Effects of Inorganic Cations on Na+ Adsorption to the Gill and Body Surface of Rainbow Trout, Oncorhynchus mykiss, in Dilute Solutions

Using the shortest practical exposure time (15 s) to 0.2 mmol∙L−1 NaCl solutions containing 22Na, over 80% of the Na+ taken up by rainbow trout, Oncorhynchus mykiss, was located on the gill or body surface (adsorption). Influx to the blood (absorption) was less than 20% of the total taken up, even though Na+ entry into the blood was detected in less than 30 s. Sodium adsorption to the fish surface was strongly reduced by micromolar concentrations of Ca2+ or by acid in the range pH 7–5. At higher NaCl concentrations (2 mmol∙L−1 in the medium, Ca2+ effects on adsorption were greatly attenuated. Aluminum had a negligible effect on adsorption at pH 4. It is concluded that adsorption has an important role in presenting Na+ to the apical surface of gill ion transporting cells, since conditions which inhibit the branchial influx of Na+ also reduce Na+ adsorption. The results suggest that adsorption is controlled by ion exchange events at gill surface ligands, and by the effects of other cations on the mobility of Na+ in the bulk water. Adsorption phenomena are likely to influence ionic regulation during normal fluctuations in environmental conditions.

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Excretion and distribution of ammonia and the influence of boundary layer acidification in embryonic rainbow trout (Oncorhynchus mykiss)

Journal of Experimental Biology ◽

10.1242/jeb.199.12.2713 ◽

1996 ◽

Vol 199 (12) ◽

pp. 2713-2723 ◽

Cited By ~ 1

Author(s):

E Rahaman-Noronha ◽

M Donnell ◽

C Pilley ◽

P Wright

Keyword(s):

Rainbow Trout ◽

Oncorhynchus Mykiss ◽

Electrical Potential ◽

Ammonia Excretion ◽

Free Water ◽

Bulk Water ◽

Water Addition ◽

Rainbow Trout Oncorhynchus Mykiss ◽

External Water ◽

Excretion Rates

This study examined ammonia excretion by embryos of the rainbow trout (Oncorhynchus mykiss). The distribution of ammonia in relation to the H+ distribution and electrical potential was determined. The influence of the pH of the unstirred layer (USL) of water next to the external surface of the embryo was also assessed. Eyed-up embryos (3540 days post-fertilization) were exposed to various external water conditions [pH 6.0, pH 10.0, 1.6 mmol l-1 NaCl, 0.0 mmol l-1 NaCl, 0.2 mmol l-1 NH4Cl, 2.5 mmol l-1 borax buffer (Na2B4O7.10H2O), 2.5 mmol l-1 Hepes, 0.1 mmol l-1 amiloride] for 30 min and ammonia excretion rates, ammonia concentration in the perivitelline fluid (PVF) and yolk, and the pH of the PVF, yolk and USL were measured. The rate of ammonia excretion was dependent, in part, on the partial pressure gradient of NH3 ( PNH3) from the PVF to the USL. Exposure to water of pH 6 increased, whereas NH4Cl or pH 10 exposure decreased, ammonia excretion rates. Elevated external Na+ levels also influenced the rate of ammonia excretion, but neither Na+-free water nor amiloride had any effect. The distribution of ammonia between the PVF and USL was dependent on the H+ distribution, but ammonia was distributed according to the electrical potential between the PVF and yolk. The USL was 0.32 pH units more acidic than the bulk water. Addition of buffer to the external water eliminated the acid USL and decreased ammonia excretion rates. We conclude that rainbow trout embryos excrete ammonia primarily as NH3, but when external Na+ levels are elevated, ammonia excretion may be independent of the PNH3 gradient. The acidic USL next to the chorion probably facilitates NH3 diffusion by maintaining the PNH3 through the conversion of NH3 to NH4+ upon entry into the USL.

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