Page 106 - The Toxicology of Fishes

86 The Toxicology of Fishes

species and found that the amount of metabolites in bile was species dependent. In each case, however,
the bile was selectively enriched in glucuronide conjugates, and all of the metabolites in water were
sulfate conjugates. These findings illustrate the potential selectivity of elimination pathways for metab-
olites as well as the inadvisability of defining metabolite profiles based solely on either bile or urine
samples. Similar considerations apply to the elimination of some metal–ligand complexes. In the little
skate (Raja erinacea) and dogfish shark, biliary elimination of methylmercury, which binds to glutathione
and other sulfur-containing biomolecules, was reduced by inhibiting the biosynthesis of glutathione or
its excretion in bile (Ballatori and Boyer, 1986).
Once secreted into the intestine, biliary conjugates may undergo enzyme-mediated hydrolysis, liber-
ating the parent compound for reabsorption and enterohepatic recirculation (Collicutt and Eales, 1974;
James, 1987; Fricker et al., 1997; Layiwola et al., 1983; Schultz et al., 2001). These processes modulate
the amount of metabolites eliminated via the feces. In addition, in situ intestinal studies in fish suggest
that conjugated metabolites of highly lipophilic compounds such as benzo(a)pyrene (benzo(a)pyrene 9-
sulfate and benzo(a)pyrene 9-glucuronide) may be absorbed intact from the GIT (James et al., 1996).
The uptake these conjugates appears to be due to their relatively low molecular weight and appreciable
lipophilicity.
Secretion of bile into the intestinal tract is stimulated by nutritional and digestive signals. Due to the
episodic nature of bile secretion and potential for xenobiotic reabsorption within the intestine, it is
difficult to assess the contribution of biliary excretion to the overall process of elimination. The gall-
bladder of a recently fed fish is generally void of bile. By necessity, therefore, chemical concentrations
in bile are generally determined using fasted animals.
Techniques for continuous collection of bile in unanaesthetized rainbow trout were described by
Schmidt and Weber (1973), Gingerich et al. (1977), and Sanz et al. (1993). Similar methods have been
developed for the skate (Raja erinacea) and dogfish shark (Squalus acanthias) (Boyer et al., 1976a,b,c).
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Basal bile flows measured using these techniques range from 50 to 200 µL hr kg in rainbow trout,
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30 to 70 µL hr kg in the skate, and 80 to 110 µL hr kg in the dogfish. Bile flow rates in mammals
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may be 50 times greater than these values (Klaassen and Plaa, 1967). Differences in the rates of biliary
elimination of bromosulfophthalein between rainbow trout and rats were found to be well correlated
with differences in bile flow rate (Gingerich et al., 1977). This finding suggests that bile formation rate
limits the rate of bromosulfophthalein excretion in bile. Fish have been shown to efficiently concentrate
a large number of xenobiotics in bile, leading to the suggestion that bile can be used as a tool for
monitoring environmental chemical exposures (Statham et al., 1976). This concentrating effect may be
a result of slow bile flow relative to the secretion of xenobiotics into bile.
In rainbow trout, the biliary excretion of two organic anions, phenolphthalein (Curtis, 1983) and
taurocholate (Curtis et al., 1986; Kemp and Curtis, 1987) increased with an increase in acclimation
temperature. Similar results were obtained for a polyaromatic hydrocarbon, benzo(a)pyrene (Curtis et
al., 1990). Hepatic blood flow has the potential to limit biliary clearance and is likely to increase with
increased temperature (assuming that hepatic blood flow as a proportion of cardiac output remains
constant). In the case of phenolphthalein, however, biliary elimination appeared to saturate at the two
lower temperatures tested (Curtis, 1983). The mechanistic basis for these observations may be related,
therefore, to changes in saturable processes responsible for chemical transport from blood to bile as well
as the rate of chemical delivery to liver.

Urinary Excretion
Fish kidneys perform two major functions: (1) ion and water balance, and (2) excretion of endogenous
and exogenous solutes. The diversity of kidney structure–function across species is enormous, but basic
features are shared by both freshwater and saltwater teleosts. This section provides an overview of fish
kidney function with an emphasis on mechanisms responsible for renal excretion of xenobiotic chemicals.
Reviews of kidney structure–function in fish are provided elsewhere (Hickman and Trump, 1969;
Pritchard, 1981; Nishimura and Imai, 1982; Pritchard and Miller, 1993; Pritchard and Renfro, 1984).
The functional unit involved with urine formation in the freshwater fish is the kidney (Figure 3.15),
consisting of an encapsulated capillary network called the glomerulus and a tubule lined with epithelial