Biochemical properties of the superficial masseter muscle in the aging rat.

In young (15 weeks) and senescent (2 years) rats, the metabolic enzyme activities and the composition of myosin heavy chain (MHC) isoforms of the superficial masseter muscle were assessed by biochemical analysis. The succinate dehydrogenase activity and phosphofructokinase activity of the masseter muscle were significantly lower in senescent rats than in young rats. A large portion of the masseter muscle was composed of MHC IIb and IId isoforms in both young and senescent rats. As compared with young rats, no significant change in the composition of MHC isoforms was found in the masseter muscle of the senescent rats. These findings indicate that aging has a significant effect on the energy supply of the superficial masseter muscle.