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Am J Physiol Cell Physiol (November 21, 2001). doi:10.1152/ajpcell.00186.2001
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Articles in PresS, published online ahead of print November 20, 2001
Am J Physiol Cell Physiol, 10.1152/ajpcell.00186.2001
Submitted on April 20, 2001
Accepted on November 12, 2001

Effect of inorganic phosphate on the unloaded shortening velocity of slow and fast mammalian muscle fibers

Jeffrey J Widrick1*

1 Exercise and Sport Science, Oregon State University, Corvallis, OR, USA

* To whom correspondence should be addressed. E-mail: jeff.widrick{at}orst.edu.

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 mM, 15 mM, 30 mM, and 0 mM added inorganic phosphate (Pi). Fifteen and 30 mM Pi 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 while 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 due to 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.




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