Effects of a Bout of Downhill Running on Skeletal Muscle Function and Ca²⁺ Handling in Mouse Extensor Digitorum Longus Muscle

Abstract

Introduction: Exercise training can increase muscle mass and improve function. However, acute unaccustomed exercise often induces muscle damage and impaires muscle strength. Although the decrease of muscle strength after exercise is due to many factors, the disruption of Ca²⁺ homeostasis also may be a factor of the muscle weakness, because intramuscular Ca²⁺ transients are important for forming myosin-actin cross-bridge, which drives muscle contraction. Here, we investigated the effects of a bout of downhill running on muscle function and intracellular calcium handling in mouse extensor digitorum longus (EDL) muscle.

Methods: Male C57BL6J mice (8-week-old) performed one session of exercise, which consisted of downhill running on a treadmill (-16% grade) at 22 m/min for two hours. Before exercise, and 24 and 48 hours after the completion of exercise session, the EDL muscles were excised from the mice. Whole EDL muscles mounted in an organ bath containing a Krebs-Ringer buffer were electrically stimulated to measure muscle contractile force ex vivo. Single EDL muscle fibers dissociated by enzymatic digestion were stained with calcium indicators, and pharmacologically stimulated to measure resting Ca²⁺ levels and Ca²⁺ transients by fluorescence imaging.

Results and Discussions: Twenty-four hours after the completion of downhill running, plasma creatine kinase and lactate dehydrogenase activities were significantly increased from baseline (p<0.05), and retuned to baseline 48 h after exercise. On the other hand, in hematoxylin-eosin stained cross sections, few damaged fibers were observed after downhill running. These data showed that the exercise protocol in this study did not induce visible muscle damage in EDL muscles. Specific force of EDL muscle significantly decreased 48 h after downhill running compared with baseline (p<0.05), but no effects on muscle fatigue properties were found. After downhill running, some muscle cells with the elevation of resting Ca²⁺ levels were found, but average values of resting Ca²⁺ level in muscle cells did not change after exercise. Previous study has shown that acute downhill running increased resting Ca²⁺ level in mouse EDL muscles with muscle damage. These data suggests that resting Ca²⁺ levels might be affected by the degree of muscle damage after exercise. The elevations of cytosolic Ca²⁺ stimulated by potassium and caffeine were significantly lower in muscle cells after downhill running compared with intact muscle cells (p<0.05), suggesting that downhill running impaired Ca²⁺ transients evoked by a stimulus. Since muscle contraction force depends on cytosolic calcium levels, our data might suggest that less Ca²⁺ transient levels relates to decreased muscle strength after downhill running.

In conclusions, a bout of downhill running induces impaired calcium release to cytosol, which may relates to muscle weakness.