Page 423 - The Toxicology of Fishes

The Osmoregulatory System 403

al., 1996; McDonald et al., 1991). The large respiratory surface of the lamellae as well as the extensive
epithelium lining the filaments represent an important area of contact between the animals and the
ambient water which facilitates an efficient exchange of respiratory gases and ions but at the same time
forms an expanded and delicate target area for toxicants.
Numerous reports have been published on the effects of toxic agents on gill structure and function,
mainly in freshwater fish, although interest in seawater fish is growing. Mallatt (1985) and Evans (1987)
summarized the main literature on gill structure available in the mid-1980s, and they emphasized the
diversity of toxic actions as well as similarities in the types of lesions induced by a large number of
toxicants. Common structural alterations include necrosis of pavement cells and chloride cells; rupture
of the epithelium; hypertrophy and hyperplasia of chloride cells, filament cells, and respiratory cells
which may lead to lamellar fusion; epithelial swelling by widening of intercellular spaces; penetration
of leucocytes from the blood into these intercellular spaces; and, in the lamellae, epithelial lifting, the
separation of the respiratory epithelium from the underlying tissue. Such alterations have been confirmed
for many toxicants (Grinwis, 1998; Nowak, 1992). More recently, several studies have shown that cell
death by toxic agents is due not only to necrosis (accidental cell death) but also apoptosis (physiologically
controlled cell death) (Wendelaar Bonga, 1997; Bury et al., 1998). In this chapter, we mainly concentrate
on the literature after 1987 and focus on the underlying action mechanisms and consequences for the
hydromineral balance. The effects of toxicants on the gills are twofold: (1) interaction (mainly inhibition)
with the ion-transporting mechanisms of the gills, which are mainly concentrated in the chloride cells,
and (2) increased permeability of the branchial epithelia to water and ions. Although the gills are mainly
affected by waterborne chemicals acting from outside, bloodborne chemicals that have entered the
circulation via the gut have also been reported to disturb branchial structure and function (Handy, 1992;
Pratap and Wendelaar Bonga, 1993).

Disturbance of Active Ion Transport Processes

The chloride cells are the most extensively studied cell types of the gills because of their key function
in ion transport activity. They are characterized by an extensive tubular system continuous with the
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basolateral membrane and containing membrane-bound, ion-translocating enzymes such as Na /K - and
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Ca -ATPase, as well as different types of exchangers and postulated ion channels in the apical and
basolateral membranes (Figure 8.2). Exposure to toxicants, particularly heavy metals, leads to a rapid
inhibition of these cells. In general, at high concentrations, this is structurally reflected by focal damage
or by necrosis and apoptosis of these cells. In response to cell death, differentiation of new cells is
commonly observed. The acceleration of cell death and cell replacement may continue for months,
although its rate in general slows down after the first week, possibly as a result of the expression in the
newly formed cells of proteins such as metallothioneins (Roesijadi, 1996), which may protect the cells
against the toxic action of the metals.
Chloride cells can be affected by waterborne as well as dietary metals, as has been shown for cadmium
in Mozambique tilapia (Pratap and Wendelaar Bonga, 1993) or copper in trout (Berntssen et al., 1999).
The toxic mechanisms involved have been elucidated for only a few metals. Copper, which is known to
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affect plasma Na ions and Na fluxes rather specifically (Laurén and McDonald, 1987; Li et al., 1998;
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Matsuo et al., 2004), has been shown to inactivate Na /K -ATPase activity (the driving force for Na ,
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–
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K , and NH transport and indirectly for other ions such as H , Ca , and, in seawater, Cl ) in vitro in
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the nanomolar range (Li et al., 1996). The result is a net loss of sodium and other ions.
The differences among species with regard to sensitivity to copper (e.g., yellow perch is rather tolerant
and rainbow trout is sensitive) have been attributed to the rate of sodium loss upon copper exposure and
not to the total amount of copper bound to the gills, which are the sites of highest copper accumulation
in fish. Copper accumulation was found to be 9 times higher in perch than in trout (Taylor et al., 2003).
At copper concentrations causing 50% mortality (96-hr LC ), cadmium is able to inhibit in vitro
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Ca -ATPase activity, the driving force for branchial and intestinal Ca uptake, in the nanomolar range
(Schoenmakers et al., 1992). This seems to be the key toxic mechanism for this metal. Reductions in
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plasma Ca concentrations after exposure to cadmium have been reported for several fish species.
Conversely, an increase in waterborne Ca , but also increased dietary calcium intake, can reduce the
2+