![]() ![]() Twitch force increased 44+/-8.7%, 47+/-7.4%, and 15+/-2.8% for the control, sham, and SCH groups, respectively, after the 1000-ms tetanic contraction. However, posttetanic potentiation observed after 100-, 500-, and 1000-ms tetanic (200 HZ) contractions was absent or drastically reduced after SCH. The time course and magnitude of staircase during stimulation at 5 HZ (for 21 s) was similar in the control, sham-operated, and SCH groups. Tetanic force was reduced by SCH, but twitch amplitude was not. In this study our purpose was to assess staircase and posttetanic potentiation in the rat gastrocnemius muscle in situ, 1 week after SCH. Spinal cord hemisection (SCH) results in atrophy of skeletal muscle and altered contractile properties. Therefore, we propose that IL-15Rα has a role in defining the phenotype of fast skeletal muscles in vivo. Consistent with a conserved functional role in humans, a genetic association was found between a SNP in the IL15RA gene and endurance in athletes stratified by sport. The alterations of physiological properties and increased resistance to fatigue in fast muscles are consistent with a shift toward a slower, more oxidative phenotype. Morphologically, fast muscles had a greater number of muscle fibers, smaller fiber areas, and a greater ratio of nuclei to fiber area. ![]() The molecular signature of these muscles included altered markers of mitochondrial biogenesis and calcium homeostasis. Fast muscles displayed fatigue resistance and a slower contractile phenotype. IL-15Rα-knockout (IL-15Rα-KO) mice ran greater distances and had greater ambulatory activity than controls. Here, we have shown that loss of IL-15Rα induces a functional oxidative shift in fast muscles, substantially increasing fatigue resistance and exercise capacity. IL-15 and IL-15Rα have been implicated in muscle phenotypes, but a role in muscle physiology has not been defined. ![]() However, IL-15Rα is not merely an IL-15 receptor subunit, as mice lacking either IL-15 or IL-15Rα have unique phenotypes. IL-15 receptor α (IL-15Rα) is a component of the heterotrimeric plasma membrane receptor for the pleiotropic cytokine IL-15.
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