Energy Metabolism in Fish Tissues Related to Osmoregulation and Cortisol Action

This is an overview of our recent studies of energy metabolism in fish brain and other organs regulated by exogenous (feeding, salinity) and endogenous (hormones) factors. To highlight our approach, we present latest results concerned osmoregulation in the gills of gilthead seabream, Sparus auratus. Our model, the seabream, is a euryhaline teleost capable of adaptation to extreme changes in environmental salinity. Treatment with cortisol allowed us to achieve circulating cortisol levels similar to those observed during osmotic adaptation and to assess how elevated hormonal levels affected simultaneously metabolic and osmoregulatory capacities of the gill tissue. Cortisol-implanted fish showed higher gill Na⁺,K⁺-ATPase activity than control fish but no changes were observed in plasma osmolality and ion levels. Plasma levels of glucose and lactate increased in cortisol-implanted fish while protein levels decreased. Cortisol treatment elicited metabolic changes in liver and brain reflecting an activation of the glycogenic and gluconeogenic potential in liver, and the glycogenic potential in brain, which are confirmatory of data obtained in previous experiments. In gills, we demonstrated that cortisol treatment elicited changes in their energy metabolism that can be summarized as a decreased capacity in the use of exogenous glucose (decreased HK activity), a decrease in the capacity of the pentose phosphate pathway (decreased G6PDH activity), and an increased glycolytic potential (increased PK activity). Observed metabolic changes in gills can be associated with those occurring in nature during osmotic adaptation in the same fish species.