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

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 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+/-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.