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 MHC2X and MHC2A compared with fibers singularly expressing MHC2A and MHCslow (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 MHC2X and MHC2A. We conclude that fiber-type differences in the reserve capacity for ATP consumption and tension cost reflect functional differences such as susceptibility to fatigue.