Effect of training with different mechanical loadings on MyHC and GLUT4 changes

GLUT4 Is Not Necessary for Overload-Induced Glucose Uptake or Hypertrophic Growth in Mouse Skeletal Muscle

GLUT4 is necessary for acute insulin- and contraction-induced skeletal muscle glucose uptake, but its role in chronic muscle loading (overload)-induced glucose uptake is unknown. Our goal was to determine whether GLUT4 is required for overload-induced glucose uptake. Overload was induced in mouse plantaris muscle by unilateral synergist ablation. After 5 days, muscle weights and ex vivo [³H]-2-deoxy-d-glucose uptake were assessed. Overload-induced muscle glucose uptake and hypertrophic growth were not impaired in muscle-specific GLUT4 knockout mice, demonstrating that GLUT4 is not necessary for these processes. To assess which transporters mediate overload-induced glucose uptake, chemical inhibitors were used. The facilitative GLUT inhibitor cytochalasin B, but not the sodium-dependent glucose cotransport inhibitor phloridzin, prevented overload-induced uptake demonstrating that GLUTs mediate this effect. To assess which GLUT, hexose competition experiments were performed. Overload-induced [³H]-2-deoxy-d-glucose uptake was not inhibited by d-fructose, demonstrating that the fructose-transporting GLUT2, GLUT5, GLUT8, and GLUT12 do not mediate this effect. To assess additional GLUTs, immunoblots were performed. Overload increased GLUT1, GLUT3, GLUT6, and GLUT10 protein levels twofold to fivefold. Collectively, these results demonstrate that GLUT4 is not necessary for overload-induced muscle glucose uptake or hypertrophic growth and suggest that GLUT1, GLUT3, GLUT6, and/or GLUT10 mediate overload-induced glucose uptake.

Lifelong Physical Activity Prevents Aging-Associated Insulin Resistance in Human Skeletal Muscle Myotubes via Increased Glucose Transporter Expression

Both aging and physical inactivity are associated with increased development of insulin resistance whereas physical activity has been shown to promote increased insulin sensitivity. Here we investigated the effects of physical activity level on aging-associated insulin resistance in myotubes derived from human skeletal muscle satellite cells. Satellite cells were obtained from young (22 yrs) normally active or middle-aged (56.6 yrs) individuals who were either lifelong sedentary or lifelong active. Both middle-aged sedentary and middle-aged active myotubes had increased p21 and myosin heavy chain protein expression. Interestingly MHCIIa was increased only in myotubes from middle-aged active individuals. Middle-aged sedentary cells had intact insulin-stimulated Akt phosphorylation however, the same cell showed ablated insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane. On the other hand, middle-aged active cells retained both insulin-stimulated increases in glucose uptake and GLUT4 translocation to the plasma membrane. Middle-aged active cells also had significantly higher mRNA expression of GLUT1 and GLUT4 compared to middle-aged sedentary cells, and significantly higher GLUT4 protein. It is likely that physical activity induces a number of stable adaptations, including increased GLUT4 expression that are retained in cells ex vivo and protect, or delay the onset of middle-aged-associated insulin resistance. Additionally, a sedentary lifestyle has an impact on the metabolism of human myotubes during aging and may contribute to aging-associated insulin resistance through impaired GLUT4 localization.

Intermuscular relationship of human muscle fiber type proportions: slow leg muscles predict slow neck muscles

INTRODUCTION

Our aim in this study was to examine whether the muscle fiber type proportions in different muscles from the same individual are interrelated.

METHODS

Samples were excised from five skeletal muscles in each of 12 human autopsy cases, and the fiber type proportions were determined by immunohistochemistry. We further examined the intermuscular relationship in fiber type proportion by reanalyzing three previously published data sets involving other muscles.

RESULTS

Subjects demonstrated a predominantly high or low proportion of type 1 fibers in all examined muscles, and the overall difference between individuals was statistically significant (P < 0.001). Accordingly, the type 1 fiber proportions in most muscles were positively correlated (median r = 0.42, range -0.03-0.80). Similar results were also obtained from the three reanalyzed data sets.

CONCLUSIONS

We suggest the existence of an across-muscle phenotype with respect to fiber type proportions; some individuals display generally faster muscles and some individuals slower muscles when compared with others.

Muscle hypertrophy in prepubescent tennis players: a segmentation MRI study

PURPOSE

To asses if tennis at prepubertal age elicits the hypertrophy of dominant arm muscles.

METHODS

The volume of the muscles of both arms was determined using magnetic resonance imaging (MRI) in 7 male prepubertal tennis players (TP) and 7 non-active control subjects (CG) (mean age 11.0 ± 0.8 years, Tanner 1-2).

RESULTS

TP had 13% greater total muscle volume in the dominant than in the contralateral arm. The magnitude of inter-arm asymmetry was greater in TP than in CG (13 vs 3%, P<0.001). The dominant arm of TP was 16% greater than the dominant arm of CG (P<0.01), whilst non-dominant arms had similar total muscle volumes in both groups (P = 0.25), after accounting for height as covariate. In TP, dominant deltoid (11%), forearm supinator (55%) and forearm flexors (21%) and extensors (25%) were hypertrophied compared to the contralateral arm (P<0.05). In CG, the dominant supinator muscle was bigger than its contralateral homonimous (63%, P<0.05).

CONCLUSIONS

Tennis at prepubertal age is associated with marked hypertrophy of the dominant arm, leading to a marked level of asymmetry (+13%), much greater than observed in non-active controls (+3%). Therefore, tennis particpation at prepubertal age is associated with increased muscle volumes in dominant compared to the non-dominant arm, likely due to selectively hypertrophy of the loaded muscles.

Detraining-induced alterations in metabolic and fitness markers after a multicomponent exercise-training program in older men

The aim of the present study was to investigate the effects of a 8-week detraining period after a 16-week multicomponent training program including strength and aerobic exercises on the main determinants of aerobic fitness, muscle strength and some metabolic markers in 24 older subjects (60.2 ± 3.0 years). The oxygen uptake at the second ventilatory threshold (VO2VT2) and at the end of exercise (VO2max), maximum voluntary contraction force (MVC) of knee extensors and some metabolic indexes, i.e., insulin sensitivity, blood lipid profile, inflammatory cytokines, and endothelial function, were evaluated at baseline and after the training and detraining periods. The training program induced significant improvements in VO2VT2 (16%, p < 0.05), VO2max (14%, p < 0.05), MVC (6.5%, p < 0.05), insulin sensitivity (16%, p < 0.05), and endothelial function (p < 0.05) but induced no significant change in lipid profile and inflammatory cytokines. Interestingly, VO2VT2 and VO2max (mL·min–1·kg–1) scores remained significantly above pretraining values after the 8-week detraining period. However, the detraining period reversed MVC values, the insulin sensitivity and endothelial function to baseline levels. To conclude, the 8-week detraining partially reversed the major components of aerobic fitness but totally abolished the gains in muscle strength and some metabolic indexes after a 16-week multicomponent training program in older men. Taken as a whole, the results of this study emphasize the importance of exercise prescriptions for older subjects and the need not to interrupt exercise-training over a prolonged period.