Pulsatile urea excretion in the ureagenic toadfish Opsanus beta: an analysis of rates and routes

ABSTRACT

This study focused on the rates and routes of urea-N and ammonia-N excretion in the ureagenic toadfish and on the possibility that urea-N excretion occurs in pulses. Experimental approaches included the following confinement in small individual containers with automated hourly sampling of water to follow temporal excretion patterns; divided chambers to separate excretion from the anterior and posterior parts of the fish; collection of urine and rectal fluid via chronic indwelling catheters; and gavage with [14C]-labelled polyethylene glycol 4000 to detect regurgitation of gastrointestinal fluids. When a standardized 'crowding' pre-treatment was employed to induce ureotelic behaviour, the fish exhibited significant elevations in the activity of glutamine synthetase in liver, kidney and gills, elevated plasma and bile urea-N levels, but unchanged ammonia-N and urea-N levels in most other body fluids. 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 duration about once every 24 h. This daily pulse did not occur by regurgitation of gut fluids, by excretion through prominent pores behind the pectoral fins or by discharge of rectal fluid or urine. Intestinal and urinary excretion accounted for less than 10 % of whole-body urea-N excretion and a negligible fraction of ammonia-N excretion. Pulsatile urea-N excretion occurred at the head end across the gills and/or body surface. Ammonia-N excretion, which was not pulsatile, also occurred largely through the head end. However, once the toadfish had been placed 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 toadfish to virtually eliminate N-waste excretion during long periods while it remains sheltered in burrows. However, during severe stress, the effects of extremely high cortisol levels overwhelm the ammonia and urea retention mechanisms, and both substances leak across the general body surface.