D. H. Robinson, G. E. McCall, FACSM

University of Puget Sound, Tacoma, WA

Skeletal muscle hypertrophy occurs in response to increased loading. In order for hypertrophy to occur, the basal lamina, the component of the extracellular matrix that directly surrounds myofibers, undergoes remodeling. This remodeling process requires the enzymatic breakdown of extracellular proteins. Matrix Metalloproteinase-9 (MMP-9) may play a vital role in remodeling the basal lamina via its ability to degrade type IV collagen, one of the most abundant proteins comprising the basal lamina. PURPOSE: To determine the myofiber hypertrophy response to functional overload of the plantaris muscle in Wild Type (WT) and MMP-9 knock out (KO) mice. METHODS: WT (n = 4) and MMP-9 KO (n = 3) mice underwent two days of plantaris functional overload via synergistic muscle ablation. Immunohistochemistry for myosin heavy chain phenotype was performed and mean CSA (n = 49-53 myofibers per plantaris) was determined for type IIa, IIb, and IIx myofibers. Independent t-tests were used to compare mean CSA for each fiber type. RESULTS: No differences were found between WT and KO myofiber CSA (p > 0.37). The mean CSAs for WT mice IIa, IIb, and IIx myofibers were 925, 1759, and 1313 µm², respectively, while the mean CSAs for KO mice IIa, IIb, and IIx myofibers were 850, 1410, 1229 µm², respectively. CONCLUSION: After two days of plantaris functional overload, mean myofiber CSAs of WT mice were similar to KO mice. However, with an increase in sample size and statistical power, the differences between WT and KO myofiber CSA may achieve significance. These preliminary data may indicate that MMP-9 is important for an optimal hypertrophy response due to MMP-9’s role in either remodeling of the ECM, or possibly other actions.

Supported by University of Puget Sound UEC student research grant.

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