Enzymes of transmitter and energy metabolism in rat middle ear and extraocular muscles

To further investigate the peculiar characteristics of the middle ear and extraocular muscles, compared to the extensively studied skeletal muscles of the limbs, activities of enzymes of transmitter and energy metabolism were measured in homogenates of these muscles from albino and pigmented rats. These activities were compared to those for a masticatory muscle and for three hindlimb muscles chosen for their preponderance of either slow oxidative, fast glycolytic, or fast oxidative glycolytic fibers. Activities of the neuromuscular transmitter enzymes choline acetyltransferase and acetylcholinesterase were relatively very high in the extraocular and middle ear muscles. The activity of malate dehydrogenase, an enzyme of oxidative energy metabolism, was as high in the extraocular, masticatory and stapedius muscles as in the oxidative hindlimb muscles, but was lower in tensor tympani. The activity of lactate dehydrogenase, an enzyme of glycolytic energy metabolism, was remarkably low in both middle ear muscles. The results are consistent with high innervation density in the extraocular and middle ear muscles, and highly oxidative metabolism in the extraocular and stapedius muscles. Metabolic differences between the stapedius and tensor tympani suggest a relatively more active role for the former in the function of the rat middle ear.