Cystic fibrosis transmembrane conductance regulator mediates sulphonylurea block of the inwardly rectifying K+ channel Kir6.1

1. Recombinant ATP-sensitive K+ channels (K-ATP channels) were heterologously expressed in the NIH3T3 mouse cell line, and the electrophysiological properties were studied using patch-clamp techniques. 2. The NIH3T3 cell lines transfected with the inwardly rectifying K+ channel Kir6.1 alone or with both Kir6.1 and cystic fibrosis transmembrane conductance regulator (CFTR) exhibited time-independent K+ currents with weak inward rectification. In contrast, no measurable K+ conductance was observed in mock-transfected cells or in cells transfected with CFTR alone. Regardless of co-transfection with Kir6.1, the transfection with CFTR produced a Cl- conductance that was activated by cell dialysis with cAMP (1 mM). The conductance was reversibly suppressed by glibenclamide (30 mu M). 3. Whole-cell currents at +60 mV were blocked in a concentration-dependent manner by Ba2+ ions with similar IC50 values: 89.3 +/- 23.3 mu M (Kir6.1 alone) and 67.3 +/- 24.9 mu M (Kir6.1-CFTR). 4. The currents recorded from Kir6.1-transfected cells were not affected by glibenclamide, whereas glibenclamide did inhibit the conductance expressed in cells co-transfected with CFTR (IC50 = 35.9 +/- 6.6 mu M). 5. In the cell-attached mode with a 150 mM K+ pipette solution, both Kir6.1- and Kir6.1-CFTR-transfected cells displayed a class of K+ channels showing weak inward rectification and a slope conductance of 50.7 +/- 1.0 and 52.4 +/- 4.9 pS, respectively. 6. In the inside-out mode, the single-channel currents recorded from both types of cells were not inhibited by intracellular ATP (1 mM). However, glibenclamide was found to Mock the single-channel activities in the co-transfected cells.