Sodium efflux and potential differences across the irrigated gill of sea water-adapted rainbow trout (Salmo gairdneri).

Sodium extrusion (J(out) (Na)) was measured across the gills of rainbow trout, Salmo gairdneri, adapted to sea water (SW) using a gill-irrigation system of small volume. The potential difference (TEP) was also measured under similar conditions. J(out) (Na) was usually between 100-250 microeq (100 g)(-1) h(-1), about an order of magnitude faster than in fresh water (FW)-adapted trout, but slower than has been reported for any other marine teleost. The TEP was between 10-11 mV, body fluids positive to SW. When the external medium was changed from SW to FW J(out) (Na) was reduced to about 25 % of the initial value, and the TEP was reduced by 40-50 mV (i.e. body fluids negative by 30-40 mV). Addition of either Na(+) or K(+) in SW concentrations reversed the changes; J(out) (Na) increased and the gill repolarized. The electrical behavior and sodium efflux in irrigated trout gill is qualitatively the same as has been reported for unanaesthetized, free-swimming fish of other species. Thus, the irrigated gill provides an adequate model for studying the mechanism of sodium extrusion in marine teleosts.