Page 95 - The Toxicology of Fishes

Toxicokinetics in Fishes 75

In mammals, an increase in feeding rate generally results in decreased gut transit time and reduced
digestibility. Both of these outcomes may contribute to reductions in xenobiotic bioavailability. Limited
information suggests that similar considerations also apply to fish; for example, the oral bioavailability
of methylmercury in catfish was found to be inversely related to meal size (McCloskey et al., 1998).
In other studies, the bioavailability of oxolinic acid to catfish was found to increase with acclimation
temperature (Kleinow et al., 1994), and that of enrofloxacin in rainbow trout decreased (Bowser et al.,
1992). The apparent disagreement between these observations may be due to competing effects of
temperature on gut transit time and digestibility. By increasing the transit time, a decrease in temper-
ature might be expected to promote bioavailability; however, an accompanying decrease in digestibility
might slow the release of chemical from the food matrix. The effect of temperature on bioavailability
may depend, therefore, on which factors limit dietary uptake for a given compound, diet, and species.
In some cases, chemical desorption from the food source may be the rate-limiting determinant of
dietary uptake; for example, desorption half-lives for hydrophobic contaminants from ingested sedi-
ments may be greater than a week, which most likely exceeds the intestinal transit time of most
organisms (Schrap, 1991).

Blood Flow—The GIT is supplied by a relatively complex vascular network (Farrell et al., 2001;
Thorarensen et al., 1991). Blood flow to the GIT as a percentage of cardiac output in resting, unfed
fish appears to vary considerably among species. Values based on the distribution of injected micro-
spheres range from less than 1% in channel catfish (Schultz et al., 1999) to nearly 20% in rainbow
trout (Barron et al., 1987a). Doppler flow probes were used to measure blood flow through the coeliaco-
mesenteric arteries of Chinook salmon (Thorarensen et al., 1993), Atlantic cod (Gadus morhua)
(Axelsson and Fritsche, 1991), and red Irish lord (Hemilepidotus hemilepidotus) (Axelsson et al., 2000).
In each case, the measured flow rate accounted for 30 to 40% of total cardiac output, leading to the
suggestion that microsphere methods underestimate blood flow to the GIT (Thorarensen et al., 1993).
In addition to the GIT, however, the coeliaco-mesenteric artery provides blood to several internal organs
including (depending on species) the liver, spleen, swim bladder, and gonads (Smith and Bell, 1975).
Blood flow to the GIT increased substantially after feeding (Axelsson and Fritsche, 1991; Axelsson et
al., 1989, 2000) and decreased during strenuous exercise (Axelsson and Fritsche, 1991). In mammals,
the increase in blood flow that occurs with feeding tends to be localized to regions that contain food,
and an increase in flow through the coeliac artery generally precedes a flow increase in the mesenteric
arteries. Similar changes probably occur in fish (Axelsson et al., 2000). The effect of these changes in
blood flow on dietary uptake of xenobiotic compounds is currently unknown. It may be speculated,
however, that an increase in blood flow would promote uptake by maintaining an inward gradient for
chemical diffusion.

The Skin
Skin Structure and Function
The skin of a fish functions as a physical barrier to separate the internal and external environments. As
such, it maintains the ionic and osmotic integrity of the internal environment, provides protection from
abrasion and disease, and participates in the exchange of respiratory gases. The skin is one of the largest
organs of the fish body (approximately 10% of body weight) and may act as an important exchange
surface for the absorption and excretion of xenobiotic chemicals.
Grossly, the structure of fish skin varies enormously among species. In nearly all cases, however, it
is possible to identify two major layers: an outer layer (epidermis) and an inner layer (dermis). These
two layers are separated from the underlying muscle by the hypodermis (also called subcutis). Accessory
structures associated with the skin include sensory receptors, scales, mucous glands, poison glands,
luminous organs, and electric organs.
The epidermis can be divided further into two layers. An outer layer made up of interlocking
stratified epithelial cells forms a tough barrier at the skin surface. Below this is the basal lamina layer,
or stratum germinativum, which is composed of highly metabolic cells that differentiate into the
various cell types found in the epidermis. Goblet cells produce mucus that is continually secreted at