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  • Title: Effect of P(i) on unloaded shortening velocity of slow and fast mammalian muscle fibers.
    Author: Widrick JJ.
    Journal: Am J Physiol Cell Physiol; 2002 Apr; 282(4):C647-53. PubMed ID: 11880253.
    Abstract:
    Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca(2+)-activating solutions containing 0, 15, 30, and 0 mM added P(i). P(i) (15 and 30 mM) had no effect on the unloaded shortening velocity (V(o)) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM P(i) did not alter V(o), whereas 30 mM P(i) reduced V(o) to 81 plus minus 1% of the original 0 mM P(i) value. This effect was readily reversible when P(i) 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 V(o) is independent of P(i). The response of the type I fibers at 30 mM P(i) is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of P(i) on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.
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