Loong Ai M, Pang Cheryl Y M, Hiong Kum C, Wong Wai P, Chew Shit F, Ip Yuen K
Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore, 117543, Republic of Singapore.
J Comp Physiol B. 2008 Mar;178(3):351-63. doi: 10.1007/s00360-007-0228-6. Epub 2007 Dec 6.
The objective of this study was to elucidate how the African lungfish, Protopterus annectens, ameliorated ammonia toxicity during 12 or 46 days of aestivation in air or in mud. Twelve days of aestivation in air led to significant increases in contents of urea, but not ammonia, in tissues of P. annectens. The estimated rate of urea synthesis increased 2.7-fold despite the lack of changes in the activities of hepatic ornithine-urea cycle enzymes, but there was only a minor change in the estimated rate of ammonia production. After 46 days of aestivation in air, the ammonia content in the liver decreased significantly and contents of urea in all tissues studied increased significantly, indicating that the fish shifted to a combination of increased urea synthesis (1.4-fold of the day 0 value) and decreased ammonia production (56% of the day 0 value) to defend against ammonia toxicity. By contrast, 12 days of aestivation in mud produced only minor increases in tissue urea contents, with ammonia contents remained unchanged. This was apparently achieved through decreases in urea synthesis and ammonia production (40 and 15%, respectively, of the corresponding day 0 value). Surprisingly, 46 days of aestivation in mud resulted in no changes in tissue urea contents, indicating that profound suppressions of urea synthesis and ammonia production (2.6 and 1.2%, respectively, of the corresponding day 0 value) had occurred. This is the first report on such a phenomenon, and the reduction in ammonia production was so profound that it could be the greatest reduction known among animals. Since fish aestivated in mud had relatively low blood pO(2) and muscle ATP content, they could have been exposed to hypoxia, which induced reductions in metabolic rate and ammonia production. Consequently, fish aestivating in mud had a lower dependency on increased urea synthesis to detoxify ammonia, which is energy intensive, than fish aestivating in air.
本研究的目的是阐明非洲肺鱼(Protopterus annectens)在空气中或泥中夏眠12天或46天期间如何缓解氨毒性。在空气中夏眠12天导致非洲肺鱼组织中尿素含量显著增加,但氨含量未增加。尽管肝脏鸟氨酸-尿素循环酶的活性没有变化,但估计的尿素合成速率增加了2.7倍,而估计的氨产生速率仅发生了轻微变化。在空气中夏眠46天后,肝脏中的氨含量显著下降,所有研究组织中的尿素含量显著增加,这表明鱼转向增加尿素合成(为第0天值的1.4倍)和减少氨产生(为第0天值的56%)相结合的方式来抵御氨毒性。相比之下,在泥中夏眠12天仅使组织尿素含量略有增加,氨含量保持不变。这显然是通过降低尿素合成和氨产生(分别为相应第0天值的40%和15%)来实现的。令人惊讶的是,在泥中夏眠46天导致组织尿素含量没有变化,这表明尿素合成和氨产生受到了深度抑制(分别为相应第0天值的2.6%和1.2%)。这是关于这种现象的首次报道,氨产生的减少非常显著,可能是已知动物中最大的减少。由于在泥中夏眠的鱼血液pO(2)和肌肉ATP含量相对较低,它们可能暴露于低氧环境,这导致代谢率和氨产生降低。因此,与在空气中夏眠的鱼相比,在泥中夏眠的鱼对增加尿素合成以解毒氨的依赖性较低,因为尿素合成需要消耗能量。