Abstract

K_ATP channels are heteromeric complexes of inwardly rectifying K⁺ channel subunits and sulfonylurea receptors (SURs). SUR2A and SUR2B, which differ within 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, [³H]P1075, to membranes from human embryonic kidney (HEK) cells transfected with murine SUR2A and 2B at 37°C. Binding to both SURs required addition of Mg²⁺ and ATP in the low micromolar range. In the presence of MgATP, micromolar concentrations of MgADP, formed by the ATPase activity of the membrane preparation, increased binding to SUR2A but inhibited binding to SUR2B. Decreasing temperatures strongly reduced [³H]P1075 binding to SUR2A, whereas binding to SUR2B was increased in a bell-shaped manner. Kinetic experiments revealed a faster dissociation of the [³H]P1075-SUR2A complex, whereas the association rate constants for [³H]P1075 binding to SUR2A and 2B were similar. Openers inhibited [³H]P1075 binding to SUR2A with potencies ≈4 times lower than to SUR2B; in contrast, glibenclamide inhibited [³H]P1075 binding to SUR2A ≈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_ATPchannels, respectively, and show that MgADP is an important modulator of opener binding to SUR. The different carboxyl termini of SUR2A and 2B lead to differences in the MgADP dependence and the thermodynamics of [³H]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 modulator binding.