Effects of ammonia loading on Porcellio Scaber: Glutamine and glutamate synthesis, ammonia excretion and toxicity
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ABSTRACTThe effects of ammonia loading in the terrestrial isopod Porcellio scaber were studied by exposing animals to atmospheres of high . Isopods show a remarkable tolerance of elevated ambient , with an LD50 of 89 Pa for a 7-day exposure. However, haemolymph total ammonia concentrations generally remained below 5 mmol l−1 (=0.37 Pa) over the range of ambient ammonia levels used (6.6–165 Pa). Following a 7-day loading period, whole-animal glutamine (Gln) and glutamate (Glu) levels increased in direct proportion to ambient , reaching values of 35 μmol g−1 fresh mass for glutamine and 12 μmol g−1 fresh mass for glutamate in 99 Pa ; these correspond to control levels of 7.5 μmol g−1 fresh mass and 5.9 μmol g−1 fresh mass, respectively. Following transfer to ammonia-free chambers, NH3 excretion rates were augmented five-to sixfold relative to non-loaded controls. Ammonia volatilization subsequently declined, approaching control levels after 8–9 days. Levels of Gln and Glu showed a concomitant decline to 13.7 μmol g−1 fresh mass and 9.2 μmol g−1 fresh mass, respectively. The results suggest that these amino acids function in ammonia sequestration and, hence, detoxification. Calculations indicate that mobilization of amino groups by deamination of accumulated Gln and Glu could explain 35 % of the increased ammonia production. Implications of NH3 volatilization for acid–base balance are discussed.
