Ammonia and urea excretion in the tidepool sculpin (Oligocottus maculosus): sites of excretion,effects of reduced salinity and mechanisms of urea transport |
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| Authors: | P A Wright P Part C M Wood |
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| Institution: | (1) Department of Zoology, University of Guelph, Guelph, Ontario, Canada, N1G 2W1;(2) Department of Ecotoxicology, Uppsala University, Norbyvägen 18A, Uppsala, S-752 36, Sweden;(3) Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1 |
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| Abstract: | Tidepool sculpins live in a variable environment where water temperature, salinity, gas tensions, and pH can change considerably with the daily tide cycle. Tidepool sculpins are primarily ammoniotelic, with 8–17% of nitrogen wastes excreted as urea. The majority of net ammonia (Jnet
amm; 85%) and urea (Jnet
urea; 74%) excretion occurred across the gill, with the remainder excreted across the skin, the kidney, and/or gut. Acute (2h) exposure to 50% seawater significantly increased Jnet
urea (2.8-fold), but reduced Jnet
amm (3.5-fold). In fish exposed to 50% seawater for 1 week, Jnet
urea returned to control values, but Jnet
amm remained slightly depressed. Unidirectional urea influx (Jin
urea) and efflux (Jout
urea) were measured using14C-urea to determine if urea was excreted across the gills by simple diffusion or by a carrier-mediated mechanism. Jin
urea increased in a linear manner with increasing urea water levels (0–11 mmol N l–1), while Jout
urea was independent of external urea concentrations. As well, Jnet
urea and Jout inurea were not significantly different from one another, indicating the absence of  back transport . Urea analogs and transport inhibitors added to the water did not have any consistent effect on unidirectional urea flux. These results demonstrate that ammonia and urea excretion rates and sites of excretion in tidepool sculpins are very similar to those found in other marine and freshwater teleosts. Urea and ammonia may play a role in osmoregulation as excretion rates and tissue levels were influenced by changes in water salinity. Finally, we found no evidence for a specific urea carrier; branchial urea excretion is likely dependent on simple diffusion. |
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| Keywords: | nitrogenous waste products gills unidirectional urea flux marine teleost osmoregulation |
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