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
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