廣鹽性魚類適應鹽度變異中腸道與肝臟麩胺酸的運輸和代謝機轉
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2016
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環境鹽度的變異已知會刺激廣鹽性魚類腸道中鈉鉀幫浦進行主動運輸以調節離子及水分的恆定；而肝臟則會進行代謝作用，以提供魚體各組織充足的能量需求。非必需胺基酸中的麩胺酸(glutamate)是生物體中含量最多的胺基酸，過去在魚類代謝機制相關的研究中，麩胺酸的運用已知與適應環境鹽度變異有關，然而麩胺酸在廣鹽性魚類腸道及肝臟的代謝調節機轉仍不明確。
本研究運用日本種青鱂魚(Oryzias latipes)作為廣鹽性魚類實驗物種。在鹽度轉移過程中，腸道和肝臟中麩醯胺酸運輸蛋白(SAT1)及麩胺酸和麩醯胺酸相關的合成酵素(GLS, GLUL)基因表現亦隨著環境鹽度增加而有顯著變化，且上述隨鹽度變化波動的運輸蛋白和酵素皆被發現表現在腸道和肝臟細胞中。此外，鹽度刺激會使麩胺酸代謝產生毒性氨，因此本研究發現與尿素合成相關的基因在腸道(CPS, OTC)和肝臟(ASL, ARG, AGMAT)中會被誘發以進行尿素生合成反應。綜合以上的結果推測：鹽度刺激會促進廣鹽性魚類腸道和肝臟中麩胺酸與麩醯胺酸的代謝和運輸，因代謝產出的含氮廢物會藉由尿素循環機制合成尿素。此適應性代謝轉換機制不僅有利於個體能量的適時供給，並能有助於維持組織細胞內的滲透壓恆定。
Environmental salinity variations have been proved to stimulate Na+-K+-ATPase (NKA) activity to regulate homeostasis in intestine of euryhaline teleosts. In addition liver can conduct metabolism for supplying intact energy demand. The non-essential amino acid, glutamate, is the most abundant amino acid in organisms. In euryhaline teleosts, glutamate accumulation and related metabolism have been observed in several vital tissues under salinity challenges. However, molecular evidences and related proper inferences regarding glutamate utilization are still in absence in teleost intestine and liver so far. In this study, we used Japanese medaka (Oryzias latipes) as a model euryhaline teleost for salinity perturbation test. Under salinity challenges, transcripts expressions of glutamine transporters (SAT1) and glutamate (Glu)/glutamine (Gln) synthesis enzymes (GSL and GLUL) in intestine and liver were obviously stimulated and found to be respectively expressed in intestinal epithelial cells and hepatocytes. Moreover glutamate deamination would further lead to NH4+ production; therefore, those NH4+-derived urea cycle candidates, carbamoyl phosphate (CPS) and ornithine transcarbamylase (OTC) in intestine, argininosuccinate lyase (ASL), arginase (ARG) and agmatinase (AGMAT) in liver, were found to be up-regulated under ambient salinity challenges. Those results inferred that Glu/Gln metabolism, subsequent transport and further urea cycle activation from metabolic nitrogenous waste in intestine and liver of euryhaline teleosts may not only be vital for intact metabolic demand, but also for body osmolality maintenance in face of salinity challenges.
Environmental salinity variations have been proved to stimulate Na+-K+-ATPase (NKA) activity to regulate homeostasis in intestine of euryhaline teleosts. In addition liver can conduct metabolism for supplying intact energy demand. The non-essential amino acid, glutamate, is the most abundant amino acid in organisms. In euryhaline teleosts, glutamate accumulation and related metabolism have been observed in several vital tissues under salinity challenges. However, molecular evidences and related proper inferences regarding glutamate utilization are still in absence in teleost intestine and liver so far. In this study, we used Japanese medaka (Oryzias latipes) as a model euryhaline teleost for salinity perturbation test. Under salinity challenges, transcripts expressions of glutamine transporters (SAT1) and glutamate (Glu)/glutamine (Gln) synthesis enzymes (GSL and GLUL) in intestine and liver were obviously stimulated and found to be respectively expressed in intestinal epithelial cells and hepatocytes. Moreover glutamate deamination would further lead to NH4+ production; therefore, those NH4+-derived urea cycle candidates, carbamoyl phosphate (CPS) and ornithine transcarbamylase (OTC) in intestine, argininosuccinate lyase (ASL), arginase (ARG) and agmatinase (AGMAT) in liver, were found to be up-regulated under ambient salinity challenges. Those results inferred that Glu/Gln metabolism, subsequent transport and further urea cycle activation from metabolic nitrogenous waste in intestine and liver of euryhaline teleosts may not only be vital for intact metabolic demand, but also for body osmolality maintenance in face of salinity challenges.
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廣鹽性魚類, 鹽度, 腸道, 肝臟, 麩胺酸, 尿素, euryhaline teleosts, salinity, intestine, liver, glutamate, urea