Page 426 - The Toxicology of Fishes

406 The Toxicology of Fishes

afferent lamellar vessels and closing of the arteriovenous anastomoses (Figure 8.1). The increased blood
flow and the recruitment of peripheral branchial lamellae that are barely perfused at rest increase not
only the effective respiratory surface but also the available surface area for the diffusion of water, ions,
and even large organic molecules (Mazeaud and Mazeaud, 1981; Randall and Perry, 1992). As a result,
all factors stimulating this adrenaline release also promote the passive loss of ions in freshwater fish and
+
the passive inflow of ions in seawater fish across the gills. In freshwater fish, diffusive losses of Na and
–
+
–
Cl can increase up to 40-fold during stress, resulting in a dramatic loss of total body Na and Cl and
further promoting the osmotic flow of water across the gills (see reviews by Mazeaud et al., 1977;
McDonald and Milligan, 1997).
Factors affecting the release of adrenaline vary from forced increased locomotor activity or a reduction
in water oxygen concentration to all kinds of social, physical, or chemical stressors. All toxic chemicals
increase branchial permeability to water and ions as soon as their impact is sufficient to evoke a stress
response, and severe stress can lead to edematous swelling of the lamellar and filamental epithelia and
even to the phenomenon of epithelial lifting (detachment of the lamellar epithelium from the underlying
tissues). As mentioned earlier, such phenomena have frequently been described in fish exposed to
inorganic or organic pollutants (Mallatt, 1985; Wendelaar Bonga, 1997). More direct evidence that these
chemicals can produce a stress response are the many reports on the elevation of plasma cortisol levels
following exposure of fish to these substances (Hontela, 1997; Pelgrom et al., 1995; Webb and Wood,
1998). Recently, Chowdhury et al. (2004) reported increased levels of plasma cortisol, glucose, lactate,
and total ammonia, all classical stress parameters, in rainbow trout exposed to waterborne cadmium.
The observed rise in aspartate aminotransferase and alanine aminotransferase in gills, kidneys, and liver
of common carp during cadmium exposure has also been ascribed to the stress response evoked by this
metal. Both enzymes are known to be involved in protein catabolism for energy production in stressed
animals (De Smet and Blust, 2001).
The recovery phase of the stress response is characterized by a compensatory increase of chloride cell
density in the gills. This explains why not only toxic metals that directly affect these cells but also
organic chemicals without specific effects on the gills increase chloride cell density (Mallatt, 1985;
Wendelaar Bonga, 1997).

The Intestine

The intestine is one of the main targets of dietary and waterborne toxic agents, and, given the important
osmoregulatory function of the intestine, it directly affects the regulation of water and ion balance. It is
therefore surprising that the intestine, as an organ of hydromineral regulation, has received little attention
from fish toxicologists, particularly when compared to the hundreds of papers on the gills. The main
reason may be that most toxicological studies are dealing with the effect of waterborne pollutants on
freshwater fish, which have low drinking rates. It should be emphasized here that, under field conditions
in neutral water, toxic metals are almost exclusively complexed or bound to particles and enter the fish
mainly as food contaminants.; thus, for reasons of ecological relevance, more attention should be paid
to the intestinal route as far as the toxicology of metals is concerned.
In freshwater fish, the role of the intestine in the control of hydromineral balance is comparable to
that of the terrestrial vertebrates, with reabsorption of calcium and magnesium being the most important
activity, complementary to the mineral-accumulating function of the gills (Flik et al., 1996). The
situation in seawater is essentially different. The high drinking rate of seawater fish, followed by
intensive reabsorption of water to offset the osmotic water loss, is an important strain on hydromineral
–
+
control as the water reabsorption is dependent on Na and Cl uptake. The role of the intestine in
hydromineral regulation is reflected by the structure of the epithelial cells. The dominating cell type,
the enterocyte, exhibits apically the appearance of cells involved in nutrient reabsorption, such as a
well-developed brush border and a well-developed lysosomal system, in addition to smooth and granular
endoplasmic reticulum and an occasional peroxisome. Along the entire length of the intestinal tract,
however, particularly in its more rectal parts, the basal parts of these cells show the typical character-
istics of cells involved in the transepithelial transport of water and ions: a basal labyrinth of folded