Calcium-independent phospholipase A₂ modulates cytosolic oxidant activity and contractile function in murine skeletal muscle cells

Phospholipase A₂ (PLA₂) activity supports production of reactive oxygen species (ROS) by mammalian cells. In skeletal muscle, endogenous ROS modulate the force of muscle contraction. We tested the hypothesis that skeletal muscle cells constitutively express the calcium-independent PLA₂ (iPLA₂) isoform and that iPLA₂ modulates both cytosolic oxidant activity and contractile function. Experiments utilized differentiated C₂C₁₂ myotubes and a panel of striated muscles isolated from adult mice. Muscle preparations were processed for measurement of mRNA by real-time PCR, protein by immunoblot, cytosolic oxidant activity by the dichlorofluorescein oxidation assay, and contractile function by in vitro testing. We found that iPLA₂ was constitutively expressed by all muscles tested (myotubes, diaphragm, soleus, extensor digitorum longus, gastrocnemius, heart) and that mRNA and protein levels were generally similar among muscles. Selective iPLA₂ blockade by use of bromoenol lactone (10 μM) decreased cytosolic oxidant activity in myotubes and intact soleus muscle fibers. iPLA₂ blockade also inhibited contractile function of unfatigued soleus muscles, shifting the force-frequency relationship rightward and depressing force production during acute fatigue. Each of these changes could be reproduced by selective depletion of superoxide anions using superoxide dismutase (1 kU/ml). These findings suggest that constitutively expressed iPLA₂ modulates oxidant activity in skeletal muscle fibers by supporting ROS production, thereby influencing contractile properties and fatigue characteristics.