Branchial Na+/K+-ATPase Expression in the European Eel (Anguilla anguilla) Following Saltwater Acclimation

The physiological adaptation of the euryhaline eel from freshwater (FW) to sea water (SW) environments is accompanied by increases in branchial chloride cell Na+/K+-ATPase activity. Branchial chloride cells are known to have unique morphological characteristics which are associated with their ability to adapt to large alterations in the external ionic environment. In FW, gill chloride cells possess an extensive tubular network which is an extension of the basolateral membrane compartment. The vast majority of Na+/K+-ATPase enzyme units have been shown to be located in the membranes of this tubular network (1). In FW environments, sodium is postulated to be retained by the pumping of this ion (by Na+/K+- ATPase) into the tubular network, where it diffuses down its concentration gradient into the internal milieu. When eels are transferred to a SW environment, the sodium gradient across the epithelium is oriented in the opposite direction, and several characteristic responses to this situation are activated (1). Chloride cells undergo extensive morphological and functional adaptations associated with the expression of Na+/K+-ATPase during the natural migration of these fish from freshwater to seawater (a change in osmolality of around 1000 mOsmol/kg). Proliferation of chloride cells within the gill epithelium occurs in such a way that where once a single chloride cell, with tight junctions between it and its respiratory cell neighbours existed, a group of chloride cells are present. The lateral spaces between these chloride cells are then partially open to the external but not internal milieu [2]. In conjunction with these changes, the apical pole of the basolateral tubular network is extended, and is in contact with the newly opened lateral spaces (2).