Inulin and mannitol are excreted at measurable rates across gills in freshwater and sea-water-adapted trout. Clearance behaviour suggests that the effluxes may reflect passive permeation, which might make such compounds suitable for monitoring permeability changes. Application of poly-L-lysine and elevation of environmental temperature made FW trout gills reversibly ‘leaky’ to Na+. Mannitol efflux paralleled that of Na+ in both experiments. However, Na+ efflux was reduced in SW trout gills, by increasing external Ca2+ and mannitol efflux was unchanged. The data suggest that these nonelectrolytes can be used to assess permeability changes that do not involve highly selective ionic channels.

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