SUMMARY

In mangrove killifish Rivulus marmoratus, urea excretion (J_urea) follows a distinct diurnal pattern with the highest rates between 12:00 h and 18:00 h. We investigated the regulating mechanisms that underlie temporal rhythms in J_urea in R. marmoratus. We hypothesized that the daily pattern of J_urea in R. marmoratus is (1) due to diurnal changes in urea synthesis rates and ultimately metabolic rate and/or (2) controlled by neuroendocrine messengers. Oxygen consumption and whole body urea content in R. marmoratus demonstrated a clear diurnal pattern with maximum rates for both parameters occurring at 12:00 h. A strong synchrony between diurnal patterns of oxygen consumption, whole body urea content and J_urea implicated metabolic regulation of the diurnal J_urea pattern. Ketanserin, a 5-HT₂ receptor antagonist, and RU-486, a cortisol receptor antagonist, were used to test the second hypothesis. Increasing antagonist concentrations of either ketanserin or RU-486 resulted in dose-dependent decreases in J_urea. Application of a single dose of either antagonist significantly decreases J_urea for up to 12 and 6 h for ketanserin and RU-48, respectively. Repeated exposure to doses of either ketanserin or RU-486 did not abolish the diurnal pattern in J_urea; however, there was a significant decrease in the amplitude of the rates. Taken together, these findings indicate that the diurnal pattern of J_urea in R. marmoratus are regulated by both metabolic and neuroendocrine factors. We propose that cortisol and 5-HT influence the absolute rate of urea excretion by altering the permeability of the gill membrane to urea and/or the rate of urea synthesis.