MCDONALD, M. Danielle; WOOD, Chris M.; WALSH, Patrick J.; RSMAS, University of Miami, Miami, USA; McMaster University, Hamilton, CANADA; RSMAS, University of Miami, Miami, USA: Pulstile urea excretion in the gulf toadfish: The role of the hypothalamic-pituitary-interrenal axis

An unusual pulsatile urea excretion mechanism (tUT) exists in the gills of the gulf toadfish that is believed to confer a chemical crypsis to this species. While studies have demonstrated the molecular and pharmacological similarly of tUT to the UT-A facilitated diffusion transport system of the mammalian kidney, the regulatory factors behind this mechanism have not yet been resolved. In mammals, AVP and glucocorticoids are two important endocrine regulators of UT-A transporters. AVT (the teleost homologue of AVP) is not involved in pulsatile urea excretion in toadfish. However, sharp, periodic declines of endogenous cortisol concentrations appear to play a permissive role in the activation of this mechanism. While pulsatile excretion of urea is consistently preceded by a decline in circulating cortisol, lowered cortisol concentrations do not always result in pulsatile excretion, suggesting that the trigger for the periodic activation of the transporter is still yet unknown. The present study focused on the potential role of the hypothalamic-pituitary-interrenal axis (HPI; the teleost homologue of the mammalian hypothalamic-pituitary-adrenal axis) in the activation of the pulsatile excretion mechanism. Circulating cortisol and 5-hydroxytryptamine (5-HT; serotonin) concentrations were manipulated by infusion and injection and the pattern of urea pulsing and the pharmacology of these effects were investigated.