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Am J Physiol Cell Physiol 282: C647-C653, 2002. First published November 21, 2001; doi:10.1152/ajpcell.00186.2001
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Vol. 282, Issue 4, C647-C653, April 2002

Effect of Pi on unloaded shortening velocity of slow and fast mammalian muscle fibers

Jeffrey J. Widrick

Department of Exercise and Sport Science, Oregon State University, Corvallis, Oregon 97331

Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca2+-activating solutions containing 0, 15, 30, and 0 mM added Pi. Pi (15 and 30 mM) had no effect on the unloaded shortening velocity (Vo) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM Pi did not alter Vo, whereas 30 mM Pi reduced Vo to 81 ± 1% of the original 0 mM Pi value. This effect was readily reversible when Pi was lowered back to 0 mM. These results are not compatible with current cross-bridge models, developed exclusively from data obtained from fast fibers, in which Vo is independent of Pi. The response of the type I fibers at 30 mM Pi is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of Pi on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.

inorganic phosphate; cross-bridge cycle; muscle fatigue; maximal shortening velocity; myosin heavy chain isoforms


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