Fast and slow skeletal muscles express a common basic profile of acetylcholinesterase molecular forms

Recent evidence suggests that the high content of acetylcholinesterase (AChE) globular form G4, characteristic of fast muscles, is controlled by phasic high-frequency activity performed by these muscles. This indicates that inactive, though still innervated, fast muscles should be devoid of their characteristic G4 pool. Accordingly, in the absence of phasic activity, both fast and slow muscles should exhibit a common basic profile of AChE molecular forms of the slow type. We first tested this hypothesis by examining the AChE content in cultures of myotubes obtained from the fusion of satellite cells originating from fast and slow muscles. These two cell populations produced AChE molecular-form profiles of the slow type characterized by modest levels of G4 together with an increased proportion of the asymmetric forms A8 relative to A12. Second, we determined the impact of muscle paralysis on the specific content of AChE molecular forms of adult rat fast and slow muscles. Complete paralysis of hindlimb muscles was achieved by chronic superfusion of tetrodotoxin (TTX) onto the sciatic nerve. Ten days after TTX inactivation, the distributions of AChE molecular forms of both fast extensor digitorum longus (EDL) and plantaris muscles were transformed into ones resembling the slow soleus, the latter showing no significant modifications in its AChE profile. Finally, we investigated the impact of nerve-mediated phasic high-frequency stimulation of TTX-inactivated fast and slow muscles on the content of AChE molecular forms. This stimulation produced a profile of AChE molecular forms similar to that observed in control EDL muscles, indicating that phasic activation counteracted the TTX-induced transformation in the distribution of AChE molecular forms in fast EDL muscles. Together, these results are consistent with the proposal that adult fast muscles constitutively express a basic profile of AChE molecular forms of the type displayed by slow muscles, onto which varying levels of G4 are added according to the amount of phasic activity performed by the muscles.