Differential Response of Muscle Fibers and Interfiber Area to In- creased and Decreased Neuromuscular Activity Levels

The purpose of the present study was to determine the relationship between the adaptations in muscle fiber size and the connective tissue content (interfiber area) in atrophying and hypertrophying skeletal muscles. Hypertrophy was induced in the cat plantaris muscle via functional overload (FO,12 weeks) induced via the removal of its major synergists. Atrophy was induced in the cat soleus muscle via a complete low-thoracic spinal cord transection (ST, 26 weeks) or spinal cord isolation of the lumbar region of the spinal cord, i.e. complete cord transections at a low thoracic and a high sacral level and bilateral deafferentation between the two transection sites (SI, 26 weeks). In addition, atrophy was induced in the rat soleus and tibialis anterior muscles via spaceflight (2 weeks) or hindlimb unloading (HU, 2 weeks). Fresh frozen tissue cross sections (20 μm thick) were stained for concanavalin A and muscle fiber area, interfiber area per fiber and the ratio of muscle fiber area : interfiber area were determined for representative regions of each muscle. FO resulted in a 133 and 139% increase in fiber area and interfiber area, respectively. In contrast, ST and SI resulted in 32 and 72% decreases in fiber area, whereas the interfiber area was unchanged. Thus, the muscle fiber area : interfiber area ratio was decreased by 37 and 69% in the soleus muscle of the ST and SI cats, respectively. For the rat soleus, there was a 39 and 28% decrease in fiber area, a 20 and 23% decrease in interfiber area, and a 24 and 10% decrease in muscle fiber area : interfiber area ratio in spaceflight and HU rats, respectively. Combined these data indicate that the relative non-muscle:muscle tissue content is maintained in hypertrophied muscles, whereas there is a disproportional loss of muscle tissue compared to nonmuscle tissue in atrophied muscles. Based on these adaptations in the connective tissue and muscle tissue elements, the strain per unit of active force would decrease in atrophied muscle and remain at control levels in hypertrophied muscle.