Identification of intestinal bicarbonate transporters involved in formation of carbonate precipitates to stimulate water absorption in marine teleost fish

Marine teleost fish precipitate divalent cations as carbonate deposits in the intestine to minimize the potential for excessive Ca2+ entry and to stimulate water absorption by reducing luminal osmotic pressure. This carbonate deposit formation, therefore, helps maintain osmoregulation in the seawater (SW) environment and requires controlled secretion of HCO3(-) to match the amount of Ca2+ entering the intestinal lumen. Despite its physiological importance, the process of HCO3(-) secretion has not been characterized at the molecular level. We analyzed the expression of two families of HCO3(-) transporters, Slc4 and Slc26, in fresh-water- and SW-acclimated euryhaline pufferfish, mefugu (Takifugu obscurus), and obtained the following candidate clones: NBCe1 (an Na+-HCO3(-) cotransporter) and Slc26a6A and Slc26a6B (putative Cl(-)/HCO3(-) exchangers). Heterologous expression in Xenopus oocytes showed that Slc26a6A and Slc26a6B have potent HCO3(-)-transporting activity as electrogenic Cl(-)/nHCO3(-) exchangers, whereas mefugu NBCe1 functions as an electrogenic Na+-nHCO3(-) cotransporter. Expression of NBCe1 and Slc26a6A was highly induced in the intestine in SW and expression of Slc26a6B was high in the intestine in SW and fresh water, suggesting their involvement in HCO3(-) secretion and carbonate precipitate formation. Immunohistochemistry showed staining on the apical (Slc26a6A and Slc26a6B) and basolateral (NBCe1) membranes of the intestinal epithelial cells in SW. We therefore propose a mechanism for HCO3(-) transport across the intestinal epithelial cells of marine fish that includes basolateral HCO3(-) uptake (NBCe1) and apical HCO3(-) secretion (Slc26a6A and Slc26a6B).