Abstract
Water permeability of trout gills (Salmo gairdneri) was studied in vitro with fish adapted to freshwater (FW) and to seawater (SW). In adapted fish, diffusional water permeability is less in SW than in FW, whereas the apparent osmotic water permeability is invariant. The ratio of osmotic to diffusional water permeabilities is greater than unity. This result demonstrates the existence of two water transfer mechanisms, diffusion and bulk transport. Osmotic permeability is increased threefold by an instantaneous change of external salinity, while diffusional permeability remains unchanged under the same circumstances. The addition of 10⁻⁶M epinephrine to the perfusion liquid leads to an increase of diffusional permeability in FW-adapted fish (+110%) and in SW-adapted individuals (+60%). Osmotic permeability increases in both cases (+30%). In the course of rapid transfer experiments, epinephrine does not modify the behavior of diffusional fluxes, while it does attenuate the osmotic flux rectification mechanis...
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