Metabolic changes induced by cold stress in rat liver mitochondria

The mechanisms involved in the metabolic changes induced by cold stress in isolated rat liver mitochondria were studied. Respiration, ATP synthesis, a nd membrane potential as well as the contents of several metabolites were d etermined in liver mitochondria from cold-exposed rats. At different times of cold exposure, the force-flux relationships showed net variation in flux (enhanced respiration, diminished ATP synthesis) with no associated variat ion in force (H+ gradient); this suggested that decoupling rather than clas sical uncoupling was involved in the effects of cold stress. The flux contr ol coefficient of the H+ leak on basal respiration was slightly increased b y 380 h of cold exposure. Cold stress also induced a diminution in total me mbrane fatty acids, Zn2+, Fe3+, ATP, and ADP/O ratios; the content of cytoc hromes c + c(l) and b oscillated. The contents of Ca2+, Na+, P-i, and cytoc hromes a + a(3) were not affected, whereas matrix ADP, AMP, K+, and Mg2+ we re markedly increased. Basal and oleic acid-stimulated respiration of mitoc hondria from cold-stressed rats was inhibited by GDP, carboxyatractyloside, or albumin. These agents did not affect basal respiration in control mitoc hondria. Western blot analysis showed enhanced expression of a protein of a bout 35 kDa, presumably the uncoupling protein 2, induced by long-term cold exposure. The overall data suggest that cold stress promoted decoupling of oxidative phosphorylation, and hence, changes in several matrix metabolite s, by increasing free fatty acids and the UCP2 content.