Mechanisms of Cl− uptake in rainbow trout: Cloning and expression of slc26a6, a prospective Cl−/HCO3− exchanger

In fresh waters, fishes continuously acquire ions to offset diffusive losses to a more dilute ambient environment and to maintain acid–base status. The objectives of the present study were to clone slc26a6, a prospective Cl−/HCO3− exchanger from rainbow trout, investigate its expression patterns in various tissues, at different developmental stages and after differential salinity exposure, and probe the mechanisms of Cl− uptake in rainbow trout embryos during development using a pharmacological inhibitor approach combined with 36Cl− unidirectional fluxes. Results showed that the cloned gene encoded a 783 amino acid protein with conserved domains characteristic of the SLC26a family of anion exchange proteins. Phylogenetic analysis of this sequence against all subfamilies of the SLC26a family demonstrated that this translated protein shared a common ancestor with other actinopterygii and mammalian SLC26a6 isoforms and thus confirmed the identity of the cloned gene. Expression of slc26a6 was detected in all tissues and developmental stages assayed but was highest in the gill of juvenile trout. In trout embryos, Cl− uptake increased significantly post-hatch and was demonstrated to be mediated via an anion exchanger specific (DIDS sensitive) pathway that was also sensitive to hypercapnia. This parallels well with the predicted function of slc26a6, and the detection of the transcript in embryos and tissues of trout. In conclusion, this study is the first report of slc26a6 in rainbow trout and functional and expression analyses indicate its likely involvement in Cl−/HCO3− exchange in two life stages of rainbow trout.