Differential sensitivity of muscle fiber types to hypoxia-induced muscle atrophy in rats relieved by 4-week resistance training

Background Hypoxia can lead to loss of skeletal muscle mass, and the sensitivity of different fiber types to hypoxia may vary.

Objective This study intends to explore the effects of resistance training on alleviating hypoxia-induced muscle atrophy of different fiber types in rats.

Methods Forty male SD rats were randomly divided into a normoxic control group, a normoxic training group, a hypoxic control group, and a hypoxic training group. The training groups took incremental weight-bearing ladder training every other day; the hypoxia groups were placed in a hypoxic chamber with an oxygen concentration of 12.4%. After 4 weeks, body composition was tested by dual-energy X-ray (DEXA); wet weights of soleus (SOL), gastrocnemius (GAS), extensor digitorum longus (EDL), and musculus biceps brachii (MBB) were weighed; muscle fiber morphology was observed after HE staining; muscle fiber cross-sectional area (FCSA) was measured after immunofluorescent staining of laminin; expressions of myosin, muscle-specific ring finger protein 1 (MuRF1), and muscle atrophy F-box protein (Atrogin1) were tested by Western blotting.

Results During the 4-week intervention, the body weight of each group was continuously increased (P < 0.05). After the intervention, the weight of the hypoxia control group(341.20±16.75) g was lower than that of the normoxic control group(377.50±10.75) g (P < 0.05); the percentage of lean body mass (LBM%) in the normoxic training group(72.54±2.09)% was higher than that of the normoxic control group(69.19±4.67)%, the LBM% in the hypoxic control group(67.08±2.55)% was lower than that in the normoxic control group, and the LBM% in hypoxic training group(70.90±1.24)% was higher than that in the hypoxic control group (P < 0.05). The percentages of wet muscle mass (WMM%) of GAS, EDL, and MBB of the hypoxic control group(6.29±0.31)%, (6.12±0.24)%, and (6.31±0.23)% respectively were lower than those of the normoxic control group(6.67±0.42)%, (6.55±0.23)%, and (6.63±0.37)% respectively; the WMM% of EDL of the normoxic training group(6.96±0.21)% was higher than that of the normoxic control group; the WMM% of EDL of the hypoxic training group(6.49±0.28)% was higher than that of the hypoxic control group (P < 0.05). The FCSAs of GAS, EDL, and MBB in the hypoxic control group were lower than those in the normoxic control group (reduced by 10.4%, 12.0%, and 10.6%, respectively), and the FCSA of EDL in the hypoxic training group was higher than that in the hypoxic control group (reduced by 11.4%) (P < 0.05). The myosin protein relative expression levels in GAS and EDL in the hypoxic control group were lower than those in the normoxic control group (GAS: 0.68±0.25 vs. 1; EDL: 0.75±0.15 vs. 1), and the level in MBB in the hypoxic training group was higher than that in the hypoxic control group (1.26±0.12 vs. 1.01±0.15) (P < 0.05). The MuRF1 protein expression levels in SOL, GAS, and EDL in the hypoxic control group were higher than those in the normoxic control group (SOL: 1.26±0.18 vs. 1; GAS: 1.47±0.21 vs. 1; EDL: 1.27±0.14 vs. 1), and the level in EDL in the hypoxic training group was lower than that in the hypoxic control group (0.74±0.11 vs. 1.27±0.14) (P < 0.05). The Atrogin1 protein relative expression levels in SOL, GAS, and EDL in the hypoxic control group were higher than those in the normoxic control group (SOL: 1.26±0.10 vs. 1; GAS: 1.36±0.21 vs. 1; EDL: 1.30±0.22 vs. 1), and the levels in SOL and EDL in the hypoxic training group were lower than those in the hypoxic control group (SOL: 0.89±0.14 vs. 1.26±0.10; EDL: 0.73±0.14 vs. 1.30±0.22) (P < 0.05).

Conclusion The designed 4-week resistance training could effectively alleviate rat skeletal muscle atrophy caused by hypoxia, particularly on EDL.