Modulation of the Kinetics of Induced Neutrophil Superoxide Generation by Fluoride

Abstract

Fluoride (F^-) is unique in that, depending upon concentration, it can either stimulate or inhibit the synthesis of the neutrophil's major intracellular microbicidal product, superoxide anion (O^- ₂). While a number of studies have delved into F^- induction of O^- ₂ generation, little has been reported on the effect of F- on the synthesis of O^- ₂ induced by other agents. In this report, we show that F^- inhibits the activation and activity of neutrophils induced either by formyl-methionyl-leucylphenylalanine (FMLP) or by phorbol myristate acetate (PMA). The extent of increase in lag period and decrease in velocity of O- ₂ synthesis was found to be directly related to the concentrations of F-and of hydrogen ion in the medium and to the length of contact time between the neutrophil and F-. Decreasing the pH of the medium, while maintaining a constant concentration of F^-, disproportionately increased the inhibitory effect of the anion over the effect of pH alone. Fluoride inhibited the responsiveness of neutrophils to FMLP more than it did their responsiveness to PMA. A gradual but partial reversal of F ^-'s inhibition of O^- ₂ synthesis could be achieved by transferring inhibited cells to F--free medium. Our study shows that the suppression of both FMLP- and PMA-induced synthesis of O^- ₂ by neutrophils is most dramatic at a reduced pH and is probably related to the intracellular accumulation of F-. On the other hand, at pH 7.2, the kinetics of the time-dependent decline in responsiveness to FMLP in the presence of F^- was similar to that seen in the absence of Ca ²⁺ . This suggests that the activation of neutrophils may also be influenced by the effect of F- on extracellular levels of Ca ²⁺. Reduction in extracellular levels of Ca²⁺ could subsequently affect intracellular stores of membrane-associated Ca²⁺ and thus inhibit the Ca²⁺-dependent activation of neutrophils by FMLP.