The impact of muscle disuse on muscle atrophy in severely burned rats

Abstract

Background: Severe burn induces a sustained hypermetabolic response, which causes long-term loss of muscle mass and decrease in muscle strength. In this study, we sought to determine whether muscle disuse has additional impact on muscle atrophy after severe burn using a rat model combining severe cutaneous burn and hindlimb unloading.

Methods: Forty Sprague-Dawley rats (≈ 300 g) were randomly assigned to sham ambulatory (S/A), sham hindlimb unloading (S/HLU), burn ambulatory (B/A), or burn hindlimb unloading (B/HLU) groups. Rats received a 40% total body surface (TBSA) full thickness scald burn, and rats with hindlimb unloading were placed in a tail traction system. At d 14, lean body mass (LBM) was determined using DEXA scan, followed by measurement of the isometric mechanical properties in the predominantly fast-twitch plantaris muscle (PL) and the predominantly slow-twitch soleus muscle (SL). Muscle weight (wt), protein wt, and wet/dry wt were determined.

Results: At d 14, body weight had decreased significantly in all treatment groups; B/HLU resulted in significantly greater loss compared with the B/A, S/HLU, and S/A. The losses could be attributed to loss of LBM. PL muscle wt and Po were lowest in the B/HLU group (<0.05 versus S/A, S/HLU, or B/A). SL muscle wt and Po were significantly less in both S/HLU and B/HLU compared with that of S/A and B/A; no significant difference was found between S/HLU and B/HLU.

Conclusions: Cutaneous burn and hindlimb unloading have an additive effect on muscle atrophy, characterized by loss of muscle mass and decrease in muscle strength in both fast (PL) and slow (SL) twitch muscles. Of the two, disuse appeared to be the dominant factor for continuous muscle wasting after acute burn in this model.

Figure 1

S/A: Sham Ambulation; S/HLU: Sham Hindlimb Unloading; B/A: Burn Ambulation; B/HLU: Burn Hindlimb Unloading. Significant difference: #: S/A vs. B/A; ‡: S/A vs. S/HLU; †: S/HLU vs. B/HLU; *: B/A vs. B/HLU; (1): p<0.05; (2): p<0.01; (3): p<0.001. 1-A: body weight at day 14. B/HLU was significantly lower than S/HLU and B/A. S/HLU, and B/A was significantly lower than S/A. 1-B: Percentage change of body weight from the original body weight. B/HLU was significantly less than S/HLU and B/A. S/HLU, and B/A was significantly less than S/A. 1-C: Daily Average Food Consumption per Gram of Body Mass. B/HLU was significantly greater than S/HLU. B/A was significantly greater than S/A.

Figure 1

S/A: Sham Ambulation; S/HLU: Sham Hindlimb Unloading; B/A: Burn Ambulation; B/HLU: Burn Hindlimb Unloading. Significant difference: #: S/A vs. B/A; ‡: S/A vs. S/HLU; †: S/HLU vs. B/HLU; *: B/A vs. B/HLU; (1): p<0.05; (2): p<0.01; (3): p<0.001. 1-A: body weight at day 14. B/HLU was significantly lower than S/HLU and B/A. S/HLU, and B/A was significantly lower than S/A. 1-B: Percentage change of body weight from the original body weight. B/HLU was significantly less than S/HLU and B/A. S/HLU, and B/A was significantly less than S/A. 1-C: Daily Average Food Consumption per Gram of Body Mass. B/HLU was significantly greater than S/HLU. B/A was significantly greater than S/A.

Figure 2. Lean Body Mass

S/A: Sham Ambulation; S/HLU: Sham Hindlimb Unloading; B/A: Burn Ambulation; B/HLU: Burn Hindlimb Unloading. Significant difference: #: S/A vs. B/A; ‡: S/A vs. S/HLU; †: S/HLU vs. B/HLU; *: B/A vs. B/HLU; (1): p<0.05; (2): p<0.01; (3): p<0.001. B/HLU was significantly less than S/HLU and B/A. S/HLU and B/A were significantly less than S/A.