Ionic and Osmotic Regulation in Rainbow Trout, Salmo Gairdneri.

Three experiments were performed using rainbow trout (Salmo gairdneri), to determine; (1)the feasibility of using an osmotic determinant to separate the ionic and osmotic components of ion-osmoregulation, (2)changes in urine and plasma concentrations and urine production associated with ionic and osmotic regulation and , (3)the energy expenditures involved in ionic and osmotic regulation. The separation of the ionic and osmotic components was accomplished by exposing fish to different but equal concentrations of ion-osmotic (sodium chloride) and osmotic (mannitol) solutions. In the first set of experiments fish were exposed to mannitol and sodium chloride concentrations of 400 to 700 mOsm. The 24-LC50's were calculated to be 767 and 175 mOsm for mannitol and sodium chloride, respectively. Mannitol's relatively lower toxicity indicated that it was possible to use it as an osmotic determinant in ion-osmo-regulation experiments utilizing fish. Plasma and urine regulatory changes in ionic and osmotic regulation were determined in a second set of experiments by exposing fish to mannitol and sodium chloride solutions and measuring plasma and urinary osmotic and ionic concentrations and urine production. Freshwater acclimated fish were transferred to test solutions of deionized water and mannitol and sodium chloride concentrations of 10, 250, and 500 mOsm. Control fish were held in freshwater. Plasma samples were taken prior to the transfer and after exposure periods of 1, 2, 4, and 7 days. Urine samples were also taken prior to transfer and then daily for 4 days after the exposure. In addition, short-term ion-osmotic adaptation was studied by exposing fish to 200 mOsm sodium chloride and freshwater and measuring plasma osmotic and ionic concentrations every 4 hours for 36 hours. Responses occurred for the most part over a short crisis period preceding establishment of new stable conditions. Plasma osmotic, sodium and potassium concentrations of hypo- and hyper- ion-osmotic exposed fish reached maximum levels after 8-24 hours, then decreased and stabilized after 7 days. Plasma calcium and magnesium concentrations of hypo-ion-osmotic exposed fish decreased initially and remained at reduced levels. Plasma osmotic concentrations of fish exposed to hyper-osmotic solutions also increased, however, there was no change in plasma sodium or potassium concentrations in these fish. As in the case of the hyper-ion-osmotic exposed fish plasma calcium and magnesium concentrations decreased in fish exposed to hypo- and hyper-osmotic solutions. There was no significant difference (p < 0.05) in urinary osmotic and ionic concentrations between fish exposed to hypo- and hyper-, ion-osmotic and osmotic solutions and the control fish. Urine production was shown to be inversely related to the external concentration, higher rates of urine production occurring at hypo-concentrations. Significantly (p < 0.05) lower rates of urine production were found after 4 days in fish exposed to ion-osmotic solutions than those in osmotic solutions at all concentrations. It was shown that plasma homeostasis was not maintained in response to changing ion-osmotic gradients and in the case of plasma osmotic concentration with changing osmotic gradients. Also, short-term plasma ionic homeostasis was maintained in response to osmotic adaptation. Recovery did not occur with a large hyper-ion-osmotic gradient when death resulted. It is suggested that ionic shifts between body compartments are an important aspect of ion-osmotic adaptation. In the last set of experiments the energetic costs of ionic and osmotic regulation was calculated from oxygen consumption data from fish exposed to freshwater, deionized water and mannitol and sodium chloride concentrations of 10, 250, and 500 mOsm. The results indicated that the cost of ionic regulation is significantly higher than the cost of osmotic regulation and that the cost of osmotic regulation is small.Ph.D.Animal PhysiologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/158289/1/8116275.pd