PULSATILE UREA EXCRETION IN THE TOADFISH (OPSANUS-BETA) IS DUE TO A PULSATILE EXCRETION MECHANISM, NOT A PULSATILE PRODUCTION MECHANISM

When subjected to a crowding/confinement protocol in the laboratory, t oadfish become facultatively ureotelic, excreting approximately 90 % o f their nitrogenous waste as urea-nitrogen (urea-N). The great majorit y of this excretion occurs via large, irregular pulses from the head r egion which occur on average once per day, with a duration of 3h or le ss. Pulses measured chemically by the appearance of urea-N in the exte rnal water were identical to those measured by assaying [C-14]urea app earance in the water from the blood plasma. Individual toadfish mainta ined plasma urea concentrations over widely differing ranges (6600-398 90 mu mol-Nl(-1)). However, independent of absolute levels, both [C-14 ]urea and total urea were distributed at ratios close to unity between the blood plasma and the water compartments of liver and white muscle . At times of pulsatile excretion, plasma urea concentration fell shar ply. These decreases, distributed throughout the tissues of the whole body, closely matched the sizes of the measured excretion pulses. Betw een pulses, plasma urea concentration increased steadily at a much slo wer rate; the rate of rise, when distributed throughout the tissues of the whole body, corresponded to the time-averaged excretion rate over the whole day. Infusion of a typical pulse amount of urea immediately after the end of a natural pulse event raised plasma urea concentrati on slightly above the pre-pulse level, but did not induce another puls e event. Plasma cortisol levels declined by approximately 60 % over th e 4h period prior to a natural pulse event and then rose quickly again once the pulse had occurred. These results indicate that urea pulses are due to activation of an excretion mechanism that rapidly clears ur ea from the blood plasma, thereby lowering stores throughout the whole body. Metabolic production of urea is continuous and is not responsib le for pulsatile excretion. The pulse event is not triggered by a spec ific plasma urea threshold, but may involve the hypothalamo-interrenal axis.