Regulation of Epithelia Solute Transport in Elasmobranch Fish

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The retention of urea for osmotic purposes in elasmobranch fish has been known for over 125 years yet we know little about solute transport across epithelia in this ancient group of fishes. Attempts have been made to understand urea transport mechanisms using gill and renal tissue, however, these studies are hampered by the complexity of the tissue used. Intestinal tissue lends itself well to transport physiology studies as it has a relatively flat surface and uniform epithelia. Recent advances have been made in establishing a viable preparation in elasmobranchs and data have consistently shown that feeding results in a net uptake of urea from the lumen of the intestine against a concentration gradient, and starvation leads to a net loss of urea to the intestinal lumen. These studies suggest significant repositioning of transport proteins in intestinal epithelia and/or remodeling of intestinal epithelia cell morphology in response to the presence of food. Either way such physiological responses would require substantial coordination through the actions of gastro-entero-pancreatic (GEP) hormones, many of which have been identified in the intestine of elasmobranch fish. While a number of studies have examined regulatory roles for these hormones in blood flow and smooth muscle contraction research investigating their role in the regulation of intestinal solute transport in elasmobranch fish is non-existent. Using isolated intestinal preparations, the proposed research will establish this tissue as a primary resource for understanding the mechanisms of solute transport in elasmobranchs. Using a classic Ussing chamber setup selective screening of specific inhibitors and agonists will identify key transport proteins involved in the movement of urea and other solutes across the intestinal epithelia. Molecular techniques will quantify mRNA expression of these proteins in fed and starved fish. These studies combined with immunohistochemical techniques will result in the development of a urea and solute transport model for elasmobranchs based on direct empirical evidence from a relatively homogeneous epithelia. Light, scanning and transmission microscopy will be used to identify morphological changes in the intestinal epithelia in response to feeding. The Ussing chamber preparation will be used to screen candidate GEP hormones for the control of urea and solute transport. Once identified, receptor expression for inhibitory and stimulatory hormones will be examined. Similarly a pancreatic in situ perfusion preparation will be established to determine if the hormones regulating solute movement across the intestine are also involved in regulating pancreatic exocrine and possibly endocrine activity. The overall goal of this proposed research program is to describe solute transport mechanisms in the elasmobranch intestine and ultimately functional evolution of vertebrate GEP hormones.**