Role of Ca2+-activated K+ channels in duodenal mucosal ion transport and bicarbonate secretion

Stimulation of muscarinic receptors in the duodenal mucosa raises cytosolic free Ca2+ concentration ([Ca2+](cyt)), thereby regulating duodenal epithelial ion transport. However, little is known about the downstream molecular targets that account for this Ca2+-mediated biological action. Ca2+-activated K+ (K-Ca) channels are candidates, but the expression and function of duodenal K-Ca channels are poorly understood. Therefore, we determined whether K-Ca channels are expressed in the duodenal mucosa and investigated their involvement in Ca2+-mediated duodenal epithelial ion transport. Two selective blockers of intermediate-conductance Ca2+-activated K+ (IKCa) channels, clotrimazole (30 mu M) and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; 10 mu M), significantly inhibited carbachol (CCh)-induced duodenal short-circuit current (I-sc) and duodenal mucosal bicarbonate secretion (DMBS) in mice but did not affect responses to forskolin and heat-stable enterotoxin of Escherichia coli. Tetraethylammonium, 4-aminopyridine, and BaCl2 failed to inhibit CCh-induced Isc and DMBS. A-23187 ( 10 mu M), a Ca2+ ionophore, and 1-ethyl-2-benzimidazolinone ( 1- EBIO; 1 mM), a selective opener of K-Ca channels, increased both I-sc and DMBS. The effect of 1- EBIO was more pronounced with serosal than mucosal addition. Again, both clotrimazole and TRAM-34 significantly reduced A23187- or 1-EBIO-induced I-sc and DMBS. Moreover, clotrimazole (20 mg/kg ip) significantly attenuated acid-stimulated DMBS of mice in vivo. Finally, the molecular identity of IKCa channels was verified as KCNN4 (SK4) in freshly isolated murine duodenal mucosae by RT-PCR and Western blotting. Together, our results suggest that the IKCa channel is one of the downstream molecular targets for [Ca2+](cyt) to mediate duodenal epithelial ion transport.