ANALYSIS OF CA2-FIBER PREPARATIONS WITH DIFFERENT PROPORTIONS OF MYOFIBRILLAR ISOFORMS( AND SR2+ ACTIVATION CHARACTERISTICS IN SKINNED MUSCLE)

To understand how the coexistence of fast and slow contractile and reg ulatory systems within single skeletal muscle fibres might affect cont ractile behaviour, fibre segments from the fast-twitch extensor digito rum longus and predominantly slow-twitch soleus muscle of the adult ra t were tied together, either in parallel or in series, and then activa ted in Ca2+- and Sr2+-buffered solutions. Experimental force-pCa and f orce-pSr relations were compared with theoretical force-pCa and force- pSr curves predicted by a model for composite fibres, which accounted for the coexistence of fast and slow myosin within the contractile uni t and enabled an estimate to be made of the relative contribution of f ast- and slow-twitch elements within the tied-fibre combinations. The contractile behaviour of a fast-twitch and a slow-twitch muscle fibre tied either in series or in parallel, were compared with the force-pCa and force-pSr data predicted from the composite fibre model. Interest ingly, the resultant force-pCa(-pSr) curves of the parallel-tied fibre combinations were well fitted with those predicted by the composite m odel. However, the experimental force-pCa(-pSr) curves of the series-t ied fibres were not well fitted by a composite curve based on the know n proportion of fast- and slow-twitch fibre components. A total force- length diagram was devised to take into account changes in the length of the fibre segments tied in series during activation, as well as pos sible differences in fibre diameter. Using this diagram it was possibl e to explain accurately the Ca2+ and Sr2+ activation curves of known f ast- and slow-twitch segments tied in series. The results from this st udy are important for the interpretation of contractile data obtained from single muscle fibres exhibiting mixed fast- and slow-twitch contr actile characteristics. Such muscle fibres have previously been identi fied in animals affected by muscular diseases (e.g. dystrophy), in mam malian extraocular muscles and in animals subjected to long-term exerc ise training.