Muscle fibers of mdx mice are more vulnerable to exercise than those of normal mice.

It is well known that eccentric exercise induces muscle damage by disrupting the sarcolemma. The aim of this study was to analyze the effects of downhill running on several locomotor and respiratory muscles of normal and mdx mice. Degenerating muscle fibers in the skeletal muscles of mice were visualized by in vivo staining with Evans blue. This dye injected intravenously stained only degenerating muscle fibers which were visible as blue fibers macroscopically and could also be seen as red fluorescent fibers microscopically. Evans blue-stained muscle fibers were either hypercontracted or degenerating. Without exercise no muscle fibers were labeled with Evans blue in the normal mice, indicating that their membranes were intact. However, even without exercise, the percentage of fibers permeable to Evans blue varied from 2% to 15% in various muscles of the mdx mice. Our downhill running protocol (i.e., running down a treadmill with a 15 degrees slope at 10 m/min) produced in normal mice only a slight (0-3%) increase in percentage of muscle fibers which were permeable to the dye compared with up to 31% in some mdx muscles.