Reserve capacity for ATP consumption during isometric contraction in human skeletal muscle fibers

Maximum velocity of the actomyosin ATPase reaction (V_max ATPase) and ATP consumption rate during maximum isometric activation (ATP_iso) were determined in human vastus lateralis (VL) muscle fibers expressing different myosin heavy chain (MHC) isoforms. We hypothesized that the reserve capacity for ATP consumption [1 - (ratio of ATP_iso to V_max ATPase)] varies across VL muscle fibers expressing different MHC isoforms. Biopsies were obtained from 12 subjects (10 men and 2 women; age 21-66 yr). A quantitative histochemical procedure was used to measure V_max ATPase. In permeabilized fibers, ATP_iso was measured using an NADH-linked fluorometric procedure. The reserve capacity for ATP consumption was lower for fibers coexpressing MHC_2x and MHC_2A compared with fibers singularly expressing MHC_2A and MHC_slow (39 vs. 52 and 56%, respectively). Tension cost (ratio of ATP_iso to generated force) also varied with fiber type, being highest in fibers coexpressing MHC_2X and MHC_2A. We conclude that fiber-type differences in the reserve capacity for ATP consumption and tension cost reflect functional differences such as susceptibility to fatigue.