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Journal of Experimental Biology 150,367-379 (1990)
Published by Company of Biologists 1990

Effects of Bicarbonate on Salt and Water Transport Across the Intestine of the Seawater EEL

MASAAKI ANDO ¹

¹ Laboratory of Physiology, Faculty of Integrated Arts and Sciences, Hiroshima University Hiroshima 730, Japan

To elucidate whether the Na⁺/K⁺/Cl^- cotransport system depends on HCO₃^- in the seawater eel intestine, the effects of HCO₃^- on the transepithelial potential difference (PD) and on net water and ion fluxes were examined. When HCO₃^- buffer was replaced with phosphate buffer, the serosa-negative PD and net Na⁺, Cl^- and water fluxes from mucosa to serosa were inhibited, indicating that the Na⁺/K⁺/Cl^- cotransport system is inhibited in phosphate-buffered solutions. Similar inhibitory effects were also observed in solutions buffered with Hepes, Tris or Tes, indicating that the inhibitory effects are not specific for the phosphate buffer but are caused by omission of the HCO₃^- buffer system. Although the HCO₃^- buffer system consists of HCO₃^- and CO₂, higher CO₂ pressure with constant HCO₃^- concentration did not enhance, but inhibited, the PD and the net water flux: this indicates that the inhibition observed after removal of the HCO₃^- buffer system is due to omission of HCO₃^- rather than CO₂. The inhibition of PD and the net water flux was greater after removal of HCO₃^- from the serosal side than from the mucosal side. Similarly, the inhibitory effects of 4,4'-diisothiocyanostilbene- 2,2'-disulphonic acid (DIDS), an inhibitor of HCO₃^- transport, were more pronounced on the serosal side than on the mucosal side. Mucosal Ba²⁺ also inhibited PD and the short-circuit current (I_sc) and enhanced the tissue resistance (R_t), presumably through partially blocking the apical K⁺ channels. However, these effects of Ba²⁺ were completely abolished after pretreatment with serosal DIDS, suggesting that Ba²⁺ and DIDS evoked the same effect. These results are combined and a possible role for HCO₃^- in the Na⁺/K⁺/Cl^- cotransport system is discussed.

Key words: HCO₃^-, Na⁺/K⁺/Cl^- cotransport, pH, water transport, eel intestine.

Accepted on February 1, 1990

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