ADAPTATION OF NITROGEN-METABOLISM TO HYPEROSMOTIC ENVIRONMENT IN AMPHIBIA
ADAPTATION OF NITROGEN-METABOLISM TO HYPEROSMOTIC ENVIRONMENT IN AMPHIBIA
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DOI：
10.1002/jez.1402150311
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发表时间：
1981-01-01
影响因子：
--
通讯作者：
BALINSKY, JB
中科院分区：
文献类型：
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作者：
BALINSKY, JB
许多种类的两栖动物，包括无尾两栖动物和乌罗德拉，都能够耐受中等盐度的环境。加拿大林蛙、绿色蟾蜍和非洲爪蛙是迄今研究过的最具泛盐度的蛙类。加拿大林蛙耐受未稀释的海水。在盐碱环境中，两栖动物表现出血浆钠和氯的升高以及细胞内钾和氯的升高。在成体中，血浆和组织尿素升高，特别是在更多的泛盐类物种中。游离氨基酸对血浆渗透压的贡献微乎其微，但在细胞内液中却非常重要。通过这些不同的方法，体液的渗透压始终保持在高于周围环境的水平。然而，幼虫两栖动物不制造尿素；加拿大林蛙可以生活在盐水中，但它们的体液渗透压保持在低于周围环境的水平。对高渗透环境的适应反应包括皮肤钠转运减少，尿流量大幅减少，以及脑下垂体后叶激素的释放。在最初的反应后，这些激素的释放减少，尿流量增加。尿素的积累缓慢，但随着适应的进行，这种物质发挥着越来越重要的作用。累积最初是由于尿素滞留，也可能是由于尿素循环的前体浓度较高而导致更多的合成。在适应的后期，尿素合成的增加是由于尿素循环酶水平的提高，特别是那些似乎具有限速作用的酶。肝脏谷氨酸脱氢酶也升高。在受到纯渗透胁迫的动物中，在不含钠的溶液中，反应与含有氯化钠的介质引起的反应相似，但不完全相同。
A large number of species of Amphibia, both Anura and Urodela, are capable of tolerating a moderately saline environment.Rana cancrivora, Bufo viridis, andXenopus laevisare among the most euryhaline frogs so far studied.Rana cancrivoracan tolerate undiluted seawater. In a saline environment, Amphibia show raised plasma sodium and chloride and raised intracellular potassium and chloride. In the adults, plasma and tissue urea are elevated, especially in the more euryhaline species. Free amino acids contribute negligibly to plasma osmolarity, but are very important in intracellular fluids. By these various means, the osmotic pressure of body fluids is always maintained at a higher level than that of the surroundings. Larval Amphibia, however, do not make urea;Rana cancrivoratadpoles can live in saltwater, but maintain the osmolarity of their body fluids below that of their surroundings.Adaptive responses to hyperosmolar environment include decreased skin sodium transport, greatly reduced urine flow, and release of posterior pituitary hormones. After the initial response, the release of these hormones declines, and urine flow increases. Accumulation of urea occurs slowly, but this substance plays an increasingly important role as adaptation proceeds. Accumulation is due initially to urea retention, and possibly to greater synthesis due to a high concentration of precursors of the urea cycle. In later stages of adaptation, increased urea synthesis is due to elevated levels of urea cycle enzymes, especially those that appear to have been rate‐limiting. Liver glutamate dehydrogenase is also elevated. In animals subjected to pure osmotic stress, in solutions not containing sodium, responses are similar to, but not identical with, those caused by a medium containing sodium chloride.