Hepatocyte heterogeneity in uptake and metabolism of malate and related dicarboxylates in perfused rat liver
Authors: STOLL Barbara , HÄUSSINGER Dieter 
Published: March 2005

In isolated perfused rat liver a near‐maximal net malate uptake of about 120 nmol g−1 min−1 was observed at influent malate concentrations above 100 μmol 1−1 and a half‐maximal uptake at about 50 μmol 1−1 in influent. 14CO2 production from added [U‐14C]malate paralleled hepatic net malate uptake, however, 14CO2 production exceeded net malate uptake by 20–25%. This was observed in antegrade as well as in retrograde perfusions and regardless of whether NH4Cl was added to the influent perfusate. Stimulation of glutamine synthesis by NH4Cl only slightly affected net malate uptake and 14CO2 production, but resulted in a marked stimulation of [14C]glutamine release from the liver.

Because [U‐14C]malate uptake by the liver (reflecting the influent/effluent concentration difference of labeled malate) could at least in part involve a malate/malate exchange mechanism, net malate uptake (as determined from the influent/effluent concentration difference of enzymatically assayable malate) may underestimate hepatic [U‐14C]malate uptake. On the other hand, during metabolic steady states 14CO2 production from added [U‐14C]malate can be considered as an upper limit estimate of [U‐14C]malate uptake by the liver. Assuming that 14CO2 production equals [U‐14C]malate uptake by the liver, extrapolation studies suggest that during maximal rates of NH4Cl‐stimulated glutamine synthesis 80–110% of the [U‐14C]malate taken up by the liver was used for glutamine synthesis. This was true for retrograde and antegrade perfusions. Similar data, i.e. a 100–130% incorporation regardless of the direction of perfusion, were obtained when [U‐14C]malate uptake was assumed to equal net malate uptake by the liver.

Substitution of Na+ in the perfusion fluid by choline abolished net malate uptake by the liver and inhibited 14CO2 production from [U‐14C]malate by more than 90%.

2‐Oxoglutarate inhibited [14C]malate uptake and [1‐14C]oxoglutarate uptake by the liver was inhibited by malate, fumarate, succinate and oxaloacetate, but not by aspartate and glutamate. Inhibition of [1‐14C]oxoglutarate uptake and of 14CO2 production from added labeled 2‐oxoglutarate by malate and fumarate seemed largely competitive. Malate, fumarate and succinate not only inhibited [1‐14C]oxoglutarate uptake, but also stimulated the release of unlabeled 2‐oxoglutarate from the liver.

The data are consistent with a predominant uptake of vascular malate by perivenous glutamine synthetase containing hepatocytes when glutamine synthesis is stimulated to Vmax values by NH4Cl. Malate and other citric acid cycle dicarboxylates, but not aspartate and glutamate, may compete with 2‐oxoglutarate for uptake into perivenous glutamine synthesizing hepatocytes. Similar to 2‐oxoglutarate uptake, also malate uptake by perfused rat liver seems largely dependent on the presence of extracellular Na+. The suggested acinar zonation of hepatic malate uptake emphasizes again the metabolic specialization of perivenous glutamine synthetase containing hepatocytes and suggests that, besides vascular 2‐oxoglutarate and glutamate, also malate and related citric acid cycle dicarboxylates can be used by these cells as a substrate for glutamine synthesis.