of lactate metabolic properties in aged and unloaded rat skeletal muscle

Skeletal muscles of elderly individuals show fatigue resistance and reduced lactate accumulation compared with those of young subjects during activities that recruit a small amount of muscle mass. To explore the mechanism underlying the functional changes in aged muscle, we focused on lactate metabolic properties, including monocarboxylate transporter (MCT) 1 and MCT4, in muscles from old and young control rats and hindlimb-suspended young rats. MCT1 expression was lower in soleus (SOL) of old rats than in SOL of young control rats, but was similar in young control and hindlimb-suspended rats. MCT4 expression was lower in extensor digitorum longus (EDL) of old rats than in that of young control rats, but did not differ between young control and hindlimb-suspended rats. The ratio of lactate dehydrogenase to citrate synthase activities was higher in SOL of hindlimb-suspended and old rats than in SOL of young control rats, and was lower in EDL of old rats than in those of young control and hindlimb-suspended rats. Our data suggest that aging causes metabolic changes that can reduce lactate accumulation during exercise and increase fatigue resistance in skeletal muscle, and that these changes result from aging rather than from inactivity.

Reduced expression of sarcalumenin and related Ca 2+-regulatory proteins in aged rat skeletal muscle

In skeletal muscle , Ca 2+-cycling through the sarcoplasm regulates the excitation–contraction–relaxation cycle. Since uncoupling between sarcolemmal excitation and fibre contraction may play a key role in the functional decline of aged muscle, this study has evaluated the expression levels of key Ca 2+-handling proteins in senescent preparations using immunoblotting and confocal microscopy. Sarcalumenin, a major luminal Ca 2+-binding protein that mediates ion shuttling in the longitudinal sarcoplasmic reticulum, was found to be greatly reduced in aged rat tibialis anterior, gastrocnemius and soleus muscle as compared to adult specimens. Minor sarcolemmal components of Ca 2+-extrusion, such as the surface Ca 2+-ATPase and the Na +–Ca 2+-exchanger, were also diminished in senescent fibres. No major changes were observed for calsequestrin, sarcoplasmic reticulum Ca 2+-ATPase and the ryanodine receptor Ca 2+-release channel. In contrast, the age-dependent reduction in the α 1S-subunit of the dihydropryridine receptor was confirmed. Hence, this report has shown that downstream from the well-established defect in coupling between the t-tubular voltage sensor and the junctional Ca 2+-release channel complex, additional age-related alterations exist in the expression of essential Ca 2+-handling proteins. This may trigger abnormal luminal Ca 2+-buffering and/or decreased plasmalemmal Ca 2+-removal, which could exacerbate impaired signaling or disturbed intracellular ion balance in aged fibres, thereby causing contractile weakness.

Systemic elevation of interleukin-15 in vivo promotes apoptosis in skeletal muscles of young adult and aged rats

In this study, we tested the hypothesis that systemic elevation of IL-15 would attenuate apoptosis in skeletal muscles of aged rats. IL-15 was administered to young adult ( n = 6) and aged ( n = 6) rats for 14 days. Apoptosis was quantified using an ELISA assay and verified through TUNEL staining of muscle sections. As expected, apoptosis was greater in muscles from aged control rats, compared to age-matched control. Apoptosis was also greater in the muscles from young adult and aged rats treated with IL-15. These increases in apoptosis were associated with decreases in muscle mass of IL-15 treated rats. These data do not support our initial hypothesis and suggest that systemic elevation of IL-15 promotes apoptosis in skeletal muscle. The proposed anti-apoptotic property of IL-15 may be specific to cell-type and/or the degree of muscle pathology present; however, additional research is required to more clearly decipher its role in skeletal muscle.

