Summary
New evidence for a tightly coupled Na+/NH +4 exchange across the gill of trout was obtained with the help of the perfused head preparation.
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1)
The rate of Na+ influx is strictly dependent on the ammonia concentration of the perfusing fluid. When the internal ammonia concentration is increased from 0 to 800 μEq/l, Na+ influx increases from 20.8±4.10 μEq/h·100 g (n=6) to 51.8±4.18 μEq/h·100g (n=6) and the net flux changes from −52.2±25.60 μEq/h·100 g (n=6) to +21.7±4.00 μEq/h·100g (n=6).
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In the absence of a pH gradient and a pCO2 gradient between the external and internal media, ammonia is found to be excreted. This ammonia excretion occurs in the form of the ammonium ion.
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A fraction of the ammonia excreted is dependent upon the presence of Na+ in the external medium. The relationship between ammonia excretion and external Na+ concentration seems to follow Michaelis Menten kinetics.
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Amiloride added to the external medium produces a significant decrease in both Na+ influx and ammonia excretion.
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Acidification of the internal medium (from 7.3 to 6.4) is followed by an important inhibition of both Na+ influx and ammonia excretion.
From these observations a model is proposed for cationic exchange through the gill epithelium. A coupled Na+/NH +4 probably occurs at the apical barrier, Na+ transport across the inner membrane being brought about by the Na+/K+ ATPase. Ammonia possibly crosses the baso-lateral membrane in the unionized form.
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Payan, P. A study of the Na+/NH +4 exchange across the gill of the perfused head of the trout (Salmo gairdneri). J Comp Physiol B 124, 181–188 (1978). https://doi.org/10.1007/BF00689179
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DOI: https://doi.org/10.1007/BF00689179