Page 102 - The Toxicology of Fishes

82 The Toxicology of Fishes

contaminant transfer in spawning yellow perch was due to the large size of its egg mass (22.3% of body
weight) and the fact that it transfers a high percentage of its limited whole-body lipid stores to the
developing ovaries (27.1%).
Reproductive life history may also influence the redistribution of lipophilic contaminants in fish. The
Chinook salmon, a semelparous (once-bearing) species, transfers most of its stored lipid into a single
spawn of eggs. By comparison, iteroparous (multiple-bearing) lake trout invest a lower proportion of
stored lipid into each reproductive effort. In a study of these two species in Lake Michigan, Miller (1993)
found that female salmon transferred 28 to 39% of their accumulated PCB body burden to the eggs
during a single spawn, but female lake trout eliminated 3 to 5% of whole-body PCBs during each of
what may be several spawns.

Other Features of Distribution
In mammals, modifications to membranes within the eye and central nervous system limit the movement
of many xenobiotics. Similarly, chemical uptake by the mammalian testis may be limited by the presence
of active transport systems and high levels of biotransformation enzymes. Membrane structure and function
within these tissues in fish are essentially unknown. Xenobiotic movements are also limited within the
placenta and mammary gland of mammals, but these tissues have no counterparts in fish. An important
consideration for fish and other poikilotherms is the impact of temperature on chemical distribution.
Reductions in temperature lower cardiac output (Barron et al., 1987a); alter membrane composition
(Crockett and Hazel, 1995); change tissue perfusion patterns (Barron et al., 1987a), xenobiotic fluid space
distribution (Van Ginneken et al., 1991), and partitioning to storage tissues (Barron et al., 1987b; Karara
and Hayton, 1989); and elicit a hypertrophic response in the intestine and liver (Das, 1967; Lee and
Cossins, 1988). For many xenobiotics, an inverse relationship exists between environmental temperature
and chemical retention (generally characterized using the terminal elimination half-life) (Bjorklund et al.,
1992; Collier et al., 1978; Jacobsen, 1989; Kasuga et al., 1984; Kleinow et al., 1994; Salte and Liestol,
1983; Van Ginneken et al., 1991; Varanasi et al., 1981). These observations may be due in part to changes
in distribution, although changes in the activities of chemical elimination pathways are likely to contribute.

Xenobiotic Elimination

Branchial Excretion
In teleosts, the most important route of elimination for neutral, water-soluble, low-molecular-weight
chemicals is across the gills. Working with the Dolly Varden char (Salvelinus malma) in a split chamber
system, Thomas and Rice (1981) showed that aromatic hydrocarbons with low to moderate lipid solubility
(log K 1 to 4) are eliminated across the gills at a greater rate than those with high lipid solubility (log
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K 4 to 7). A similar finding was reported for goldfish exposed to a series of substituted phenols with
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log K values ranging from 1 to 5 (Nagel and Urich, 1980). Erickson and McKim (1990b) compiled data
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from several studies involving guppies (Bruggeman et al., 1984; Gobas et al., 1989; Konemann and van
Leeuwen, 1980; Opperhuizen et al., 1985) and found that elimination rates declined linearly with chemical
log K across a wide range (3 to 8) of values (Figure 3.13). These rate calculations were based, however,
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on retained chemical residues and may have reflected elimination by branchial and non-branchial routes.
Current models of chemical flux at fish gills (Erickson and McKim, 1990b) suggest that this
dependence of branchial elimination on chemical lipophilicity (or hydrophobicity) is due largely to
chemical binding in blood. The effect of this binding is to lower the diffusion gradient across the gill
epithelium by reducing the concentration of chemical in blood that is in a “free” (diffusing) form. Direct
tests of this hypothesis for high log K compounds are difficult to perform due to very low chemical
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concentrations in expired water and the tendency of these compounds to adsorb to tubings, glassware,
and other experimental apparatus. Dvorchik and Maren (1972) injected dogfish sharks (Squalus acanthias)
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with [ C]-DDT and collected samples of expired water but were unable to detect any radioactivity
eliminated by this route. More recently, Fitzsimmons et al. (2001) developed a method using continuous
column extraction of expired water, coupled with high-resolution mass spectrometry, to measure