Metabolic transitions in rat jaw muscles during postnatal development.

The program of acquisition of adult metabolic phenotypes was studied in three jaw muscles in order to determine the link between muscle metabolism and functional development. During early postnatal stages, there were similar transitions in the masseter, anterior digastric, and internal pterygoid muscles with respect to fiber growth, fiber type composition, and whole muscle energy metabolism. Oxidative capacity, as judged by the activities of the enzymes succinate dehydrogenase (SDH), malate dehydrogenase (MDH), and beta-hydroxyacyl CoA dehydrogenase (beta OAC), rose sharply after birth to reach near maximal levels by 3 weeks. The capacities for glycolytic metabolism represented by lactate dehydrogenase (LDH), and for high-energy phosphate metabolism represented by adenylokinase (AK) and creatine kinase (CK) activities, rose gradually, not reaching peak values until 6 weeks or later. Thus, the maturation of oxidative metabolism preceded that of glycolytic metabolism in the developing jaw muscles. This was documented for individual fibers in the masseter muscle. Differential metabolic maturation among the jaw muscles was evident beyond 3 weeks. All three jaw muscles attained their specific adult fiber-type profile by about 6 weeks. This maturation program differed from that of hindlimb muscles [Nemeth et al., J Neurosci 9:2336-2343, 1989] and diaphragm muscle [Kelly et al., J Neurosci 11:1231-1242, 1991], reflecting their differential energy demands for contractile performance.