Mechanism of the protective effect of hypothermia on ammonia toxicity in astrocytes

Abstract

Ammonia is a key factor in the pathogenesis of hepatic encephalopathy (HE). Acute ammonia treatment causes energy failure of astrocytes, which are able to compensate partly by increased anaerobic metabolism as a means of making up for the energetic shortfall. As hypothermia offers protection from severe encephalopathy and lactate accumulation in liver failure, we investigated the mechanism by which hypothermia protects against ammonia toxicity by multinuclear NMR spectroscopy. 12 h exposure to 5 mm NH4CL decreased the phosphocreatine (PCr)/creatine (Cr) and ATP/ADP ratios to 65 and 76% of control, increased synthesis and release of glutamine to 200–250% and led to a significant stimulation of glycolytic activity reflected by increased uptake and consumption of glucose and accumulation of de novo synthesized intra‐ and extracellular lactate to 161 and 230% of control. The protective effect of mild hypothermia was evident from inhibiton of lactate accumulation and restoration of ammonia‐induced depletion of PCr/Cr. Moderate hypothermia led to an increase of PCr/Cr ratio and inhibited lactate synthesis to 14% of normothermic control, but did not prevent the ATP decrease. While hypothermia inhibited glycolytic flux, intracellular glutamine remained elevated. The results suggest that hypothermia‐induced protection against ammonia toxicity results from reduction of cellular energy demand leading to inhibition of anaerobic glucose metabolism and a compensatory stimulation of mitochondrial energy production.Acknowledgements: Funded by CIHR Canada.