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Transport Mechanism of Methionine In The Intestinal Tract Of Rainbow Trout (Oncorhynchus mykiss)



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DL-Methionine (DL-Met) and its corresponding hydroxy analogue (DL-MHA) have been increasingly used in animal feeds. In mammalian and avian species, DL-Met is known to be transported by amino acid transporters, while DL-MHA is transported by monocarboxylate transporters. However, the characterization of transporters responsible for transport of these Met products in the fish intestine has not been studied. Therefore, this thesis focuses on understanding the transport mechanism of DL-Met and DL-MHA in different intestinal segments of rainbow trout (Oncorhynchus mykiss) using radiolabeled substrates and gene expression. Firstly, both sodium-dependent and independent DL-Met transport were characterized in Ussing chambers at low (µM) and high concentrations (mM) in triploid and diploid trout intestine at pyloric caeca (PC), midgut (MG), and hindgut (HG) regions. DL-[14C]Met radiolabeled isotope fluxes demonstrated a Na+-dependent high-affinity (Km in µM ranges) and low-affinity (Km in mM ranges) transport mechanism across the intestine, associated with apical ASCT2 and B0AT1-like transporters at low and high concentrations, respectively. Gene expression detected the presence of transporters y+LAT1 and LAT4, which might play a role in facilitating re-influx/efflux Met from the basolateral side of intestinal epithelial cells. Secondly, the dependence of DL-MHA transport was investigated in both sodium and proton conditions. The results indicated that there was intestinal segmental segregation of DL-[14C]MHA flux, revealing different transport mechanisms along trout intestine. Specifically, the apical DL-MHA influx was mediated by sodium-requiring systems in all regions, which associated with SMCTs. Basolateral efflux in PC and MG regions seemed to be proton-independent, but basolateral efflux in HG tended to be proton-dependent, which associated with MCT9 and MCT1, respectively. Thirdly, the transport rates of radioisotopic DL-Met and DL-MHA were compared to partially explain the effectiveness of the use of these Met sources in animal feeds. The comparison showed that DL-Met flux rates were significantly higher than DL-MHA throughout the intestinal segments at both low and high concentrations in physiological conditions. This probably has economic implications in selecting a suitable Met form to supplement in fish diets. Overall, DL-Met and DL-MHA transport in trout intestine followed different pathways along the intestinal tract. Similar to mammals, the transport of both Met substrates were dependent on sodium. In addition, DL-Met transport was more efficient than DL-MHA. These findings are of importance from both a physiological and nutritional perspective.



Methionine, Intestine, Rainbow Trout, Transport Kinetics, Radioactive Isotope Flux



Doctor of Philosophy (Ph.D.)


Veterinary Biomedical Sciences


Veterinary Biomedical Sciences


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