Responses of cleaved caspase‐8 and −12 apoptotic pathways to 12 week treadmill exercise in aging rat skeletal muscle

Previously our laboratory demonstrated that exercise training (ET) attenuated age‐induced muscle fiber atrophy, caspase‐3 cleavage, and pro‐apoptotic Bcl‐2 signaling in the rat white gastrocnemius. However, cleavage of caspase‐3 and apoptosis may also be stimulated by inflammatory cytokines (e.g. TNF‐α), the fas/cytokine pathway, and ER/Ca2+ stress pathway via caspase‐8 and caspase‐12 activation, respectively. Therefore, the purpose of this study was to test the hypothesis that 12 weeks of exercise training would protect against caspase‐8 cleavage, upstream fas/cytokine signaling, and caspase‐12 cleavage in the rat white gastrocnemius. Three and 31 mo. old Fischer 344 x Brown Norway rats were assigned to young sedentary (YS), young exercise (YE), old sedentary (OS), and old exercise (OE) groups. ET groups ran on a treadmill for 45 min/day, 5 d/wk for 12 wks. TNF‐α, TNFR1, cleaved caspase‐8, cleaved caspase‐12, hydroperoxides, and Hsp70 levels were assesssed. Cleaved caspase‐8 levels were elevated (+91.7%) in the OS group, but not in the OE group. Upstream TNFR1 was lower in ET young (−62%) and old (−25%) rats, without a reduction in TNF‐α. In contrast, cleaved caspase‐12 was not affected by aging or ET. ET decreased hydroperoxide levels (−19.8%) and increased Hsp70 (+46.7%) in the OE group vs. OS. These data indicate that protective effects of ET against age‐induced apoptosis in white muscle were related to upstream modulation of oxidative stress, Hsp70, and possibly the fas/cytokine pathway, but not the ER/Ca2+ stress pathway.

Opposite pathobiochemical fate of pyruvate kinase and adenylate kinase in aged rat skeletal muscle as revealed by proteomic DIGE analysis

Sarcopenia is the drastic loss of skeletal muscle mass and strength during ageing. In order to better understand the molecular pathogenesis of age-related muscle wasting, we have performed a DIGE analysis of young adult versus old rat skeletal muscle. Proteomic profiling revealed that out of 2493 separated 2-D spots, 69 proteins exhibited a drastically changed expression. Age-dependent alterations in protein abundance indicated dramatic changes in metabolism, contractile activity, myofibrillar remodelling and stress response. In contrast to decreased levels of pyruvate kinase (PK), enolase and phosphofructokinase, the mitochondrial ATP synthase, succinate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase and adenylate kinase (AK) were increased in senescent fibres. Higher expression levels of myoglobin and fatty acid binding-protein indicated a shift to more aerobic-oxidative metabolism in a slower-twitching aged fibre population. The drastic increase in alphaB-crystallin and myotilin demonstrated substantial filament remodelling during ageing. An immunoblotting survey of selected muscle proteins confirmed the pathobiochemical transition process in aged muscle metabolism. The proteomic analysis of aged muscle has identified a large cohort of new biomarkers of sarcopenia including opposite changes in PK and AK, which might be useful for the design of improved diagnostic procedures and/or therapeutic strategies to counteract ageing-induced muscle degeneration.

Age-dependent upregulation of p53 and cytochrome c release and susceptibility to apoptosis in skeletal muscle fiber of aged rats: Role of carnitine and lipoic acid

Mitochondrial dysfunction has been implicated in the regulation of myofiber loss during aging, possibly by apoptotic pathways. However, the mitochondrial-mediated pathway of apoptosis by cytochrome c in skeletal muscle remains ambiguous. To understand this, we have studied the upstream and downstream events of cytochrome c release, and assessed the efficacy of carnitine and lipoic acid cosupplementation. The results show that elevated levels of cytosolic cytochrome c activate apoptosis in aged rats, and was confirmed further by in vitro caspase-3 assay. Interestingly, the exogenous addition of cytochrome c results in a much higher increase of caspase-3 activity in aged treated rats than age-matched control rats, strongly suggesting that cytochrome c is a limiting factor for caspase-3 activation in the cytosol. Carnitine and lipoic acid supplement decreased apoptosis in aged rats by maintaining mitochondrial membrane integrity and thereby preventing further loss of cytochrome c in vivo. Furthermore, the upregulation of p53 observed in aged rats is attributed to the loss of outer mitochondrial membrane integrity and subsequent release of cytochrome c through BH3-only proteins. In conclusion, the p53-dependent activation of the mitochondrial-cytochrome c pathway of apoptosis in the present study suggests the existence of cross talk between mitochondria and nucleus. However, the exact molecular mechanism remains to be explored. Oral supplements of carnitine and lipoic acid play an antiapoptotic role in aged rat skeletal muscle by protecting mitochondrial membrane integrity.

