Subconductance states of the cardiac K(ATP) channel revealed by partial block with glybenclamide.

Currents carried by single ATP-sensitive potassium channels (K(ATP) channels) were recorded in membrane patches isolated from adult rat ventricular myocytes. Channel currents were blocked completely by ATP at millimolar concentrations and by glybenclamide at micromolar concentrations. However, at lower glybenclamide concentrations (1-1000 nM), a partial block, manifest as a subconductance state, was often seen. At concentrations of 100-300 nM the mean size of the subconductance state was 33+/-2.7 pS (175 mM potassium in the pipette; n=13). The size of the conductance substate varied slightly with the concentration of glybenclamide, (42 pS at 1 nM, 34 pS at 100 nM and 31 pS at 1 microM), while the open time of the subconductance state decreased with increasing glybenclamide concentration (n=4). ATP (4 mM) completely blocked both the main conductance state of the channel and the subconductance state induced by glybenclamide. Submaximal concentrations of ATP also appeared to induce subconductance states, but these could not be resolved into discrete conductance levels. The observation that subconductance states can be induced by low concentrations of glybenclamide may have implications for models of how the binding of glybenclamide is translated into closure of the Kir6.2 pore.