Cellular and molecular contractile function in aged human skeletal muscle is altered by phosphate and acidosis and partially reversed with an ATP analog.

Skeletal muscle fatigue occurs, in part, from accumulation of hydrogen (H) and phosphate (P); however, the molecular basis through which these ions inhibit function is not fully understood. Therefore, we examined the effects of these metabolites on myosin-actin cross-bridge kinetics and mechanical properties in skeletal muscle fibers from older (65-75 years) adults. Slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibers were examined under control (5 mM P, pH 7.0) and fatigue (30 mM P, pH 6.2) conditions at maximal calcium-activation (5 mM ATP) and rigor (0 mM ATP). In MHC I and IIA fibers, fatigue decreased force per fiber size (23-37%), which was accompanied by reduced strongly bound myosin head characteristics (number and/or stiffness; 21-47%) and slower cross-bridge kinetics (longer myosin attachment times (22-46%) and reduced rates of force production (20-33%)) compared with control. MHC I myofilaments became stiffer with fatigue, a potential mechanism to increase force production. In rigor, which causes the myosin that can bind actin to be strongly bound, fatigue decreased force per fiber size (32-33%) in MHC I and IIA fibers, indicating less force was generated per cross-bridge. By replacing ATP with 2-deoxy-ATP (dATP), the fatigue-induced slowing of cross-bridge kinetics in MHC I and IIA fibers was reversed and reduced force production in MHC I fibers was partially improved, revealing potential mechanisms to help mitigate fatigue in older adults. Overall, our results identify novel fiber type-specific changes in cross-bridge kinetics, force per cross-bridge, and myofilament stiffness that help explain fatigue in older adults.