Interaction of K(ATP) channel modulators with sulfonylurea receptor SUR2B: implication for tetramer formation and allosteric coupling of subunits.

Sulfonylurea receptors (SURs) are subunits of ATP-sensitive K+ channels (K-ATP channels); they mediate the channel-closing effect of sulfonylureas such as glibenclamide and the channel-activating effect of K-ATP channel openers such as the pinacidil analog P1075. We investigated the inhibition by MgATP and P1075 of glibenclamide binding to SUR2B, the SUR subtype in smooth muscle. To increase specific binding, experiments were also performed using SUR2B(Y1206S), a mutant with higher affinity for glibenclamide than for the wild-type (K-D = 4 versus 22 nM, respectively) but otherwise exhibiting similar pharmacological properties. In the absence of MgATP, [H]glibenclamide binding to both SURs was homogenous. MgATP inhibited [H-3]glibenclamide binding to both SURs to 25% by reducing the apparent number of glibenclamide binding sites, leaving the affinity unchanged. In the absence of MgATP, P1075 inhibited [H-3]glibenciamide binding in a monophasic manner with K-i approximate to1 muM. In the presence of MgATP (1 mM), inhibition was biphasic with one K-i value resembling the true affinity of P1075 for SUR2B (2-6 nM) and the other resembling K, in the absence of MgATP (approximate to1 muM). The data show that (1) MgATP induces heterogeneity in the glibenclamide sites; (2) the high-affinity glibenclamide sites remaining with MgATP are linked to two classes of P1075 sites; and (3) P1075 interacts specifically with SUR2B also in the absence of MgATP. The data are discussed with the assumption that SUR2B, expressed alone, forms tetramers; that MgATP induces allosteric interactions between the subunits; and that mixed SUR2B-glibenclamide-P1075 complexes can exist at equilibrium.