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Oxygen, carbon dioxide and ammonia exchange in the gastrointestinal tract of teleost fish Jung, Hyewon (Ellen)

Abstract

The gastrointestinal tract (GIT) of teleost fish is a multifunctional organ system involved in digestion, assimilation of nutrients, immune responses, neuroendocrine functions and osmoregulation. Many of these processes have implications for gas exchange, acid-base regulation and ion regulation. However, there is a current lack of knowledge on the basic physiology of the three respiratory gases - oxygen, carbon dioxide and ammonia - in the GIT. My thesis expands our current understanding of the basic profiles of the three respiratory gases in the GIT lumen, their potential exchange with the vascular system, and the changes associated with feeding. In addition, the potential influence of osmoregulatory functions on these parameters was investigated by conducting a comparative study between freshwater and seawater species; the rainbow trout (Oncorhynchus mykiss) was used as a freshwater model species for in depth investigations and the English sole (Parophrys vetulus) as a comparative seawater species. A combination of in vivo, in situ and in vitro techniques was used to assess these areas at the whole animal, tissue and cellular levels. Based on findings from 5 research chapters, I conclude that both freshwater rainbow trout and seawater English sole have nearly anoxic, hypercapnic and high ammonia environments in the GIT lumen in both fasting and fed conditions. Feeding increases carbon dioxide and ammonia levels, but the GIT epithelia regulates the diffusion of the three respiratory gases into the vascular system regardless of feeding status. The different osmoregulatory requirements in seawater versus freshwater did not greatly affect these conclusions. Furthermore, the intestinal tissues have the capacity to handle and transport two nitrogenous products: urea and ammonia. Throughout the GIT, there were expressions of both a potential NH₄⁺ transporter (NKCC) and NH₃ channels (Rhbg, Rhcg2), but ammonia transport was independent of the PNH3 gradient. The thesis also highlights the importance of the stomach in absorbing ammonia despite a reverse PNH3 gradient at physiologically relevant pH levels.

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Attribution-NonCommercial-NoDerivatives 4.0 International