Influence of fatty acids on energy metabolism

Abstract

The uncoupling-like effect of fatty acids was further substantiated in experiments with perfused rat livers by 2 ways; firstly the kinetics of changes in metabolic rates (O2 consumption, ketogenesis, fatty acid oxidation) were analyzed; secondly subcellular contents of adenine nucleotides and pH gradients across the mitochondrial membrane were determined following fractionation of freeze-fixed and dried tissues in non-aqueous solvents. The relaxation kinetics of the increase in O2 consumption following fatty acid infusion revealed 2 components, a rapid one with a half-time around 10 s and a slow one with a half-time of more than 100 s. The rapid component was similar to the kinetics of fatty acid oxidation (ketogenesis and 14CO2 production from labeled fatty acids) whereas the half-time of the slow component was in the range of half-times observed with the increase in O2 consumption following addition of carbonylcyanide p-trifluoromethoxyphenylhydrazone. In the presence of fatty acids, the cytosolic ATP concentrations and ATP/ADP ratios decreased, whereas the corresponding parameters for the mitochondrial space were either increased (oleate) or decreased (octanoate). The effects of oleate were dependent on the albumin concentrations in the perfusate. The normally large difference between cytosolic and mitochondrial ATP/ADP ratios became smaller. Similar observations were obtained with uncoupling agents. The pH gradient across the mitochondrial membrane as calculated from the subcellular distribution of 5,5 dimethyl[2-14C]oxazolidine-2,4-dione was inversed following the addition of both carbonylcyanide p-trifluoromethoxyphenylhydrazone and fatty acids, i.e. the mitochondrial matrix became more acidic than the cytosol. The pH gradient was not affected when oleate was added in the presence of high albumin concentrations. The increase in hepatic O2 consumption due to octanoate or oleate is, in part, caused by a mechanism similar to uncoupling of oxidative phosphorylation. This mechanism seems not to be an artifact of isolated systems; it may be of physiological importance for processes in which reducing equivalents are removed independently of the ATP demand of the hepatocyte.