The role of sodium ion in superoxide (O-2) generation by human peripheral neutrophils was investigated. Cells were activated by exposure to the synthetic tripeptide, N-formyl-methionyl-leucyl-phenylalanine (FMLP), and O-2 release was assessed by ferricytochrome c reduction after 5 min of incubation at 37°C in the presence of FMLP 4 × 10-8 M. In the absence of monovalent cations (isotonic glucose), negligible O-2 generation occurred.
There was a progressive increase in the magnitude of FMLP-induced O-2 generation with increasing Na+ concentration up to 90 mM, where the response was noted to plateau. Varying the K+ concentration (1 to 10 mM) had no effect on the amount of O-2 produced in the presence of Na+ 140 mM.
FMLP also stimulated 22Na+ and 45Ca2+ uptake by the cells in a dose- and time-dependent fashion. FMLP-induced 22Na+ uptake appeared to be independent of the external Ca2+ concentration (0 to 4 mM). In contrast, there was a progressive decrease in the magnitude of the FMLP-induced increase in 45Ca2+ uptake as the Na+ concentration was reduced by replacement with choline+ or glucose. These studies support a requirement for Na+ in FMLP-induced O-2 generation and suggest that a Na+ influx may underlie the nature of this requirement. The data are also consistent with the hypothesis that a Na+ influx may precede the Ca2+ influx in the FMLP-induced activation sequence.
This work was supported in part by Grant 8030 from the Medical Research Information System, United States Public Health Service Grant AM 19349, and an Arthritis Foundation Clinical Research Center Grant.