title: Mechanisms of ion and acid-base regulation at the gills of freshwater fish.
abstract: This review examines the branchial mechanisms utilized by freshwater fish to regulate internal acid-base status and presents a model to explain the underlying basis of the compensatory processes. Rainbow trout, Oncorhynchus mykiss, and brown bullhead, Ictalurus nebulosus, were examined under a variety of experimental treatments which induced respiratory and metabolic acid-base disturbances. Acid-base regulation was achieved by appropriate adjustments of Na+ and Cl- net fluxes across the gills which, in turn, were accomplished by variable contributions of three different branchial mechanisms: 1) differential changes in Na+ and Cl- diffusive effluxes, 2) changes in internal substrate (H+, HCO3-) availability, and 3) morphological adjustments to the gill epithelium. Differential diffusive efflux of Na+ over Cl- was involved only during periods of metabolic alkalosis. The importance of internal substrate availability was demonstrated using a two-substrate model. According to the model, ionic flux rates (J(in)Cl-, J(in)Na+) are determined not only by the concentration of the external ion (Na+, Cl-) but also by the concentration of the internal counterion (H+, HCO3-). This system provides for an "automatic negative feedback" to aid in the compensation of metabolic acid-base disturbances. Morphological alteration of the gill epithelia and the associated regulation of chloride cell (CC) fractional area is an essential third mechanism which is especially important during respiratory acid-base disturbances. Specifically, fish vary the availability of the CC associated Cl-/HCO3- exchange mechanism by physical covering/uncovering of CCs by adjacent pavement cells.
authors.affiliation: Department of Biology, University of Ottawa, Ontario, Canada.
authors.forename: G G
authors.initials: GG
authors.lastname: Goss
authors.forename: S F
authors.initials: SF
authors.lastname: Perry
authors.forename: C M
authors.initials: CM
authors.lastname: Wood
authors.forename: P
authors.initials: P
authors.lastname: Laurent
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology*; D000818:Animals; D005880:Gills / Q000502:physiology*; D007059:Ictaluridae / Q000502:physiology*; D014337:Trout / Q000502:physiology*