Interactions between glutamine metabolism and cell‐volume regulation in perfused rat liver
Authors: HÄUSSINGER Dieter , LANG Florian , BAUERS Kathrin , GEROK Wolfgang 
Published: March 2005

In the presence of near‐physiological glutamine concentrations, exposure of perfused rat liver to hypotonic perfusion media switched glutamine balance across the liver from net release to net uptake. This was due to both stimulation of flux through glutaminase and inhibition of flux through glutamine synthetase. Conversely, during exposure to hypertonic media, net glutamine release from the liver increased due to inhibition of glutaminase flux and slight stimulation of flux through glutamine synthetase. The effect of perfusate osmolarity on glutaminase flux was observed at an NH4Cl concentration (0.5 mM) sufficient for near‐maximal ammonia stimulation of glutaminase. This indicates the involvement of different mechanisms of glutaminase flux control by extracellular osmolarity changes and ammonia. The effects of anisotonicity on flux through glutamine‐metabolizing enzymes were fully reversible. Glutamine (0.6 mM) stimulated urea synthesis from NH4Cl (0.5 mM) during hypotonic and normotonic conditions.

Exposure to hypotonic and hypertonic media led, after initial liver‐cell swelling and shrinkage, respectively to volume‐regulatory K+ fluxes which largely restored the initial liver‐cell volume despite the continuing osmotic challenge. Even after completion of cell‐volume regulatory K+ fluxes, the effects of perfusate osmolarity on hepatic glutamine metabolism persisted. This indicates that in anisotonicity the liver cell is left in an altered metabolic state, even after completion of volume‐regulatory responses.

During perfusion with isotonic media, addition of glutamine (3 mM) led to an increase of liver mass by about 4% within 2 min, which was accompanied by a net K+ uptake by the liver. Thereafter, the new steady state of increased liver mass was maintained throughout glutamine infusion. When the liver mass had reached this new steady state, a net release of K+ from the liver of about 3 μmol/g liver was observed during the following 10 min. Withdrawal of glutamine was followed by a slow reuptake of K+ and the liver mass returned to its initial value. Following exposure to glutamine (3 mM), the intracellular glutamine concentration (as calculated from glutamine tissue levels, taking into account the extracellular space determined with the [3H]inulin technique) rose from about 1 mM to 30–35 mM within about 12 min, indicating a 10–12‐fold concentrative uptake of glutamine into the liver cells and an osmotic challenge for the hepatocyte. When intracellular glutamine had reached its steady‐state concentration, net K+ efflux from the liver was also terminated.

It is concluded that (a) hepatic glutamine metabolism is under the control of liver‐cell volume and/or volume‐regulatory responses, thereby involving flux changes through glutaminase and glutamine synthetase, i.e. alterations of intercellular glutamine cycling; (b) volume‐regulatory mechanisms may be signals for switching carbohydrate utilization to glutamine utilization as respiratory fuel and vice versa; and (c) the concentrative uptake of glutamine into liver cells induces cell swelling and activates volume‐regulatory K+‐release mechanisms. Thus, activation of glutaminase flux by glutamine may not only be due to increased intramitochondrial glutamine concentrations, but also due to alterations of cell volume. It is hypothesized that cell‐volume‐regulatory mechanisms, which are activated during the concentrative amino‐acid uptake into liver cells as it occurs in the postprandial state, are themselves not‐yet‐recognized signals modulating the metabolic state of the hepatocyte.