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 G(4), characteristic of fast muscles, is control led by phasic high-frequency activity performed by these muscles. This indicates that inactive, though still innervated, fast muscles should be devoid of their characteristic G(4) pool. Accordingly, in the abse nce of phasic activity, both fast and slow muscles should exhibit a co mmon basic profile of AChE molecular forms of the slow type. We first tested this hypothesis by examining the AChE content in cultures of my otubes obtained from the fusion of satellite cells originating from fa st and slow muscles. These two cell populations produced AChE molecula r-form profiles of the slow type characterized by modest levels of G(4 ) together with an increased proportion of the asymmetric forms Ag rel ative to A(12). Second, we determined the impact of muscle paralysis o n the specific content of AChE molecular forms of adult rat fast and s low muscles. Complete paralysis of hindlimb muscles was achieved by ch ronic superfusion of tetrodotoxin (TTX) onto the sciatic nerve. Ten da ys after TTX inactivation, the distributions of AChE molecular forms o f 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 investig ated the impact of nerve-mediated phasic high-frequency stimulation of TTX-inactivated fast and slow muscles on the content of AChE molecula r forms. This stimulation produced a profile of AChE molecular forms s imilar to that observed in control EDL muscles, indicating that phasic activation counteracted the TTX-induced transformation in the distrib ution of AChE molecular forms in fast EDL muscles. Together, these res ults are consistent with the proposal that adult fast muscles constitu tively express a basic pro tile of AChE molecular forms of the type di splayed by slow muscles, onto which varying levels of G(4) are added a ccording to the amount of phasic activity performed by the muscles.