Mg 2+ sensitizes K ATP channels to inhibition by DIDS: dependence on the sulphonylurea receptor subunit

ATP‐sensitive potassium channels (K ATP channels) consist of pore‐forming Kir6.x subunits and of sulphonylurea receptors (SURs). In the absence of Mg 2+ , the stilbene disulphonate, DIDS, irreversibly inhibits K ATP channels by binding to the Kir subunit. Here, the effects of Mg 2+ on the interaction of DIDS with recombinant K ATP channels were studied in electrophysiological and [ 3 H]‐glibenclamide binding experiments. In inside‐out macropatches, Mg 2+ (0.7 m M ) increased the sensitivity of K ATP channels towards DIDS up to 70 fold (IC 50 =2.7 μ M for Kir6.2/SUR2B). Inhibition of current at DIDS concentrations 10 μ M was irreversible. Mg 2+ sensitized the truncated Kir6.2Δ26 channel towards inhibition by DIDS only upon coexpression with a SUR subunit (SUR2B). The effect of Mg 2+ did not require the presence of nucleotides. [ 3 H]‐glibenclamide binding to SUR2B(Y1206S), a mutant with improved affinity for glibenclamide, was inhibited by DIDS. The potency of inhibition was increased by Mg 2+ and by coexpression with Kir6.2. In the presence of Mg 2+ , DIDS inhibited binding of [ 3 H]‐glibenclamide to Kir6.2/SUR2B(Y1206S) with IC 50 =7.9 μ M by a non‐competitive mechanism. Inhibition was fully reversible. It is concluded that the binding site of DIDS on SUR that is sensed by glibenclamide does not mediate channel inhibition. Instead, Mg 2+ binding to SUR may allosterically increase the accessibility and/or reactivity of the DIDS site on Kir6.2. The fact that the Mg 2+ effect does not require the presence of nucleotides underlines the importance of this ion in modulating the properties of the K ATP channel.British Journal of Pharmacology (2002) 137 , 429–440. doi: 10.1038/sj.bjp.0704905