SLC26 family members are anionic transporters involved in Cl^- and HCO₃^- absorption or secretion in epithelia. SLC26A9, preferentially expressed in the lung, is a poorly characterized member of this family. In this study, we investigated the transport properties of human SLC26A9 to determine its functional and pharmacological characteristics. SLC26A9 protein expression results in the appearance of an anionic current exhibiting an apparently linear current/voltage relationship and increases in ³⁶Cl influxes and effluxes. The sequences of conductivity, Cl^- >I^- > NO₃^- ≧ gluconate > SO₄ ^2- and selectivity (P_x/P_CI), I^- > NO₃^- > Cl^- > gluconate > SO₄^2- are found. Cl^- channel inhibitors DIDS and NS 3623 inhibit SLC26A9 associated currents while the specific CFTR inhibitor (CFTR(inh)-172) or glybenclamide has little effect. Elevation of intracellular cAMP (a CFTR activator) is also ineffective whereas increasing intracellular calcium blocks the SLC26A9 associated currents. The HCO₃^- conductance mediated by the SLC26A9 protein expression is low and no intracellular pHi changes are detectable under conditions favoring a Cl^-/HCO₃^- exchange. However, the presence of HCO₃^-/CO₂ stimulates the Cl^--transporting activity of SLC26A9 in Xenopus laevis oocytes or SLC26A9-transduced COS-7 cells. As an important initial step in characterizing SLC26A9 function, we conclude that SLC26A9 is a Cl^- channel and we suggest that HCO₃^- acts as a modulator of the channel. SLC26A9 physiological role in airway epithelia and its potential interaction with CFTR remain to be elucidated.

We investigated the possible functional- and physical protein-interactions between two airway Cl(-) channels, SLC26A9 and CFTR. Bronchial CFBE41o- cell lines expressing CFTR(WT) or CFTR(ΔF508) were transduced with SLC26A9. Immunoblots identified a migrating band corresponding to SLC26A9 present in whole-cell lysates as on apical membrane of cells grown on polarized filters. CFTR levels were increased by the presence of SLC26A9 in both CFTR(WT) and CFTR(ΔF508) cell lines. In CFBE41o- cells and CFBE41o-/CFTR(WT) cells transduced with SLC26A9, currents associated to the protein expression were not detected. However, the forskolin (FK)-stimulated currents were enhanced in SLC26A9-transduced cells compared to control cells. Therefore, the presence of SLC26A9 resulted in an increase in CFTR activity (same % of CFTR((inh)-172) or GlyH-101 inhibition in both groups). In CFBE41o-/CFTR(ΔF508) cells transduced with SLC26A9 (at 27°C), a current associated to the protein expression was also lacking. FK-stimulated currents and level of CFTR((inh)-172) inhibition were not different in both groups. The presence of SLC26A9 in Xenopus oocytes expressing CFTR also enhanced the FK-stimulated currents as compared to oocytes expressing CFTR alone. This stimulation was mostly linked to CFTR. An enhancement of FK-stimulated currents was not found in oocytes co-expressing SLC26A9 and CFTR(ΔF508). In conclusion, in both protein expression systems used, SLC26A9 stimulates CFTR activity but not that of CFTR(ΔF508). Our co-immunoprecipitation studies demonstrate a physical interaction between both anion channels. We propose as an alternative hypothesis (not exclusive) to the known SLC26A9-STAS domain/CFTR interaction, that SLC26A9 favors the biogenesis and/or stabilization of CFTR, leading to stimulated currents.

Branchial sodium uptake was measured across secondary lamellae (SL) using the isolated-perfused head preparation. This study was correlated with histological examination.Freshwater trout, Sulmo guirdneri, were exposed to various artificial media:distilled water (DW), fresh water (FW: NaCl < 200 pMA; C a + + = 1.5 mMA), FW + 5 mM CaCl2 and FW + 10 mM CaC12. These environments altered the number and forms of "chloride cells" (CC) located in SL and were related to changes in maximal transport rate of Na+. An increase in a specific type of CC (round) was associated with a n increase in V, , , , and a decrease in round cells was associated with a decrease in V, , , .

Cichlids of the genus Oreochromis are fish of economic importance in African countries. They tolerate brackish water, however, with great variations between species. In this work, two species, both from the Ivory Coast but of different origins, O. niloticus (field and laboratory strains) and O. aureus (field strain) were compared during osmotic challenges (10, 20 and 30%o salinity) in order to provide physiological support for their specific behaviour when confronted with natural hypertonic environments. Tolerance to salinity was assessed by correlated observations on gill structure, plasma sodium levels and gill Na+/K+ ATPase activity. In fresh water (FW), all fish presented a gill epithelium structure characteristic of FW stenohaline fish: no chloride cells (CC) on the lamellae and few CC on the filaments. An increase in external salinity induced the proliferation of CC on filaments, a feature typical of seawater teleosts. This change in gill structure was accompanied by an increase of gill Na+/K+ ATPase activity. In the most tolerant strains, plasma Na+ did not change, indicating successful ion regulation in the hypertonic media. With regard to potential interest of field strains in fish culture, O. aureus acclimated more easily to brackish water than O. niloticus. Interestingly, O. niloticus, kept for several generations in the laboratory, performed best in our challenge studies. Plasma Na+ levels and gill CC proliferation upon transfer to an isotonic medium may be the parameters of choice when testing these fish for their response to a salinity change.