Stimulus-response coupling in the human neutrophil. Transmembrane potential and the role of extracellular Na+

Abstract

Receptor-ligand interactions at the surface of the human neutrophil induce lysosomal enzyme release and the generation of O2.-, responses which are anteceded by changes in the membrane potential (delta psi) as measured by [3H]-triphenylmethylphosphonium ion distribution. Surface stimuli (immune complexes, concanavalin A) initiated a rapid (less than 10 s) hyperpolarization response by both normal and cytochalasin B-treated cells. Replacement of extracellular Na+ with either K+ or choline depressed O2.- generation and lysosomal enzyme release in neutrophils exposed to concanavalin A or immune complexes. Replacement of Na+ with K+ led to a substantial fall in resting membrane potential, whereas replacement of Na+ with choline did not. Thus, depression of O2.- generation and lysosomal enzyme release in Na+-free medium were specifically due to a lack of extracellular Na+ and not to depolarization of the membrane. Although it has been shown that extracellular Na+, and possibly an influx of Na+, is required for optimal neutrophil function, neither depolarization nor Na+ influx per se was sufficient to activate fully these cells, since the Na+ ionophore, monensin, was not an effective stimulus for beta-glucuronidase release or O2.- generation. The hyperpolarization response to neutrophils exposed to immune complexes and to concanavalin A was greatly diminished in both high [K+] and [choline] buffers. Thus, extracellular Na+ was required for an optimal membrane potential response to receptor-ligand interaction. Since O2.- generation and lysosomal enzyme release in response to the Ca2+ ionophore, A23187, were also reduced in the absence of extracellular Na+, it was concluded that extracellular Na+ was also required after induction of Ca2+ fluxes. Ouabain (1 mM) had no effect on O2.- generation, lysosomal enzyme release or the hyperpolarization response to immune complexes, indicating that the hyperpolarization observed on stimulation cannot be due to the action of the electrogenic pump, (Na+ + K+)-ATPase. The experiments indicate that extracellular Na+ is required (1) in the delta psi response triggered by receptor-ligand interaction, and (2) at a step(s) subsequent to Ca2+ fluxes and common to O2.- generation and lysosomal enzyme release.