title: Freshwater fish gill ion transport: August Krogh to morpholinos and microprobes.
abstract: August Krogh proposed that freshwater fishes (and other freshwater animals) maintain body NaCl homoeostasis by extracting these ions from the environment via separate Na(+) /NH(4)(+) and Cl(-) /HCO(3)(-) exchangers in the gill epithelium. Subsequent data from other laboratories suggested that Na(+) uptake was more probably coupled to H(+) secretion via a vesicular proton pump (V-ATPase) electrically coupled to a Na(+) channel. However, despite uncertainty about electrochemical gradients, evidence has accrued that epithelial Na(+) /H(+) exchange indeed may be an alternative pathway for Na(+) uptake. The specific pathways for Na(+) uptake may be species and environment specific. An apical Cl(-) /HCO(3)(-) exchanger is generally accepted for most species (some species do not extract Cl(-) from freshwater), but the relative roles of anion exchanger-like (SLC4A1) vs. pendrin-like (SLC26Z4) exchangers are unknown, and also may be species specific. Most recently, data have supported the presence of an apical Na(+) + Cl(-) cotransporter (NCC-type), despite thermodynamic uncertainty. Ammonia extrusion may be via NH(3) diffusing through the paracellular junctions or NH(4) (+) substitution on both basolateral and apical ionic exchangers (Na(+) + K(+) -ATPase; Na(+) + K(+) + Cl(-) - cotransporter; and Na(+) /H(+) exchanger), but recent evidence suggests that Rhesus-glycoproteins mediate both basolateral and apical movement of ammonia.
authors.affiliation: Department of Biology, University of Florida, Gainesville, FL 32611, USA. devans@ufl.edu
authors.forename: D H
authors.initials: DH
authors.lastname: Evans
mesh_terms: D000641:Ammonia / Q000378:metabolism; D000818:Animals; D016764:Cell Polarity; D002712:Chlorides / Q000378:metabolism; D004848:Epithelium / Q000033:anatomy & histology / Q000378:metabolism; D005399:Fishes / Q000033:anatomy & histology / Q000378:metabolism*; D005880:Gills / Q000033:anatomy & histology / Q000378:metabolism*; D006706:Homeostasis; D007473:Ion Channels / Q000378:metabolism; D017136:Ion Transport / Q000502:physiology*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D006180:Proton-Translocating ATPases / Q000378:metabolism; D011522:Protons; D012964:Sodium / Q000378:metabolism
chemical_list: D002712:Chlorides; D007473:Ion Channels; D026901:Membrane Transport Proteins; D011522:Protons; D000641:Ammonia; D012964:Sodium; D006180:Proton-Translocating ATPases