Environmental Impacts on the Physiological Mechanisms Controlling Xenobiotic Transfer across Fish Gills

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Abstract
Fish physiologists have provided the basic information on gill morphology, gill function, and vascular dynamics with which to understand branchial flux of gases, water, and ions. In addition, pharmacologists and toxicologists, working in the area of drug action, have characterized the physicochemical attributes of xenobiotic chemicals that determine their rate of movement across biological membranes. Recently, aquatic toxicologists have applied this information to the question of what mechanisms control the movement of organic chemicals across fish gills and how exchange is affected by chemical properties. This research on gill transfer mechanisms was extended to consider environmental conditions that have been reported to influence chemical exchange (e.g., dissolved and/or suspended organic material, hydrogen ion concentration [pH], dissolved O₂, content, and water temperature). Mathematical models were developed that predict gill exchange as a function of basic processes such as water flow across the gills, blood flow through the gills, partitioning of the chemical between water and blood, and diffusion between blood and water across gill epithelia. Such mechanistic models can predict the effects of environmental conditions on exchange rates of xenobiotics. To fully develop a predictive capability for xenobiotic uptake and distribution by fish, it will be necessary to incorporate these gill models into emerging, physiologically based models for the entire animal.

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