PULSATILE UREA EXCRETION IN THE UREAGENIC TOADFISH OPSANUS-BETA - AN ANALYSIS OF RATES AND ROUTES

This study focused on the rates and routes of urea-N and ammonia-N exc retion in the ureagenic toadfish and on the possibility that urea-N ex cretion occurs in pulses, Experimental approaches included the followi ng: confinement in small individual containers with automated hourly s ampling of water to follow temporal excretion patterns; divided chambe rs to separate excretion from the anterior and posterior parts of the fish; collection of urine and rectal fluid via chronic indwelling cath eters; and gavage with [C-14]-labelled polyethylene glycol 4000 to det ect regurgitation of gastrointestinal fluids, When a standardized 'cro wding' pre-treatment was employed to induce ureotelic behaviour, the f ish exhibited significant elevations in the activity of glutamine synt hetase in liver, kidney and gills, elevated plasma and bile urea-N lev els, but unchanged ammonia-N and urea-N levels in most other body flui ds, Unencumbered ureotelic fish confined in small containers excreted 82% of their waste-N as urea-N and 18% as ammonia-N; almost all (94%) of this urea-N excretion occurred in a single pulse of less than 3 h d uration about once every 24h, This daily pulse did not occur by regurg itation of gut fluids, by excretion through prominent pores behind the pectoral fins or by discharge of rectal fluid or urine, Intestinal an d urinary excretion accounted for less than 10% of whole-body urea-N e xcretion and a negligible fraction of ammonia-N excretion, Pulsatile u rea-N excretion occurred at the head end across the gills and/or body surface, Ammonia-N excretion, which was not pulsatile, also occurred l argely through the head end, However, once the toadfish had been place d in divided chambers, urea-N excretion became continuous rather than pulsatile, and ammonia-N excretion increased greatly, A severe stress response was indicated by high levels of plasma cortisol, and the skin , which lacks scales, became a significant route of both ammonia-N and urea-N excretion, We speculate that the normal adaptive significance is that ureotelism facilitates cryptic behaviour, allowing the toadfis h to virtually eliminate N-waste excretion during long periods while i t remains sheltered in burrows, However, during severe stress, the eff ects of extremely high cortisol levels overwhelm the ammonia and urea retention mechanisms, and both substances leak across the general body surface.