Background: Local anesthetics (LAs) have been shown to inhibit human polymorphonuclear neutrophil (hPMN) functions in vitro, but mechanisms are poorly understood. In this study the authors determined how LAs affect superoxide anion production of hPMNs primed with platelet-activating factor (PAF). The authors studied which pharmacologic properties of LAs are important for this action and assessed the LA site of action within the PAF signaling pathway.
Methods: Metabolic activity of primed and/or activated hPMNs were measured using the cytochrome-c assay. hPMNs were incubated with several LAs for 1 h to assess interference with PAF signaling. Using protein kinase C (PKC) inhibitors, the PKC activator phorbol myristate acetate (PMA), and the phospholipase C (PLC) antagonist U-73122, we studied involvement of PKC and PLC in the priming process. Pertussis toxin (PTX) was used to characterize the G proteins mediating this pathway. Combined administration of lidocaine with PMA or PTX was used to determine the LA site of action within the priming pathway.
Results: Platelet-activating factor effectively primed hPMNs. Ester LAs (tetracaine and benzocaine) exerted the most profound inhibitory effect on PAF-primed hPMNs, whereas inhibitory potency of amide LAs increased with decreased charged fraction. The major PAF-induced priming pathway is PLC- and PKC-dependent and mainly Gq-mediated. The main target site for LA in this pathway is located upstream of PKC.
Conclusions: Local anesthetics in clinically relevant concentrations inhibit superoxide anion production of PAF-primed hPMNs. Effects on priming by these compounds might explain, at least in part, the previously unexplained difference between concentrations of LAs required for their antiinflammatory action in vitro and in vivo. This study suggests a target site for LAs within a Gq-coupled signaling pathway.