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Does the presence of a seawater gill morphology induced by dietary salt loading affect Cl uptake and acid–base regulation in freshwater rainbow trout Oncorhynchus mykiss

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The goal of this study was to determine the effect of the changes in gill morphology induced by dietary salt feeding on several aspects of gill function in rainbow trout Oncorhynchus mykiss maintained in fresh water with specific emphasis on Cl uptake (J INCl) and acid–base regulation. The addition of 11% NaCl to the diet caused J INCl to be reduced by c. 45% from 214·4 ± 26·7 to 117·3 ± 17·4 µmol kg−1 h−1 (mean ±s.e.). Rates of Cl efflux (J OUTCl), net Cl flux (J NETCl), J NETNa+ and plasma levels of Na+ or Cl were unaffected by salt feeding. On the basis of significant effect of the salt diet on decreasing the maximal uptake rate of Cl (J MAXCl), it would appear that internal salt loading caused a decrease in the number of functional ion transport proteins involved in Cl uptake (e.g. Cl–HCO3 exchangers) and decreased the transporting capacity of existing proteins. The acid–base regulating capacity of control fish and salt‐loaded fish was assessed by monitoring arterial blood acid–base status [partial pressure of CO2 (PCO2), pH and HCO3 ] during exposure to external hypercapnia (nominally 7·5 mm Hg). Both groups of fish exhibited typical compensatory responses to sustained hypercapnia consisting of the gradual accumulation of plasma HCO3 and thus metabolic restoration within 24 h of the initial respiratory acidosis elicited by hypercapnia. Overall, the results demonstrate that while Cl uptake capacity is reduced in salt‐fed fish, there is no associated alteration in acid–base regulating capability.
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Document Type: Regular Paper

Publication date: 01 February 2012

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