Effect of long-term muscle paralysis on human single fiber mechanics

Maximal force is unaffected by emphysema-induced atrophy in extensor digitorium longus

Patients with chronic obstructive pulmonary disease (COPD) demonstrate a limited exercise capacity. It is unknown whether muscle fiber atrophy and subsequent decrease in force production contributes to this functional limitation. Therefore, the purpose of this investigation was to determine whether emphysema-induced muscle fiber atrophy leads to a reduction in locomotory muscle force production. Maximal muscle force production and fiber cross-sectional area were measured in the almost exclusively fast-twitch extensor digitorium longus muscles at 4 and 8 months following saline (control, n=8/time period) or elastase (emphysema, n=15/time period) instillation in the lungs of hamsters. Excised lung volume increased 145 and 161% with emphysema at 4 and 8 months, respectively (both P<0.01). Muscle mass, maximal force, and fiber cross-section were unaltered at 4 months. However, absolute mass (-15%) and fiber cross-sectional area (-18%) were reduced at 8 months (both P<0.01). Surprisingly, maximal force was preserved in emphysema animals. These data demonstrate that maximal muscle force may be preserved in the face of emphysema-induced fiber atrophy.

Single muscle fiber function with concurrent exercise or nutrition countermeasures during 60 days of bed rest in women

There is limited information on skeletal muscle properties in women with unloading and countermeasure programs to protect the unloading-induced atrophy. The current investigation tested the hypothesis that a concurrent aerobic and resistance exercise training program would preserve size and contractile function of slow- and fast-twitch muscle fibers. A secondary objective was to test the hypothesis that a leucine-enriched high-protein diet would partially attenuate single fiber characteristics. Vastus lateralis muscle biopsies were obtained before and on day 59 of bed rest from a control (BR; n = 8), nutrition (BRN; n = 8), or exercise (BRE; n = 8) group. Single muscle fibers were studied for diameter, peak force (Po), contractile velocity, and power. Those in the BR group had a decrease ( P < 0.05) in myosin heavy chain (MHC) I diameter (−14%), Po(−35%), and power (−42%) and MHC IIa diameter (−16%) and Po(−31%; P = 0.06) and an increase ( P < 0.05) in MHC hybrid fibers. Changes in size and function of MHC I (−19 to −44%) and IIa (−21% to −30%) fibers and MHC distribution in BRN individuals were similar to results in the BR group. In BRE conditions, MHC I and IIa size and contractile function were preserved during bed rest. These data show that the concurrent exercise program preserved the myocellular profile of the vastus lateralis muscle during 60-day bed rest. To combat muscle atrophy and function with long-term unloading, the exercise prescription program used in this study should be considered as a viable training program for the upper leg muscles, whereas the nutritional intervention used cannot be recommended as a countermeasure for skeletal muscle.

Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway

Ribosomal S6 kinase 1 (S6K1) is a downstream component of the mammalian target of rapamycin (mTOR) signaling pathway and plays a regulatory role in translation initiation, protein synthesis, and muscle hypertrophy. AMP-activated protein kinase (AMPK) is a cellular energy sensor, a negative regulator of mTOR, and an inhibitor of protein synthesis. The purpose of this study was to determine whether the hypertrophy/cell growth-associated mTOR pathway was downregulated during muscle atrophy associated with chronic paraplegia. Soleus muscle was collected from male Sprague-Dawley rats 10 wk following complete T(4)-T(5) spinal cord transection (paraplegic) and from sham-operated (control) rats. We utilized immunoprecipitation and Western blotting techniques to measure upstream [AMPK, Akt/protein kinase B (PKB)] and downstream components of the mTOR signaling pathway [mTOR, S6K1, SKAR, 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor (eIF) 4G and 2alpha]. Paraplegia was associated with significant soleus muscle atrophy (174 +/- 8 vs. 240 +/- 13 mg; P < 0.05). There was a reduction in phosphorylation of mTOR, S6K1, and eIF4G (P < 0.05) with no change in Akt/PKB or 4E-BP1 (P > 0.05). Total protein abundance of mTOR, S6K1, eIF2alpha, and Akt/PKB was decreased, and increased for SKAR (P < 0.05), whereas 4E-BP1 and eIF4G did not change (P > 0.05). S6K1 activity was significantly reduced in the paraplegic group (P < 0.05); however, AMPKalpha2 activity was not altered (3.5 +/- 0.4 vs. 3.7 +/- 0.5 pmol x mg(-1) x min(-1), control vs. paraplegic rats). We conclude that paraplegia-induced muscle atrophy in rats is associated with a general downregulation of the mTOR signaling pathway. Therefore, in addition to upregulation of atrophy signaling during muscle wasting, downregulation of muscle cell growth/hypertrophy-associated signaling appears to be an important component of long-term muscle loss.