The impairment of hepatic ammonia detoxification in the diseased liver is a key event in the pathogenesis of hepatic encephalopathy. Hepatic ammonia consumption by the synthesis of both urea and glutamine (Gln) is critical for keeping systemic ammonia levels low. In the intact liver lobule, urea synthesis and Gln synthesis are anatomically arranged in series[2-8] (Fig. 1A). Through the use of bidirectional (antegrade and retrograde) rat liver perfusion, the concept of intercellular Gln cycling [i.e., the glutaminase (GLNase)-catalyzed deamidation of Gln in periportal hepatocytes and the glutamine synthetase (GS)–catalyzed resynthesis of Gln in pericentral scavenger cells] has been established.[2, 9] Under normal conditions, periportal Gln breakdown and pericentral Gln synthesis are well balanced. Under pathological conditions such as sepsis, liver cirrhosis, and CCl4 or acetaminophen intoxication, a scavenger cell defect essentially contributes to the development of hyperammonemia,[1, 4-6, 10] which is potentially life-threatening.
Modeling of hepatic urea metabolism started more than 30 years ago, but it remained limited because acinar compartmentation had not yet been elucidated. Later, acinar compartmentalization of ammonia metabolism was integrated into the modeling of the hepatic contribution to systemic acid-base homeostasis. Recently, spatial-temporal models (STMs) of liver tissue have been developed.[14-16] These models consider hepatocytes and sinusoids of a liver lobule as well as the principles of how individual cells interact in order to establish functional tissue microarchitecture. They can be used to simulate, for example, the destruction and regeneration process of a liver lobule after the administration of CCl4, but they lack the ability to simulate spatial-temporal profiles of metabolites.
To bridge this gap, a two-compartment metabolic model (MM) of ammonia, urea, and Gln metabolism was developed (Fig. 1B), and it was integrated into an STM of liver regeneration. Moreover, the published version of the STM, comprising only a single liver lobule, was extended to a group of seven lobules. To achieve this goal, the following strategy was applied (Fig. 2):
- Mouse liver perfusion experiments were performed with different concentrations of ammonia and Gln in antegrade and retrograde directions of both intact and impaired tissue, and the effluent concentrations of ammonia, Gln, and urea were thereby monitored. The resulting perfusion data, complemented by available biological knowledge (e.g., the stoichiometry of metabolic reactions), were used to establish an MM of ammonia detoxification in the intact murine liver.
- The recently established STM for liver regeneration after CCl4-induced damage in mice was applied to simulate the volumes of GS-active, GS-inactive, and necrotic compartments after CCl4 intoxication and during regeneration.
- The MM and the STM were coupled to form an integrated metabolic spatial-temporal model (IM), which allowed the prediction of ammonia detoxification in damaged and regenerating livers.
- For validation, the simulation results of the IM were compared to in vivo data for rats and mice. This resulted in good agreement between experimental and simulated data.
- The investigation was completed with a simplified model of blood circulation with three ammonia-detoxifying compartments in order to estimate the contributions of the (damaged) liver and extrahepatic tissues to ammonia metabolism in vivo.