The sleeper Bostrichthys sinensis (Family Eleotridae) stores glutamine andreduces ammonia production during aerial exposure

Bostrichthys sinensis inhabits brackish water, living in the crevices of th e river mouths of Shang Xi and Guangdong, China. In its natural habitat, it may encounter aerial exposure frequently during low tides, and it usually remains quiescent in the absence of water. Upon aerial exposure in the labo ratory, the ammonia excretion rate decreased to one-fourth that of the subm erged control. Although all the enzymes of the ornithine-urea cycle were de tected in the liver of this fish, the activity of hepatic carbamoyl phospha te synthetase was too low for the cycle to be functioning. Indeed, ammonia accumulated in the tissues and was not converted to urea. Results indicate that ammonia produced through amino acid catabolism was detoxified to gluta mine during the first 24 h of aerial exposure. The excess amount of glutami ne stored in the muscle during this period could account approximately for the reduction in ammonia equivalent excreted. There was indeed a significan t increase in the activity of glutamine synthetase from the liver of specim ens exposed to terrestrial conditions. In contrast to the production of ala nine, formation of glutamine is energetically expensive. Since B. sinensis remained relatively inactive on land, the reduction in energy demand for mu scular activity might provide it with the opportunity to exploit glutamine formation as a means to detoxify ammonia. After 72 h of aerial exposure, B. sinensis reduced internal ammonia production, possibly through reductions in proteolysis and amino acid catabolism, to avoid excessive accumulation o f ammonia.