[Polymorphism of acetylcholinesterase and myosin during development of fast and slow muscles denervated in the newborn rabbit].

The rabbit Semimembranosus proprius (SMp) and Semimembranosus accessorius (SMa) muscles represent good models for studying the transformations of muscle properties during postnatal differentiation. In the adult, these muscles are homogeneous in slow twitch (SMp) and fast twitch (SMa) fibers, respectively. However, they are heterogeneous at birth and express their adult characteristics from two months onwards. During this period we studied the influence of motor innervation on the development of their properties, particularly at the level of acetylcholinesterase (AChE) molecular forms and myosin slow (LCs) and fast (LCf) light chains. The postnatal alteration of SMa and SMp muscles was characterized by the disappearance of the neonatal heterogeneity and the acquisition of the homogeneous fast or slow fiber type pattern. The fibers of these muscles denervated at birth were altered differently: dramatic atrophy of fast twitch fibers whatever the muscles studied, preservation of SMp slow twitch fiber characteristics and fatty degeneration of SMa. At birth, both muscles presented a similar pattern of myosin fast and slow LC. In control muscles, the alteration of fiber populations to homogeneous types led to the disappearance of supernumerary chains from 15 days onwards. In the slow muscle, neonatal denervation prevented LCf disappearance. In the fast muscle, denervation influenced essentially the installation of LCf which was delayed by 15 days. At birth, the polymorphism of AChE was similar in SMp and SMa muscles. One month after denervation, the specific activity of AChE was twice that of the control. Its polymorphism was not much disturbed, while in the adult denervation induced a large increase in AChE specific activity (x 10) and particularly a great alteration in its polymorphism according to the fast or slow muscle fiber types.