Ip Yuen K, Zubaidah Ramdzan M, Liew Pei C, Loong Ai M, Hiong Kum C, Wong Wai P, Chew Shit F
Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore.
Physiol Biochem Zool. 2004 Mar-Apr;77(2):242-54. doi: 10.1086/383499.
The African sharptooth catfish Clarias gariepinus lives in freshwater, is an obligatory air breather, and exhibits high tolerance of environmental ammonia. This study aimed at elucidating the strategies adopted by C. gariepinus to defend against ammonia toxicity during ammonia exposure. No carbamoyl phosphate synthetase (CPS) I or III activities were detected in the liver or muscle of the adult C. gariepinus. In addition, activities of other ornithine-urea cycle (OUC) enzymes, especially ornithine transcarbamylase, were low in the liver, indicating that adult C. gariepinus does not have a "functional" hepatic OUC. After being exposed to 50 or 100 mM NH4Cl for 5 d, there was no induction of hepatic OUC enzymes and no accumulation of urea in tissues of the experimental animals. In addition, the rate of urea excretion remained low and unchanged. Hence, ammonia exposure did not induce ureogenesis or ureotely in C. gariepinus as suggested elsewhere for another obligatory air-breathing catfish of the same genus, Clarias batrachus, from India. Surprisingly, the local C. batrachus did not possess any detectable CPS I or III activities in the liver or muscle as had been reported for the Indian counterpart. There were no changes in levels of alanine in the muscle, liver, and plasma of C. gariepinus exposed to 50 or 100 mM NH4Cl for 5 d; neither were there any changes in the glutamine levels in these tissues. Yet even after being exposed to 100 mM NH4Cl for 5 d, there was no significant increase in the level of ammonia in the muscle, which constitutes the bulk of the specimen. In addition, the level of ammonia accumulated in the plasma was relatively low compared to other tropical air-breathing fishes. More importantly, for all NH4Cl concentrations tested (10, 50, or 100 mM), the plasma ammonia level was maintained relatively constant (2.2-2.4 mM). These results suggest that C. gariepinus was able to excrete endogenous ammonia and infiltrated exogenous ammonia against a very steep ammonia gradient. When exposed to freshwater (pH 7.0) with or without 10 mM NH4Cl, C. gariepinus was able to excrete ammonia continuously to the external medium for at least 72 h. This was achieved while the plasma NH4+ and NH3 concentrations were significantly lower than those of the external medium. Diffusion trapping of NH3 through boundary layer acidification can be eliminated as the pH of the external medium became more alkaline instead. These results represent the first report on a freshwater fish (C. gariepinus) adopting active excretion of ammonia (probably NH4+) as a major strategy to defend against ammonia toxicity when exposed to environmental ammonia.
非洲尖齿胡鲶（Clarias gariepinus）生活在淡水中，是一种 obligatory air breather（此处可能有误，推测为“ obligate air breather”，即专性空气呼吸者），并且对环境氨具有较高的耐受性。本研究旨在阐明非洲尖齿胡鲶在氨暴露期间抵御氨毒性所采用的策略。在成年非洲尖齿胡鲶的肝脏或肌肉中未检测到氨甲酰磷酸合成酶（CPS）I 或 III 的活性。此外，肝脏中其他鸟氨酸 - 尿素循环（OUC）酶的活性，尤其是鸟氨酸转氨甲酰酶的活性较低，这表明成年非洲尖齿胡鲶没有“功能性”的肝脏 OUC。在暴露于 50 或 100 mM NH₄Cl 5 天后，实验动物的肝脏 OUC 酶没有诱导产生，组织中也没有尿素积累。此外，尿素排泄率仍然较低且未发生变化。因此，如其他地方针对来自印度的同属另一种专性空气呼吸鲶鱼——印度胡鲶（Clarias batrachus）所表明的那样，氨暴露并未在非洲尖齿胡鲶中诱导尿素生成或尿素排泄。令人惊讶的是，当地的印度胡鲶在肝脏或肌肉中未检测到任何可检测到的 CPS I 或 III 活性，正如对印度同类的报道那样。暴露于 50 或 100 mM NH₄Cl 5 天的非洲尖齿胡鲶的肌肉、肝脏和血浆中的丙氨酸水平没有变化；这些组织中的谷氨酰胺水平也没有任何变化。然而，即使在暴露于 100 mM NH₄Cl 5 天后，构成样本大部分的肌肉中的氨水平也没有显著增加。此外，与其他热带空气呼吸鱼类相比，血浆中积累的氨水平相对较低。更重要的是，对于所有测试的 NH₄Cl 浓度（10、50 或 100 mM），血浆氨水平保持相对恒定（2.2 - 2.4 mM）。这些结果表明，非洲尖齿胡鲶能够在非常陡峭的氨梯度下排泄内源性氨并渗透外源性氨。当暴露于含有或不含有 10 mM NH₄Cl 的淡水（pH 7.0）中时，非洲尖齿胡鲶能够持续向外部介质排泄氨至少 72 小时。这是在血浆 NH₄⁺和 NH₃浓度显著低于外部介质的情况下实现的。随着外部介质的 pH 变得更碱性，通过边界层酸化对 NH₃的扩散捕获可以消除。这些结果代表了关于淡水鱼（非洲尖齿胡鲶）在暴露于环境氨时采用主动排泄氨（可能是 NH₄⁺）作为抵御氨毒性的主要策略的首次报道。
