Characterisation of the Rat SK4/IK1 K+ Channel

Background and Aims: The Ca 2+ -activated K + channel rSK4 is the rat homologue of the human SK4/IK1 (KCNN4) channel. In colonic mucosa rSK4 plays a key role during acetylcholin-induced secretion. This study was aimed to characterize the properties of the rat SK4 channel.Methods: Electrophysiological measurements were performed on rSK4 expressing Xenopus laevis oocytes and rat colonic crypts. Intracellular Ca 2+ activity was assessed by Oregon Green fluorescence measurements.Results: The 10 pS rSK4 expressed in oocytes was Ca 2+ -sensitive and inhibited by calmodulin antagonists. 1-ethyl-2-benzimidazolinone (1-EBIO), a known activator of SK4/IK1 channels, also activated rSK4. 1-EBIO affected the current neither at saturating Ca 2+ activities nor under Ca 2+ -free conditions, but increased the Ca 2+ sensitivity of rSK4. rSK4 was strongly activated by cytosolic ATP. However, PKA itself, PKA inhibitors and mutation of the PKA phosphorylation site (S332A) did not affect channel activity. The PKC activator 1,2-dioctanoyl-sn-glycerol and the PKC inhibitor bisindolylmaleimide also failed to influence rSK4.Conclusion: The Ca 2+ -sensitive rSK4 is activated by 1-EBIO probably via facilitation of the Ca 2+ -calmodulin-rSK4 interaction. The strong ATP-activation of rSK4 is likely to be caused by phosphorylation via a yet unknown kinase and might involve additional subunits.