Changes in Recombinant ATP-sensitive K+ Currents by Taurine Expressed in Xenopus Oocytes

Abstract

ATP-sensitive potassium (KATP) channels in different tissues usually share a common pore-forming Kir6.2 subunit, but possess different types of SUR subunit. KATP channels in the beta cell are composed of Kir6.2/SUR1, the skeletal and cardiac type of Kir6.2/SUR2A, and the vascular smooth muscle type of Kir6.2/SUR2B. We have compared the effect of taurine, which is a sulfonic aminoacid that is present particularly in mammalian excitable tissues at high concentrations, on the activities of KATP channels with distinct SUR submits and their sulfonylurea sensitivities, using recombinant KATP channels expressed in Xenopus laevis oocytes. Intracellular taurine inhibited all the above three types of KATP channels in dose-dependent manners. They were well fit to the Hill equation with similar potencies, showing IC50 (50% inhibitory concentration of the channel activity) of 10.6 ± 1.34 mM for Kir6.2/SUR1, 12.3 ± 1.9 mM for Kir6.2/SUR2A, and 12.6 ± 1.3 mM for Kir6.2/SUR2B. The Hill coefficients (h) were 4.5, 3.9,and 4.5, respectively. However, the taurine inhibiton did not appear for Kir6.2△C63 currunt. In the presenece of 10 mmol/L taurine, the sensitivities of the three channel types to glibenclamide were enhanced to a similar extent compared to those in the absence of taurine. Sensitivity of the channels to gliclazied, another sulfonylurea, was not increaed in the presence of taurine. From the above results, it might be speculated that the inhibitory effect of taurine on KATP channel activity is not related with different SUR subunits. The binding sites for glibenclamide on SURs, but not Kir6.2, might be critical for the taurine action.