Metabolic efficiency of liver mitochondria in rats with decreased thermogenesis

We have studied changes in hepatic mitochondrial efficiency induced by 24-h fasting or acclimation at 29 degrees C, two conditions of reduced thermogenesis. Basal and palmitate-induced proton leak, which contribute to mitochondrial efficiency, are not affected after 24-h fasting, when serum free triiodothyronine decreases significantly and serum free fatty acids increase significantly. In rats at 29 degrees C, in which serum free triiodothyronine and fatty acids decrease significantly, basal proton leak increases significantly, while no variation is found in palmitate-induced proton leak. The present results indicate that mitochondrial efficiency in the liver is not related to a physiological decrease in whole body thermogenesis.