We examined the effect of 28 days of overload on mammalian target of rapamycin (mTOR) and extracellular signal–regulated kinase (ERK) signaling in young adult (Y; 6-month old) and aged (O; 30-month ...old) Fischer 344 × Brown Norway rats subjected to bilateral synergist ablation (SA) of two thirds of the gastrocnemius muscle or sham surgery (CON). Although plantaris (PLA) muscle hypertrophy was attenuated by aging, mTOR phosphorylation was 44% and 35% greater in Y SA and O SA compared with CON (p = .038). Ribosomal protein S6 phosphorylation was 114% and 24% higher in Y SA and O SA compared with CON (p = .009). Eukaryotic initiation factor 2Bϵ phosphorylation was 33% and 9% higher in Y SA and O SA compared with CON (p = .04). Translational signaling in young adult and aged plantaris muscle is equally responsive to chronic overload.
The purpose of this study was to compare the extent of neural activation assessed by the central activation ratio (CAR) versus activation estimated from T2 magnetic resonance imaging (MRI) and ...neuromuscular electrical stimulation (NMES).
Seven college-age individuals volunteered for this study. CAR was determined by manually superimposing a train of NMES (50 Hz, 450-mus biphasic pulses) for 1 s during a maximal voluntary effort. The MRI-NMES method assessed activation by stimulating the knee extensors for 3 min in a 2 s on, 2 s off cycle. T2 MR images were taken at rest and after NMES was administered. Theoretical maximal torque (TMT) of the knee extensors was calculated based on the MRI-NMES activation data. The TMT was then divided by the maximal voluntary isometric contraction (MVIC) of each subject to determine the extent of neural activation during a MVIC.
The results for CAR reveal the percent activation (mean +/- SD) of the quadriceps femoris during a MVIC was 92 +/- 7% for the right thigh and 96 +/- 4% for the left thigh. The MRI-NMES method estimated that MVIC could be achieved if 75 +/- 14% of the knee extensors on the right thigh and 74 +/- 14% on the left thigh were activated. These results are similar to findings that showed MVIC could be achieved by stimulating 71% of the knee extensors.
We conclude that CAR overestimates the extent of neural activation during an MVIC because the 3D shape of the thigh is altered. This will change electric current flow to the axonal motor neuron branches and limit the artificially evoked torque, thereby resulting in an overestimation of CAR.
This study examined the effects of altering surface neuromuscular electrical stimulation (SNMES) parameters on the specific tension of the quadriceps femoris muscle. Seven able-bodied subjects had ...magnetic resonance images taken of both thighs prior to and immediately after four SNMES protocols to determine the activated muscle cross-sectional area (CSA). The four protocols were: (1) research (RES, 100 Hz, 450 micros, and amplitude set to evoke 75% of maximal voluntary isometric torque, MVIT); (2) pulse duration (PD, 100 Hz, 150 micros, same current as in RES); (3) frequency (FREQ, 25 Hz, 450 micros, and same current as in RES); (4) amplitude (AMP, 100 Hz, 450 mus, and current set to evoke the average of the initial torques of PD and FREQ, 45 +/- 9% of MVIT). Reducing the amplitude of the current from 75 to 45% of MVIT did not alter specific tension, 25 +/- 8 N/cm2, suggesting that the amplitude probably affects torque and the area of activated muscle proportionally. Shortening the pulse duration from 450 to 150 micros caused specific tension to drop from 25 +/- 6 to 20 +/- 6 N/cm2 (P < 0.05), indicating that pulse duration increased torque and the activated CSA disproportionally. Alternatively, reducing the frequency from 100 to 25 Hz decreased specific tension from 25 +/- 6 to 17 +/- 4 N/cm2 (P < 0.05), suggesting that the frequency increased torque without affecting the activated CSA. Clinicians who administer SNMES should be aware of the magnitude of adaptations to a given amplitude, pulse duration, and frequency.
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The purpose of this study was to assess activation of the mTOR signaling pathway in young and aging rats in response to chronic muscle overload. Young (6 mo; n = 16) and older (30 mo; n ...= 23) male rats (F344×BN) were subjected to 4 weeks of bilateral surgical ablation (SA) of two‐thirds of the gastrocnemius muscle to promote compensatory hypertrophy in the plantaris (PLA) and soleus (SOL) or a sham surgery (CON). A significant interaction (p= 0.00) was found between group (SA vs. CON) and age (6mo. vs. 30mo.) when examining the normalized muscle weights (muscle weight/final body weight) of the PLA and SOL. The PLA muscle mass (mean ± SD) following SA was heavier in young rats when compared to aging rats (1.34 ± 0.07 g muscle weight/g body weight vs. 0.79 ± 0.08; p=0.00). An equivalent pattern was observed in the SOL in response to SA (0.52 ± 0.04 vs. 0.34 ± 0.04; p=0.00). Phosphorylation of p70
S6K
in the SOL and PLA was similar in both young and aging rats when comparing SA (n=4) versus CON (n=4) (SOL: 6 mo; 0.21 ± 0.14 A.U. vs. 0.10 ± 0.10, 30mo; 0.20 ± 0.19 vs. 0.10 ± 0.06, PLA: 6 mo; 0.04 ± 0.02 vs. 0.02 ± 0.02, 30 mo; 0.06 ± 0.06 vs. 0.05 ± 0.01). These data indicate that aging muscle in rats is less responsive to hypertrophy in the PLA and SOL, which may be caused by impairment in mTOR signaling.
Supported by USDA 58‐1950‐7‐707 and NIA AG‐25270
Abstract only
We examined activation of the mammalian target of rapamycin (mTOR) and mitogen‐activated protein kinase (MAPK) signaling pathways in adult (Y; 6 mo old; n = 16) and aged (O; 30 mo old; ...n = 16) male rats (Fischer 344 x Brown Norway) subjected to chronic overload‐induced muscle hypertrophy of the plantaris (PLA) and soleus (SOL) muscles. Animals underwent either 4 weeks of bilateral surgical ablation (SA) of two‐thirds of the gastrocnemius muscle or sham surgery (CON). Animals were subjected to SA to promote compensatory hypertrophy in the PLA and SOL muscles. Muscle weights of the Y CON (434 ± 39 mg; mean ± SD; p < 0.05) and Y SA (506 ± 56 mg) rats were significantly heavier than the O CON (353 ± 18 mg) and O SA (402 ± 41 mg) rats. There was a significant interaction between group and age for phosphorylated mTOR. Phosphorylated mTOR was higher in the Y SA group (4129 ± 674 A.U.; mean ± SE; p < 0.05) than in the Y CON (1582 ± 674), O SA (2063 ± 630) and O CON (2667 ± 728) groups. The ratio of phosphorylated to total mTOR was significantly higher in both SA groups than in their respective controls (0.107 ± 0.105 A.U. vs. ‐0.297 ± 0.112; p < 0.05). We also observed a significant effect of age on total p44 MAPK. It was higher in the O (17969 ± 1242; p < 0.05) than the Y (13342 ± 1242) rats. These data indicate mTOR signaling is attenuated in an aging animal model of chronic overload‐induced muscle hypertrophy. Supported by USDA 58‐1950‐7‐707, NIA AG‐25270 and DK007651.
Thesis (M.S.)--University of Georgia, 2006.
Directed by Gary A. Dudley. Includes an article submitted to Archives of physical medicine and rehabilitation. Includes bibliographical references (leaves ...70-75).
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