title: Effect of P(i) on unloaded shortening velocity of slow and fast mammalian muscle fibers.
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.
authors.affiliation: Department of Exercise and Sport Science, Oregon State University, Corvallis, OR 97331, USA. jeff.widrick@orst.edu
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Widrick
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D007536:Isomerism; D008322:Mammals; D009119:Muscle Contraction / Q000187:drug effects*; D018763:Muscle Fatigue / Q000187:drug effects; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000737:chemistry / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology; D018995:Myosin Heavy Chains / Q000032:analysis / Q000737:chemistry; D010710:Phosphates / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D010710:Phosphates; D000255:Adenosine Triphosphate; D018995:Myosin Heavy Chains; D002118:Calcium