Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes
Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes
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DOI：
10.1111/jfb.12279
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发表时间：
2014-03-01
影响因子：
2
通讯作者：
Ip, Y. K.
中科院分区：
文献类型：
--
作者：
Chew, S. F.;Ip, Y. K.
随着呼吸空气能力的发展，一些鱼类可以从水中出现，在陆地上漫游，甚至在干旱时钻入泥中。呼吸空气的鱼类有修改鳃形态和形态测量和辅助呼吸器官，这将倾向于减少鳃氨排泄。由于氨的毒性，呼吸空气的鱼类，特别是两栖类，配备了各种策略，以减轻氨毒性在浮现或氨暴露。这些策略可分为（1）增强氨排泄和减少氨进入，（2）将氨转化为毒性较小的产物用于积累和随后的排泄，（3）减少氨产生和避免氨积累，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值和减少鳃和皮肤的NH3渗透性，理论上是最有效的策略，以保持低的内部氨浓度。NH3挥发涉及某些上皮表面的碱化，并且需要防止NH3回流的机制。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。夏眠的非洲肺鱼将氨解毒为尿素，并在唤醒后排出累积的尿素。在一些呼吸空气的鱼类中，通过抑制氨基酸催化剂和蛋白水解，或通过部分氨基酸催化剂导致丙氨酸形成，来减少氨的产生。其他人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，其他一些人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，呼吸空气的鱼类改善氨毒性的反应是多种多样的，由物种的行为和它所生活的环境的性质决定。
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.