Submitted on October 18, 2007
Accepted on January 19, 2008
Identification of intestinal bicarbonate transporters involved in formation of carbonate precipitates to stimulate water absorption in marine teleost fish
Yukihiro Kurita1, Tsutomu Nakada1, Akira Kato1, Hiroyuki Doi2, Abinash C. Mistry1, Min-Hwang Chang3, Michael F. Romero3, and Shigehisa Hirose1*
1 Biological Sciences, Tokyo Institute of Technology, Yokohama, Japan
2 Shimonoseki Academy of Marine Science, Shimonoseki Marine Science Museum "Kaikyokan", shimonoseki, Japan
3 Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota, United States
* To whom correspondence should be addressed. E-mail: shirose{at}bio.titech.ac.jp.
Marine teleost fish precipitate divalent cations as carbonate deposits in their intestine to minimize the potential for excessive calcium entry and to stimulate water absorption by reducing lumen 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 calcium 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 freshwater- and seawater-acclimated euryhaline pufferfish, mefugu (Takifugu obscurus), and obtained the following candidate clones: NBCe1 (a sodium/bicarbonate cotransporter) and Slc26a6A & B (putative Cl-/HCO3- exchangers). Heterologous expression in Xenopus oocytes showed that Slc26a6A and Slc26a6B have potent bicarbonate transporting activity as electrogenic Cl-/nHCO3- exchangers, while mefugu NBCe1 functions as an electrogenic Na+/nHCO3- cotransporter. The expression of NBCe1 and Slc26a6A were highly induced in the intestine in SW and that of Slc26a6B was high in the intestine under both SW and FW both, suggesting their involvement in bicarbonate secretion and carbonate precipitate formation. Immunohistochemistry showed staining on the apical (Slc26a6A & B) 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).
