Severe burns and trauma, as “primary” injury, lead to a systemic hypercatabolic and hypermetabolic response characterized by a significant loss of lean body mass and a negative nitrogen balance. Patients without subsequent complications may fully recover; however, often times critically ill patients end up being subjected to a “second hit,” usually superimposed infections that prolong the inflammatory response and the associated systemic hypercatabolic and hypermetabolic response. Such metabolic abnormalities, when lingering, are correlated with a dramatic loss of lean body mass and increased risk of death by multiple organ dysfunction syndrome. There is currently little basic information on the effect of the second hit on the metabolic response of the host. In the current studies, we are investigating the effect of a standard burn injury followed by experimentally induced infection, namely cecal ligation and puncture (CLP), on the hepatic metabolic response. Since the liver orchestrates many of the critical pathways responsible for carbohydrate and amino acid metabolism in the body, it provides a unique window into the systemic metabolic response. More specifically, rats were subjected to a full-thickness scald burn on the dorsal skin corresponding to 20% of the total body surface area (TBSA), followed by CLP two days later. Some animals received a “sham-burn,” where animals were anesthetized and prepped but not burned, as well as a “sham-CLP” where animals underwent laparotomy but no cecal damage. Totally four different groups were investigated: Burn/CLP, Burn/Sham-CLP, Sham-Burn/CLP and Sham-Burn/Sham-CLP. Livers were retrieved at different time points extending up to 10 days from each animal group (n=3 per group), and perfused in an ex vivo perfusion system to measure hepatic metabolic rates in a well-defined environment, thus providing a comprehensive picture of intrinsic changes in hepatic metabolism [1-3]. An additional variable, overnight fasting prior to perfusion, was also investigated in a subset of animals because all prior published studies used fasted animals. Net metabolic fluxes for the important metabolites entering and exiting the liver were used as the primary measurements and applied to a stoichiometric based methodology incorporating flux balance and metabolic pathway analyses (elementary mode analysis) in order to determine intracellular fluxes and active pathways. Pathway-based thermodynamic constraints were further introduced into the model.
Burn-only animals (prior to CLP induction) that were in a fed state prior to perfusion exhibited a significant down-regulation in urea production, and a slight increase in beta-hydroxybutyrate production and oxygen consumption. Fasting prior to perfusion did not affect the burn-induced responses resulting in increased beta-hydroxybutyrate production and oxygen consumption, but reversed the urea response such that burn significantly increased urea synthesis. In addition, in burned animals, fasting prior to perfusion led to increased glutamine uptake. Finally, fasting increased lactate uptake in sham-treated animals, but not in burned animals. These results suggest that fasting alters specific aspects – but not all - of the metabolic response to burn injury, and care must be taken to define the nutritional state of the animal model used in such studies. Conversely, burn injury alters the response to fasting, which indicates that the normal adaptations to the periodicity of food intake are altered by burn injury. We are in the process of extending these studies to the sequential burn-CLP double hit model. We plan to identify intracellular fluxes and critical pathways in burn and CLP livers compared to their sham counterparts, in order to both quantify and qualify the effects of these traumas on liver metabolism. These studies will provide clues about specific nutrients which can be used to diminish the physiologic stress and the hypermetabolic response.
1. Banta, S., et al., Contribution of gene expression to metabolic fluxes in hypermetabolic livers induced through burn injury and cecal ligation and puncture in rats. Biotechnology and Bioengineering, 2007. 97(1): p. 118-137.
2. Lee, K., et al., Metabolic Flux Analysis of Postburn Hepatic Hypermetabolism. Metabolic Engineering, 2000. 2(4): p. 312-327.
3. Lee, K., et al., Profiling of dynamic changes in hypermetabolic livers. Biotechnology and Bioengineering, 2003. 83(4): p. 400-415.
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