Iron load and redox stress in skeletal muscle of aged rats

Loss of skeletal muscle mass (sarcopenia) is a major contributor to disability in old age. We used two-dimensional gel electrophoresis and mass spectrometry to screen for changes in proteins, and cDNA profiling to assess transcriptional regulations in the gastrocnemius muscle of adult (4 months) and aged (30 months) male Sprague-Dawley rats. Thirty-five proteins were differentially expressed in aged muscle. Proteins and mRNA transcripts involved in redox homeostasis and iron load were increased, representing novel components that were previously not associated with sarcopenia. Tissue iron levels were elevated in senescence, paralleling an increase in transferrin. Proteins involved in redox homeostasis showed a complex pattern of changes with increased SOD1 and decreased SOD2. These results suggest that an elevated iron load is a significant component of sarcopenia with the potential to be exploited clinically, and that mitochondria of aged striated muscle may be more vulnerable to radicals produced in cell respiration.

Atrophy-related ubiquitin ligases, atrogin-1 and MuRF1 are up-regulated in aged rat Tibialis Anterior muscle

A phenotypic feature of aging is skeletal muscle wasting. It is characterized by a loss of muscle mass and strength. Age-related loss of muscle mass occurs through a reduction in the rate of protein synthesis, an increase in protein degradation or a combination of both. However, the underlying mechanism is still poorly understood. To test the hypothesis that the ubiquitin-proteasome pathway contributes to this phenomenon, we studied MuRF1 and atrogin-1 expression in Tibialis Anterior muscle of aged rats. These two E3 ligases are considered as sensitive markers of muscle protein degradation by the ubiquitin-proteasome system. Our results revealed that, in skeletal muscle of aged rats, the decline in muscle mass is accompanied by an increase in the level of oxidized proteins and ubiquitin conjugates (90%) whereas the functionality of the proteasome remains constant compared to young rats. Furthermore, the level of both MuRF1 and atrogin-1 mRNA is markedly up-regulated in aged muscle (respectively ×2 and ×2.5). Taken together these data argue for the involvement of the ubiquitin-proteasome pathway in sarcopenia of fast-twitch muscle, in particular through increased expression of MuRF1 and atrogin-1. Moreover, we observed a decrease in the IGF-1/Akt signalling pathways and elevated level of TNFα mRNA in aged rat muscle. Therefore, IGF-1/Akt and TNFα represent potential mediators implicated in the regulation of MuRF1 and atrogin-1 genes during aging.

Oxidative Stress and DNA Single Strand Breaks in Skeletal Muscle of Aged Rats: Role of Carnitine and Lipoicacid

The exposure of biological system to various conditions of oxidative stress is the major contributor for aging process. Oxidative stress in turn increases the cellular levels of oxidatively modified proteins, lipids and nucleic acids resulting in a loss of physical activity and metabolic integrity. In this study, we evaluated the role of L-carnitine and DL-alpha-lipoic acid in minimizing oxidant generation and macromolecular damage in skeletal muscle of aged rats. We found that the oxidant generation was increased in aged rat skeletal muscle when compared to young rats. There was a simultaneous increase in the levels of lipid peroxidation, protein carbonyl content and DNA strand breaks in aged rat skeletal muscle. Administration of L-carnitine (300 mg/kg body wt/day) and DL-alpha-lipoic acid (100 mg/kg body wt/day) to aged rats for 30 days, decreased the oxidant generation, lipid peroxidation, protein carbonylation and DNA strand breaks. We concluded that co-administration of carnitine and lipoic acid to aged rats has the potential to prevent oxidative stress mediated macromolecular damage in skeletal muscle of aged rats by their putative role as efficient antioxidants.

