Key Points
- To investigate the individual and combined effects of inorganic phosphate and hydrogen ions on the mechanical properties and ATPase activity of skeletal muscle fibers.
- Glycerinated rabbit psoas muscle fibers were fully activated with calcium (pCa 4–5) at 10 °C across varying pH levels (7.0 to 6.0) and phosphate concentrations (3 to 20 mM).
- Contraction velocity was determined via isotonic load clamps and step length changes, while ATPase activity was quantified spectrophotometrically using an enzyme-coupled NADH depletion assay.
- Increasing phosphate from 3 to 20 mM at pH 7.0 reduced isometric tension by approximately 20% and ATPase activity by 15–20%, with no effect on maximum contraction velocity.
- Lowering pH from 7.0 to 6.0 at 3 mM phosphate decreased isometric tension by 45% and reduced both maximum velocity and ATPase activity by 25–30%.
- Combined proton and phosphate exposure produced additive, reversible inhibition of tension and kinetics, maintaining an unchanged force-velocity curvature parameter of 0.20.
Structured PICO
PPopulationGlycerinated rabbit psoas muscle fibres
IInterventionIncreased phosphate (from 3 to 20 mM) and decreased pH (from 7 to 6)
CComparatorBaseline conditions (pH 7.0 and 3 mM phosphate)
OOutcomeMechanics and energetics of muscle contraction (isometric tension, maximum contraction velocity, ATPase activity)surrogate
Increased levels of phosphate and protons explain much of the inhibition of isometric tension, contraction velocity, and ATPase activity observed during moderate muscle fatigue.