Metabolic Fate and Distribution of ^<15>N-Ammonia in an Ammonotelic Amphibious Fish, Periophthalmus modestus, Following Immersion in ^<15>N-Ammonium Sulfate : a Long Term Experiment

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Incorporation of ^<15>N-ammonia into five nitrogenous components (Ammonia-N, Amide-N, Amino-N, Urea-N and Protein-N) in tissue was compared among the blood, brain, liver, gill, skin and muscle of the mudskipper, Periophthalmus modestus, following immersion in 15 mM ^<15>N-ammonium sulfate (99.7 atom%) dissolved in diluted sea water for 24 to 168 hr. Total net ^<15>N-uptake (μmol-^<15>N/g wet mass) into tissue was greater in the order of the brain, liver, gill, muscle, skin and blood. Among the components in each tissue, amount of ^<15>N in the form of Amino-N was the highest during the first 24 hr. Subsequently, that of Protein-N greatly increased, particularly in the liver, gill and brain. Amount of ^<15>N in the form of Urea-N was negligible throughout the experimental period. In addition to having the highest ammonia content, the muscle showed the highest ^<15>N-Concentrations (36-38 atom% excess) in Ammonia-N and Amide-N during the course of immersion. Amount of ^<15>N in the form of Ammonia-N in the blood remained at the lowest levels among the tissues examined until 96 hr at which the nitrogen in the muscle reached equilibrium with ^<15>N. It may be concluded that under ammonia loading conditions, the muscle plays the most important role not only in trapping a large amount of ammonia, but also in producing glutamine and other amino acids.
1995-04-15