Ion Regulation by the Rainbow Trout, Salmo gairdneri, in Ion-Poor Water

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Abstract
This study describes the effects on hard-water-adapted rainbow trout of prolonged exposure (≥ 1 wk) to very low external ion levels, those characteristic of very dilute soft water (NaCl = 0.06, Ca²⁺ = 0.025 mMol/liter). Net losses of NaCl but not of Ca²⁺ occurred. The gills were the predominant site of the net NaCl losses. Rates of loss declined with time, but initial ion balance was not restored except in animals maintained at threefold higher NaCl levels. The compensatory reduction in ion loss rate occurred because of an initial decline in branchial efflux and a later increase in both Na and Cl transport. Urine flow rate, an indirect measure of branchial water permeability, increased significantly as a result of low ion exposure but subsequently declined to levels typical of normal hard water. In contrast, mannitol clearance, a measure of branchial paracellular channel permeability, was unaffected by soft-water exposure. The chelating agent EDTA, used as a means of removing Ca²⁺ bound to the gill surface, was effective at 2 mMol/liter in increasing ionic permeability of both hard-water- and soft-wateradapted fish. The latter, however, showed a significantly smaller effect of EDTA treatment, which suggests increased branchial Ca²∇ binding activity.