Molecular and functional characterization of the small Ca2+-regulated K+ channel (rSK4) of colonic crypts

Colonic crypt cells possess basolateral Ca(2+)regulated Kf channels which support C1- secretion by providing the necessary driving force. The pharmacological characteristics of these channels were examined in Ussing chamber experiments of rat and rabbit colon mucosa by the use of blockers. The chromanol 293B, a blocker of K(V)LQT1 channels, and clotrimazole (CTZ), a blocker of small Ca2+-activated K+ channels, blocked stimulated C1- secretion completely. Small-conductance Ca2+-activated K+ channels (SK) in excised basolateral patches of rat colonic crypts were inhibited concentration dependently by the imidazoles CTZ, NS004 and NS1619 and activated by 1-EBIO. These properties are similar to those of the known human SK channel (hSK4). hSK4-expressing Xenopus laevis oocytes showed ionomycin-activated and CTZ-inhibited K+ currents. When P2Y(2) receptors were coexpressed these currents were also activated by ATP. The concentration/response curve was identical to that of rat SK channels. In human colonocytes (T84) exposed to hSK4 antisense probes, but not to sense probes, carbachol-induced K+ currents were attenuated. With RT-PCR an hSK4 could be demonstrated in human colon and in T84 colonocytes. By homology cloning the SK of the rat colon (rSK4) was identified. This protein has a high homology to hSK4 and mouse IKI. These data indicate that the Ca2+-activated and imidazole-inhibited basolateral K+ current in the colon is caused by SK4 channels.