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 s upport C1- secretion by providing the necessary driving force. The pharmaco logical characteristics of these channels were examined in Ussing chamber e xperiments of rat and rabbit colon mucosa by the use of blockers. The chrom anol 293B, a blocker of K(V)LQT1 channels, and clotrimazole (CTZ), a blocke r of small Ca2+-activated K+ channels, blocked stimulated C1- secretion com pletely. Small-conductance Ca2+-activated K+ channels (SK) in excised basol ateral patches of rat colonic crypts were inhibited concentration dependent ly by the imidazoles CTZ, NS004 and NS1619 and activated by 1-EBIO. These p roperties are similar to those of the known human SK channel (hSK4). hSK4-e xpressing Xenopus laevis oocytes showed ionomycin-activated and CTZ-inhibit ed K+ currents. When P2Y(2) receptors were coexpressed these currents were also activated by ATP. The concentration/response curve was identical to th at of rat SK channels. In human colonocytes (T84) exposed to hSK4 antisense probes, but not to sense probes, carbachol-induced K+ currents were attenu ated. With RT-PCR an hSK4 could be demonstrated in human colon and in T84 c olonocytes. By homology cloning the SK of the rat colon (rSK4) was identifi ed. This protein has a high homology to hSK4 and mouse IKI. These data indi cate that the Ca2+-activated and imidazole-inhibited basolateral K+ current in the colon is caused by SK4 channels.