Effect of Aging on the Expression of Peroxisome Proliferator-Activated Receptor γ and the Possible Relation to Insulin Resistance

Background: The morbidity of insulin resistance tends to increase with aging. However, studies on molecular mechanisms underlying insulin resistance in aging process is still in paucity. Objective: The effect of aging on peroxisome proliferator-activated receptor γ (PPARγ) expression, and in addition, the possible association of PPARγ expression with insulin resistance in aging processwere investigated. Methods: The minimal model technique (MMT) based on frequently sampled intravenous glucose tolerance test was adopted and SI and glucose effectiveness of young and aged rats were compared. RT-PCR and Western blot were used to determine the expression of PPARγ at mRNA and protein level in adipose tissue and skeletal muscle, as well as in human colic omentum, respectively. Results: MMT result implied existence of various degrees of insulin resistance in aged rats. The expression of PPARγ at both mRNA and protein levels in adipose tissue of aged rats dramatically decreased; consequently, the expression of its target gene lipoprotein lipases mRNA also markedly decreased compared with those in young rats. Furthermore, the level of PPARγ mRNA and glucose transporter-4 mRNA in skeletal muscle of aged rats attenuated significantly. The expression of PPARγ mRNA in omental adipose tissue of old men was significantly decreased, accompanied by a tendency of higher insulin resistance index, compared with those of the young. Conclusion: Aging may be associated with diminished PPARγ expression affecting insulin resistance in the aged individuals.

Detection of sequence-specific tyrosine nitration of manganese SOD and SERCA in cardiovascular disease and aging

Nitration of protein tyrosine residues (nY) is a marker of oxidative stress and may alter the biological activity of the modified proteins. The aim of this study was to develop antibodies toward site-specific nY-modified proteins and to use histochemistry and immunoblotting to demonstrate protein nitration in tissues. Affinity-purified polyclonal antibodies toward peptides with known nY sites in MnSOD nY-34 and of two adjacent nY in the sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA2 di-nY-294,295) were developed. Kidneys from rats infused with ANG II with known MnSOD nY and aorta from atherosclerotic rabbits and aging rat skeletal and cardiac sarcoplasmic reticulum with known SERCA di-nY were used for positive controls. Staining for MnSOD nY-34 was most intense in distal renal tubules and collecting ducts. Staining of atherosclerotic aorta for SERCA2 di-nY was most intense in atherosclerotic plaques. Aging rat skeletal muscle and atherosclerotic aorta and cardiac atrium from human diabetic patients also stained positively. Staining was decreased by sodium dithionite, which chemically reduces nitrotyrosine to aminotyrosine, and the antigenic nY-peptide blocked staining for each respective nY site but not for the other. As previously demonstrated, immunoblotting failed to detect these modified proteins in whole tissue lysates but did when the proteins were concentrated. Immunohistochemical staining for specific nY-modified tyrosine residues offers the ability to assess the effects of oxidant stress associated with pathological conditions on individual proteins whose function may be affected in specific tissue sites.

