Role of plasma membrane transport in hepatic glutamine metabolism

Abstract

1. In livers of fed rats and in perfused livers supplied with a physiological portal glutamine concentration of 0.6 mM, the mitochondria1 and cytosolic glutamine concentrations are 20 mM and 7 mM, respectively, thus, the mitochondrial/cytosolic glutatnine concentration gradient is 2 ~ 3.2. Uptake and release of glutamine by periportal and perivenous hepatocytes occurs predominantly by an N a -dependent transport system (so-called system 'N').3. Histidine in near-physiological concentrations inhibits both glutamine uptake by periportal hepatocytes and its release by perivenous hepatocytcs. This is not due to an inhibition of glutamine-metabolizing enzymes by histidine or its metabolites. With physiological portal glutamine concentrations (0.6 mM), stimulation of glutaminase flux or of glutamine transaminasc flux is followed by a decrease of hepatic glutamine levels to about 80% or 30%, respectively, glutamine levels are further decreased to 50% or 20% in the presence of histidine.4. When glutamine is synthesized endogeneously (no glutamine added), the histidine-induced inhibition of glutitmine release is paralleled by a 210%, increase or the hepatic tissue level of glutaminc.5. In experiments with and without iiiethioninc sulfoximine and in the absence of added glutamine, the glutamine content in the small perivenous hepatocyte population containing glutamine synthetase is estimatcd t o be about 3.5 pmol/g wet weight and that in the periportal hepatocytes as low as 0.3 pmol/g wet weight.6. In contrast to the prevailing view, it is concludcd that glutamine transport across the plasma mernbranc of hepatocytes is a potential regulatory site in glutamine degradation and synthcsis, especially under the influence of efrectors like histidine.Glutamine uptake into the hepatocytes occurs by an Nadependent transport system 'N', also shared by other amino acids likc histidine or asparagine [l -31 (for review see [4]). It has bccn shown that histidine and asparagine inhibit glutamine transport across the liver plasma inembrane [2, 31. In addition, facilitated diffusion allows hepatocytes to relcasc glutaniine when intracellular concentrations are elevated [3].It is assumed in the literature that glutamine transport across the plasma membrane of hepatocytes is not ratc-controlling for its catabolism [3 -61 and that regulation of hcpatic glutamine breakdown occurs at thc level of glutamine-metabolizing enzymes. However. these previous studies focused on glutaminc transport rather than its metabolism and they did not take into account the recently discovered hepatocyte hctcrogeneity with glutaminase being periportal [7] and glutamine synthetase being perivenous [7, 81 and the activity of glutamine transaminases in pcriportal as well perivenous hepatocytcs [9]. This unique distribution of glutamine-mctabolizing enzymes results in an intercellular glutamine cycle [7] (for review see [lo]): while glutamine is taken up by periportal hepatocytes, it is released by perivenous hepatocytes into the

Hormonal regulation of amino acid transport system N in primary cultures of rat hepatocytes

“…The existence of a hormonal control of amino acid transport system N supports the recent findings of Haussinger et al [6] which point to a specific role of glutamine transport in glutamine metabolism. In view of the intercellular compartmentation of glutamine metabolism, i. e. the preferential utilization and catabolism of glutamine in zones 1, 2 and part of zone 3 (the 'afferent urea compartment' [30]) 130, 311 and the exclusive synthesis of glutamine in the inner part of zone 3 (the 'efferent glutamine compartment' [30]) [31,321, it seems reasonable to assume that the respective hepatocytes differ in the extent of uptake and release of glutamine.…”

“…The hormone-induced stimulation of system N was prevented by cycloheximide. 6. The induced uptake of glutamine was inhibited by excess amounts of asparagine and histidine but not of These results clearly differentiate the hormonal regulation of system N from that of system A.…”

“…In view of the intercellular compartmentation of glutamine metabolism, i. e. the preferential utilization and catabolism of glutamine in zones 1, 2 and part of zone 3 (the 'afferent urea compartment' [30]) 130, 311 and the exclusive synthesis of glutamine in the inner part of zone 3 (the 'efferent glutamine compartment' [30]) [31,321, it seems reasonable to assume that the respective hepatocytes differ in the extent of uptake and release of glutamine. While Haussinger et al [6] demonstrated the participation of Na+-dependent system N in uptake and release of glutamine, Fafournoux et al [33] suggested that release of this amino acid might preferably by mediated via a N+-independent transport component. These assumptions imply a somewhat different distribution of individual components mediating glutamine transport among hepatocytes located in different zones of the liver acinus.…”

“…As demonstrated by Haussinger et al [6] this transport system provides a potential regulatory site of glutamine metabolism particularly in the presence of physiological concentrations of histidine, which acts as a strong competitive inhibitor [3, 61. The high capacity of system N found in the normal fed animal [3] was found to be elevated in streptozotocin-induced diabetes [7] while starvation for 48 h seemed to be without effect The regulatory mechanisms of system N are not well understood. Like system A it was found to be stimulated by deprivation in vitro of cells with respect to amino acids [3, 9, 101.…”

Inhibitors of protein kinase C block the α1‐adrenergic refractoriness induced by phorbol 12‐myristate 13‐acetate, vasopressin and angiotensin II

“…This metabolic pathway has several sites of control [49 -561 including the plasma membrane transport of glutamine [50], glutaminase activity [54, 551, citrulline synthesis [24, 561, conversion of glutamate into aspartate [51] and the activity of arginosuccinate synthetase [49]. The stimulation of glutaminase activity seems to be a key step in the hormonal modulation of this pathway [54, 551.…”