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 (Isc) and enhanced the tissue resistance (Rt), 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.

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