Aging augments interstitial K+concentrations in active muscle of rats

Skeletal muscle performance declines with advancing age, and the underlying mechanism is not completely understood. A large body of convincing evidence has demonstrated a crucial role for interstitial K+ concentration ([K+]o) in modulating contractile function of skeletal muscle. The present study tested the hypothesis that during muscle contraction there is a greater accumulation of [K+]o in aged compared with adult skeletal muscle. Twitch muscle contraction was induced by electrical stimulation of the sciatic nerves of 8- and 32-mo-old Fischer 344 x Brown Norway rats. Levels of [K+]o were measured continuously by a microdialysis technique with the probes inserted into the gastrocnemius muscle. Stimulation at 1, 3, and 5 Hz elevated muscle [K+]o by 52, 64, and 88% in adult rats, and by 78, 98, and 104% in aged rats, respectively, and the increase was significantly higher in aged than in adult rats. Recovery for [K+]o, as measured by the time for [K+]o to recover by 20 and 50% from peak response after stimulation, was slower in aged rats. Ouabain (5 mM), a specific inhibitor of the Na+-K+ pump, was added in the perfusate to inhibit the reuptake of K+ into the cells to assess the role of the pump in the overall K+ balance. Ouabain elevated muscle [K+]o at rest, and the effect was significantly attenuated in aged animals. The present data demonstrated an augmented [K+]o in aged skeletal muscle compared with adult skeletal muscle, and the data suggested that an alteration in the function of the Na+-K+ pump may contribute, in part, to the deficiency in K+ balance in skeletal muscle of aged rats.

Overexpression of Id2 Results in a Decrease in MyoD in Proliferating C2C12 Myoblasts

Inhibitor of differentiation protein-2 (Id2) is a dominant negative helix-loop-helix (HLH) protein, and a positive regulator of proliferation, in various cells. Id2 may also have a role in apoptosis, because the N-terminal region induces apoptosis in myeloid 32d.3 cells. Furthermore, elevated levels of Id2 and apoptotic markers have been identified in aged rat skeletal muscles. The role of Id2 in skeletal muscle cells is unknown. PURPOSE Because Id2 is elevated in aged skeletal muscle, the objective of this study was to determine if overexpression of Id2 would alter MyoD levels in proliferating C2C12 myoblasts, a mouse satellite cell line. METHODS The full length Id2 gene was generated by PCR amplification from cDNA of rat hindlimb muscles. Primers with Nde1 and HindIII restrictions sites were used to subclone Id2 into a pDNR-1r Donor Vector that contained two loxP sites. Cre recombinase mediated the transfer of the Id2 fragment located between the two loxP sites of the Id2-pDNR-1r donor vector, to the loxP acceptor vector pLP-IRES2-eGFP to yield Id2-IRES-eGFP. The internal ribosomal entry sequence (IRES) allows both the Id2 gene and the enhanced green fluorescent protein (eGFP) gene to be translated simultaneously from a single bicistronic mRNA. For control experiments, pDNR-1r and Luciferase-pDNR-1r vectors were recombined with the pLP-IRES-eGFP vector to generate pDNR-IRES-eGFP and Luc-IRES-eGFP expression vectors. RESULTS Luciferase activity in Luc-IRES-eGFP transfected cells is significantly (p = 0.0001) higher (81,568-fold increase) compared to the average luciferase activity of C2C12 cells, pDNR-IRES-eGFP and Id2-IRES-eGFP transfected myoblasts. Florescence activated cell sorter (FACS) analysis revealed that Id2 expression is significantly (p > 0.05) elevated (383% & 275%) in Id2-IRES-eGFP transfected C2C12 myoblasts. Id2 expression was significantly (p > 0.05) elevated (383%) in Id2-IRES-eGFP transfected C2C12 myoblasts through western analysis. FACS analysis showed that MyoD expression was significantly (p > 0.05) lower (20%) in C2C12 myoblasts transfected with Id2. There were no significant differences in E47/E12 expression between groups. CONCLUSION These data suggest that the overexpression of Id2 in vitro maybe responsible for the decrease of MyoD, because Id2 has been shown to preferentially bind to Class A basic HLH proteins E47 and E12. This would leave MyoD without a dimerization partner and subject to proteolysis. Supported by NIH R01AG021530

Insulin-dependent glycogen synthesis is delayed in onset in the skeletal muscle of food-deprived aged rats

