The marble goby Oxyeleotris marmoratus activates hepatic glutamine synthetase and detoxifies ammonia to glutamine during air exposure

Ammonia levels in various tissues of the marble goby Oxyeleotris marmoratus remained constant throughout a 72 h period of air exposure. The rate of am monia excretion in these experimental fish decreased to approximately one-f ifth of that of the submerged control. Ammonia was not converted to urea du ring air exposure because there were no significant increases in urea conte nt in the tissues. Also, urea excretion rate was lowered to one-fiftieth th at of the submerged fish. After 24 h of air exposure, there was a significa nt increase in muscle glutamine content, which peaked at 48 h. The increase in glutamine content could account for the decreases in the amounts of amm onia and urea excretion during air exposure, The specific activities of hep atic glutamate dehydrogenase (amination) and glutamine synthetase in these experimental fish increased threefold and thirtyfold, respectively, in comp arison with the submerged controls. Thus, O. marmoratus appears to be the f irst known teleost that responds to air exposure by activating hepatic glut amine synthetase to detoxify internally produced ammonia.