CYTOSKELETAL STRUCTURE OF SKELETAL-MUSCLE - IDENTIFICATION OF AN INTRICATE EXOSARCOMERIC MICROTUBULE LATTICE IN SLOW-TWITCH AND FAST-TWITCHMUSCLE-FIBERS

We used immunochemical quantification and indirect immunofluorescence to investigate the cell content, distribution, and organization of mic rotubules in adult fat slow-twitch soleus and fast-twitch vastus later alis muscles. An immunoblotting assay demonstrated that the soleus mus cle (primarily Type I fibers) was found to have a 1.7-fold higher rela tive content of alpha-tubulin compared with the superficial portion of the vastus lateralis muscle (primarily Type IIb fibers). Both physiol ogical muscle types revealed a complex arrangement of microtubules whi ch displayed oblique, longitudinal, and transverse orientations within the sarcoplasmic space. The predominance of any one particular orient ation varied significantly from one muscle tissue section to another. Nuclei were completely surrounded by a dense net-like structure of mic rotubules. Both muscle fiber types were found to possess a higher dens ity of microtubules in the sub-sarcolemmal region. These microtubules followed the contour of the sarcolemma in slightly contracted fibers a nd showed a fine punctate appearance indicative of a restricted distri bution. The immunofluorescence results indicate that microtubules are associated with the sarcolemma and therefore may form a part of the me mbrane cytoskeletal domain of the muscle fiber. We conclude that the m icrotubule network of the adult mammalian skeletal muscle fiber consti tutes a bone fide component of the exosarcomeric cytoskeletal lattice domain along with the intermediate filaments, and as such could theref ore participate in the mechanical integration of the various organelle s of the myofibers during the contraction-relaxation cycle.