HORSE SPECIES SYMPOSIUM: The aging horse: Effects of inflammation on muscle satellite cells.

With improvements in care, the equine population is living longer, remaining active, and competing at increasingly older ages. Both advancing age and exercise result in increased concentrations of circulating and local cytokines, including IL-1β, IL-6, and tumor necrosis factor-α. Athletic endeavors in the aged horse may further increase the proinflammatory environment in muscle, decreasing the ability to react appropriately to exercise. Poor response to exercise limits the athletic ability of geriatric horses, thus reducing their useful life span and potentially increasing the risk of injury. Satellite cells are muscle stem cells that reside adjacent to muscle fibers in skeletal muscle and are at least partially responsible for maintenance of muscle mass and muscle hypertrophy. In the adult animal, these cells normally exist in a quiescent state, becoming active, proliferating, and differentiating in response to specific stimuli. Growth factors and cytokines present during hypertrophy and following exercise affect satellite cell activity. Whereas the specific effects of cytokines on equine satellite cells are not well established, cytokines can influence satellite cell and myoblast proliferation and differentiation both positively and negatively. Understanding the effects of cytokines on equine satellite cell function will provide insight into the mechanisms responsible for the poor response to exercise in the aged horse. The proinflammatory environment in aged horses may inhibit exercise induced satellite cell activity, thereby diminishing exercise-induced hypertrophy. As more horses are surviving and competing into their 20s, more research is required to understand the response of these animals to exercise during normal aging.