Skeletal muscle fiber has a great ability to hypertrophy during growth and in response to exercise stimuli, and atrophy during aging and in response to disuse. Because the muscle fiber is a multinucleated cell, the region of cytoplasm governed by a single myonucleus (myonuclear domain; MND) is a very important factor for understanding muscle plasticity. Although the MND size varies with fiber type, metabolic property and species, it was considered that the size was maintained constantly during muscle adaptation. Recently, however, there have been many studies demonstrating the variability of the MND size. In some of these studies, it is hypothesized that muscle hypertrophy is achieved by increase in protein synthesis rate until the ‘ceiling’ of MND size and subsequent addition of myonuclei by satellite cell activation. On the other hand, during muscle atrophy, the myonuclei seems to be long lasting as ‘muscle memory’ storing information about previous size to prepare for recovery of muscle fiber. Understanding the variability of MND size more deeply would provide fundamental insights into the mechanism of skeletal muscle plasticity.

We investigate the effects of training and detraining on the satellite cell activation in thoroughbred horse muscles after an exhaustive exercise. Six horses were subjected to conventional training for 18 weeks and detraining for 6 weeks. Before training (Pre), after 10-week and 18-week training (10Tr and 18Tr), and after 6-week detraining (DTr), an incremental exercise test (IET) was performed on inclined treadmill to measure VO₂max and the velocity at a plasma lactate of 4 mmol/l (VLA4). Biopsy samples from gluteus medius muscle was obtained before and at 1 minute (1min), 3 hour (3hr), 6 hour (6hr) and 1 day after each IET. Number of muscle satellite cell were counted in type identified muscle fibers by immuno-histochemical stain images. The levels of mRNA expressions were determined using real time RT-PCR system. The number of satellite cells in 10Tr was significantly higher in type IIa fibers (0.31±0.10) than Pre (0.15±0.06). As compared to each value before IET, IL-6 mRNA expression (fold change) increased remarkably at 6hr after IET in 10Tr (x 2290.2) and 18Tr (x 2304.2), but not in both Pre (x 260.0) and DTr (x 853.3). IGF-I and Myogenin mRNA expressions were significantly increased at 1 day after IET in 18Tr (x 6.6 and x 3.3), but not in both Pre and DTr. These results suggested that the increased reactivity of satellite cells by training for 18 weeks is almost disappeared after detraining for 6 week, as well as VO₂max and VLA4.