Regulation of lactate dehydrogenase in tilapia (Oreochromis mossambicus) gills during acclimation to salinity challenge

Upon salinity challenge, euryhaline teleosts immediately regulate the functions of many ion transporters and enzymes in gill mitochondrion-rich (MR) cells in order to maintain their internal homeostasis. A large amount of energy is necessary to operate these transporters and enzymes; however, the mechanism of energy metabolism in fish gills is still unclear. In the present study, tilapia (Oreochromis mossambicus) was used as a model animal to investigate the roles of lactate dehydrogenase (LDH) in the energy metabolism of fish gills during acclimation to seawater (SW) challenge. Results of LDH isozyme gel electrophoresis and Western blotting demonstrated that LDH1 is the major form and LDH5 the minor form expressed in tilapia gill epithelial cells. Immunocytochemical experiments indicated that both LDH1 and LDH5 were expressed in MR cells (using Na⁺-K⁺-ATPase as a marker). Protein expression levels of LDH1 and citrate synthase (CS) increased immediately in the 1st hour of acclimation to SW, while that of LDH5 was significantly enhanced during the 1st-3rd h. Taken together, an energy metabolism model in gill epithelial cells during SW acclimation is proposed: the tricarboxylic acid cycle in MR cells is stimulated to produce adenosine triphosphate (ATP) which initially triggers salt secretion; conversion of pyruvate to lactate is subsequently stimulated to enhance anaerobic respiration to produce additional energy.