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From the Department of Molecular Biophysics and Physiology, Rush Presbyterian St. Luke's Medical Center, Chicago, Illinois 60612

H⁺ currents in human neutrophils, rat alveolar epithelial cells, and several mammalian phagocyte cell lines were studied using whole-cell and excised-patch tight-seal voltage clamp techniques at temperatures between 6 and 42°C. Effects of temperature on gating kinetics were distinguished from effects on the H⁺ current amplitude. The activation and deactivation of H⁺ currents were both highly temperature sensitive, with a Q₁₀ of 6–9 (activation energy, E_a, 30–38 kcal/mol), greater than for most other ion channels. The similarity of E_a for channel opening and closing suggests that the same step may be rate determining. In addition, when the turn-on of H⁺ currents with depolarization was fitted by a delay and single exponential, both the delay and the time constant (_act) had similarly high Q₁₀. These results could be explained if H⁺ channels were composed of several subunits, each of which undergoes a single rate-determining gating transition. H⁺ current gating in all mammalian cells studied had similarly strong temperature dependences. The H⁺ conductance increased markedly with temperature, with Q₁₀ 2 in whole-cell experiments. In excised patches where depletion would affect the measurement less, the Q₁₀ was 2.8 at >20°C and 5.3 at <20°C. This temperature sensitivity is much greater than for most other ion channels and for H⁺ conduction in aqueous solution, but is in the range reported for H⁺ transport mechanisms other than channels; e.g., carriers and pumps. Evidently, under the conditions employed, the rate-determining step in H⁺ permeation occurs not in the diffusional approach but during permeation through the channel itself. The large E_a of permeation intrinsically limits the conductance of this channel, and appears inconsistent with the channel being a water-filled pore. At physiological temperature, H⁺ channels provide mammalian cells with an enormous capacity for proton extrusion.

Key Words: proton channels • ion channels • pH • microglia • Q₁₀

Abbreviations: _a, activation energy; E_H, Nernst potential for H⁺; g_H, H⁺ chord conductance; HBC, hydrogen-bonded chain; I_H, extrapolated H⁺ current amplitude; V_hold, holding potential; V_rev, measured reversal potential

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