On the Mechanism of Sodium Extrusion across the Irrigated Gill of Sea Water-Adapted Rainbow Trout (Salmo gairdneri)

Sodium efflux (J_out^Na) across the irrigated trout gill was rapid in sea water (SW), but only about 25 % as large in fresh water (FW). The difference correlated with a change in the potential difference across the gill (TEP). The latter was about +10 mV (blood positive) in SW, but –40 mV in FW. Both flux and electrical data indicated that gills in this fish are permeable to a variety of cations including Na⁺, K⁺, Mg²⁺, choline, and Tris. They are less permeable to anions; P_Na:P_K:P_Cl was estimated to be 1:10:0.3, and P_Cl > P_gluconate. The TEP was shown to be a diffusion potential determined by these permeabilities and the extant ionic gradients in SW, FW as well as in other media. J_out^Na appeared to be diffusive in all of the experiments undertaken. Exchange diffusion need not be posited, and the question of whether there is an active component remains open.