Page 89 - The Toxicology of Fishes

Toxicokinetics in Fishes 69

positively correlated with that of oxygen at two temperatures (10 and 20°C) and two levels of forced
swimming. There was no indication of change in uptake efficiency for either methylmercury or oxygen.
Similarly, Murphy and Murphy (1971) demonstrated a positive correlation between oxygen consumption
and uptake of waterborne dichlorodiphenyltrichloroethane (DDT) by the mosquito fish (Gambusia affinis)
at 5 and 20°C. The ratio of DDT uptake rate to fish weight was proportional to the ratio of oxygen
consumption to fish weight at each of the two temperatures. The authors concluded that the uptake of
DDT across the gills was influenced by changes in respiratory volume. Black et al. (1991) found that
the branchial uptake of three moderately hydrophobic compounds—benzo(a)pyrene, 2,2′,5,5′-tetrachlo-
robiphenyl, and naphthalene—changed in direct proportion to oxygen consumption when rainbow trout
were subjected to an acute reduction in temperature from 17°C to 8°C (Figure 3.6A). The decrease in
oxygen consumption was accompanied by reductions in ventilation volume and ventilation rate (Figure
3.6B). Finally, it is well known that chemical diffusion rates in solution increase with temperature.
Similar changes are expected to occur in biological membranes and would be important if diffusion
across the gill epithelium limited the rate of branchial flux. Fish may also respond to prolonged changes
in temperature by altering the molecular structure of the gill epithelium, changing its permeability to
xenobiotic compounds.

The Gastrointestinal Tract
Gastrointestinal Tract Structure and Function
The gastrointestinal tract (GIT) of fishes functions in digestion, nutrient absorption, and excretion and
as a barrier to the external environment. For some groups of fishes, the GIT also plays a role in
osmoregulation, buoyancy, ion regulation, and placental nutrition. Diverse structural adaptations in
different fish species reflect a wide array of digestive strategies. Gross differences among species include
the presence or absence of an acid-secreting stomach, presence or absence of a gizzard, large differences
in GIT length, and an absence or varying number of blind diverticula called pyloric ceca that project
from the intestine near the pylorus of the stomach. Detailed reviews of fish GIT structure and function
are provided elsewhere (Kapoor et al., 1975; Kleinow and James, 2001; Smith, 1989).
Common to most species, the buccal cavity is followed by a short and distensible esophagus; some
type of stomach; proximal, middle, and distal intestine; and finally a rectal region. Along its length, the
GIT changes histologically. Major features of the esophagus include numerous goblet cells, which
provide lubrication, and primary, secondary, and tertiary folds, which allow distension during the
ingestion and swallowing of food. A true stomach, when present, often contains cardiac glands that
extend between the lamina propria and columnar epithelial cells lining the stomach lumen.
The structure of the intestine, including its diameter and the abundance of goblet cells and longitudinal
folds, varies along its length. Villi and crypt regions, as classically defined in the mammalian intestine,
are not evident in many fishes but are replaced by the longitudinal folds which exhibit varying degrees
of branching (Field et al., 1978; Kapoor et al., 1975). As with mammals, the intestinal lumen of most
fishes is lined by a simple columnar epithelium that possesses a brush border. Pyloric ceca are lined
with columnar absorptive cells and are histologically similar to the adjacent intestine.
The proximal intestine (including, when present, pyloric ceca) provides the greatest surface area for
absorption due to the relative abundance and length of longitudinal folds. Comparisons among species
suggest that total absorptive surface area varies inversely with diet quality (Horn, 1989). For a given
species, intestinal length, mucosal weight, and total gut surface area may vary inversely with acclimation
temperature (Lee and Cossins, 1988).
The absorptive surface of the intestine consists of several structural and functional layers. On the
lumen margin, overlying the mucosal epithelium, is an unstirred layer consisting of water and a layer
of mucus. The mucus is an organic-based permeable gel that is constantly renewed by goblet cells of
the mucosa. The thickness of the mucus layer changes by region and is generally greatest in distal
sections of the intestine. The composition of mucus may also differ among intestinal regions (Trevisan,
1979) and species (Jirge, 1970). A third, largely functional layer is interposed between the epithelial
cell surface and the unstirred layer. Often referred to as the acid microclimate, this layer is a cell
membrane charge barrier associated with the epithelial cells. The absorptive pathway extends through