Physiological and molecular analysis of the interactive effects of feeding and high environmental ammonia on branchial ammonia excretion and Na+ uptake in freshwater rainbow trout

Alex M. Zimmer, C. Michele Nawata, Chris M. Wood

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62 Citations (Scopus)

Abstract

Recently, a "Na+/NH4+ exchange complex" model has been proposed for ammonia excretion in freshwater fish. The model suggests that ammonia transport occurs via Rhesus (Rh) glycoproteins and is facilitated by gill boundary layer acidification attributable to the hydration of CO2 and H+ efflux by Na+/H+ exchanger (NHE-2) and H+-ATPase. The latter two mechanisms of boundary layer acidification would occur in conjunction with Na+ influx (through a Na+ channel energized by H+-ATPase and directly via NHE-2). Here, we show that natural ammonia loading via feeding increases branchial mRNA expression of Rh genes, NHE-2, and H+-ATPase, as well as H+-ATPase activity in juvenile trout, similar to previous findings with ammonium salt infusions and high environmental ammonia (HEA) exposure. The associated increase in ammonia excretion occurs in conjunction with a fourfold increase in Na+ influx after a meal. When exposed to HEA (1.5 mmol/l NH4HCO3 at pH 8.0), both unfed and fed trout showed differential increases in mRNA expression of Rhcg2, NHE-2, and H+-ATPase, but H+-ATPase activity remained at control levels. Unfed fish exposed to HEA displayed a characteristic reversal of ammonia excretion, initially uptaking ammonia, whereas fed fish (4 h after the meal) did not show this reversal, being able to immediately excrete ammonia against the gradient imposed by HEA. Exposure to HEA also led to a depression of Na+ influx, demonstrating that ammonia excretion can be uncoupled from Na+ influx. We suggest that the efflux of H+, rather than Na+ influx itself, is critical to the facilitation of ammonia excretion.

Original languageEnglish
Pages (from-to)1191-1204
Number of pages14
JournalJournal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology
Volume180
Issue number8
DOIs
StatePublished - Oct 29 2010

Fingerprint

molecular analysis
Oncorhynchus mykiss
Fresh Water
Ammonia
excretion
rainbow
ammonia
uptake mechanisms
H-transporting ATP synthase
Proton-Translocating ATPases
Fish
Fishes
Trout
Acidification
effect
trout
acidification
Meals
Boundary layers
fish

Keywords

  • Ammonia excretion
  • Feeding
  • High environmental ammonia (HEA)
  • Rainbow trout
  • Rhesus glycoproteins

ASJC Scopus subject areas

  • Physiology
  • Ecology, Evolution, Behavior and Systematics
  • Animal Science and Zoology
  • Biochemistry
  • Endocrinology

Cite this

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title = "Physiological and molecular analysis of the interactive effects of feeding and high environmental ammonia on branchial ammonia excretion and Na+ uptake in freshwater rainbow trout",
abstract = "Recently, a {"}Na+/NH4+ exchange complex{"} model has been proposed for ammonia excretion in freshwater fish. The model suggests that ammonia transport occurs via Rhesus (Rh) glycoproteins and is facilitated by gill boundary layer acidification attributable to the hydration of CO2 and H+ efflux by Na+/H+ exchanger (NHE-2) and H+-ATPase. The latter two mechanisms of boundary layer acidification would occur in conjunction with Na+ influx (through a Na+ channel energized by H+-ATPase and directly via NHE-2). Here, we show that natural ammonia loading via feeding increases branchial mRNA expression of Rh genes, NHE-2, and H+-ATPase, as well as H+-ATPase activity in juvenile trout, similar to previous findings with ammonium salt infusions and high environmental ammonia (HEA) exposure. The associated increase in ammonia excretion occurs in conjunction with a fourfold increase in Na+ influx after a meal. When exposed to HEA (1.5 mmol/l NH4HCO3 at pH 8.0), both unfed and fed trout showed differential increases in mRNA expression of Rhcg2, NHE-2, and H+-ATPase, but H+-ATPase activity remained at control levels. Unfed fish exposed to HEA displayed a characteristic reversal of ammonia excretion, initially uptaking ammonia, whereas fed fish (4 h after the meal) did not show this reversal, being able to immediately excrete ammonia against the gradient imposed by HEA. Exposure to HEA also led to a depression of Na+ influx, demonstrating that ammonia excretion can be uncoupled from Na+ influx. We suggest that the efflux of H+, rather than Na+ influx itself, is critical to the facilitation of ammonia excretion.",
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AU - Zimmer, Alex M.

