Sensitivity of Kir6.2-SUR1 currents, in the absence and presence of sodium azide, to the K_ATP channel inhibitors, ciclazindol and englitazone

Two electrode voltage clamp and single channel recordings were used to investigate the actions of various ATP-sensitive K(+) (K_ATP) channel inhibitors on cloned K_ATP channels, expressed in Xenopus oocytes and HEK 293 cells. Oocytes expressing Kir6.2 and SUR1 gave rise to inwardly rectifying K⁺ currents following bath application of 3 mM sodium azide. Inside-out recordings from non-azide treated oocytes demonstrated the presence of K_ATP channels which were activated by direct application of 3 mM azide and 0.1 mM Mg-ATP. Tolbutamide inhibited azide-induced macroscopic Kir6.2-SUR1 currents, recorded from Xenopus oocytes, with an IC₅₀ value similar to native K_ATP channels. Ciclazindol and englitazone also inhibited these currents in a concentration-dependent manner, but with relative potencies substantially less than for native K(ATP) channels. Single channel currents recorded from inside-out patches excised from oocytes expressing Kir6.2-SUR1 currents were inhibited by tolbutamide, Mg-ATP, englitazone and ciclazindol, in the absence of azide, with potencies similar to native K_ATP channels. In the presence of azide, Kir6.2-SUR1 currents were inhibited by englitazone and tolbutamide but not ciclazindol. Single channel currents derived from Kir6.2?26, expressed in HEK 293 cells, were inhibited by ciclazindol and englitazone irrespective of the absence or presence of SUR1. In conclusion, heterologously expressed Kir6.2 and SUR1 recapitulate the pharmacological profile of native pancreatic beta-cell K_ATP channels. However, currents induced by azide exhibit a substantially reduced sensitivity to ciclazindol. It is likely that ciclazindol and englitazone inhibit K_ATP currents by interaction with the Kir6.2 subunit.