ATP-sensitive K⁺ channels in rat colonic epithelium

ATP-sensitive K⁺ (K_ATP) channels couple the metabolic state of a cell to its electrical activity. They consist of a hetero-octameric complex with pore-forming Kir6.x (Kir6.1, Kir6.2) and regulatory sulfonylurea receptor (SUR) subunits. Functional data indicate that K_ATP channels contribute to epithelial K⁺ currents at colonic epithelia. However, their molecular identity and their properties are largely unknown. Therefore, changes in short-circuit current (I _sc) induced by the K_ATP channel opener pinacidil (5 10⁻⁴ mol l⁻¹) were measured in Ussing chambers under control conditions and in the presence of different blockers of K_ATP channels. The channel subunits expressed by the colonic epithelium were identified by immunohistochemistry and by RT-PCR. The K⁺ channel opener, when administered at the serosal side, induced an increase in I _sc consistent with the induction of transepithelial Cl⁻ secretion after activation of basolateral K⁺ channels, whereas mucosal administration of pinacidil resulted in a negative I _sc. The increase in I _sc evoked by serosal pinacidil was inhibited by serosal administration of glibenclamide (5 10⁻⁴ mol l⁻¹) and gliclazide (10⁻⁶ mol l⁻¹), but was resistant even against a high concentration (10⁻² mol l⁻¹) of tolbutamide. In contrast, none of these inhibitors (administered at the mucosal side) reduced significantly the negative I _sc induced by mucosal pinacidil. Instead, pinacidil inhibited Cl⁻ currents across apical Cl⁻ channels in basolaterally depolarized epithelia indicating that the negative I _sc induced by mucosal pinacidil is due to a transient inhibition of Cl⁻ secretion. In mRNA prepared from isolated colonic crypts, messenger RNA for both pore-forming subunits, Kir6.1 and Kir6.2, and two regulatory subunits (SUR1and SUR2B) was found. Expression within the colonic epithelium was confirmed for these subunits by immunohistochemistry. In consequence, K_ATP channels are present in the basolateral membrane of the colonic epithelium; their exact subunit composition, however, has still to be revealed.