Biochemistry of the anterior, medial, and posterior genioglossus in the aged rat.

Age-related tongue weakness may contribute to swallowing deficits in the elderly. One contributing factor may be an alteration in muscle-fiber-type properties with aging. However, it is not clear how muscle fiber types within the aged tongue may vary from those found in young adults, or how fiber types may vary across the anteroposterior axis of the extrinsic tongue muscles. We examined the myosin heavy chain (MHC) composition of anterior, medial, and posterior sections of the genioglossus muscle (GG) in ten old male Fischer 344/Brown Norway rats and compared findings to previously reported data from young adult male rats. Significant differences (p < 0.01) between young adult and old rats were found in the distribution of MHC isoforms along the anteroposterior axis of the muscle. In the anterior, medial, and posterior regions, there was a significantly smaller proportion of type IIb MHC in the old rat GG muscles, while the proportion of type IIx MHC was significantly greater. In the medial region, the proportion of type I MHC was found to be significantly greater in the old rats. Thus, we found a shift to more slowly contracting muscle fibers in the aged rat tongue.