Role of Na-K-ATPase in chloride cell function

In seawater eels the efflux of sodium (and chloride) across the gill is directly proportional to the activity of Na-K-ATPase in homogenates of gill filaments. The rate of ion movement, however, is substantially greater than at the temperature of seawater. Na-K-ATPase is localized predominantly on the basolateral surface of the chloride cell so that ouabain inhibits from the blood side rather than from the apical or mucosal surface of chloride cells. Chloride, rather than sodium, is probably the actively transported ion species, and an attractive hypothesis for active chloride transport is one that invokes the cotransport of chloride with sodium across the basolateral membrane, the energy for which is supplied indirectly by the operation of the Na-K-ATPase pump. Exposure to freshwater sharply dissociates ion movements from Na-K-ATPase activity, possibly by changing the permeability of cell membranes to chloride movements.