Abstract
In eutherian mammals, fluid secretion is essential for intestinal function. This is driven by electrogenic Cl- secretion, which involves a NaK2Cl cotransporter (NKCC1) in the enterocyte basolateral membrane and the cystic fibrosis transmembrane conductance regulator ( CFTR) in the apical membrane. However, in the possum ileum, NKCC1 expression is low and secretagogues stimulate electrogenic HCO3- secretion driven by a basolateral NaHCO3 cotransporter (pNBCe1). Here we investigated whether electrogenic anion secretion occurs in possum duodenum and jejunum and determined the role of CFTR in possum intestinal anion secretion. Prostaglandin E-2 (PGE(2)) and forskolin stimulated a large increase in ileal short-circuit current (I-sc), consistent with electrogenic HCO3- secretion, but had little effect on the duodenal and jejunal I-sc. Furthermore, 5-nitro-2-(3-phenylpropylamino) benzoic acid ( NPPB) and N-(2-naphthalenyl)-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] glycine hydrazide (GlyH101) inhibited cloned possum CFTR in cultured cells and the PGE(2)-stimulated ileal I-sc, implicating CFTR in ileal HCO3 secretion. Consistent with this, CFTR is expressed in the apical membrane of ileal crypt and lower villous cells, which also express pNBCe1 in the basolateral membrane. In contrast, duodenal and jejunal CFTR expression is low relative to the ileum. Jejunal pNBCe1 expression is also low, whereas duodenal and ileal pNBCe1 expression are comparable. All regions have low NKCC1 expression. These results indicate that cAMP-dependent electrogenic Cl- secretion does not occur in the possum small intestine because of the absence of CFTR and NKCC1. Furthermore, CFTR functions as the apical anion conductance associated with HCO3- secretion and its distribution limits electrogenic HCO3- secretion to the ileum.