RESPONSES OF SKELETAL MUSCLE SIZE AND ANABOLISM ARE REPRODUCIBLE WITH MULTIPLE PERIODS OF UNLOADING/RELOADING.

AIM: Mechanical unloading has long been understood to contribute to rapid and substantial adaptations within skeletal muscle, most notably muscle atrophy. Studies have often demonstrated that many of the alterations resulting from disuse are reversed with a reintroduction of load and have supported the concept of muscle plasticity. We hypothesized that adaptations during disuse and recovery was a repeatable/reproducible phenomenon, which we tested with repeated changes in mechanical load.

METHODS: Rats were assigned to one of the following five groups: animals undergoing one or two bouts of hindlimb unloading (28-d), with or without recovery (56-d), or control. Following the completion of their final time point, posterior crural muscles were studied.

RESULTS: Muscle sizes were lower following 28-d of disuse but fully recovered with a 56-d reloading period, regardless of the number of disuse/recovery cycles. Mixed protein fractional synthesis rates consistently reflected mass and loading conditions (supported by anabolic signaling); whereas the myofibrillar protein synthesis response varied among muscles. Amino acid concentrations were assessed in the gastrocnemius free pool, and did not correlate with muscle atrophy associated with mechanical unloading. Muscle collagen concentrations were higher following the second unloading period and remained elevated following 56-d of recovery.

CONCLUSION: Anabolic responses to alterations in load are preserved throughout multiple perturbations, but repeated periods of unloading may cause additive strain to muscle structure (collagen). This study suggests that while mass and anabolism are reproducibly reflective of the loading environment, repeated exposure to unloading and/or reloading may impact the overall structural integrity of muscle.