The ionoregulatory responses to hypoxia in the freshwater rainbow trout Oncorhynchus mykiss

Physiol Biochem Zool. Mar-Apr 2010;83(2):343-55. doi: 10.1086/648566.


We utilized the rainbow trout, a hypoxia-intolerant freshwater teleost, to examine ionoregulatory changes at the gills during hypoxia. Progressive mild hypoxia led first to a significant elevation (by 21%) in J(Na)(influx) (measured with 22Na), but at 4-h hypoxia when PCO2 reached approximately 110 mmHg, there was a 79% depression in J(Na)(influx). Influx remained depressed during the first hour of normoxic recovery but was restored back to control rates thereafter; there were no significant changes in J(Na)(efflux) or J(Na)(net). A more prolonged (8 h) and severe hypoxic (approximately 80 mmHg) exposure induced a triphasic response whereby J(Na)(influx) was significantly elevated during the first hour, as during mild hypoxia, but returned to control rates during the subsequent 3 h. Thereafter, rates started to gradually increase and remained significantly elevated by about 38% through to 8 h of hypoxia. A similar triphasic trend was observed with J(Na)(efflux) but with larger changes than in J(Na)(influx), such that negative Na+ balance occurred during the hypoxic exposure. Net K+ loss rates to the water approximately doubled. There were no significant alterations in ammonia excretion rates in either of the hypoxia regimes. Branchial Na+/K+-ATPase activity did not change during 4 h at PO2 approximately 80 mmHg or return to normoxia; H+-ATPase activity also did not change during hypoxia but was significantly depressed by approximately 75% after 6 h of normoxic recovery. Scanning electron microscopy revealed that within 1 h of exposure to PO2 approximately 80 mmHg, exposed mitochondria-rich cell (MRC) numbers increased by 30%, while individual MRC exposed surface area and total MRC surface area both increased by three- to fourfold. MRC numbers had decreased below control levels by 4 h of hypoxia, but surface exposure remained elevated by approximately twofold, a response that persisted through 6 h of normoxic recovery. Environmental hypoxia induces complex changes in gill ionoregulatory function in this hypoxia-intolerant species that are very different from those recently reported in the hypoxia-tolerant Amazonian oscar.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptation, Physiological / physiology
  • Ammonia / metabolism
  • Animals
  • Gills / anatomy & histology
  • Gills / metabolism
  • Gills / physiology
  • Hypoxia / physiopathology*
  • Microscopy, Electron, Scanning
  • Mitochondria / metabolism
  • Mitochondria / ultrastructure
  • Oncorhynchus mykiss / physiology*
  • Sodium / metabolism
  • Sodium / physiology
  • Sodium-Potassium-Exchanging ATPase / metabolism
  • Sodium-Potassium-Exchanging ATPase / physiology


  • Ammonia
  • Sodium
  • Sodium-Potassium-Exchanging ATPase