Little is known about nitrogenous waste (N waste) handling and excretion (J_{N waste}) during the complex life cycle of the sea lamprey (Petromyzon marinus), an extant jawless fish that undergoes a complete metamorphosis from a filter-feeding larva (ammocoete) into a parasitic juvenile that feeds on the blood of larger, jawed fishes. Here, we investigate the ammonia- and urea-handling profiles of sea lampreys before, during, and after metamorphosis. The rates of ammonia excretion (J_amm) and urea excretion (J_urea) significantly decreased after the onset of metamorphosis, with the lowest rates observed during midmetamorphosis. Near the completion of metamorphosis, rates of J_{N waste} ($J_{\mathrm{N waste}}=J_{a\mathrm{mm}}+J_{u\mathrm{rea}}$) significantly increased as sea lampreys entered the juvenile period. Feeding juvenile lampreys had greater than 10- to 15-fold higher J_amm and fivefold higher J_urea compared to nonfed juveniles, which corresponded to higher postprandial (postfeeding) concentrations of plasma ammonia and urea. The routes of J_amm and J_urea completely diverged following metamorphosis. In larvae, J_amm was equally split between branchial (gills) and extrabranchial (skin plus renal) pathways, but following metamorphosis, >80% of ammonia was excreted via the gills in nonfeeding juvenile lampreys, and >95% of ammonia was excreted via the gills in adult sea lampreys. Urea, on the other hand, was predominantly excreted via extrabranchial routes and, to a lesser extent, the gills in larvae and in nonfeeding juveniles. In adults, however, virtually all urea was excreted via urine. Reverse transcription polymerase chain reaction and in silico analyses also indicated that a urea transporter encoded by a slc4a2-like gene is present in lampreys. The branchial expression of this transporter is modulated throughout sea lamprey life history, as it is higher in the larvae and steadily decreases until the adult stage. We conclude that the divergent pathways of J_amm and J_urea during the sea lamprey life cycle reflect changes in their habitat, lifestyle, and diet. Further, the near-complete reliance on renal routes for J_urea in adult sea lampreys is unique among fishes and may reflect the ancestral condition of how this N waste product was handled and excreted by the earliest vertebrates.

中文翻译：

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海七鳃鳗生命周期中氨和尿素产生和排泄的不同途径

在海七鳃鳗 ( Petromyzon marinus ) 的复杂生命周期中，对含氮废物 (N 废物) 的处理和排泄 ( J _{N 废物}) 知之甚少，这是一种现存的无颌鱼，从滤食性幼虫 (ammocoete) 经历完全变态变成寄生幼鱼，以较大的有颚鱼的血为食。在这里，我们研究了海七鳃鳗在变态之前、期间和之后的氨和尿素处理曲线。变态开始后氨排泄率（J _amm）和尿素排泄率（J _urea）显着下降，在变态中期观察到的速率最低。在变态完成附近，J率_{N 废物}(${Ĵ}_{\mathrm{氮废物}}={Ĵ}_{\mathrm{一个}\mathrm{毫米}}+{Ĵ}_{你\mathrm{雷亚尔}}$) 随着海七鳃鳗进入幼年期而显着增加。与未喂食的幼鱼相比，喂养幼鱼的J _amm高 10 到 15 倍以上，_尿素J高5 倍，这对应于血浆氨和尿素的餐后（喂食后）浓度较高。J _amm和J _urea的路线在变态后完全分歧。在幼虫中，J _amm在鳃（鳃）和鳃外（皮肤加肾）途径之间平均分配，但在变态后，> 80% 的氨在非摄食的幼年七鳃鳗中通过鳃排出，> 95% 的氨在成年海中通过鳃排出七鳃鳗。另一方面，尿素主要通过鳃外途径排泄，其次是幼虫和非摄食幼鱼的鳃。然而，在成年人中，几乎所有的尿素都是通过尿液排出的。逆转录聚合酶链反应和计算机分析还表明，由slc4a2编码的尿素转运蛋白类基因存在于七鳃鳗中。这种转运蛋白的鳃表达在整个海七鳃鳗的生活史中受到调节，因为它在幼虫中较高，并且在成虫阶段逐渐减少。我们得出结论，海七鳃鳗生命周期中J _amm和J _urea的不同途径反映了其栖息地、生活方式和饮食的变化。此外，成年海七鳃鳗几乎完全依赖肾脏途径获取J_尿素，这在鱼类中是独一无二的，这可能反映了最早的脊椎动物如何处理和排泄这种 N 废物的祖先状况。