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We have examined the effects of changing extracellular pH on the function of nicotinic acetylcholine receptors from Torpedo californica using ion flux and electrophysiological methods. Agonist-induced cation efflux from vesicles containing purified, reconstituted receptors showed a monotonic dependence on external hydrogen ion concentration with maximal fluxes at alkaline pH and no agonist-induced efflux at pH's less than approximately 5. A similar pH dependence was measured for the peak agonist-activated membrane currents measured in microelectrode voltage-clamped Xenopus oocytes induced to express Torpedo receptor through mRNA injection. Half-maximal inhibition occurred at a similar pH in both systems, in the range of pH 6.5-7.0. Single-channel currents from Torpedo ACh receptors measured in patch-clamp recordings were also reduced in amplitude at acid pH with an apparent pKa for block of less than 5. Measurements of channel kinetics had a more complicated dependence on pH. The mean channel open time determined from patch-clamp measurements was maximal at neutral pH and decreased at both acid and alkaline pH's. Thus, both channel permeability properties and channel gating properties are affected by the extracellular pH.
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