The total rate of N-waste excretion (MN) in juvenile tambaqui living in ion-poor Amazonian water comprised 85 % ammonia-N (MAmm-N) and 15 % urea-N (MUrea-N). Both occurred mainly across the gills with only ~5 % of MAmm-N and ~39 % of MUrea-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 MAmm-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), MAmm-N declined, and recovered thereafter, similar to the response seen in other hypoxia-tolerant teleosts; MUrea-N did not change. However, during gradual hypoxia, MAmm-N remained constant, but MUrea-N eventually fell. The acute temperature sensitivities of MAmm-N and MN were low from 28 °C (acclimation) to 33 °C (Q10 ~1.5), but high (~3.8) from 33 to 38 °C, relative to MO2 (~1.9 throughout). In contrast, MUrea-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, MO2 and MUrea-N increased in parallel with velocity (up to 3.45 body lengths s−1), but MAmm (and thus MN) 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. MAmm-N peaked at 4–6 h, and MUrea-N at 6–9-h post-feeding, with an additional peak in MAmm-N only at 21 h. During subsequent fasting, MN 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. © 2016, Springer-Verlag Berlin Heidelberg.
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