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1 School of Biomedical Sciences, Victoria University, Melbourne, Victoria, Australia
2 School of Zoology, Faculty of Science and Technology, La Trobe University, Bundoora, Victoria, Australia
* To whom correspondence should be addressed. E-mail: Craig.Goodman{at}vu.edu.au.
In this study we investigated whether the previously established differences between fast and slow twitch single skeletal muscle fibers of the rat, in terms of MHC isoform composition and contractile function, are also detectable in E-C coupling events. To this end, we compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from soleus (SOL), extensor digitorum longus (EDL) and the white region of the sternomastoid (SM) muscle to t-system depolarisation induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization induced force response (maxDIFR; normalized to the maximum Ca2+-activated force in that fiber) and the number of responses elicited until the force declined by 75 % of maxDIFR (75%R-D). The mean maxDIFR values for IIB EDL and IIB SM fibers were not statistically different and both were greater than the mean maxDIFR for type I SOL fibers. The mean 75%R-D value for IIB EDL fibers was greater than that for type I SOL fibers as well as for IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned, rat single muscle fibers are related to both MHC-based fiber type and the muscle of origin.
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