Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore.
J Fish Biol. 2014 Mar;84(3):603-38. doi: 10.1111/jfb.12279. Epub 2014 Jan 20.
With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.
随着呼吸空气能力的发展，一些鱼类可以离开水，在陆地上进行短途旅行，甚至在干旱时期钻入泥中。呼吸空气的鱼类改变了鳃的形态和形态计量以及辅助呼吸器官，这往往会减少鳃部氨的排泄。由于氨是有毒的，呼吸空气的鱼类，尤其是两栖鱼类，配备了各种策略来减轻出水面或暴露于氨时的氨毒性。这些策略可以分为 (1) 增强氨排泄和减少氨进入，(2) 将氨转化为更具毒性的产物进行积累和随后排泄，(3) 减少氨的产生和避免氨积累，以及 (4) 在细胞和组织水平上耐受氨。主动氨排泄与降低环境 pH 值以及降低鳃和皮肤 NH₃通透性相结合，在理论上是维持低内部氨浓度的最有效策略。NH₃挥发涉及某些上皮表面的碱化，需要防止 NH₃反流的机制。尿素合成是一个能量密集的过程，因此在呼吸空气的硬骨鱼中并不常见。非洲肺鱼通过将氨解毒为尿素来解毒氨，并且在觉醒后排出积累的尿素。一些呼吸空气的鱼类通过抑制氨基酸分解和蛋白水解，或者通过部分氨基酸分解导致丙氨酸形成，从而减少氨的产生。其他鱼类可以通过增加肝脏和肌肉中谷氨酰胺的合成来减缓氨的积累。然而，一些鱼类在细胞和组织水平上对氨具有高耐受性，包括大脑中的组织。总之，呼吸空气的鱼类为减轻氨毒性而做出的反应是多种多样的，这取决于物种的行为和它生活的环境的性质。
