EFFECTS OF FEEDING AND CONFINEMENT ON NITROGEN-METABOLISM AND EXCRETION IN THE GULF TOADFISH OPSANUS-BETA

In order to elucidate further the cues for, and the biochemical mechan isms of, the transition to ureogenesis in the gulf toadfish Opsanus be ta, experiments on 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 proport ion as ammonia (61%), than previously published 'control' measurements , Feeding of unconfined toadfish elevated total nitrogen excretion app roximately threefold, with little change in the proportion of urea ver sus ammonia, During the first 24 h of confinement of unfed toadfrsh, a bsolute levels of urea excretion remained constant while ammonia excre tion rates fell to near zero, so that toadfish became 90% ureotelic, W hen fed prior to confinement, urea excretion rates remained constant f or the first 24 h, and the bulk of the nitrogen was excreted as ammoni a (80%); excretion of the excess dietary nitrogen took up to 48 h to c omplete, If pre-adapted to confinement and then fed, toadfish excreted only about 55% of their nitrogenous waste as ammonia, and excretion o f excess dietary nitrogen was completed by 24 h, Elevations of hepatic glutamine synthetase (GNS) activities accompanied confinement and wer e shown to be almost exclusively in the cytosolic compartment and to b e correlated with a decrease in the ratio of hepatic levels of glutama te:glutamine. These GNS activity increases also appear to account in p art for the decrease in the percentage of ammoniotely in toadfish unde r conditions of nitrogen loading after confinement, However, additiona l means of regulating total nitrogen excretion (e.g. changes in protei n 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.