Acidosis and Phosphate Directly Reduce Myosin's Force-Generating Capacity Through Distinct Molecular Mechanisms

Area of Science:

• Muscle physiology
• Biochemistry
• Molecular mechanisms of muscle contraction

Background:

• Muscle fatigue is associated with elevated intracellular acidosis (high [H+]) and phosphate (Pi).
• The precise molecular mechanisms by which acidosis and Pi impair myosin function during muscle contraction remain incompletely understood.

Purpose of the Study:

• To investigate the direct effects of acidosis and elevated Pi on the force-generating capacity of myosin.
• To elucidate the distinct molecular mechanisms through which acidosis and Pi contribute to muscle fatigue.

Main Methods:

• Utilized a laser trap assay to measure the force generated by individual myosin ensembles.
• Examined the effects of acidosis (pH 6.5 vs. 7.4) and elevated Pi, alone and in combination, on myosin's force production and ATPase rate.

Main Results:

• Acidosis reduced myosin's average force-generating capacity by 20%, attributed to decreased force per interaction and increased negative force events.
• Elevated Pi under acidic conditions similarly reduced force, primarily by eliminating high force-generating events due to accelerated myosin detachment from actin.
• Acidosis significantly slowed myosin's ATPase rate (~90%), while Pi addition partially restored it under acidic conditions.

Conclusions:

• Acidosis and Pi exert distinct effects on the myosin cross-bridge cycle, contributing to their synergistic impact on muscle force.
• These findings provide novel molecular insights into how acidosis and Pi impair muscle contraction during fatigue.