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.

Cortisol-sensitive urea transport across the gill basolateral membrane of the gulf toadfish (Opsanus beta)

“…Final ethanol concentration in the individual solutions did not exceed 0.1% (vol/vol). The effects of urea analogs (acetamide, thiourea, N-methylurea) were measured at twofold higher concentrations (0.8 mmol/l) than urea (0.4 mmol/l), in accordance with earlier published studies (47,71,74).…”

Modulation of Rh glycoproteins, ammonia excretion and Na+ fluxes in three freshwater teleosts when exposed chronically to high environmental ammonia

“…Previously, urea was thought to permeate cell membranes solely by simple diffusion through cell membranes (Wood, 1993;Wright et al, 1995b) but abundant evidence now suggests the occurrence of a specialized facilitated diffusion-type urea transporter (UT) in teleost gills (for review, see McDonald et al, 2012). We found a marked increase in branchial mRNA expression of UT during HEA, occurring only in goldfish (Fig.10).…”

Effects of temperature and salinity on oxygen consumption and ammonia excretion of juvenile miiuy croaker, Miichthys miiuy (Basilewsky)

“…The OCR and AER of miiuy croaker were calculated from the following equations (Wright et al 1995;Valverde et al 2006): …”