Acidosis (low pH) is the oldest putative agent of muscular fatigue, but the molecular mechanism underlying its depressive effect on muscular performance remains unresolved. Therefore, the effect of low pH on the molecular mechanics and kinetics of chicken skeletal muscle myosin was studied using in vitro motility (IVM) and single molecule laser trap assays. Decreasing pH from 7.4 to 6.4 at saturating ATP slowed actin filament velocity (Vactin) in the IVM by 36%. Single molecule experiments, at 1 μM ATP, decreased the average unitary step size of myosin (d) from 10 ± 2 nm (pH 7.4) to 2 ± 1 nm (pH 6.4). Individual binding events at low pH were consistent with the presence of a population of both productive (average d = 10 nm) and nonproductive (average d = 0 nm) actomyosin interactions. Raising the ATP concentration from 1 μM to 1 mM at pH 6.4 restored d (9 ± 3 nm), suggesting that the lifetime of the nonproductive interactions is solely dependent on the [ATP]. Vactin, however, was not restored by raising the [ATP] (1–10 mM) in the IVM assay, suggesting that low pH also prolongs actin strong binding (ton). Measurement of ton as a function of the [ATP] in the single molecule assay suggested that acidosis prolongs ton by slowing the rate of ADP release. Thus, in a detachment limited model of motility (i.e., Vactin ∼ d/ton), a slowed rate of ADP release and the presence of nonproductive actomyosin interactions could account for the acidosis-induced decrease in Vactin, suggesting a molecular explanation for this component of muscular fatigue
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