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Department of Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin 53226
Sarcomere
lesions were previously observed with reloading of rat adductor longus
muscles after spaceflight and hindlimb unloading (HU).
Spaceflown rats displayed more lesioned fibers in the
"slow-fiber" region, suggesting a damage-susceptible fiber type.
Unloading induces fast myosin expression in some slow fibers,
generating hybrid fibers. We examined whether lesion damage differed
among slow-, hybrid-, and fast-fiber types in HU-reloaded adductor
longus muscles. Temporal HU for 5, 8, 11, 14, and 17 days revealed that hybrid fiber percent, detected by antimyosin immunostaining, peaked at
29 ± 12% by 14 days. A 14-day HU followed by 12-14 h of
voluntary reloading was performed to induce lesions.
2 analysis showed that slow
fibers were preferentially damaged, accounting for 92 ± 5% of
lesioned fibers; hybrid and fast fibers accounted for 7 ± 4 and
<0.5%, respectively. Atrophy did not explain differential lesion
damage across fiber types, as slow and hybrid fibers atrophied to a
similar extent. Because active myofiber contractions are requisite for
lesion formation, selective recruitment of slow fibers most likely
explains their damage susceptibility.
hindlimb unloading; voluntary reloading; eccentric contraction; hybrid fibers; recruitment
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