Greatly elevated urea excretion after air exposure appears to be carrier mediated in the slender lungfish (Protopterus dolloi).
Greatly elevated urea excretion after air exposure appears to be carrier mediated in the slender lungfish (Protopterus dolloi).
复制标题
在细长肺鱼（Protopterus dolloi）中，接触空气后尿素排泄量大大增加似乎是载体介导的。
DOI：
10.1086/432919
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
Ip,YuenK
中科院分区：
文献类型：
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作者：
Wood,ChrisM;Walsh,PatrickJ;Chew,ShitF;Ip,YuenK
在水生条件下，杜氏原鳍鱼是排氨的， 只排出少量的尿素氮然而，回到水中后， 在空气中夏眠30天后，肺鱼只排出少量的 氨-N，还有大量的尿素-N类似的模式也可以看到 在陆地化21-30天后， 空气暴露，但始终保持湿润。两种治疗后， 尿素氮排泄是双相的，立即增加，然后下降， 最后，第二次较大的增加，在约12小时达到峰值，并可能延长 之后的几天。第二个月期间的尿素氮排泄率 峰值达到2，000 - 6，000 μmol N kg−1h−1，比 大多数鱼类，仅与已知使用UT-A的物种的比率相当 型易化扩散尿素转运蛋白。分室研究和 [3 H]-PEG-4000（a）清除率的测量 肾小球滤过和细胞旁扩散标记物）和两种结构 脲类似物（[14 C]-乙酰胺和 [14C]- 硫脲），以表征两个峰 尿素氮排泄量较小的第一个峰几乎与 在头部（包括内鳃和外鳃）和 体隔室（包括泌尿道开口），仅伴有适度的 [14 C]-乙酰胺清除率的增加等于 [14C]- 硫脲清除率，并且可以通过 大幅度但持续时间较短的增加， [3H]- PEG-4000清除率（约为 地面速率）。尿素氮中延迟的、大得多的第二个峰 排泄代表两个隔室的外排增加，但发生 主要（72%）通过身体而不是头部区域。第二个高峰是 伴随着[14 C]-乙酰胺的大量增加 清除率，但[14 C]-硫脲仅略微进一步升高 间隙乙酰胺与硫脲的渗透率比是典型的 其他鱼类中的UT-A型转运蛋白。 [3H]- PEG-4000清除率此时稳定， 大约是陆地速率的两倍，尿素及其类似物的排泄速率 比可以解释的要大很多倍 [3H]- PEG-4000清除。我们得出结论，第一个 峰值可以通过尿排泄和细胞旁扩散的增加来解释 在重新淹没时穿过鳃，而第二个峰值可归因于 UT-A型易化扩散的延迟和延长激活 机制，主要是在皮肤，也许也在鳃上皮细胞。
Under aquatic conditions,Protopterus dolloiis ammoniotelic, excreting only small amounts of urea‐N. However, upon return to water after 30 d estivation in air, the lungfish excretes only small amounts of ammonia‐N but massive amounts of urea‐N. A similar pattern is seen after 21–30 d of terrestrialization, a treatment in which the lungfish is air exposed but kept moist throughout. After both treatments, the time course of urea‐N excretion is biphasic with an immediate increase, then a fall, and finally a second larger increase that peaks at about 12 h and may be prolonged for several days thereafter. Urea‐N excretion rates during the second peak reach 2,000–6,000 μmol N kg−1h−1, two to three orders of magnitude greater than rates in most fish and comparable only to rates in species known to employ UT‐A type facilitated diffusion urea transporters. Divided chamber studies and measurements of the clearance rates of [3H]‐PEG‐4000 (a glomerular filtration and paracellular diffusion marker) and two structural analogs of urea ([14C]‐acetamide and [14C]‐thiourea) were performed to characterize the two peaks of urea‐N excretion. The smaller first peak was almost equally partitioned between the head (including internal and external gills) and the body compartment (including urinary opening), was accompanied by only a modest increase in [14C]‐acetamide clearance equal to that in [14C]‐thiourea clearance, and could be accounted for by a large but short‐lasting increase in [3H]‐PEG‐4000 clearance (to about fivefold the terrestrial rate). The delayed, much larger second peak in urea‐N excretion represented an elevated efflux into both compartments but occurred mainly (72%) via the body rather than the head region. This second peak was accompanied by a substantial increase in [14C]‐acetamide clearance but only a modest further rise in [14C]‐thiourea clearance. The acetamide to thiourea permeability ratio was typical of UT‐A type transporters in other fish. [3H]‐PEG‐4000 clearance was stable at this time at about double the terrestrial rate, and excretion rates of urea and its analogs were many fold greater than could be accounted for by [3H]‐PEG‐4000 clearance. We conclude that the first peak may be explained by elevated urinary excretion and paracellular diffusion across the gills upon resubmergence, while the second peak is attributable to a delayed and prolonged activation of a UT‐A type facilitated diffusion mechanism, primarily in the skin and perhaps also in branchial epithelia.