Page 391 - The Toxicology of Fishes

Liver Toxicity 371

exposure route and obviated the solvent effect seen after intraperitoneal injections. Necrotic hepatocytes
were more abundant in the high-dose group (3 mL CCl per kg), and a loss of cytoplasmic basophilia
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with swelling characterized the affected hepatocytes. In rats used as biological controls for the experi-
ment, the researchers found a quantity of necrosis not approximated by trout even after greater doses
of the toxicant. Also, with respect to the chronic toxicity of CCL , trout appear to be less sensitive. In
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rainbow trout, chloroform enhanced the hepatocarcinogenicity of aflatoxins (Kotsanis and Metcalfe,
1991), while in medaka chronic exposure to chloroform was not hepatocarcinogenic (Toussaint et al.,
2001a). The apparent higher resistance of fish to CCl -induced injury might be because fish liver lacks
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the metabolic machinery capable of bioactivating CCl . Evidence against this includes the report of
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14
increased lipid peroxidation and C-labeled CCl residues in trout liver 6 hours following exposure to
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a single intraperitoneal injection of 1 mL CCl per kg (Statham et al., 1978). Also, in mullet (Mugil
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cephalus) and croaker (Micropogonias undulates), increased lipid peroxidation occurred as a result of
CCl treatment (Wofford and Thomas, 1988). Further, in vitro studies have provided strong evidence
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that fish liver cells metabolize CCl to free radicals (Rabergh and Lipsky, 1997); also, Toussaint et al.
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(2001a) found that chloroform is metabolized in medaka liver.
A second hypothesis that could explain why CCl is less hepatotoxic in trout would be the presence
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of higher amounts of antioxidants or free-radical scavengers such as glutathione. When trout and rat
were compared after exposure to monochlorobenzene (Dalich and Larson, 1985), rat hepatic glutathione
was about three times that of trout; therefore, it seems unlikely that the lack of susceptibility of trout to
CCl can be explained by excessive amounts of antioxidant. In addition, Toussaint et al. (2001b) showed
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that CCl treatment of trout hepatocytes resulted in a serious depletion of cellular glutathione levels.
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Droy (1988) considered the possibility that interspecies differences in oxygen tension and glu-
tathione distribution patterns might explain the trout response to CCl . Glutathione is effective against
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CCl -induced lipid peroxidation in rat liver but only in the presence of oxygen, when CCL O is
3
2
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readily formed (Burk et al., 1983). The relative abundance of glutathione in portal vs. centrolobular
areas also contributes to the protection seen in the portal hepatocytes of rodent liver (Smith et al.,
1979); therefore, glutathione appears to protect rat liver portal hepatocytes from CCl because of the
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preferential distribution of glutathione and the likely ability of this compound to scavenge CCL O 2
3
but not CCl . Droy (1988) reported a refractiveness in trout to the hepatotoxic effects of CCl , with
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3
scattered individual hepatocyte necrosis being the typical response. Perhaps the inability of CCl to
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produce appreciable toxicity in trout liver could be due to comparatively high oxygen tension. Under
these conditions, CCl would be converted to CCL O , which would then combine with glutathione
3
3
2
and be rendered inert.
Exposure of rodents to CCL produces a mosaic of altered hepatocytes in centrolobular zones and an
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apparent sparing of hepatocytes in the periportal zone, the more oxygenated zone. Zonal reactions are
not seen following exposure of fish to CCl . When carrier-solvent-associated changes are removed from
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consideration, CCl is not a particularly strong hepatotoxicant in fishes. Scattered individual hepatocyte
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necrosis is the usual response. Differences in CCL metabolism, levels of glutathione, metabolic attack
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on the parent compound, oxygen tension, and absence of the Kupffer cell in trout have been suggested
as possible reasons underlying this differential response.
Taken together, this body of work indicates that livers of fishes respond to reference hepatotoxicants
but that the response is quantitatively different from that of rodents. The pattern of morphological
alteration is different, likely reflecting innate differences in hepatic architecture and quantitative as well
as qualitative differences in metabolism of parent compounds. Additional effort is needed to better
characterize hepatotoxicity in fishes. Increased understanding of structure (especially architecture), of
the roles of hepatocytes and nonhepatocytic cell types in toxic responses, and of metabolism will enhance
our ability to interpret alterations following exposure to drugs, personal-care products, pollutants, and
biotoxins.
As we shall see in the subsequent section, elegant and detailed ultrastructural studies in livers, isolated
primary cells, and fish-derived cell lines have made it possible for us to demonstrate hepatocellular
adaptation following exposure to reference hepatotoxicants or develop an appreciable assemblage of
individual environmentally relevant compounds as is suggested by studies from wild fish collected at
heavily contaminated sites.