Chemotactic Factor-Induced Generation of Superoxide Radicals by Human Neutrophils: Evidence for the Role of Sodium

Abstract

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.