Page 94 - The Toxicology of Fishes

74 The Toxicology of Fishes

Because of its location, high surface area, and role in nutrient uptake (Gauthier and Landis, 1972;
Noaillac-Depeyre and Gas, 1976, 1979), the proximal intestine is the most important site for xenobiotic
absorption in fish. It is also clear that other regions of the teleost intestine are capable of significant
chemical uptake (Kleinow et al., 1992; Nichols et al., 2004a). Regionally selective absorption has been
demonstrated with both organic chemicals and metals (Pentreath, 1976; Shears and Fletcher, 1983).
Many xenobiotics are absorbed concurrently or in conjunction with specific nutrient classes. Digestive
processes liberate xenobiotics from the food matrix, transport them to the mucosal epithelium, and
control their presentation to the absorptive surface. Gut uptake of lipophilic contaminants has been
strongly associated with digestion and assimilation of dietary lipid. Mechanical mixing, pancreatic
lipases, and bile salts disperse dietary lipid and contribute to the formation of an equilibrium phase
consisting of small mixed micelles (Kleinow and James, 2001). Lipophilic contaminants reside in the
interior of these micelles, while the hydrophilic exterior allows the micelles to remain soluble in the
aqueous milieu of the luminal contents. Micelles provide a packaging and transport phase that can
traverse the unstirred water layer of the mucosa. Upon reaching the enterocytes, the micelles are
dissociated by the combined action of pH changes and mucosal lipases, liberating stored xenobiotic.
This process creates a highly localized concentration gradient of both xenobiotic and fatty acids across
the absorptive epithelium.
Changes in the character of the ingesta can facilitate or hinder the absorption of xenobiotics; for
example, moderate levels of dietary lipid appear to facilitate the absorption of lipophilic contaminants,
but low or very high lipid diets are associated with reduced uptake efficiency (Andrews et al., 1978; Van
Veld, 1990). High levels of dietary fat may overwhelm the ability of the system to digest and remove
this material. Unprocessed lipid would then provide a partitioning phase for lipophilic compounds within
the gut lumen. In addition, qualitative features of dietary lipid such as fatty acid chain length and
composition may determine xenobiotic solubility in micelles and subsequent systemic availability (Doi
et al., 2000).
Dietary lipids absorbed by the enterocytes are exported as lipoproteins to interstitial spaces of the
lamina propria and to the vasculature. Mammals release these products as chylomicrons and very-low-
density lipoproteins. Fish appear instead to release very-low-density lipoproteins, high-density lipopro-
teins, and free fatty acids (Babin and Verneir, 1989; Iijima et al., 1985, 1990). Because fish do not
possess a lymphatic system, the potential for xenobiotics in lymph to bypass the liver, as seen in
mammals, does not exist. This feature may contribute to the hepatic extraction of some xenobiotics
before they reach the systemic circulation.
Gastric Evacuation and Gut Transit Time—Considerable effort has been expended to characterize rates
and patterns of gastric evacuation in fish. Exponential decay models have been successfully employed
to describe gastric evacuation in several species (Brodeur and Pearcy, 1987; He and Wurtsbaugh, 1993;
Persson, 1986; Ruggerone, 1989). This approach suggests that the gastric emptying rate at any time
point is proportional to the amount of food remaining in the stomach. Generally, the rate constant for
gastric emptying increases with acclimation temperature and decreases with food particle size (Jobling,
1987; Windell et al., 1976). Meal size has been reported to increase, decrease, and have no effect on
gastric evacuation rates. These apparent differences may be due in part to the use of dry and wet weights
to express stomach contents data (Elliott, 1991). Variable relationships also exist between dietary
composition and gastric emptying; increased dietary lipid has been shown to delay gastric emptying in
some cases (Windell et al., 1969) but not others (Persson, 1982).
Relatively little is known of total gut transit times in fish. When blennies (Blennius pholis) were fed
a single meal of fresh lugworms, the time required for intestinal emptying ranged from 20 to 30 hours
and tended to increase with meal size (Grove and Crawford, 1980). Gut transit times for a stomachless
species, the bighead carp (Aristichthys nobilis), ranged from about 7 to 13 hours and decreased with
increasing body size (Opuszynski and Shireman, 1991). The time required for rainbow trout to completely
digest a meal of live fathead minnows was less than 48 hours; however, a small amount of material was
retained within the intestines between feedings (Nichols et al., 2004a). This finding suggests that complete
evacuation of one meal depends on the consumption of additional meals. Gut transit times tend to
decrease with increased acclimation temperature (Hofer et al., 1982; Shrable et al., 1969).