Constancy of masseter muscle structure and function with age in F344 rats.

Senile muscle atrophy is a characteristic feature of advancing age. Despite the growing body of knowledge about weight-bearing muscles in rodents and man, there is relatively little such information on the muscles of mastication. Therefore, the primary goal here was to develop a masseter muscle preparation from male Fischer 344 rats suitable for studying contractile characteristics in vitro. And, secondarily, the goal was to examine this preparation for evidence of age-related changes in muscle composition and function in rats aged 4--24 months. Histochemical analysis of the composition of the four anatomical regions (branches) of the masseter revealed a mixture of rapidly contracting, fatiguable and fatigue-resistant muscle fibres with no significant differences between branches. Fibre type and size were determined with myosin ATPase, NADH-TR and toluidine blue staining of quick- frozen muscle sections. No significant changes in fibre type or fibre area were found with increasing age. One branch of the masseter, the anterior deep masseter, is sufficiently thin (less than 0.8 mm thickness) for adequate diffusion of oxygen and nutrients in studies of isometric contractile properties in vitro. Contraction time, half-relaxation time, dry weight:wet weight ratio and maximum force per unit area were found to be similar in muscles of young and old rats. Our results demonstrate that the anterior deep masseter of the rat is a suitable preparation for investigating masseter function in vitro. The surprising absence of age-related changes in composition and function is consistent with some, but not all, data on ageing rodent limb muscles. The results suggest that masticatory muscle performance is preserved with age in rats.