Localization of alpha 7 integrins and dystrophin suggests potential for both lateral and longitudinal transmission of tension in large mammalian muscles

Non-primate mammalian muscles with fascicles above 35 mm in length are composed predominantly of arrays of short, non-spanning muscle fibres, which terminate within the belly of the muscle fascicle at one or both ends. We have previously described the morphological form of various muscle-to-muscle and muscle-to-matrix junctions which are likely involved in tension transmission within one such muscle - the guinea pig sternomastoid muscle (Young et al. 2000). Here, we use immunohistochemistry to investigate the cell adhesion molecules present at these junctions. We find strong immunoreactivity against the alpha 7B integrin subunit and dystrophin, and slight reactivity against the alpha 7A integrin at all intrafascicular fibre terminations (IFTs), as well as at the muscle-tendon junction (MTJ). Tenascin, the sole ligand for alpha 9 beta 1 integrin, was absent from IFTs but present at the MTJ, suggesting the two sites are molecularly distinct. In addition to their expression at junctional sites, alpha 7B integrin and dystrophin were also expressed ubiquitously along the non-junctional sarcolemma, suggesting potential involvement in diffuse lateral transmission of tension between adjacent fibres. We conclude that the distribution of alpha 7 beta 1 integrins and dystrophin in series-fibred muscles suggests they are involved in transmission of tension from intrafascicularly terminating fibres to neighbouring fibres lying both in-series and in-parallel, via the extracellular matrix (ECM).