Role of serum myostatin during the lactation period.

Myostatin, also known as GDF-8 (Growth/Differentiation Factor-8), is a member of the TGF-beta superfamily that negatively regulates skeletal muscle mass in mammals. Mutation of the myostatin gene in mice, cattle, and humans causes a massively developed skeletal muscle, characterized by muscle hypertrophy and hyperplasia. Although myostatin is predominantly expressed in skeletal muscle tissue, several recent studies have shown the presence of myostatin protein in blood and suggested a possible role for circulating myostatin in the regulation of skeletal muscle mass. In the present study, we examined changes in the levels of active form myostatin (13 kDa) in serum after birth by Western blot analysis to predict the role of serum myostatin in early postnatal muscle growth in the rat. Interestingly, the amount of active form myostatin in serum increased after birth and then decreased along with ageing after weaning. To clarify the role of increased serum myostatin during the postnatal period, we administrated follistatin, an inhibitor of myostatin activity, into postnatal rats intraperitoneally just after birth. Follistatin-administration during the postnatal period caused selective hypertrophy of type II muscle fibers in the soleus muscle. These results demonstrate that myostatin in serum acts on skeletal muscle and negatively regulates early postnatal muscle growth.