ACUTE AND CHRONIC EFFECTS OF TEMPERATURE, AND OF NUTRITIONAL STATE, ON ION HOMEOSTASIS AND ENERGY-METABOLISM IN TELEOST HEPATOCYTES

Short- and long-term effects of temperature on ion flux and energy tur nover were studied in hepatocytes from thermally acclimated trout and roach. In trout hepatocytes K+ efflux was insensitive towards acute ex posure to low temperature but was downregulated during cold acclimatio n of the fish so as to balance the uncompensated decreased K+(Rb+) upt ake of the cells. In contrast, both K+(Rb+) uptake and K+ efflux of ro ach hepatocytes were temperature sensitive in the short term. These ac ute effects, however, were offset during cold acclimation by a near pe rfect compensation of both fluxes leading to re-establishment of ion f lux homeostasis at the original level. Our findings, based on a new me thod permitting the simultaneous monitoring of K+ efflux and uptake in the same cell population, provide experimental verification of two of the three possible strategies, recently discussed by Cossins et al. ( 1995), by which the ionic steady state of fish cells may adjust to acu te and chronic temperature change. By comparing hepatocytes from two g roups of trout, one kept on a maintenance diet (ration I)! the other f ed ad libitum (ration II), we discovered striking effects of nutrition al state on the absolute levels as well as on the temperature relation ships of K+ uptake and protein synthetic activity. Both of these funct ions in the hepatocytes increased in the ration II fed as compared to the ration I fed trouts, but the increase of protein synthetic activit y was greater and more uniform at the three experimental temperatures than that of KC uptake. Moreover, protein synthetic activity proved to be considerably more temperature sensitive than K+ uptake and, in con trast to the latter, showed a compensatory response after cold acclima tion.