AU - Nawata, C. Michele

AU - Wood, Chris M.

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N2 - Recently, a "Na+/NH4+ exchange complex" model has been proposed for ammonia excretion in freshwater fish. The model suggests that ammonia transport occurs via Rhesus (Rh) glycoproteins and is facilitated by gill boundary layer acidification attributable to the hydration of CO2 and H+ efflux by Na+/H+ exchanger (NHE-2) and H+-ATPase. The latter two mechanisms of boundary layer acidification would occur in conjunction with Na+ influx (through a Na+ channel energized by H+-ATPase and directly via NHE-2). Here, we show that natural ammonia loading via feeding increases branchial mRNA expression of Rh genes, NHE-2, and H+-ATPase, as well as H+-ATPase activity in juvenile trout, similar to previous findings with ammonium salt infusions and high environmental ammonia (HEA) exposure. The associated increase in ammonia excretion occurs in conjunction with a fourfold increase in Na+ influx after a meal. When exposed to HEA (1.5 mmol/l NH4HCO3 at pH 8.0), both unfed and fed trout showed differential increases in mRNA expression of Rhcg2, NHE-2, and H+-ATPase, but H+-ATPase activity remained at control levels. Unfed fish exposed to HEA displayed a characteristic reversal of ammonia excretion, initially uptaking ammonia, whereas fed fish (4 h after the meal) did not show this reversal, being able to immediately excrete ammonia against the gradient imposed by HEA. Exposure to HEA also led to a depression of Na+ influx, demonstrating that ammonia excretion can be uncoupled from Na+ influx. We suggest that the efflux of H+, rather than Na+ influx itself, is critical to the facilitation of ammonia excretion.

AB - Recently, a "Na+/NH4+ exchange complex" model has been proposed for ammonia excretion in freshwater fish. The model suggests that ammonia transport occurs via Rhesus (Rh) glycoproteins and is facilitated by gill boundary layer acidification attributable to the hydration of CO2 and H+ efflux by Na+/H+ exchanger (NHE-2) and H+-ATPase. The latter two mechanisms of boundary layer acidification would occur in conjunction with Na+ influx (through a Na+ channel energized by H+-ATPase and directly via NHE-2). Here, we show that natural ammonia loading via feeding increases branchial mRNA expression of Rh genes, NHE-2, and H+-ATPase, as well as H+-ATPase activity in juvenile trout, similar to previous findings with ammonium salt infusions and high environmental ammonia (HEA) exposure. The associated increase in ammonia excretion occurs in conjunction with a fourfold increase in Na+ influx after a meal. When exposed to HEA (1.5 mmol/l NH4HCO3 at pH 8.0), both unfed and fed trout showed differential increases in mRNA expression of Rhcg2, NHE-2, and H+-ATPase, but H+-ATPase activity remained at control levels. Unfed fish exposed to HEA displayed a characteristic reversal of ammonia excretion, initially uptaking ammonia, whereas fed fish (4 h after the meal) did not show this reversal, being able to immediately excrete ammonia against the gradient imposed by HEA. Exposure to HEA also led to a depression of Na+ influx, demonstrating that ammonia excretion can be uncoupled from Na+ influx. We suggest that the efflux of H+, rather than Na+ influx itself, is critical to the facilitation of ammonia excretion.

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KW - Rhesus glycoproteins

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