Skeletal muscle function and structure change as we age. Humans typically experience a loss of muscle mass or muscle weakness which can greatly reduce mobility and stability. While much is known about aging skeletal muscle in humans and rodents, less is known about horses, which are rather athletic animals that are living longer due to advancements in veterinary care and retirement programs. Researchers from the University of Florida decided to explore how aging effects skeletal muscles of horses. To do this, they examined gluteus medius (speed and locomotion) and triceps brachii (mainly postural) muscles in sedentary Quarter Horses aged 2 years old (young) or 17-25 years old (i.e. aged). Their results were recently published in the Journal of Applied Physiology.
The team characterized the fiber type of each muscle as well as the number and function of mitochondria. With aging in humans and rodents, there is a decline in the function and/or number of these energy producing organelles. This video is a nice review of the muscle fiber types and role of mitochondria:
Examining muscle biopsies, the research team found that the number of mitochondria were decreased in the triceps, but not gluteal muscles with aging. They also found changes in the muscle fiber type with aging. A high proportion of type II fibers (like in gluteus muscle) gives horses their speed whereas more type I fibers play a role in maintaining posture (like in triceps). With aging, the gluteus muscle develops more type I fibers and the triceps muscle develops more type IIA fibers with less IIX fiber types in both muscles. Although the activities of some mitochondrial enzymes also declined with aging, it was not associated with a decline in mitochondrial function. The authors attributed this lack of measurable mitochondrial dysfunction to either the horses not being old enough for dysfunction to exist yet (20 years horse = about 65 years human) or perhaps that aging in horse skeletal muscle is different from that of rodents and humans. Either way, understanding how our equine friends age is fascinating.
Li C, White SH, Warren LK, Wohlgemuth SE. Effects of aging on mitochondrial function in skeletal muscle of Quarter Horses. Journal of Applied Physiology. Article in press June 9, 2016. doi: 10.1152/japplphysiol.01077.2015