Page 36 - The Toxicology of Fishes
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16 The Toxicology of Fishes
because the larger size of these molecules reduces their ability to penetrate lipid membranes (Opperhuizen
et al., 1985) or because their high hydrophobicity results in them being bound to even larger organic
substances in the exposure water (Black and McCarthy, 1988). For the partially ionized chemicals, lower
uptake rates occur because the charged forms diffuse less readily across lipid membranes. For both of
these latter two groups of chemicals, accumulation rates are limited primarily by their diffusion across
the epithelium rather than by water and blood flows.
Uptake of toxic chemicals from water via the skin will generally be much less important than uptake
via the gills because the skin of fish typically provides less surface area, a thicker and less permeable
diffusion barrier, slower transport of water to the exchange surface, less blood flow, and no counter-
current flow of water and blood. McKim et al. (1996) reported uptake of chlorinated ethanes via skin
to be only a few percent of total uptake in adult rainbow trout and channel catfish (Ictalurus punctatus).
For smaller fish, however, uptake via skin can be important (Lien et al., 1994; Saarikoski et al., 1986),
because, as size decreases, skin is generally more permeable and has an increasing surface area relative
to the gills. This is especially true for fish embryos and larvae, whose gills are not well developed and
for which respiration can be largely via their more permeable skin. Although many of the same general
principles apply whether uptake is via skin or gill, details regarding epithelial permeabilities, transport
mechanisms, and the chemical microenvironment differ between gill and skin and are not well charac-
terized. The relative importance of gill vs. skin generally is not known or even explicitly considered in
chemical risk assessments, but it should be remembered that there will be a skin component of uptake
of waterborne chemicals that could be significant and might exhibit bioavailability relationships different
from the gill.
Accumulation via Diet
When chemicals in water partition strongly into solid phases or exist as dissolved forms that are poorly
absorbed at gill and skin surfaces, the major route of exposure to fish can be via ingestion of contaminated
food (or sediment for some species) and subsequent absorption within the gastrointestinal tract (GIT).
The contents of the GIT represent an extension of the external environment, albeit one that is substantially
modified by the process of digestion. Structural and functional features of the GIT that contribute to
dietary uptake of xenobiotic chemicals were reviewed by Kleinow and James (2001). Briefly, these
features include:
1. The large absorptive surface area is increased in many species by the presence of blind
diverticula called pyloric ceca (Buddington and Diamond, 1987).
2. One or a few epithelial cell layers separate the contents of the gut lumen from elements of the
blood circulation.
3. A high degree of tissue vascularization is present; blood perfusion of the GIT increases
following the consumption of a meal and is probably controlled at a regional level so blood
flow is directed to gut segments where digestion is occurring (Axelsson et al., 2000).
4. The contact time between gut contents and the gut epithelium is relatively long; total gut transit
times in fish vary widely but generally range from a few hours to a day or more.
5. Ingested food items are broken down by the combined effects of acidification, enzymatic action,
and physical disruption, releasing chemical contaminants to the liquid environment within the
GIT. The rate of digestion and efficiency of nutrient uptake depend in turn on factors such as
temperature, feeding frequency, meal size, and food digestibility.
The processes that control chemical flux between gut contents and the general circulation are not as
well understood as those occurring at the gills. In general, compounds for which dietary uptake is an
important route of uptake tend to remain associated with components of the meal until these components
are taken up by the fish. Mixed micelles resulting from the digestion of dietary lipid sequester lipophilic
organic compounds. These micelles are subsequently broken down at the surface of intestinal epithelium,
releasing their contents within the aqueous boundary layer. Metals that possess high affinity for protein