Mechanotherapy does not Enhance Recovery Following Muscle Disuse Atrophy in Female Rats.

BACKGROUND: Recovery from skeletal muscle atrophy is important for regaining function. We previously showed that mechanotherapy enhances regrowth of muscle in adult male rats by elevating protein synthesis.

OBJECTIVE: The purpose of this study was to determine if female rats also respond to mechanotherapy during regrowth after disuse-induced atrophy.

HYPOTHESIS: Mechanotherapy in female rats augments muscle regrowth after atrophy by enhancing protein synthesis.

METHODS: Adult (10 month) F344/BN female rats were randomly assigned to one of four groups: weight-bearing (WB; N=6), hindlimb suspension (HS; N=9), reambulation after HS without mechanotherapy (RA; N=9), or RA with mechanotherapy (RAM; N=10). HS lasted for 14 and RA for 7 days. Starting the day of RA, mechanotherapy was applied to the right gastrocnemius muscle at a load of 4.5N for 30 minutes every other day (4 bouts total). Rats received deuterium oxide (D2 O) for determination of myofibrillar protein and RNA synthesis (Ksyn) and degradation (Kdeg). We analyzed gastrocnemius muscles for myonuclear number (dystrophin-DAPI), satellite cell (SC) number (Pax7), and fiber type and cross-sectional area (CSA; laminin-myosin heavy chain). We used a one-way ANOVA to determine statistical significance at p<0.05. Values are mean ± SD.

RESULTS: Muscle wet weight (g) was lower with HS (933 ± 56) compared to WB (1140 ± 86) and greater in RA (1012 ± 64) and RAM (1030 ± 41) compared to HS. However, no difference was observed between RA and RAM. Mean fiber CSA (µm2 ) was significantly lower in HS (1877 ± 244) compared to WB (2395 ± 248), while RA (2124 ± 287) was not different from WB or HS. RAM CSA (2013 ± 182) was lower than WB and not different from HS or RA. Myofibrillar Ksyn (mg/day) was lower in HS (13 ± 2.1) compared to WB (22 ± 2.2). RA (32 ± 3.2) and RAM (30 ± 4.3) were higher than HS and WB, but no difference was observed between RA and RAM. Myofibrillar Kdeg (1/t ) was greater in HS (0.03 ± 0.004) compared to WB (0.02 ± 0.005), and RA (0.04 ± 0.007) and RAM (0.04 ± 0.005) were higher than HS and WB, but not different from each other. RNA Ksyn (%/day) was not different between WB (2 ± 0.2) and HS (2 ± 0.3), whereas RE (3 ± 0.4) and REM (4 ± 0.6) were both higher that WB and HS, but not different from each other. There were no differences in fiber type distribution between the groups. CSA of all fiber types was lower with HS, whereas no differences were observed between RA, RAM and HS. We did not find a significant difference in myonuclear number or centrally located nuclei between the groups. The number of SCs (per/fiber) was lower in HS (0.03 ± 0.01) and RA (0.04 ± 0.01) compared to WB (0.05 ± 0.008) and was higher in RAM (0.05 ± 0.02) compared to HS.

CONCLUSION: Muscle recovery from atrophy in female rats occurs with reambulation and elevated protein turnover is the underlying anabolic mechanism. We therefore conclude that the additional load using mechanotherapy does not enhance muscle recovery in females, which is different than the response previously published for males.