Hydrosoluble Nutrient and Toxicant Absorption by Terrestrial Vertebrates

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Many nutrients, natural toxins, and manmade toxins and drugs are hydrosoluble (soluble in water). Efficient transport of hydrosoluble chemicals across the vertebrate small intestine is generally thought to depend on membrane transport proteins (mediated transport), but some birds can efficiently transport hydrosoluble chemicals through the gaps between cells (i.e., by a passive paracellular pathway). This research will (1) test the generality of this finding in other birds, mammals, and reptiles spanning a broad size range (0.05 kg - 50 kg), (2) test mechanisms that may explain differences among species in the importance of paracellular transport of hydrosoluble chemicals, and (3) determine hydrosoluble absorption of toxins by the paracellular pathway. Physiological, pharmacokinetic, and microscopy techniques will be used to test these hypotheses. The differential absorption of carbohydrate probes, which range in size from 150 -666 Daltons, will be used to assess how the gap between cells restricts transport according to molecular size. Differences in the absorption of two forms of glucose (D- and L-glucose) will provide information about the relative rates of glucose absorption by the mediated versus passive pathways. The effects of a nutrient (sugar) in the gut on absorption will be determined. Intestinal tissues will be examined via light and electron microscopy to characterize structural features (e.g., intestinal surface area and density of tight junctions at the gaps between adjacent cells) that may contribute to variation in paracellular absorption. Another study will measure absorption of a man-made hydrosoluble toxin (the herbicide glyphosate) and a natural hydrosoluble phytochemical (anthocyanin) in two species. Movement of the toxin glyphosate along the paracellular path between (not through) intestinal cells will be directly observed using autoradiography. The proposed studies have important implications for vertebrate ecology, digestive physiology, evolutionary physiology and ecotoxicology.