Cortisol in teleosts: dynamics, mechanisms of action, and metabolic regulation

Cortisol is the principal corticosteriod in teleost fishes and its plasma c oncentrations rise dramatically during stress. The relationship between thi s cortisol increase and its metabolic consequences are subject to extensive debate. Much of this debate arises from the different responses of the man y species used, the diversity of approaches to manipulate cortisol levels, and the sampling techniques and duration. Given the extreme differences in experimental approach, it is not surprising that inconsistencies exist with in the literature. This review attempts to delineate common themes on the p hysiological and metabolic roles of cortisol in teleost fishes and to sugge st new approaches that might overcome some of the inconsistencies on the ro le of this multifaceted hormone. We detail the dynamics of cortisol, especi ally the exogenous and endogenous factors modulating production, clearance and tissue availability of the hormone. We focus on the mechanisms of actio n, the biochemical and physiological impact, and the interaction with other hormones so as to provide a conceptual framework for cortisol under restin g and/or stressed states. Interpretation of interactions between cortisol a nd other glucoregulatory hormones is hampered by the absence of adequate ho rmone quantification, resulting in correlative rather than causal relations hips. The use of mammalian paradigms to explain the teleost situation is generall y inappropriate. The absence of a unique mineralocorticoid and likely minor importance of glucose in fishes means that cortisol serves both glucocorti coid and mineralocorticoid roles; the unusual structure of the fish glucoco rticoid receptor may be a direct consequence of this duality. Cortisol affe cts the metabolism of carbohydrates, protein and lipid. Generally cortisol is hyperglycaemic, primarily as a result of increases in hepatic gluconeoge nesis initiated as a result of peripheral proteolysis. The increased plasma fatty acid levels during hypercortisolaemia may assist to fuel the enhance d metabolic rates noted for a number of fish species. Cortisol is an essent ial component of the stress response in fish, but also plays a significant role in osmoregulation, growth and reproduction. Interactions between corti sol and toxicants may be the key to the physiology of this hormone, althoug h cortisol's many important housekeeping functions must not be ignored. Com bining molecular approaches with isolated cell systems and the whole fish w ill lead to an improved understanding of the many faces of this complex hor mone in an evolutionary and environmental framework.