2-SIDED ACTION OF PROTONS ON AN INWARD RECTIFIER K+ CHANNEL (IRK1)

A cloned inwardly rectifying potassium channel IRK1, expressed in Xeno pus oocytes was found to be sensitive to an extracellular acidic pH le vel of below 6, achieved by buffering with a membrane-impermeable buff er, phthalate. The voltage dependency of the suppressive effect of pH on the macroscopic current suggested that the location of the proton-s ensitive site was at approximate to 5% of the distance from the outer entrance to the port. The single-channel conductance was reduced by pr otonation of the channel on the extracellular side. The external proto n-binding site appears to consist of a single class of negatively char ged groups with a pK of around 4.6. An intracellular acidic pH, buffer ed with membrane-permeable acetate, was found to inhibit, in a voltage -independent manner, the macroscopic IRKI current with an approximate apparent FK of 5.6 and an approximate apparent Hill coefficient of 3.3 . The single-channel activity was abolished by intracellular acidifica tion down to pH 5.0.