Independent stimulation of membrane potential changes and the oxidative metabolic burst in polymorphonuclear leukocytes

Abstract

Early events of stimulus-response coupling in polymorphonuclear leukocytes (PMNL) reportedly include membrane depolarization as a necessary antecedent to oxidative responses. However, depolarization by nonspecific means (ionophores) is insufficient to elicit an oxidative burst. This apparent conflict might be related to whether depolarization is due to membrane receptor-mediated stimulation of PMNL. We used two fluorescent probes and dual laser flow cytometry to monitor both membrane potential and oxidative product formation in individual PMNL, following stimulation by phorbol myristate acetate (PMA) or formylmethionyl-leucyl-phenylalanine (fMLP). Dipentylindocarbocyanine [di-I-C5(3)] is one of a family of dyes that partition between cells and aqueous media as a function of transmembrane potential. The dye appeared stable in the presence of PMNL oxidative products. Oxidation of intracellularly trapped, nonfluorescent dichlorofluorescin (DCFH) to fluorescent dichlorofluorescein (DCF) provided a quantitative assessment of oxidative metabolism (H2O2 production) of stimulated PMNL. Incubating PMNL with both fluorochromes resulted in stable red [di-l-C5(3)] and green (DCF) fluorescence when examined on a Cytofluorograph. Upon stimulation by 0.03 to 0.1 nmol/L PMA, PMNL showed a unimodal apparent depolarization (decrease in di-l-C5(3) fluorescence). Oxidative activity (increased DCF fluorescence) was first seen at a concentration of PMA of 0.17 nmol/L, higher than that required to elicit depolarization. This oxidative burst appeared as a dose-dependent, graded response. Thus, receptor-mediated membrane depolarization, although antecedent to the onset of the oxidative burst, was not in itself sufficient to trigger the oxidative metabolic response. When PMNL were isolated by centrifugation through Ficoll-metrizoate, fMLP caused an apparent depolarization of a variable subpopulation of the cells. However, such purified PMNL appeared relatively unstable and often depolarized spontaneously. PMNL studied without centrifugation through Ficoll-metrizoate were stable. Moreover, fMLP stimulation of such cells did not cause membrane depolarization but did stimulate a two- to six-fold increase in DCFH oxidation. Apparently, membrane depolarization may antecede oxidative responses in PMNL, but appears to depend upon the method of cell preparation and the nature of the stimulus.