Walsh, P.J. & Milligan, C.L. (1995). Effects of feeding and confinement on nitrogen metabolism and excretion in the gulf toadfish Opsanus beta. J. Exp. Biol. 198:1559-1566.
In order to elucidate further the cues for, and the biochemical mechanisms of, the transition to ureogenesis in the gulf toadfish, Opsanus beta, experiments of the effects of feeding (i.e.nitrogen loading) were carried out. Baseline nitrogen excretion rates were first measured on solitary toadfish in large water volumes (i.e. unconfined conditions). These nitrogen excretion rates were higher, and had a higher proportion as ammonia (61%), than previously published 'control' measurements. Feeding of unconfined toadfish elevated total nitrogen excretion approximately threefold, with little change in the proportion of urea versus ammonia. During the first 24h of confinement of unfed toadfish, absolute levels of urea excretion remained constant while ammonia excretion rates fell t near zero, so that toadfish became 90% ureotelic. When fed prior to confinement, urea =excretion rates remained constant for the first 24h, and the bulk of the nitrogen was excreted as ammonia(80%); excretion of the excess dietary nitrogen took up to 48h to complete. If pre-adapted to confinement and then fed, toadfish excreted only about 55% of their nitrogenous waste as ammonia, and excretion of excess dietary nitrogen was completed by 24h. Elevations of hepatic glutamine synthetase (GNS) activities accompanied confinement and were shown to be almost exclusively in the cytosolic compartment and to be correlated with a decrease in the ratio of hepatic levels of glutamate:glutamine. These GNS activity increase also appear to account in part for the decrease in the percentage of ammoniotely in toadfish under conditions of nitrogen loading after confinement. However, additional means of regulating total nitrogen excretion (e.g. changes in protein turnover rates) and the degree of ureogenesis versus ammoniogenesis (e.g. N-acetylglutamate stimulation of carbamoylphosphate synthetase) must be postulated to account fully for changes in nitrogen excretion rates and activation of ureogenesis under some circumstances.