Abstract
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 degrees 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, Ea, approximately 30-38 kcal/mol), greater than for most other ion channels. The similarity of Ea 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 (tauact) 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 degrees C and 5.3 at <20 degrees 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 Ea 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.