An alternate pathway of cAMP-stimulated Cl- secretion across the NKCC1-null murine duodenum

Background & Aims: Adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated anion secretion across the duodenal epithelium requires the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane-and anion uptake proteins in the basolateral membrane. NKCC1, the epithelial Na+/K+/2Cl(-) cotransporter, is the major protein responsible for Cl- uptake. In this study. We evaluate the role of NKCC1 in determining the relative rates of transepithelial Cl- and HCO3- secretion during cAMP stimulation of the duodenum. Methods: bicarbonate and chloride secretion across duodenal mucosa was measured in. USSing chambers by pH stat and Cl-36 flux methods using mice with either gene-targeted deletion of NKCC1 (NKCC1-/-) or bumetanide blockade of NKCC1. Results: Total anion secretion stimulated by forskolin treatment of NKCC1-null duodenum resulted from approximately equivalent rates of electrogenic chloride, electrogenic bicarbonate, and electroneutral bicarbonate secretion. Evaluation of the alternate chloride secretory pathway indicated chloride uptake by a basolateral membrane anion exchange process with characteristics consistent with the anion exchanger isoform AE2. Conclusions: Chloride Uptake by basolateral anion exchanger activity (AE2) supports intracellular cAMP-stimulated chloride secretion in the NKCC1-null duodenum. A model for the alternate chloride secretion pathway is proposed whereby chloride uptake via AE2 is coupled to basolateral NaHCO3 co-transport to support CFTR-mediated chloride and bicarbonate secretion.