Coexpression with the inward rectifier K(+) channel Kir6.1 increases the affinity of the vascular sulfonylurea receptor SUR2B for glibenclamide.

ATP-sensitive K(+) channels are closed by the hypoglycemic sulfonylureas like glibenclamide (GBC) and activated by a class of vasorelaxant compounds, the K(+) channel openers. These channels are octamers of Kir6.x and sulfonylurea receptor (SUR) subunits with 4:4 stoichiometry. The properties of the opener-sensitive K(+) channel in the vasculature are well matched by the SUR2B/Kir6.1 channel; however, the GBC sensitivity of the recombinant channel is unknown. In binding experiments we have determined the affinity of GBC for SUR2B and the SUR2B/Kir6.1 channel and compared the results with the channel blocking potency of GBC. All experiments were performed in whole transfected human embryonic kidney cells at 37 degrees C. The equilibrium dissociation constants (K(D)) of GBC binding to SUR2B and to the SUR2B/Kir6.1 complex were determined to be 32 and 6 nM, respectively; the K(D) value of the opener P1075 (N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine) ( approximately 5 nM) was, however, not affected by cotransfection. In whole cell voltage-clamp experiments, GBC inhibited the SUR2B/Kir6.1 channel with IC(50) approximately 43 nM. The data show that, in the intact cell: 1) SUR2B, previously considered to be a low-affinity SUR, has a rather high affinity for GBC; 2) coexpression with the inward rectifier Kir6.1 increases the affinity of SUR2B for GBC; 3) the recombinant channel exhibits the same GBC affinity as the opener-sensitive K(+) channel in vascular tissue; and 4) the K(D) value of GBC binding to the octameric channel is 7 times lower than the IC(50) value for channel inhibition. The latter finding suggests that occupation of all four GBC sites per channel is required for channel closure.