title: 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.
abstract: 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 NH₄⁺ 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 NH₄⁺ 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.
authors.affiliation: Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore. dbsipyk@nus.edu.sg
authors.forename: Yuen K
authors.initials: YK
authors.lastname: Ip
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Wilson
authors.forename: Ai M
authors.initials: AM
authors.lastname: Loong
authors.forename: Xiu L
authors.initials: XL
authors.lastname: Chen
authors.forename: Wai P
authors.initials: WP
authors.lastname: Wong
authors.forename: Inês L S
authors.initials: IL
authors.lastname: Delgado
authors.forename: Siew H
authors.initials: SH
authors.lastname: Lam
authors.forename: Shit F
authors.initials: SF
authors.lastname: Chew
mesh_terms: D000064:Acclimatization / Q000502:physiology; D000595:Amino Acid Sequence; D000641:Ammonia / Q000097:blood / Q000378:metabolism; D000643:Ammonium Chloride / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D003001:Cloning, Molecular; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000502:physiology*; D005618:Fresh Water; D005880:Gills / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D010471:Perches / Q000502:physiology*; D010802:Phylogeny; D012333:RNA, Messenger / Q000378:metabolism; D012623:Seawater; D016415:Sequence Alignment; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D012333:RNA, Messenger; D000643:Ammonium Chloride; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D000641:Ammonia; D025262:Vacuolar Proton-Translocating ATPases; D000254:Sodium-Potassium-Exchanging ATPase