Reduction in the rates of protein and amino acid catabolism to slow down the accumulation of endogenous ammonia: A strategy potentially adopted by mudskippers (Periophthalmodon schlosseri and Boleophthalmus boddaerti) duringaerial exposure in constant darkness

This study was designed to elucidate the strategies adopted by mudskippers to handle endogenous ammonia during aerial exposure in constant darkness. U nder these conditions, specimens exhibited minimal locomotory activity, and the ammonia and urea excretion rates in both Periophthalmodon schlosseri a nd Boleophthalmus boddaerti decreased significantly. As a consequence, ammo nia accumulation occurred in the tissues of both species of mudskipper. A s ignificant increase in urea levels was found in the liver of P. schlosseri after 24 h of aerial exposure, but no similar increase was seen in the tiss ues of B. boddaerti. It is unlikely that these two species of mudskipper de toxified ammonia to urea during aerial exposure since B. boddaerti does not possess a complete ornithine-urea cycle (OUC) and, although all the OUC en zymes were present in P. schlosseri, the activity of carbamoyl phosphate sy nthetase present in the liver mitochondria was too low to render the OUC fu nctional for ammonia detoxification. Peritoneal injection of (NH4Cl)-N-15 i nto P. schlosseri showed that this mudskipper was capable of incorporating some of the labelled ammonia into urea in its liver. However, aerial exposu re did not affect this capability and did not induce detoxification of the accumulated ammonia to urea. Mudskippers exposed to terrestrial conditions and constant darkness did, however, show significant decreases in the total free amino acid content in the liver and blood, in the case of P. schlosse ri and in the muscle of B. boddaerti. No changes in the alanine or glutamin e content of the muscle were found in either species. Analyses of the balan ce between me reduction in nitrogenous excretion and the increase in nitrog enous accumulation further revealed that these two species of mudskipper we re capable of reducing their protein and amino acid catabolic rates. Such a daptations constitute the most efficient way to avoid the build-up of inter nal ammonia, and would render unnecessary the detoxification of ammonia thr ough energetically expensive pathways. This finding may be the first report of a teleost fish showing a reduction in proteolysis and amino acid catabo lism in response to aerial exposure.