The effect of chronic cortisol elevation on urea metabolism and excretion in the rainbow trout (Oncorhynchus mykiss)

The objective of this study was to investigate the possible involvement of cortisol in controlling urea metabolism and excretion in the ammoniotelic rainbow trout (Oncorhynchus mykiss). Trout fitted with dorsal aortic and internal urinary catheters received either no implant (control), or were implanted with coconut oil (sham), cortisol in coconut oil, RU486, a glucocorticoid receptor blocker, in coconut oil, or cortisol+RU486 in coconut oil, and monitored over 72 h. Rainbow trout treated with cortisol (+/-RU486) had similarly elevated plasma cortisol concentrations that were six fold greater than in control and sham fish. Elevated circulating cortisol concentrations caused a three-fold rise in plasma and urine urea concentrations, which was blocked by RU486. Similarly, a positive correlation between plasma cortisol and plasma urea concentrations was observed in fish treated with cortisol alone but not in fish treated with cortisol+RU486. Cortisol treatment caused an elevation in branchial (two fold higher) and urinary (three fold higher) excretion rates of urea compared to sham-implanted fish, which was prevented by treatment with RU486. However, as branchial and renal clearance were unaffected, there appears to be no stimulation or inhibition of urea excretion mechanisms in the gill or kidney separate from effects due to changes in plasma urea concentrations. Thus, cortisol and glucocorticoid receptors appear to be involved in the regulation of endogenous urea production but not in the control of urea excretory mechanisms in the ammoniotelic trout.