Reinnervation of a denervated slow muscle triggers high extrajunctional expression of the asymmetric molecular forms of acetyicholinesterase

Expression of acetylcholine receptor and of the asymmetric molecular forms of acetylcholinesterase (AChE) in the extrajunctional regions of rat muscles is suppressed during early postnatal development. In mature muscles, the extrajunctional synthesis of acetylcholine receptor, but not of the asymmetric molecular forms of AChE, becomes reactivated after denervation. The hypothesis that a denervated muscle needs reinnervation in order to revert transiently to an immature state characterized by high extrajunctional production of the asymmetric AChE forms, was examined in rat muscles recovering after nerve crush. Molecular forms of AChE were analysed by velocity sedimentation. Activity of the asymmetric A12 AChE form in the extrajunctional regions of the slow soleus (SOL) muscle increased during the first week after reinnervation to about 9 times its control level, remained high for about one week, and declined towards normal thereafter. If the nerve was crushed close to the muscle and reinnervation occured very rapidly, the extrajunctional increase of the A₁₂ AChE form still occured but was less pronounced than after late reinnervation. In contrast, a transient paralysis of the SOL muscle due to acetylcholine receptor blockade by α-bungarotoxin, followed by spontaneous recovery of muscle activity after 3–5 days, did not revert AChE regulation into an immature state. Disuse of the SOL muscle caused by leg immobilization, which is known to change the tonic pattern of neural stimulation of the SOL muscle into a phasic one, did not prevent the reversion of AChE regulation during reinnervation. This indicates that neural stimulation pattern is not crucial for this reversion. In contrast to slow SOL, the fast extensor digitorum longus muscle did not revert to an immature state in respect to AChE regulation after reinnervation. This muscle type-specific response may be due to intrinsic differences between the myogenic cells of slow and fast muscle fibres. © 1995 Wiley-Liss, Inc.