Nitrogen metabolism in tambaqui (Colossoma macropomum), a neotropical model teleost: hypoxia, temperature, exercise, feeding, fasting, and high environmental ammonia

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• 作者：Chris M. Wood ; José Gadelha de Souza Netto…
• 关键词：Ammonia ; Urea ; Protein ; Endogenous fraction ; Exogenous fraction ; Oxygen consumption ; Nitrogen quotient ; Transepithelial potential
• 刊名：Journal of Comparative Physiology B
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：187
• 期：1
• 页码：135-151
• 全文大小：
• 刊物类别：Biomedical and Life Sciences
• 刊物主题：Animal Physiology; Biomedicine, general; Human Physiology; Zoology; Biochemistry, general;
• 出版者：Springer Berlin Heidelberg
• ISSN：1432-136X
• 卷排序：187

文摘

The total rate of N-waste excretion (M_N) in juvenile tambaqui living in ion-poor Amazonian water comprised 85 % ammonia-N (M_Amm-N) and 15 % urea-N (M_Urea-N). Both occurred mainly across the gills with only ~5 % of M_Amm-N and ~39 % of M_Urea-N via the urine. Tambaqui were not especially tolerant to high environmental ammonia (HEA), despite their great resistance to other environmental factors. Nevertheless, they were able to maintain a continued elevation of M_Amm-N during and after 48-h exposure to 2.5 mmol L−1 HEA. The normally negative transepithelial potential (−18 mV) increased to −9 mV during the HEA period, which would help to reduce branchial NH₄+ entry. During 3 h of acute environmental hypoxia (30 % saturation), M_Amm-N declined, and recovered thereafter, similar to the response seen in other hypoxia-tolerant teleosts; M_Urea-N did not change. However, during gradual hypoxia, M_Amm-N remained constant, but M_Urea-N eventually fell. The acute temperature sensitivities of M_Amm-N and M_N were low from 28 °C (acclimation) to 33 °C (Q10 ~1.5), but high (~3.8) from 33 to 38 °C, relative to \( M_{{{\text{O}}_{ 2} }} \) (~1.9 throughout). In contrast, M_Urea-N exhibited a different pattern over these temperature ranges (Q10 2.6 and 2.1, respectively). The nitrogen quotient (NQ = 0.16–0.23) was high at all temperatures, indicating a 60–85 % reliance on protein to fuel aerobic metabolism in these fasting animals. During steady-state aerobic exercise, \( M_{{{\text{O}}_{ 2} }} \) and M_Urea-N increased in parallel with velocity (up to 3.45 body lengths s−1), but M_Amm (and thus M_N) remained approximately constant. Therefore, the NQ fell progressively, indicating a decreasing reliance on protein-based fuels, as work load increased. In group feeding trials using 45 % protein commercial pellets, tambaqui excreted 82 % (range 39–170 %) of the dietary N within 24 h; N-retention efficiency was inversely related to the ration voluntarily consumed. M_Amm-N peaked at 4–6 h, and M_Urea-N at 6–9-h post-feeding, with an additional peak in M_Amm-N only at 21 h. During subsequent fasting, M_N stabilized at a high endogenous rate from 2 through 8 days post-feeding. Possible reasons for the high wasting of protein-N during both fasting and feeding are discussed.