Page 390 - The Toxicology of Fishes

370 The Toxicology of Fishes

Responses of Fish Liver to Reference Hepatotoxicants
During the early stages of development of new animal models for toxicological research, there is a
validation phase that demonstrates that the animal responds to compounds of proven toxicity (in this
case, reference hepatotoxicants) and that the response has features similar to those seen in established
models (usually rodents). Studies investigating three reference hepatotoxicants—acetaminophen (AP),
allyl formate (AF), and carbon tetrachloride (CCl )—were reviewed by Hinton et al. (2001). The
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interested reader is referred to that review, as we provide only a brief summary here.

Summary of Acetaminophen Review
Although AP produces centrolobular necrosis in rodent liver, exposed rainbow trout responded with
focal rather than zonal necrosis and the extent of necrosis was less. Trout exposed to AP responded with
an increase in serum glutamic pyruvic transaminase (SGPT) in a dose-dependent manner (Droy, 1988)
but one that was at least an order of magnitude lower than reported for hamsters and rats. Other parameters
of hepatic toxicity failed to show a response relationship in AP-exposed trout. Responses of mullet
(Mugil cephalus) were similar (Thomas and Wofford, 1984); for example, exposure to AP failed to cause
significant depletion of glutathione stores, and the authors concluded that mullet liver failed to produce
a sufficiently toxic metabolite. Because fish have the capacity to form certain reactive intermediates of
AP, differences in metabolism were not regarded as the sole explanation for the reduced levels and the
different patterns of hepatotoxicity observed. AP deserves further analysis in fish liver toxicity. It is an
example of a reference hepatotoxicant that produces discrete but lowered responses, the pattern and
characteristics of which will likely lead to improved understanding of fish liver toxicity in general.

Summary of Allyl Formate Review
Allyl formate is the only reference hepatotoxicant studied in fish that has selective periportal zonal
toxicity in rodent liver. In rainbow trout, the magnitude of toxic response following AF exposure was
appreciable, with morphologic, biochemical, and functional changes elicited (Droy et al., 1989). Ultra-
structural analysis of AF-induced hepatotoxity showed that endothelial cells of sinusoids were targeted
and that the hemorrhagic necrosis likely involved early lethal toxicity to this cell type. When the
concentration was lowered but the route of administration maintained, enhanced morphologic response
in perivenous areas suggested a zonal response. AF hepatotoxicity is likely mediated via alcohol dehy-
drogenase, but histochemical studies of trout liver indicated no preferential localization of this enzyme
(Schar et al., 1985). Similarly, glutathione localization in trout liver is homogeneous and likely would
not lead to a heterogeneous response to AF. AF response in trout was appreciable, and further analysis
of its time course over a broader range of lowered concentrations will likely prove fruitful in determining
the role of signaling between endothelial cells and hepatocytes in fish liver toxicity.

Summary of Carbon Tetrachloride Review

The classic example of biotransformation-mediated generation of toxicity in mammalian liver is the
cytochrome P450-dependent conversion of CCl and chloroform to free radicals such as ·CCl and
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CCl OO·, which then initiate lipid peroxidation, acute liver injury (e.g., necrosis), and, under chronic
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conditions, liver neoplastic changes (Treinen-Moslen, 2001). In rodents, CCl is metabolized to CCl ,
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which rapidly combines with oxygen to form the very reactive CCL O (Packer et al., 1978); therefore,
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different free-radical species of CCl are present under aerobic and anaerobic conditions in rat liver.
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Hepatic injury is maximal in centrolobular areas of mammalian liver where oxygen tension is lowest
(Burk et al., 1984a,b). Also, hypoxia potentiates covalent binding of metabolites to rat liver microsomal
lipids and proteins (Shen et al., 1982).
Fish appear to be more resistant to the toxic action of CCl . In the study by Droy et al. (1988), acute
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oral dosing of fish with CCl was selected because this method of delivery was related to a more natural
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