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+ (KCa) channels are candidates, but the expression and function of duodenal KCa channels are poorly understood. Therefore, we determined whether KCa 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 μM) and 1-[(2-chlorophenyl)diphenylmethyl]-1 H-pyrazole (TRAM-34; 10 μM), significantly inhibited carbachol (CCh)-induced duodenal short-circuit current ( Isc) 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 μM), a Ca2+ ionophore, and 1-ethyl-2-benzimidazolinone (1-EBIO; 1 mM), a selective opener of KCa channels, increased both Isc 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 Isc 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.
Molecular Mechanisms of Calcium-sensing Receptor-mediated Calcium Signaling in the Modulation of Epithelial Ion Transport and Bicarbonate Secretion