Cystic fibrosis transmembrane conductance regulator in the gills of the climbing perch, Anabas testudineus, is involved in both hypoosmotic regulation during seawater acclimation and active ammonia excretion during ammonia exposure
This study aimed to clone and sequence the cystic fibrosis transmembrane conductance regulator (cftr) from, and to determine the effects of seawater acclimation or exposure to 100 mmol l−1 NH4Cl in freshwater on its mRNA and protein expressions in, the gills of Anabas testudineus. There were 4,530 bp coding for 1,510 amino acids in the cftr cDNA sequence from A. testudineus. The branchial mRNA expression of cftr in fish kept in freshwater was low (<50 copies of transcript per ng cDNA), but significant increases were observed in fish acclimated to seawater for 1 day (92-fold) or 6 days (219-fold). Branchial Cftr expression was detected in fish acclimated to seawater but not in the freshwater control, indicating that Cl− excretion through the apical Cftr of the branchial epithelium was essential to seawater acclimation. More importantly, fish exposed to ammonia also exhibited a significant increase (12-fold) in branchial mRNA expression of cftr, with Cftr being expressed in a type of Na+/K+-ATPase-immunoreactive cells that was apparently different from the type involved in seawater acclimation. It is probable that Cl− excretion through Cftr generated a favorable electrical potential across the apical membrane to drive the excretion of $$ {\text{NH}}_{4}^{ + } $$ against a concentration gradient through a yet to be determined transporter, but it led to a slight loss of endogenous Cl−. Since ammonia exposure also resulted in significant decreases in blood pH, [HCO3−] and [total CO2] in A. testudineus, it can be deduced that active $$ {\text{NH}}_{4}^{ + } $$ excretion could also be driven by the exit of HCO3− through the apical Cftr. Furthermore, A. testudineus uniquely responded to ammonia exposure by increasing the ambient pH and decreasing the branchial bafilomycin-sensitive V-type H+-ATPase activity, which suggests that its gills might have low NH3 permeability.