Metabolic Adaptations of a Lower Vertebrate to Long-Term Hypothermic Hypoxia Provide Clues to Successful Clinical Liver Preservation

Abstract

This study was designed to determine whether the metabolic adaptations developed by frogs to tolerate natural events of hypothermic hypoxia would precondition its liver forex vivoorgan storage. The metabolic responses of the frog,Rana castabiena,were compared to those of a mammalian system (rat) throughout a prolonged period of organ storage. Livers from rats and frogs were flushed and stored in UW solution at 5°C for periods of 24–96 h. In frog livers, ATP was maintained high and constant over the first 24 h of storage; values ranged from 2.7 to 3.0 μmol/g. Even after 96 h cold storage, ATP remained > 1.0 μmol/g. In contrast, ATP levels in stored rat livers dropped rapidly, and by 4 h ATP was 1.2 μmol/g. In terms of anaerobic endproduct accumulation, lactate levels rose 5.8 μmol/g in frog liver (over 96 h) and by 8.6 μmol/g in rat liver (over 24 h). This difference in flux through glycolysis was also reflected in relative rates of carbohydrate catabolism (i.e., glucose + lactate production). The rate of carbohydrate catabolism for frog liver was 0.74 μmol/g/h compared to 2.26 μmol/g/h for rat liver; a Q10value of 6.2 was estimated for livers fromR. castabiena.An assessment of glycolytic enzyme activities revealed that key differences in the responsiveness of pyruvate kinase to allosteric modifiers may have been responsible for the marked drop in the rate of anaerobic energy production in frog tissues. Although the concept of depressed metabolism in a lower vertebrate is not new, the data presented in this study demonstrate that a depressed metabolic state can be achieved in isolated livers fromR. castabienasimply through cold exposure. With respect to clinical relevance, the results of this study indicate that energetics of stored livers can be maintained effectively through an efficient reduction in energy use in combination with a slow, yet continuous, rate of energy production facilitated by glycolysis.