The enzyme creatine kinase (CK) is important in maintaining the energy supply of skeletal muscles, acting as a kind of ATP buffer. As a kinase, it catalyzes the reversible transfer of a phosphoryl group from phosphocreatine to ADP to yield creatine and ATP. In the rat, CK activity in the postnatally developing diaphragm has been found to increase significantly with maturation. Its activity in the costal diaphragm muscle nearly doubles between the neonate and the adult. This increase corresponds to the change from muscle fiber types with a low energy requirement to types with a high energy requirement (Watchko et al., 1996).
Thyroid hormones are known to be extremely important chemical messengers during development, especially in such processes as amphibian metamorphosis. Finkelstein et al. (1992) investigated the neural and hormonal control of the development of the sheep diaphragm and found that thyroid hormones are important in its differentiation and development. The authors put forth that because substantial development of the diaphragm occurs postpartum, neural activity in utero is not sufficient to account for the developmental changes. They found that in the sheep diaphragm, contractile specialization is mostly complete at birth, although the ratios of fiber type and the myosin isozymes continue to change postnatally. The high level of thyroid hormome in the embryonic sheep is implicated in this in utero maturation of the fast-twitch muscle. Thyroid hormones may also be responsible for the differentiation of the twitch characteristics of the fast and slow fiber types.