Sarcomere lesion damage occurs mainly in slow fibers of reloaded rat adductor longus muscles

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. chi2 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.