Introduction

Different environmental salinities are known to exert profound effects on the expression and activity of specific proteins associated with ion transport in migratory or estuarine aquatic vetebrates. The osmoregulatory adaptation of the ion transporting capacity is essential for the survival of teleost fish which inhabit both freshwater (FW) and seawater (SW) environments and in particular for catadromous and anadromous species which migrate between the two environments during their life cycle. The principles of osmoregulation and adaptation of teleosts to environments of differing salinity are generally accepted to involve concerted responses controlling the rate of drinking and the subsequent regulation and sometimes reversal of ion transport in the secretory/absorptive epithelia of the gill, gut, kidney and bladder. These processes ensure that the osmolality of the plasma varies little although there can be several orders of magnitude changes in the salinity of the environment.

When teleost fish, such as the European eel (Anguilla anguilla), are in an hypo-osmotic medium such as FW, they continually gain water across permeable body surfaces, primarily the gills, and this influx is balanced by the production of large quantities of dilute urine by the renal system. Salt losses from the body are reduced by the low body surface permeability and the absorption of ions from the food in the intestine and an efficient reabsorption of ions from the tubular fluid and urine in the kidney and bladder, respectively. Furthermore, some ions, and in particular calcium, are actively absorbed across the gills. When, as part of their natural life cycle, eels enter SW they encounter a change in the environmental osmolality of around 1000 mOsmol/kg.