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MUCOSAL BIOLOGY

Na⁺/Ca²⁺ exchange regulates Ca²⁺-dependent duodenal mucosal ion transport and HCO₃^– secretion in mice

Division of Gastroenterology, Department of Medicine, School of Medicine, University of California, San Diego, San Diego, California

Submitted 24 August 2004 ; accepted in final form 12 October 2004

Stimulation of muscarinic receptors in duodenal mucosa raises intracellular Ca²⁺, which regulates ion transport, including HCO₃^– secretion. However, the underlying Ca²⁺ handling mechanisms are poorly understood. The aim of the present study was to determine whether Na⁺/Ca²⁺ exchanger (NCX) plays a role in the regulation of duodenal mucosal ion transport and HCO₃^– secretion by controlling Ca²⁺ homeostasis. Mouse duodenal mucosa was mounted in Ussing chambers. Net ion transport was assessed as short-circuit current (I_sc), and HCO₃^– secretion was determined by pH-stat. Expression of NCX in duodenal mucosae was analyzed by Western blot, and cytosolic Ca²⁺ in duodenocytes was measured by fura 2. Carbachol (100 µM) increased I_sc in a biphasic manner: an initial transient peak within 2 min and a later sustained plateau starting at 10 min. Carbachol-induced HCO₃^– secretion peaked at 10 min. 2-Aminoethoxydiphenylborate (2-APB, 100 µM) or LiCl (30 mM) significantly reduced the initial peak in I_sc by 51 or 47%, respectively, and abolished the plateau phase of I_sc without affecting HCO₃^– secretion induced by carbachol. Ryanodine (100 µM), caffeine (10 mM), and nifedipine (10 µM) had no effect on either response to carbachol. In contrast, nickel (5 mM) and KB-R7943 (10–30 µM) significantly inhibited carbachol-induced increases in duodenal mucosal I_sc and HCO₃^– secretion. Western blot analysis showed expression of NCX1 proteins in duodenal mucosae, and functional NCX in duodenocytes was demonstrated in Ca²⁺ imaging experiments where Na⁺ depletion elicited Ca²⁺ entry via the reversed mode of NCX. These results indicate that NCX contributes to the regulation of Ca²⁺-dependent duodenal mucosal ion transport and HCO₃^– secretion that results from stimulation of muscarinic receptors.

sodium-calcium exchange

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