RESPONSE OF COMPRESSED SKINNED SKELETAL-MUSCLE FIBERS TO CONDITIONS THAT SIMULATE FATIGUE

During fatigue, muscles become weaker, slower, and more economical at producing tension. Studies of skinned muscle fibers can explain some b ut not all of these effects, and, in particular, they are less economi cal in conditions that simulate fatigue. We investigated three factors that may contribute to the different behavior of skinned fibers. 1) S kinned fibers have increased myofilament lattice spacing, which is rev ersible by osmotic compression. 2) A myosin subunit becomes phosphoryl ated during fatigue. 3) Inosine 5'-monophosphate (IMP) accumulates dur ing fatigue. We tested the response of phosphorylated and unphosphoryl ated single skinned fibers (isometric tension, contraction velocity, a nd adenosinetriphosphatase activity) to changes in lattice spacing (0- 5% dextran) and IMP (0-5 mM) in the presence of altered concentrations of P-i (3-25 mM), H+ (pH 7-6.2), and ADP (0-5 mM). The response of ma ximally activated skinned fibers to the direct metabolites of ATP hydr olysis is not altered by osmotic compression, phosphorylating myosin s ubunits, or increasing IMP concentration. These factors, therefore, do not explain the discrepancy between intact and skinned fibers during fatigue.