Changes in membrane fluidity of gilthead seabream Sparus auratus L., 1758 hepatocytes subjected to falling temperatures

We present a study on membrane fluidity dynamics in hepatocytes of gilthead seabream Sparus auratus L., 1758 when exposed to thermal variations. A group of 15 specimens was acclimated to a control physiological temperature (17 °C) during one week; the fish were then subjected to diminishing temperatures during the following week, reaching 7.5 °C, and a subgroup was maintained for two weeks, reaching a final temperature of 6 °C. The temperature drop resulted in an anisotropy change from 0.2205 (r) under control conditions to 0.164 (r) at 7.5 °C, after one week. No significant differences between this group and the fish later kept at lower temperatures, reaching 6 °C, were observed after one week; this can be interpreted as a thermal adaptation of the membranes to colder conditions. The adaptive fluidity changes at temperatures of 17, 7.5, and 6 °C were coincident with the observed lessening of Na+ / K+-ATP-ase enzymatic activity as temperatures fell. Therefore, it is possible that after a decrease in water temperature, common in farmed fish under winter conditions, membrane fluidity changes could be correlated with decreasing enzymatic activity in membrane ATP-ases, as an adaptive strategy.