Ammonia and urea excretion in the tidepool sculpin (Oligocottus maculosus): sites of excretion, effects of reduced salinity and mechanisms of urea transport

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 (J(net) amm; 85%) and urea (J(net) 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 J(net) urea (2.8-fold), but reduced J(net) amm (3.5-fold). In fish exposed to 50% seawater for 1 week, J(net) urea returned to control values, but J(net) amm remained slightly depressed. Unidirectional urea influx (J(in) urea) and efflux (J(out) urea) were measured using(14)C-urea to determine if urea was excreted across the gills by simple diffusion or by a carrier-mediated mechanism. J(in) urea increased in a linear manner with increasing urea water levels (0-11 mmol N l(-1)), while J(out) urea was independent of external urea concentrations. As well, J(net) urea and J(out 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.