Oxygen and substrate dependence of hepatic cellular respiration: Sinusoidal oxygen gradient and effects of ethanol in isolated perfused liver and hepatocytes

Abstract

The oxygen dependence of hepatic cellular respiration was studied by employing simultaneous organ spectrophotometry of cytochromes and hemoglobin, the latter used as an intrasinusoidal optical oxygen probe. The K_m of cytochrome aa₃ for oxygen was found to be 6.8 μM in the isolated perfused liver and 0.3 μM in suspensions of isolated hepatocytes. The results indicate that the sinusolid-to-cell pO₂ gradient is about 5 torr. Optical determination of the average effective pO₂ indicates that the axial sinusoidal O₂ profile does not conform to zero-order O₂ uptake in the liver. Because of extensive NAD⁺ reduction, ethanol increases the thermodynamic driving force of oxidative phosphorylation, and it also increased the oxygen consumption in both the perfused liver and the hepatocyte suspension, but had no effect on the grade of steady-state cytochrome aa₃ reduction, the cellular energy state [ATP]/[ADP]·[P_i], or the K_m of cytochrome aa₃ for oxygen. The results indicate that hepatic energy metabolism is oxygen independent at very low O₂ concentrations, but that the sinusoidal axial O₂ concentration is anomalous, probably due to the spatial arrangement of the metabolizing systems.