Page 421 - The Toxicology of Fishes

8

The Osmoregulatory System

Sjoerd E. Wendelaar Bonga and Robert A.C. Lock

CONTENTS
Introduction............................................................................................................................................401
The Gills ................................................................................................................................................402
Disturbance of Active Ion Transport Processes...........................................................................403
Disturbance of Permeability to Water and Ions ..........................................................................405
The Intestine...........................................................................................................................................406
Heavy Metals................................................................................................................................407
Organic Pollutants........................................................................................................................408
The Kidneys...........................................................................................................................................409
Toxic Agents and Endocrine Control of Osmoregulation.....................................................................410
References .............................................................................................................................................411

Introduction
The gills and the gut are the main entry routes for toxic agents in teleost fish, and this makes these
organs important targets for such chemicals. Gills and gut are also important organs for the regulation
of water and ions in fish, and this is the main reason why many toxic agents, either waterborne or present
in the food, are well known for disturbing hydromineral regulation in fish. This is readily reflected in
2+
2+
+
–
changed osmolarity and the concentration of the main ions (in particular Na , Cl , Ca , and Mg ) in
the blood plasma. Effective hydromineral control is of utmost importance in fish, as most of these animals
–1
are living either in freshwater, with an osmolarity in general less than 5 mOsmol L and thus far below
–1
that of the blood plasma (300 to 325 mOsmol L ), or in seawater, with an osmolarity about three times
–1
higher than in the blood of seawater-dwelling fish (950 to 1050 mOsmol L ). The resulting osmotic
and ionic gradients across the branchial epithelia form the driving force for passive water and ion
movements between the fish and the water. These movements—the outflow of ions and osmotic uptake
of water in freshwater fish and the inflow of ions and osmotic water loss in seawater fish—are counter-
acted by an effective control of permeability to water and ions of the integumental epithelia, particularly
in freshwater conditions (Eddy, 1981; Evans, 1987). Nevertheless, compensation of the resulting water
and ion flows requires a constant action of the principal water and ion regulatory organs, the gills, skin,
and kidneys, in both freshwater and seawater. The functions of the three organs differ substantially
between freshwater and seawater fish, and this is one of the reasons why the impact of many toxic agents
is dependent on water salinity. In freshwater, the outflow of ions over the gills is offset by active ion-
exchange processes in the gills and intestine, whereas the osmotic inflow of water is compensated by
the excretion of large volumes of urine with a low osmolarity. In a marine environment, the osmotic
loss of water is compensated by drinking seawater and the subsequent secretion of monovalent ions via
the gills and monovalent and divalent ions via the kidneys. The latter processes also redress the passive
inflow of ions in seawater fish.

401