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¹ Department of Physiology, University of Geneva Medical Center, CH-1211 Geneva 4, Switzerland
² Department of Physiology and Laboratory of Cellular and Molecular Physiology, Semmelweis University and Hungarian Academy of Sciences, H-1444 Budapest, Hungary

Address correspondence to Dr. Nicolas Demaurex, Department of Physiology, University of Geneva Medical Center, 1 Michel-Servet, CH-1211 Geneva 4, Switzerland. Fax: (41) 22-379-5402; email: Nicolas.Demaurex{at}medecine.unige.ch

The NADPH–oxidase is a plasma membrane enzyme complex that enables phagocytes to generate superoxide in order to kill invading pathogens, a critical step in the host defense against infections. The oxidase transfers electrons from cytosolic NADPH to extracellular oxygen, a process that requires concomitant H⁺ extrusion through depolarization-activated H⁺ channels. Whether H⁺ fluxes are mediated by the oxidase itself is controversial, but there is a general agreement that the oxidase and H⁺ channel are intimately connected. Oxidase activation evokes profound changes in whole-cell H⁺ current (I_H), causing an approximately -40-mV shift in the activation threshold that leads to the appearance of inward I_H. To further explore the relationship between the oxidase and proton channel, we performed voltage-clamp experiments on inside-out patches from both resting and phorbol-12-myristate-13-acetate (PMA)-activated human eosinophils. Proton currents from resting cells displayed slow voltage-dependent activation, long-term stability, and were blocked by micromolar internal [Zn²⁺]. I_H from PMA-treated cells activated faster and at lower voltages, enabling sustained H⁺ influx, but ran down within minutes, regaining the current properties of nonactivated cells. Bath application of NADPH to patches excised from PMA-treated cells evoked electron currents (I_e), which also ran down within minutes and were blocked by diphenylene iodonium (DPI). Run-down of both I_H and I_e was delayed, and sometimes prevented, by cytosolic ATP and GTP--S. A good correlation was observed between the amplitude of I_e and both inward and outward I_H when a stable driving force for e^- was imposed. Combined application of NADPH and DPI reduced the inward I_H amplitude, even in the absence of concomitant oxidase activity. The strict correlation between I_e and I_H amplitudes and the sensitivity of I_H to oxidase-specific agents suggest that the proton channel is either part of the oxidase complex or linked by a membrane-limited mediator.

Key Words: physiology • NADPH–oxidase • zinc • phagocyte • patch-clamp

Abbreviations used in this paper: CGD, chronic granulomatous disease; DPI, diphenylene iodonium chloride; NADPH, nicotinamide adenine dinucleotide phosphate; NBT, p-nitro-blue tetrazolium chloride; phox, phagocyte oxidase.

¹ The empirical equations that can be applied to estimate the H⁺ current threshold potential (E_threshold) under different E_H (E_rev) conditions in resting and PMA treated eosinophils are E_threshold = 0.79 E_rev + 23 mV and E_threshold = 0.63 E_rev - 22 mV, respectively (DeCoursey, 2003).

² The ability of NBT to capture electrons directly from the oxidase (Briggs et al., 1975) might also account for the increased correlation and larger e^- currents observed in the presence of NBT.

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