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

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 [Formula: see text] (~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, [Formula: see text] 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.