Propionic acid stimulates superoxide generation in human neutrophils

Short-chain carboxylic acids are the metabolic by-products of pathogenic anaerobic bacteria and are found at sites of infection in millimolar quantities. We previously reported that propionic acid, one of the short-chain carboxylic acids, induces an increase in intracellular Ca²⁺ ([Ca²⁺](i)) in human neutrophils. Here we investigate the effect of propionic acid on superoxide generation in human neutrophils. Propionic acid (10 mM) induced inositol 1,4,5-trisphosphate (IP₃) formation and a rapidly transient increase in [Ca²⁺](i), but not superoxide generation, whereas 1 μM formylmethionyl-leucyl-phenylalanine (fMLP), a widely used neutrophil-stimulating bacterial peptide, stimulated not only IP₃ formation and Ca²⁺ mobilization but also superoxide generation. The IP₃ level induced by propionic acid was slightly lower than that induced by fMLP. The transient increase in [Ca²⁺](i) induced by propionic acid immediately returned to the basal level, whereas a sustained increase in [Ca²⁺](i), which was higher than the basal level, following a transient increase in [Ca²⁺](i) was induced by fMLP. The peak level induced by propionic acid was lower than that with fMLP. In the absence of extracellular Ca²⁺ thapsigargin, a potent inhibitor of endoplasmic reticulum Ca²⁺-ATPase, induced an increase in [Ca²⁺](i) even after propionic acid stimulation, but not after fMLP. The Ca²⁺ ionophore A23187 and thapsigargin induced superoxide generation by themselves. Propionic acid enhanced the superoxide generating effect of A23187 and thapsigargin. These results suggest that Ca²⁺ mobilization induced by propionic acid is much weaker than that with fMLP, and propionic acid is able to generate superoxide in the presence of a Ca²⁺ ionophore and a Ca²⁺ influx activator.