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Characterization and modulation of volatile fatty acid transport in the gastrointestinal tract of an herbivorous teleost

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Title: Characterization and modulation of volatile fatty acid transport in the gastrointestinal tract of an herbivorous teleost
Authors: Titus, Eric Ellis
Issue Date: 1991
Abstract: Symbiotic microbial fermentation in the intestine of the fishes currently stands as an unexplored field in animal physiology. Herbivorous and omnivorous fish consume large quantities of vegetative matter high in structural polysaccharides which undergo microbial fermentative catabolism to form nutritive volatile fatty acids. This study was designed to investigate the assimilation of microbial fermentation products in the gut of the tilapia, Oreochromis mossambicus, an herbivorous/omnivorous teleost. Transport pathways for the volatile fatty acids (VFA) were investigated at the subcellular level, i.e. the brush border and basolateral membranes, and at the cellular level, in order to assemble a transintestinal transport model for the passage of VFA. In order to investigate the assimilation of these nutrients at the organismal level the modulation of intestinal VFA transport was investigated as a function of dietary manipulation. The occurrence of VFA in intestinal contents of the tilapia were analyzed by high performance liquid chromatography. Concentrations of acetate ranging from 15 mM to 18 mM were found throughout the length of the intestine, as were trace amounts of propionate. Characterization of acetate transport across the brush border membrane revealed that there is an apparent anion exchange mechanism which is independent of sodium and specific for volatile fatty acids and bicarbonate. Transport kinetic parameters determined that this mechanism operate optimally at acetate concentrations approximating those found in the gut lumen. Characterization of acetate transport across the basolateral membrane revealed a qualitatively identical anion exchange mechanism whereby intracellular VFA exit the cell in exchange for bicarbonate from the blood. In vitro acetate transport studies across intact intestinal sheets revealed significant inhibition of net transepithelial acetate flux in the presence of acetazolamide, an inhibitor of the carbonic anhydrase-mediated generation of intracellular bicarbonate. A transintestinal transport model for VFA is proposed whereby specific anion exchangers work in concert, resulting in a net transmural flux of luminal VFA in exchange for serosal and intracellular bicarbonate. This process is driven by a downhill lumen-to-blood VFA concentration gradient and supplemented by the intracellular generation of bicarbonate by carbonic anhydrase. Nutrient transport modulation was investigated by raising separate populations of tilapia on diets varying in relative concentrations of protein and carbohydrate. The fish raised on diets high in carbohydrate exhibited enhanced rates of glucose uptake across the brush border membrane compared to fish raised on the low carbohydrate/high protein diet, but there was no significant difference in transport rates for proline or acetate between the two diet groups. This specific up-regulation of glucose flux is suggestive that the herbivorous fish is more dependent on glucose than the other two nutrients as a source of calories, and can modulate the uptake rate of this nutrient in accordance with its availability. A comprehensive overview of symbiotic microbial fermentation in the different vertebrate classes is presented, along with a discussion of intestinal nutrient transport modulation, in order to place the current study into perspective within the broad fields of animal nutrition and digestive physiology.
Description: Thesis (Ph. D.)--University of Hawaii at Manoa, 1991.
Includes bibliographical references (leaves 130-133)
xii, 136 leaves, bound ill. 29 cm
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections:Ph.D. - Zoology

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