Page 422 - The Toxicology of Fishes

402 The Toxicology of Fishes

The exchange and secretion of ions are essential not only for compensating the passive ion flows
between fish and the ambient water and the excretion of superfluous ions extracted from food, but also
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+
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for the elimination of metabolically produced ions, such as H , NH , and HCO . The gills have a
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particularly important role in this respect, as they function as organs for the exchange of ions and
respiratory gases as well as for acid–base regulation (Evans, 1987; McDonald et al., 1991). Thus, when
considering the actions of toxic agents on hydromineral regulation in fish, the effects of these substances
on gills, gut, and kidneys have to be taken into account.
In this chapter, we review the most important aspects of the interactions of toxic agents with these
osmoregulatory systems on the basis of a selection of the available literature and with an emphasis on
those aspects that are typical for fish as aquatic organisms. We discuss the effects of direct interaction
of toxic agents with the osmoregulatory epithelia, as well as the indirect effects resulting from the actions
of toxic chemicals as stressors and as disruptors of endocrine cells. Like all external factors with the
capacity to disturb internal equilibria of animals or with an impact experienced as threatening, toxic
agents can evoke a stress response in animals. In fish, much more than in the terrestrial vertebrates,
stressors affect the hydromineral balance of the fish. This is an aspect of fish that has long been ignored
or underestimated in studies on toxic mechanisms and toxic effects in these animals (Wendelaar Bonga,
1997).

The Gills

We consider here the direct and indirect effects of toxic agents on the hydromineral and acid–base
balance that can be attributed to damage of the branchial structure and alterations in branchial function.
Gills consist of two sets of four branches, with numerous rows of filaments; each filament has two rows
of lamellae, which represent the respiratory surface. The afferent arteries to the gills open into blood
sinuses in the lamellae—flat spaces lined by cytoplasmic extensions of the pillar cells. The spaces are
drained by efferent arterioles, which anastomose with a central venous sinus in the filament, drained by
venolymphatic vessels (Figure 8.1). Whereas the lamellae are covered by a uniform one- to two-layer
flat epithelium of respiratory cells, the filament epithelium is multilayered and consists mainly of filament
cells, with an apical layer of flattened pavement cells with mucus cells and chloride cells. The chloride
cells are the main location of ion-dependent ATPase activity as well as ion channels and other mechanisms
involved in transcellular and paracellular ion transport. There are notable indications that H –ATPase
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activity is also present in respiratory cells and/or pavement cells (see reviews by Evans, 1987; Flik et

water flow

L efa

v

e a
FIGURE 8.1 Simplified diagram of a branchial filament with lamellae (L; their numbers vary from 30 to several hundred
per filament, depending on species). The blood enters the filament via a branch of the afferent gill arch artery (a) and enters
the vascular spaces of the lamellae. These are drained by efferent lamellar arteries, which join the efferent filament artery
(efa), which opens into the efferent arch artery (e). Part of the blood enters the central venous sinus of the filament, which
is drained by a venolymphatic vessel (v).