Page 92 - The Toxicology of Fishes

72 The Toxicology of Fishes

40

20
AMOUNT IN ANIMAL (mg) 6
10

4

2

1
0 20 40 60 80 100
HOURS

FIGURE 3.8 Simulated effect of gut absorption rate on whole-animal chemical kinetics. Simulation conditions: one-
compartment open model; dose, 50 mg; elimination half-life, 20 hours; absorption half-life, 7 hours (solid line) or 70 hours
(dashed line).

calculation of which mathematically factors for the effect of chemical elimination, including biotrans-
formation. Dietary absorption efficiencies have been reported for a number of halogenated organic
chemicals in several fish species (Burreau et al., 1997; Gobas et al., 1988; Muir and Yarechewski, 1988;
Niimi and Oliver, 1988; Opperhuizen and Sijm, 1990). Gobas et al. (1988) compiled much of this
information and related measured absorption efficiencies to chemical log K (Figure 3.9). At log K ow
ow
values less than 6, reported values range from 30 to 70%, averaging about 50%. A log K -dependent
ow
decrease in absorption efficiency is apparent for compounds with log K values greater than 7. When
ow
a compound is administered as a single dietary dose, the rate of absorption influences chemical concen-
trations achieved in tissues. At a fixed rate of elimination, an increase in the rate of absorption results
in relatively higher maximum concentrations. Decreasing the rate of absorption results in relatively lower
maximum concentrations that tend to be maintained for longer periods of time (Figure 3.8). When
multiple doses of a compound are administered in the diet, the rate of absorption does not generally
impact steady-state concentrations.
Bengtsson et al. (1979) fed bleaks (Alburnus alburnus) a technical mixture of chlorinated paraffins
(CPs) and found that dietary uptake was inversely related to carbon-chain length and the extent of
chemical chlorination. In a subsequent study, dietary uptake and accumulation of CPs by rainbow trout
was variably related to the extent of chlorine substitution, depending on carbon-chain length (Fisk et
al., 1996). Chlorination of short-chain-length compounds appears to increase accumulation by reducing
biotransformation. In contrast, high levels of chlorine substitution may limit the diffusion of CPs with
longer chain lengths across the mucosal membrane and perhaps incorporation into micelles. In several
fish species, dietary uptake of high-molecular-weight CPs (MW > 600) was low or nonexistent (Bengts-
son et al., 1979; Lombardo et al., 1975; Zitko, 1974). In general, lipophilic contaminants are more readily
taken up from the diet if they have a molecular weight less than 600 (Bruggeman et al., 1984; Niimi
and Oliver, 1988), low extent of chlorine substitution (Tanabe et al., 1982), and small molecular volume
(<0.25 nm ) (Niimi and Oliver, 1988).
3
Dietary uptake of lipophilic contaminants may depend on the existing contaminant burden in the fish.
14
14
Doi et al. (2000) showed that the uptake of [ C]-3,3′,4,4′-tetrachlorobiphenyl ([ C]-PCB 77) from in
situ perfused catfish intestines was reduced by dietary pretreatment with unlabeled PCB 77. The cause
of this decrease was unclear but did not appear to be related to changes in first-pass intestinal metabolism.
In vivo studies with catfish have also demonstrated lower dietary uptake of PCBs following preexposure
in the diet (Hansen et al., 1976) or exposure to contaminated sediments (Dabrowska et al., 1996). Kinetic