A nose-to-nose comparison of the physiological and molecular responses of rainbow trout to high environmental ammonia in seawater versus freshwater

Chris M. Wood, C. Michele Nawata

Research output: Contribution to journalArticle

44 Scopus citations

Abstract

Steelhead rainbow trout acclimated to either freshwater (FW) or seawater (SW) were exposed to high environmental ammonia (HEA, 1000μmoll -1 NH 4HCO 3, pH7.8-8.0) for 24h. SW trout restored ammonia excretion more rapidly (3-6h versus 9-12h in FW), despite higher production rates and lower plasma pH. Plasma total ammonia levels stabilized at comparable levels below the external HEA concentration, and blood acid-base disturbances were small at both salinities. The electrochemical gradients for NH 4 + entry (FNH 4 +) were the same in the two salinities, but only because FW trout allowed their transepithelial potential to rise by -15mV during HEA exposure. Elevation of plasma [cortisol] during HEA exposure was more prolonged in SW fish. Plasma [glucose] increased in SW, but decreased in FW trout. Plasma [urea-N] also decreased in FW, in concert with elevated urea transporter (UT) mRNA expression in the gills. Of 13 branchial transporters, baseline mRNA expression levels were higher for Rhcg1, NHE2, NKCC1a and UT, and lower for NBC1 and NKA-1a in SW trout, whereas NKA-1b, NHE3, CA2, H +-ATPase, Rhag, Rhbg and Rhcg2 did not differ. Of the Rh glycoprotein mRNAs responding to HEA, Rhcg2 was greatly upregulated in both FW and SW, Rhag decreased only in SW and Rhcg1 decreased only in FW. H+-ATPase mRNA increased in FW whereas NHE2 mRNA increased in SW; NHE3 did not respond, and V-type H +-ATPase activity declined in SW during HEA exposure. Branchial Na +,K +- ATPase activity was much higher in SW gills, but could not be activated by NH 4 +. Overall, the more effective response of SW trout was explained by differences in physical chemistry between SW and FW, which greatly reduced the plasma NH 3 tension gradient for NH 3 entry, as well as by the higher [Na +] in SW, which favoured Na +-coupled excretion mechanisms. At a molecular level, responses in SW trout showed subtle differences from those in FW trout, but were very different than in the SW pufferfish. Upregulation of Rhcg2 appears to play a key role in the response to HEA in both FW and SW trout, and NH 4 + does not appear to move through Na +,K +-ATPase.

Original languageEnglish (US)
Pages (from-to)3557-3569
Number of pages13
JournalJournal of Experimental Biology
Volume214
Issue number21
DOIs
StatePublished - Nov 1 2011

Keywords

  • Ammonia transport
  • Cortisol
  • Gene expression
  • K -ATPase
  • Na
  • Oncorhynchus mykiss
  • PNH
  • Rhesus glycoprotein
  • Transepithelial potential

ASJC Scopus subject areas

  • Animal Science and Zoology
  • Ecology, Evolution, Behavior and Systematics
  • Molecular Biology
  • Physiology
  • Insect Science
  • Aquatic Science

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