Effects of environmental salinity on branchial permeability of rainbow trout, Salmo gairdneri.

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Abstract
The gill fluxes of various nonelectrolytes were measured in freshwater- and seawater-adapted trout (S. gairdneri). The studies were performed in vitro with a perfused-head preparation. Different transepithelial pathways were specified according to the physicochemical characteristics of the permeant molecules: for hydrophilic and lipophilic molecules of small molecular radius, a transcellular pathway in the respiratory cells of the secondary lamellae; for hexose, all paracellular pathways in the gill epithelium and for hexose polymers (inulin, dextran), a transcellular pathway in the Cl cells of the primary lamellae. The selectivity of the respiratory cells as a function of the liposolubility of the molecule tested was low. The decrease of this selectivity in the course of saltwater adaptation taken together with the modification of lipid composition of membranes and the effect of adrenaline [epinephrine] on the branchial permeability suggested that nonelectrolytes diffuse through a lipid phase able to form hydrogen bonds. The high permeability of gills to hexose polymers of high MW suggests a vesicular transport, especially in fish adapted to freshwater.