ATP-sensitive K+ channel modulator binding to sulfonylurea receptors SUR2Aand SUR2B: Opposite effects of MgADP

K-ATP channels are heteromeric complexes of inwardly rectifying K+ channel subunits and sulfonylurea receptors (SURs). SUR2A and SUR2B, which differ w ithin the carboxyl terminal exon 38, are characteristic for the cardiac and smooth muscle type channels, respectively. Here we compare binding of the tritiated K-ATP channel opener, [H-3]P1075, to membranes from human embryon ic kidney (HEK) cells transfected with murine SUR2A and 2B at 37 degrees C. Binding to both SURs required addition of Mg2+ and ATP in the low micromol ar range. In the presence of MgATP, micromolar concentrations of MgADP, for med by the ATPase activity of the membrane preparation, increased binding t o SUR2A but inhibited binding to SUR2B. Decreasing temperatures strongly re duced [H-3]P1075 binding to SUR2A, whereas binding to SUR2B was increased i n a bell-shaped manner. Kinetic experiments revealed a faster dissociation of the [H-3]P1075-SUR2A complex, whereas the association rate constants for [H-3]P1075 binding to SUR2A and 2B were similar. Openers inhibited [H-3]P1 075 binding to SUR2A with potencies approximate to 4 times lower than to SU R2B; in contrast, glibenclamide inhibited [H-3]P1075 binding to SUR2A appro ximate to 8 times more potently than to SUR2B. The data suggest that SUR2A and 2B represent the opener receptors of cardiac and vascular smooth muscle K-ATP channels, respectively, and show that MgADP is an important modulato r of opener binding to SUR. The different carboxyl termini of SUR2A and 2B lead to differences in the MgADP dependence and the thermodynamics of [H-3] P1075 binding, as well as in the affinities for openers and glibenclamide, underlining the importance of this part of the molecule for K-ATP channel m odulator binding.