WILLIAMS, K.L.; ASHLEY-ROSS, M.A.*: Effects of metamorphosis and training on fiber types of hindlimb muscles in the tiger salamander

Both endurance training and aging have been shown to alter fiber types in human athletes. Salamanders are an excellent model to parse these two effects on muscle fiber types due to the obvious metamorphosis from aquatic larva to terrestrial adult. Tiger salamanders, Ambystoma tigrinum, were procured as larvae and assigned to one of four groups. Half of the animals were induced to metamorphose by addition of thyroxine to their aquarium water, while the other half were left untreated. Within those two groups, half of the salamanders were trained to walk for 20 minutes at a time, three times per week, while the other half were untrained. Animals were trained for 8 weeks. Hindlimb muscles were removed, frozen in isopentane cooled in liquid nitrogen, and subsequently sectioned on a cryostat. Serial sections were stained for myofibrillar ATPase (acid preincubation, pH 4.6) and succinate dehydrogenase. In general, muscle fibers from metamorphosed salamanders stained more darkly than those from larvae for both mATPase and SDH, indicating slower myosin and more oxidative capacity. However, training also exerts a distinct effect, in that trained metamorphs had fibers of a range of sizes in their muscles (some large, some small), while untrained metamorphs had only small fibers in their hindlimb muscles. Training also engendered an unexpected result in the larvae: the majority of muscle fibers from trained larvae had prominent voids in their interiors, making the fibers resemble toroids. Neither untrained larvae nor untrained metamorphs had toroid-shaped muscle fibers. Some muscle fibers from trained metamorphosed animals contained voids, though they constituted a small percentage of the total number of fibers. Thus, the voids in muscle fibers of trained animals may be related to the hypertrophic response.