Insulin resistance with aging may be responsible for impaired glycogen synthesis in the skeletal muscle of aged rats and contribute to the well-known decreased ability to respond to stress with aging. For this reason, to assess the ability of the skeletal muscle to utilize glucose for glycogen synthesis during aging, the time course of glycogen synthesis was continuously monitored by 13C nuclear magnetic resonance for 2 h in isolated [ 13C] glucose-perfused gastrocnemius–plantaris muscles of 5-day food-deprived adult (6–8 months; n=10) or 5-day food-deprived aged (22 months; n=8) rats. [ 13C] glucose (10 mmol/L) perfusion was carried out in the presence or absence of an excess of insulin (1 μmol/L). Food deprivation only decreased glycogen level in adult rats (8.9±2.4 μmol/g in adults vs. 35.6±2.4 μmol/g in aged rats; P<.05). In the presence of an excess of insulin, muscle glycogen synthesis was stimulated in both adult and aged muscles, but the onset was delayed with aging (40 min later). In conclusion, this study highlights the important role of glycogen depletion in stimulating glycogen synthesis in muscles. Consequently, the absence of glycogen depletion in response to starvation in aged rats may be the origin of the delay in insulin-stimulated glycogen synthesis in the skeletal muscle. Glycogen synthesis clearly was not impaired with aging.

Beta2-agonist administration increases sarcoplasmic reticulum Ca2+-ATPase activity in aged rat skeletal muscle.

Aging is associated with a slowing of skeletal muscle contractile properties, including a decreased rate of relaxation. In rats, the age-related decrease in the maximal rate of relaxation is reversed after 4-wk administration with the beta2-adrenoceptor agonist (beta2-agonist) fenoterol. Given the critical role of the sarcoplasmic reticulum (SR) in regulating intracellular Ca2+ transients and ultimately the time course of muscle contraction and relaxation, we tested the hypothesis that the mechanisms of action of fenoterol are mediated by alterations in SR proteins. Sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) kinetic properties were assessed in muscle homogenates and enriched SR membranes isolated from the red (RG) and white (WG) portions of the gastrocnemius muscle in adult (16 mo) and aged (28 mo) F344 rats that had been administered fenoterol for 4 wk (1.4 mg/kg/day ip, in saline) or vehicle only. Aging was associated with a 29% decrease in the maximal activity (Vmax) of SERCA in the RG but not in the WG muscles. Fenoterol treatment increased the Vmax of SERCA and SERCA1 protein levels in RG and WG. In the RG, fenoterol administration reversed an age-related selective nitration of the SERCA2a isoform. Our findings demonstrate that the mechanisms underlying age-related changes in contractile properties are fiber type dependent, whereas the effects of fenoterol administration are independent of age and fiber type.

Poster 46: Immunohistochemical evidence of immature myosin expression in aged rat skeletal muscle

Objective: To evaluate 2 hindlimb skeletal muscles in young and aged rats for presence of immature myosin heavy chain (MHC) isoforms. Design: Randomized controlled trial. Setting: Basic science laboratory. Animals: Fisher 344 Brown Norway F1 hybrid rats, ages 12 and 36 months. Interventions: The soleus and extensor digitorum longus (EDL) muscles of young (12mo old) and aged (36mo old) sedentary rats were excised, quick frozen, and stained immunohistochemically for the developmental MHC isoform. Muscle cross-sections were then evaluated for the number of developmental positive fibers per unit area. Main Outcome Measures: Number of muscle fibers per unit cross-sectional area expressing developmental MHC. Results: The soleus muscles of the aged rats showed significantly greater numbers of developmental MHC positive cells when compared with the soleus of the 12-month-old rats. The EDL of the 36-month-old rats exhibited noticeably greater numbers of developmental MHC positive cells than in the young animals in half of the muscles studied. Conclusions: There is evidence of skeletal muscle regeneration occurring in the soleus of the aged rats in the midst of the fiber atrophy that accompanies aging. Such changes in the EDL also occur, but appear to be less common. The muscle specific differences observed in the expression of developmental MHC with age may be influenced by the fiber type and function differences of this muscle as compared with the soleus. Evaluation of the coexpression of the developmental MHC with the mature MHCs (slow, fast) is currently under investigation.