Differential effects of hypothermal stress on lactate metabolism in fresh water- and seawater-acclimated milkfish, Chanos chanos.

The milkfish Chanos chanos, an economically important cultured marine species in Southeast Asia, exhibits stenothermal and euryhaline characteristics and huge mortality usually occurs during extreme cold weather in winter. Under conditions beyond optimal temperatures, ectothermic species experience an increase in anaerobic glycolysis. To better understand the hypothermal acclimation response of this tropical species, the lactate metabolic profiles of freshwater (FW)- and seawater (SW)-acclimated milkfish were compared under control (optimal temperature; 28 °C) and hypothermal treatment (18 °C) conditions. In this study, the lactate dehydrogenase (LDH) isoform genes, ldha and ldhb, were identified in milkfish livers and muscles, respectively. The LDH is a bidirectional enzyme that triggered the conversion of pyruvate to lactate via anaerobic glycolysis as LDH exhibits the reductase activity (LDH-R), while via the reverse direction as LDH exhibits the oxidase activity (LDH-O). The hypothermal stress significantly upregulated the LDH-R activity in the muscles and the monocarboxylate transporter activity in both muscles and livers, of SW- and FW-acclimated milkfish. The levels of blood lactate, however, decreased in SW-acclimated milkfish. Under hypothermal stress, anaerobic metabolism increased in the muscles of both FW and SW individuals, whereas the liver of SW-acclimated milkfish showed better acute phase capacity to utilize blood lactate than FW-acclimated milkfish. Taken together, in the present study, the major functions of the bidirectional enzyme LDH were identified according to its LDH-O and LDH-R activities. Furthermore, environmental salinities were found to affect the acute anaerobic metabolic strategies of euryhaline teleosts under hypothermal stress and were correlated with their hypothermal tolerance ability.