Lactate and ornithine improve substrate hydrogen supply in isolated hepatocytes.

The influence of lactate and ornithine on the mitochondrial redox state as indicated by the 3-hydroxybutyrate/acetoacetate ratio was studied in isolated hepatocytes. The load on the hepatic energy metabolism when lactate and/or ornithine are additionally present can be higher than in the sole presence of octanoic acid. This was revealed by enhanced oxidative turnover rates and higher 3-hydroxybutyrate/acetoacetate ratios at identical rates of respiration. During gluconeogenesis from lactate, such an improved energy supply takes place through more intense oxidation of octanoic acid in the citric-acid cycle. In addition to octanoate oxidation, however, ornithine supplies substrate hydrogen through its catabolism to glutamate as was demonstrated by comparative studies in isolated rat liver mitochondria. The ambivalent character of lactate and ornithine with regard to loading and supplying the energy metabolism seems to substantially account for energetic homoeostasis of the liver.