Enhanced Hypo-Osmoregulation Induced by Warm-Acclimation in Antarctic Fish is Mediated by Increased Gill and Kidney Na+ /K+ - ATPase Activities

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Abstract
Serum osmolality and serum inorganic ion concentrations were studied in two antarctic fish species, Trematomus bernacchii and T. newnesi, during 5 weeks of acclimation to 4 ˚C and compared with control values for groups acclimated to-1.5 ˚C. Acclimation to 4 ˚C significantly decreased the serum osmolality of both species, thereby increasing their seawater-to-extracellular fluid (ECF) osmotic gradient. The decline in osmolality with acclimation to 4 ˚C was accompanied by significant and rapid losses of Na+ and Cl− during the first 14 days of acclimation and was maintained throughout the study period. At day 35 of acclimation, the lipid composition and microsomal Na+ /K+-ATPase specific activities at 4 ˚C and 37 ˚C were determined in membranes from gill, kidney, liver and muscle tissues. No warm-induced decrease in fatty acid unsaturation was found in the tissues of either species. In the gills and kidneys of both species, the Na+ /K+-ATPase activities assayed at 4 ˚C were increased after acclimation to 4 ˚C. The Na+ /K+-ATPase activities at 37 ˚C increased at the higher acclimation temperature in T. newnesi kidneys and T. bernacchii gills, but in both species there was no compensation to temperature in the liver, regardless of assay temperature. Muscle Na+ /K+-ATPase activity decreased in response to warm-acclimation in T. bernacchii and T. newnesi assayed at 4 ˚C and 37 ˚C, respectively. During acclimation to 4 ˚C, the discontinuity in the Arrhenius plot of the Na+ /K+-ATPase activities of T. newnesi gill moved to a lower temperature, whereas that of kidney remained unchanged. The results indicate that acclimation to 4 ˚C induced a decrease in serum osmolality which resulted from the positive compensation of Na+ /K+-ATPase in osmoregulatory tissues. The enhancement in Na+ /K+-ATPase activity at 4 ˚C suggests that energy expenditure in antarctic fish may be lessened, in part, by maintaining a reduced seawater-to-ECF osmotic gradient.