Ip Y K, Chew S F, Randall D J
Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore.
Physiol Biochem Zool. 2004 Sep-Oct;77(5):768-82. doi: 10.1086/422057.
Most tropical fishes are ammonotelic, producing ammonia and excreting it as NH3 by diffusion across the branchial epithelia. Hence, those air-breathing tropical fishes that survive on land briefly or for an extended period would have difficulties in excreting ammonia when out of water. Ammonia is toxic, but some of these air-breathing fishes adopt special biochemical adaptations to ameliorate the toxicity of endogenous ammonia accumulating in the body. The amphibious mudskipper Periophthalmodon schlosseri, which is very active on land, reduces ammonia production by suppressing amino acid catabolism (strategy 1) during aerial exposure. It can also undergo partial amino acid catabolism, leading to the accumulation of alanine (strategy 2) to support locomotory activities on land. In this case, alanine formation is not an ammonia detoxification process but reduces the production of endogenous ammonia. The snakehead Channa asiatica, which exhibits moderate activities on land although not truly amphibious, accumulates both alanine and glutamine in the muscle, with alanine accounting for 80% of the deficit in reduction in ammonia excretion during air exposure. Unlike P. schlosseri, C. asiatica apparently cannot reduce the rates of protein and amino acid catabolism and is incapable of utilizing partial amino acid catabolism to support locomotory activities on land. Unlike alanine formation, glutamine synthesis (strategy 3) represents an ammonia detoxification mechanism that, in effect, removes the accumulating ammonia. The four-eyed sleeper Bostrichyths sinensis, which remains motionless during aerial exposure, detoxifies endogenous ammonia to glutamine for storage. The slender African lungfish Protopterus dolloi, which can aestivate on land on a mucus cocoon, has an active ornithine-urea cycle and converts endogenous ammonia to urea (strategy 4) for both storage and subsequent excretion. Production of urea and glutamine are energetically expensive and appear to be adopted by fishes that remain relatively inactive on land. The Oriental weatherloach Misgurnus anguillicaudatus, which actively burrows into soft mud during drought, manipulates the pH of the body surface to facilitate NH3 volatilization (strategy 5) and develops high ammonia tolerance at the cellular and subcellular levels (strategy 6) during aerial exposure. Hence, with regard to excretory nitrogen metabolism, modern tropical air-breathing fishes exhibit a variety of strategies to survive on land, and they represent a spectrum of specimens through which we may examine various biochemical adaptations that would have facilitated the invasion of the terrestrial habitat by fishes during evolution.
大多数热带鱼类是排氨型的，产生氨并通过鳃上皮扩散以NH3的形式排出。因此，那些能在陆地上短暂或长期生存的呼吸空气的热带鱼类，出水后在排泄氨方面会有困难。氨是有毒的，但其中一些呼吸空气的鱼类采取了特殊的生化适应措施来减轻体内积累的内源性氨的毒性。两栖弹涂鱼中华大弹涂鱼在陆地上非常活跃，在暴露于空气中时，通过抑制氨基酸分解代谢来减少氨的产生（策略1）。它也可以进行部分氨基酸分解代谢，导致丙氨酸积累（策略2）以支持在陆地上的运动活动。在这种情况下，丙氨酸的形成不是一个氨解毒过程，而是减少了内源性氨的产生。亚洲鳢虽然不是真正的两栖动物，但在陆地上表现出适度的活动能力，它在肌肉中积累丙氨酸和谷氨酰胺，在暴露于空气中时，丙氨酸占氨排泄减少量不足部分的80%。与中华大弹涂鱼不同，亚洲鳢显然不能降低蛋白质和氨基酸的分解代谢速率，也无法利用部分氨基酸分解代谢来支持在陆地上的运动活动。与丙氨酸的形成不同，谷氨酰胺的合成（策略3）代表一种氨解毒机制，实际上可以去除积累的氨。四眼睡鱼中华乌塘鳢在暴露于空气中时保持不动，将内源性氨解毒为谷氨酰胺进行储存。细长的非洲肺鱼多氏原鳍鱼可以在陆地上的黏液茧中夏眠，它有活跃的鸟氨酸-尿素循环，并将内源性氨转化为尿素（策略4）进行储存和随后的排泄。尿素和谷氨酰胺的产生在能量上是昂贵的，似乎被那些在陆地上相对不活跃的鱼类所采用。东方泥螈在干旱期间会积极钻入软泥中，通过调节体表pH值来促进NH3挥发（策略5），并在暴露于空气中时在细胞和亚细胞水平上形成高氨耐受性（策略6）。因此，关于排泄性氮代谢，现代热带呼吸空气的鱼类表现出多种在陆地上生存的策略，它们代表了一系列标本，通过这些标本我们可以研究各种生化适应，这些适应在进化过程中可能促进了鱼类对陆地栖息地的入侵。
