FIGURE 8.

Model to show the hypothetical role of NBCn2 in small intestine epithelium. In the apical membrane, NHEs (e.g., NHE2 and NHE3) in concert with SLC26 transporters mediate NaCl absorption. The NaCl absorption via this pathway involves the action of carbonic anhydrase (CA) in both the luminal side and the cytosol. The presence of NBCn2 in the apical membrane likely provides an alternate pathway for the absorption of Na⁺ and the direct uptake of luminal HCO₃^–. In the basolateral membrane, NaCl extrusion is mediated by the coupled action of Na⁺-K⁺ pump and Cl^– channel ClC-2 (Lipecka et al., 2002; Pena-Munzenmayer et al., 2005). Kir7.1 expressed in the basolateral membrane provides a pathway for the cycling of K⁺ (Partiseti et al., 1998; Nakamura et al., 1999).