*4.3. Myotube Formation*

In the present study, lactate increased the myotube diameter and protein content in C2C12 cells. These results are supported by the previous report that lactate caused the activation of anabolic signals for hypertrophy and myogenesis in skeletal muscle cells [18]. Our previous data also showed that extracellular lactate increased the diameter of C2C12 myotubes in a dose-dependent manner [19]. In addition, the present study demonstrated that extracellular lactate caused the extension of the myotube length and the increase of the myonuclei number. Therefore, it was suggested that lactate may stimulate the fusion of myoblasts, which results in myotube formation. This contributes to muscle hypertrophy and regeneration [5–7].

It has been a debatable argumen<sup>t</sup> that various metabolites, including lactate, may be involved in exercise-associated skeletal muscle hypertrophy [31]. In the present study, cell culture experiments demonstrated lactate induces an increase in muscle mass even though no myotube contraction is observed. On the other hand, lactate may also stimulate the hypertrophic effects of physical activity on skeletal muscle, since mice moved freely immediately after the administration of lactate in the present study. Additional results are needed to elucidate the difference and interaction between lactate and muscle contraction in skeletal muscle hypertrophy.
