CYTOPLASMIC ACIDOSIS INDUCES MULTIPLE CONDUCTANCE STATES IN ATP-SENSITIVE POTASSIUM CHANNELS OF CARDIAC MYOCYTES

We studied the effect of cytoplasmic acidosis on the ionic conducting states of ATP-sensitive potassium channels in heart ventricular cells of guinea pigs and rabbits by using a patch-clamp technique with insid e-out patch configuration. Under normal conditions (pH 7.4), the chann el alternated between a closed state and a main open state in the abse nce of nucleotides on the cytoplasmic side. As internal pH was reduced below 6.5, the single channel current manifested distinct subconducta nce levels. The probability of the appearance of these subconductance levels was pH dependent with a greater probability of subconductance s tates at lower pH. A variance-mean amplitude analysis technique reveal ed two subconductance levels approximately equally spaced between the main open level and the closed level (63 and 33%). A current-voltage p lot of the two subconductance levels and the main level showed that th ey had similar reversal potentials and rectification properties. An in trinsic flickering gating property characteristic of these-ATP-sensiti ve channels was found unchanged in the 63% subconductance state, sugge sting that this subconductance state and the main conductance state sh are similar ion pore properties (including ion selection and block) an d similar gating mechanisms. The appearance of the subconductance stat es decreased as ionic strength was increased, and the subconductance s tates were also slightly voltage dependent, suggesting an electrostati c interaction between the protons and the negative surface charge in t he vicinity of the binding sites, which maybe close to the inner entra nce of the ion pore. Proteolytic modification of the channel on the cy toplasmic side with trypsin did not abolish the subconductance levels. External acidosis did not induce subconductance levels. These results suggest that protons bound to the negatively charged group at the inn er entrance of the channel ion pore may induce conformational changes, leading to partially reduced conductance states.