LOCATION OF THE SULFONYLUREA RECEPTOR AT THE CYTOPLASMIC FACE OF THE BETA-CELL MEMBRANE

1 In insulin-secreting cells the location of the sulphonylurea receptor was examined by use of a sulphonylurea derivative representing the glibenclamide molecule devoid of its cyclohexyl moiety (compound III) and a benzenesulphonic acid derivative representing the glibenclamide molecule devoid of its cyclohexylurea moiety (compound IV). At pH 7.4 compound IV is only present in charged form. 2 Lipid solubility declined in the order tolbutamide >compound III >compound IV. 3 The dissociation constant (K-D) for binding of compound IV to the sulphonylurea receptor in HIT-cells (pancreatic beta-cell line) was similar to the K-D value for tolbutamide and fourfold higher than the K-D value for compound III. 4 In mouse pancreatic beta-cells, drug concentrations inhibiting adenosine 5'-triphosphate-sensitive K+ channels (K-ATP-channels) half-maximally (EC(50)) were determined by use of the patch-clamp technique. When the drugs were applied to the extracellular side of outside-out or the intracellular side of inside-out membrane patches, the ratio of extracellular to intracellular EC(50) values was 281 for compound IV, 25.5 for compound III and 1.2 for tolbutamide. 5 In mouse pancreatic beta-cells, measurement of K-ATP-channel activity in cell-attached patches and recording of insulin release displayed much higher EC(50) values for compound TV than inside-out patch experiments. A corresponding, but less pronounced difference in EC(50) values was observed for compound III, whereas the EC(50) values for tolbutamide did not differ significantly. 6 It is concluded that the sulphonylurea receptor is located at the cytoplasmic face of the beta-cell plasma membrane. Receptor activation is induced by the anionic forms of sulphonylureas and their analogues.