The effect of osmotic and ion-osmotic stresses on the blood and urine composition and urine flow of rainbow trout (Salmo gairdneri).

Changes in urine and plasma concentrations (sodium, potassium, magnesium, calcium and total osmotic) and urine production were determined in fish exposed to various concentrations of an ionically active substance, sodium chloride, and a non-electrolyte, mannitol, as well as freshwater. Responses occurred for the most part over a short crisis period preceding establishment of new stable conditions. It was shown that plasma homeostasis was not maintained in response to changing ion-osmotic and osmotic gradients. Urinary osmotic and ionic concentrations were unaffected and urine production was shown to be inversely related to the external concentration. It is suggested that ionic shifts between body compartments are an important aspect of ion-osmotic adaptation.