Two-sided action of protons on an inward rectifier K+ channel (IRK1).

A cloned inwardly rectifying potassium channel, IRK1, expressed in Xenopus oocytes was found to be sensitive to an extracellular acidic pH level of below 6, achieved by buffering with a membrane-impermeable buffer, phthalate. The voltage dependency of the suppressive effect of pH on the macroscopic current suggested that the location of the proton-sensitive site was at approximately 5% of the distance from the outer entrance to the pore. The single-channel conductance was reduced by protonation of the channel on the extracellular side. The external proton-binding site appears to consist of a single class of negatively charged groups with a pK of around 4.6. An intracellular acidic pH, buffered with membrane-permeable acetate, was found to inhibit, in a voltage-independent manner, the macroscopic IRK1 current with an approximate apparent pK of 5.6 and an approximate apparent Hill coefficient of 2.3. The single-channel activity was abolished by intracellular acidification down to pH 5.0.