Membrane skeleton of innervated and denervated fast- and slow-twitch muscle

We used confocal microscopy and immunoblotting to study membrane skeletal p roteins of fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles of the adult rat, In the extensor digitorum longus (EDL), beta-spe ctrin concentrates in costameres. whereas dystrophin is enriched at costame res but is also present in intercostameric regions. In the soleus, beta-spe ctrin and dystrophin underlie much of the sarcolemma, and intercostameric r egions are difficult to detect. The EDL sarcolemma reorganizes following de nervation to resemble soleus sarcolemma, but denervation does not significa ntly affect the latter. Consistent with these observations, soleus contains similar amounts of dystrophin but more beta-spectrin than EDL. Denervation increases beta-spectrin levels only in the EDL and dystrophin levels in bo th muscles. Denervation does not affect beta-fodrin, a beta-spectrin homolo g expressed in embryonic myofibers. Thus, neuromuscular activity controls s arcolemmal organization and the levels of beta-spectrin and dystrophin, but not postnatal downregulation of beta-fodrin, The differences in organizati on of the sarcolemma may underlie the differential susceptibility of fast a nd slow myofibers to dystrophinopathies. (C) 2000 John Wiley & Sons, Inc.