ENHANCED HYPO-OSMOREGULATION INDUCED BY WARM-ACCLIMATION IN ANTARCTICFISH IS MEDIATED BY INCREASED GILL AND KIDNEY NA+ K+-ATPASE ACTIVITIES/

Serum osmolality and serum inorganic ion concentrations were studied i n two antarctic fish species. Trematomus bernacchii and T. newnesi, du ring 5 weeks of acclimation to 4 degrees C and compared with control v alues for groups acclimated to -1.5 degrees C. Acclimation to 4 degree s C significantly decreased the serum osmolality of both species, ther eby increasing their seawater-to-extracellular fluid (ECF) osmotic gra dient. The decline in osmolality with acclimation to 4 degrees C was a ccompanied by significant and rapid losses of Na+ and C1(-) 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 degrees C and 37 degrees C wer e 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, t he Na+/K+- ATPase activities assayed at 4 degrees C were increased aft er acclimation to 4 degrees C. The Na+/K+-ATPase activities at 37 degr ees C increased at the higher acclimation temperature in T. newnesi ki dneys and T. bernacchii gills, but in both species there was no compen sation to temperature in the liver, regardless of assay temperature, M uscle Na+/K+-ATPase activity decreased in response to warm-acclimation in T. bernacchii and T. newnesi assayed at 4 degrees C and 37 degrees C, respectively. During acclimation to 4 degrees C, the discontinuity in the Arrhenius plot of the Na+/K+-ATPase activities of T. newnesi g ill moved to a lower temperature, whereas that of kidney remained unch anged, The results indicate that acclimation to 4 degrees C induced a decrease in serum osmolality which resulted from the positive compensa tion of Na+/K+-ATPase in osmoregulatory tissues. The enhancement in Na +/K+-ATPase activity at 4 degrees C suggests that energy expenditure i n antarctic fish may be lessened, in part, by maintaining a reduced se awater-to-ECF osmotic gradient.