Figure 1: Example of running behavior of a normal and myotonic mouse during all of week 8. The distance run is quantified for each 10 min interval as a function of time during the day. Each day depicted is from 8 am one morning until 8 am the following morning.

Mentions: Examples of the temporal pattern of running over the course of each day during the eighth week are depicted in Figure 1. Both normal and myotonic mice did the vast majority of their wheel running between 8 pm and 8 am, as expected for nocturnal animals. Normal mice almost always ran for multiple consecutive 10-min periods, typically 15–20 consecutive periods at the onset of the night, and then fewer consecutive periods as the night went on. In contrast, myotonic mice only ran only a few times per night, and for short periods of time per episode (typically no more than three consecutive 10-min periods).

van Lunteren E, Moyer M, Cooperrider J, Pollarine J - Front Physiol (2011)

Bottom Line: During the eighth week, myotonic mice were running significantly less than normal mice (322 ± 177 vs 5058 ± 1253 m/day, P = 0.025).Furthermore, there were considerable reductions in consecutive running times (18.8 ± 1.5 vs 59.0 ± 3.7 min, P < 0.001) and in the distance per consecutive running period (58 ± 38 vs 601 ± 174 m, P = 0.048) in myotonic compared with normal animals.Conclusion/Significance: These findings indicate that CLC-1 chloride deficient myotonia in mice markedly impairs spontaneous exercise activity, with reductions in both total distance and consecutive running times.

Affiliation: Cleveland Department of Veterans Affairs Medical Center Cleveland, OH, USA.

Abstract: Background: Genetic deficiency of the muscle CLC-1 chloride channel leads to myotonia, which is manifested most prominently by slowing of muscle relaxation. Humans experience this as muscle stiffness upon initiation of contraction, although this can be overcome with repeated efforts (the "warm-up" phenomenon). The extent to which CLC-1 deficiency impairs exercise activity is controversial. We hypothesized that skeletal muscle CLC-1 chloride channel deficiency leads to severe reductions in spontaneous exercise. Methodology/Principal Findings: To examine this quantitatively, myotonic CLC-1 deficient mice were provided access to running wheels, and their spontaneous running activity was quantified subsequently. Differences between myotonic and normal mice in running were not present soon after introduction to the running wheels, but were fully established during week 2. During the eighth week, myotonic mice were running significantly less than normal mice (322 ± 177 vs 5058 ± 1253 m/day, P = 0.025). Furthermore, there were considerable reductions in consecutive running times (18.8 ± 1.5 vs 59.0 ± 3.7 min, P < 0.001) and in the distance per consecutive running period (58 ± 38 vs 601 ± 174 m, P = 0.048) in myotonic compared with normal animals. Conclusion/Significance: These findings indicate that CLC-1 chloride deficient myotonia in mice markedly impairs spontaneous exercise activity, with reductions in both total distance and consecutive running times.