Musclin has been described as a muscle-derived secretory peptide, responsive to 
insulin in vivo, and inducing insulin resistance in vitro. Because muscle fibers 
display very different metabolic properties and insulin sensitivity, we tested 
the hypothesis that musclin expression could depend on myofiber type. Musclin 
mRNA was detected at high level in fast gastrocnemius and plantaris muscles, but 
only as traces in soleus, a slow-twitch muscle. A single fiber analysis showed 
that musclin was produced by muscle fibers themselves, almost exclusively type 
IIb fibers. Slow to fast transition of soleus phenotype after hindlimb 
suspension increased musclin mRNA levels, whereas fast to slow transition of 
plantaris phenotype after functional overload decreased musclin mRNA levels. 
This clearly suggests that musclin transcription is strongly related to 
fast-glycolytic phenotype. We conclude that musclin is produced by myocytes in a 
highly fiber-type specific manner and that physiological changes in type IIb MHC 
lead to coordinated musclin expression.