Maintenance of skeletal muscle mass is essential for overall health, functionality and quality of life, and it is critical to elucidate the fundamental mechanisms underlying the maintenance. Skeletal muscle has been known to cause disuse atrophy during long-term recumbence and to recover by exercise. Although there are many studies about skeletal muscle regulatory factors, such as IGF-1, FGF, HGF, and IL-6, that are known to affect skeletal muscle mass, mechanism of exercise on muscle cell proliferation is still unclear. Our previous study using rat soleus muscle indicated that myosin heavy chain (MHC) and HSP 70 proteins were significantly increased by the exercise after disuse atrophy. It is well known that the mechanical stress by exercise raises intracellular calcium level in the muscle cell, therefore we examined effects of increase in intracellular Ca2+ on MHC I, HSP 70, and IL-6 mRNA expression level using real-time PCR method in C2C12 skeletal myoblasts. The C2C12 cells which were differentiated into myocyte in D-MEM containing 2% FBS was incubated with ionomycin in 6 or 24 hr. First, we measured MHC I mRNA level and it was significantly increased by the 6 hr incubation with ionomycin compared with control. 24 hr incubation with ionomycin significantly upregurated HSP 70 mRNA level, although 6 hr incubation with it did not affect the mRNA level. IL-6 mRNA level was significantly increased by the 6 hr incubation of ionomycin. Second, we examined the effect of IGF-1 on MHC I, HSP 70, and IL-6 mRNA expression levels. IL-6 mRNA level was significantly increased by the 24 hr incubation with IGF-1, although MHC I and HSP 70 mRNA levels were not increased. Third, we examined the effect of IL-6 on MHC I, HSP 70, and IL-6 mRNA expression levels. The mRNA levels of IL-6 and HSP 70 were significantly increased by the 24 hr incubation with IL-6, although MHC I mRNA level was not increased. Thus, it is considered that activation of Ca2+-dependent signaling pathways, such as calcineurin, could act as a major factor upregulating MHC I mRNA levels. Therefore, we confirmed that effect of calcineurin inhibitor on ionomycin induced increase in mRNA level of MHC I. Ionomycin induced upregulation of MHC I mRNA level was significantly attenuated by the application of FK506. These results indicate that activation of Ca2+-calmodulin/calcineurin mediated signaling pathways might have essential roles for upregulation of MHC I mRNA level.