The linkage between Na+ uptake and ammonia excretion in rainbow trout: Kinetic analysis, the effects of (NH4)2SO4 and NH4HCO3 infusion and the influence of gill boundary layer pH

Annika Salama, Ian J. Morgan, Chris M. Wood

Research output: Contribution to journalArticle

54 Scopus citations

Abstract

The nature of the linkage between between branchial ammonia excretion (J(Amm)) and unidirectional Na+ influx (J(in)/(Na)) was studied in the freshwater rainbow trout (Oncorhynchus mykiss). Arterial plasma total [ammonia], P(NH3) and J(Amm) were all elevated approximately threefold by intravascular infusion for 24 h with either 70 mmol l-1 (NH4)2SO4 or 140 mmoll-1 NH4HCO3 at a rate of approximately 400 μmol kg-1 h-1. Both treatments markedly stimulated J(in)/(Na). NH4HCO3 induced metabolic alkalosis in the blood plasma, whereas (NH4)2SO4 caused a slight metabolic acidosis. Experiments with Hepes-buffered water (5 mmol l-1) under control conditions demonstrated that increases in gill boundary layer pH were associated with decreases in both J(in)/(Na) and J(Amm). Thus, the stimulation of J(in)/(Na) caused by ammonium loading was not simply a consequence of a Na+-coupled H+ extrusion mechanism activated by internal acidosis or by alkalosis in the gill boundary layer. Indeed, there was no stimulation of net acidic equivalent excretion accompanying NH4HCO3 infusion. Michaelis-Menten kinetic analysis by acute variation of water [Na+] demonstrated that both infusions caused an almost twofold increase in J(max)/(Na) but no significant change in K(m), indicative of an increase in transporter number or internal counterion availability without an alteration in transporter affinity for external Na+. The increase in J(in)/(Na) was larger with (NH4)2SO4 than with NH4HCO3 infusion and in both cases lower than the increase in J(Amm). Additional evidence of quantitative uncoupling was seen in the kinetics experiments, in which acute changes in J(in)/(Na) of up to threefold had negligible effects on J(Amm) under either control or ammonium-loaded conditions. In vitro measurements of branchial Na+/K+-ATPase activity demonstrated no effect of NH4+ concentration over the concentration range observed in vivo in infused fish. Overall, these results are consistent with a dominant role for NH3 diffusion as the normal mechanism of ammonia excretion, but indicate that ammonium loading directly stimulates J(in)/(Na), perhaps by activation of a non-obligatory Na+/NH4+ exchange rather than by an indirect effect (e.g. Na+-coupled H+ excretion) mediated by altered internal or external acid-base status.

Original languageEnglish (US)
Pages (from-to)697-709
Number of pages13
JournalJournal of Experimental Biology
Volume202
Issue number6
StatePublished - Mar 1 1999

Keywords

  • (NH)SO infusion
  • Ammonia excretion
  • Branchial ion exchange
  • Gill boundary layer pH
  • Na uptake
  • Na/K-ATPase
  • NH diffusion
  • NHHCO infusion
  • Oncorhynchus mykiss
  • Rainbow trout

ASJC Scopus subject areas

  • Agricultural and Biological Sciences(all)
  • Agricultural and Biological Sciences (miscellaneous)

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