Study on the free amino acid levels in the hemolymph, gill, hepatopancreas and muscle of Penaeus monodon exposed to elevated ambient ammonia.

Tiger shrimp Penaeus monodon following 24 h exposure to 0.002 (control), 0.072 and 0.718 mM ammonia were examined for the free amino acid (FAA), ammonia and urea levels in the hemolymph, gill, hepatopancreas and muscle. Control shrimps contained total FAA in hemolymph (1.19 µmol ml(-1)), gill (21.81 µmol g(-1)), hepatopancreas (100.81 µmol g(-1)) and muscle (239.54 µmol g(-1)). Glycine and arginine were the major contributors to the total FAA pool, and made up of 90% of the total FAA in the muscle of P. monodon. The total FAA level in the hemolymph increased directly with ambient ammonia, whereas the total FAA level in the hepatopancreas was inversely related to ambient ammonia. No significant difference of total FAA was observed in the gill and muscle among the shrimps in three treatments. Ammonia level increased by 160% in hemolymph, 105% in gill, 236% in hepatopancreas and 68% in muscle for the shrimps exposed to 0.718 mM ammonia. Urea and ornithine in the hepatopancreas increased by 107 and 1446%, whereas arginine level in the hepatopancreas decreased by 50% for the 0.718 mM ammonia-exposed shrimps. Decreases of arginine and other FAA with a concomitant increase of ornithine and urea level in the hepatopancreas indicated catabolism of FAA and ureogenesis. Increases of ammonia, urea, taurine, glutamine, proline, alanine, glycine and asparagine in the hemolymph revealed a intracellular osmoregulation for P. monodon under the stress of ambient ammonia at 0.718 mM.