Differences in cytochrome c oxidase capacity in smooth muscle of opossum esophagus and lower esophageal sphincter.

The tonic contraction that defines the lower esophageal sphincter is exclusively aerobic, whereas the rhythmic contractions that characterize the smooth muscle of the esophageal body can be partially sustained anaerobically. Also, relative mitochondrial mass is greater in the sphincter than it is in the body of the esophagus. Full-thickness strips of the muscularis propria of the opossum esophagus were examined as to cytochrome c oxidase capacity to test the hypothesis that differences in oxidative metabolism exist among different levels of the smooth-muscled esophagus. Homogenates were examined as to rate of cytochrome c oxidation, deoxyribonucleic acid content, and protein content. The rate of cytochrome c oxidation was greatest at the top of the smooth muscle segment and declined distally to reach the lowest value in the lower esophageal sphincter. Deoxyribonucleic acid content did not differ among levels examined. Protein/deoxyribonucleic acid ratios were lower in the sphincter than at other levels of the smooth-muscled esophagus. The results indicate that the sphincter muscle has a lower capacity for oxidative metabolism than does the body, that a gradient exists along the esophageal body in oxidative capacity, and that smooth muscle cells in the sphincter are likely to be smaller than are those of the esophageal body. The gradient in oxidative capacity along the esophageal body may be related to other gradients that have been described in this region.