Adaptive regulation of alanine transport in hepatic plasma membrane vesicles from the endotoxin-treated rat

Regulation of expression of the SN1 transporter during renal adaptation to chronic metabolic acidosis in rats

During chronic metabolic acidosis, renal glutamine utilization increases markedly. We studied the expression of the system N1 (SN1) amino acid transporter in the kidney during chronic ammonium chloride acidosis in rats. Acidosis caused a 10-fold increase in whole kidney SN1 mRNA level and a 100-fold increase in the cortex. Acidosis increased Na(+)-dependent glutamine uptake into basolateral and brush-border membrane vesicles (BLMV and BBMV, respectively) isolated from rat cortex (BLMV, 219 +/- 66 control vs. 651 +/- 180 pmol. mg(-1). min(-1) acidosis; BBMV, 1,112 +/- 189 control vs. 1,652 +/- 148 pmol. mg(-1). min(-1) acidosis, both P < 0.05). Na(+)-independent uptake was unchanged by acidosis in BLMV and BBMV. The acidosis-induced increase in Na(+)-dependent glutamine uptake was eliminated by histidine, confirming transport by system N. SN1 protein was detected only in BLMV and BBMV from acidotic rats. After recovery from acidosis, SN1 mRNA and protein and Na(+)-dependent glutamine uptake activity rapidly returned to control levels. These data provide evidence that regulation of expression of the SN1 amino acid transporter is part of the renal homeostatic response to acid-base imbalance.

Mechanisms of the protective effect of L-alanine to D-galactosamine-induced hepatocellular injury: comparative studies of L-alanine and pyruvate

The addition of L-alanine reduced lactate dehydrogenase leakage from primary cultured rat hepatocytes treated with galactosamine (D-gal), while D-alanine and other amino acids did not. However, the mechanisms have not yet been entirely clarified. In this study, we used various inhibitors of metabolism, i.e., aminooxyacetate, oligomycin, and quinolinic acid, to examine the relation between this protective effect and the metabolism of L-alanine. Quinolinic acid (10 mM) did not affect the hepatoprotective effect of L-alanine, while oligomycin (0.1 mug/ml) and aminooxyacetate (1 mM) eliminated the hepatoprotective effect of L-alanine. L-Alanine also increased the albumin secretion by cultured hepatocytes treated with D-gal, while pyruvate had little effect. It was revealed that the intracellular content of pyruvate did not increase as a result of addition of L-alanine. These results are consistent with the hypothesis that L-alanine metabolism is important for hepatoprotection, but pyruvate cannot be used as a substitute for L-alanine.

Posttranscriptional regulation of ATA2 transport during liver regeneration

The recent cloning of ATA2, a cDNA displaying characteristics identical to the System A transporter, has provided the first molecular tool for study of System A-mediated amino acid transport in liver. Despite the 233 +/- 9 and 472 +/- 11% increase in System A transport activity following partial hepatectomy at 6 and 12 h, respectively, the steady-state level of ATA2 mRNA did not show a corresponding marked increase. Examination of the kinetic properties of System A following partial hepatectomy revealed a K(m) of 0.26 +/- 0.04 mM which is consistent with the reported K(m) for ATA2. These results indicate that a System A transporter present in regenerating liver and ATA2 are identical, but that the increase in System A activity following partial hepatectomy does not result from an increase in steady-state levels of ATA2 mRNA. These observations suggest that ATA2-mediated transport of amino acids is regulated at the posttranscriptional level.

Endotoxin-stimulated macrophages decrease bile acid uptake in WIF-B cells, a rat hepatoma hybrid cell line

Endotoxemia leads to cytokine-mediated alterations of the hepatocellular sodium-taurocholate-cotransporting polypeptide (ntcp). We hypothesized that stimulated macrophages are essential transducers for down-regulating hepatocellular bile salt uptake in response to endotoxin (lipopolysaccharide [LPS]) exposure. Using an in vitro model, we exposed mouse macrophages (IC-21 cell line) to LPS for 24 hours. Concentrations of cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, and IL-6 increased 10.6-fold, 12.5-fold, and 444-fold, respectively, in LPS-conditioned IC-21 medium (CM) versus unconditioned IC-21 medium (UM). WIF-B rat hepatoma hybrid cells were incubated with either CM or UM or treated directly with medium containing recombinant TNF-alpha, IL-1beta, and IL-6. [(3)H]Taurocholate ([(3)H]TC) uptake decreased in WIF-B cells exposed to either TNF-alpha (54% of control), IL-1beta (78%), IL-6 (55%) as single additives, or in triple combination (TCC) (43%). A virtually identical decrease was observed after exposing WIF-B cells to CM (52%, P <.001). LPS had no direct effect on [(3)H]TC uptake. CM treatment did not decrease L-alanine transport in WIF-B cells. Blocking antibodies against TNF-alpha, IL-1beta, and IL-6 restored the diminished [(3)H]TC uptake in cells exposed to TCC and CM to 87% and 107% of controls, respectively. Northern blotting revealed that ntcp messenger RNA (mRNA) expression was significantly reduced in WIF-B cells after exposure to CM, and in primary rat hepatocytes exposed to CM or TNF-alpha (68%, 14%, and 29% of control, respectively). We conclude that macrophages and their ability to secrete the cytokines TNF-alpha, IL-1beta, and IL-6 may be essential in mediating the endotoxin-induced cholestatic effect of decreased hepatocellular bile salt uptake.

Effects of LPS on transport of indocyanine green and alanine uptake in perfused rat liver

Lipopolysaccharide (LPS) initiates cholestasis. Whether this process is mediated by tumor necrosis factor-alpha (TNF-alpha) and whether the cholestatic response to LPS is associated with intrahepatic accumulation of possibly toxic substances are under debate. To study these questions the hepatic uptake and biliary excretion of indocyanine green (ICG) was examined in the isolated perfused rat liver 18 h after intravenous treatment of rats with either saline, 1 mg/kg body wt LPS, or LPS and intraperitoneal pentoxifylline (POF) (n = 6 in each group). POF inhibits TNF-alpha release after LPS administration. LPS induced a typical acute-phase response with increased mRNA for acute-phase proteins, reduced albumin mRNA, and increased hepatic uptake of alanine. Intrinsic hepatic clearance of ICG in controls (1.01 +/- 0.05 ml. min(-1). g liver(-1)) was similarly decreased by LPS alone (0.62 +/- 0.04 ml. min(-1). g(-1); P = 0.002 vs. control) or combined with POF (0.66 +/- 0.06 ml. min(-1). g(-1)). A kinetic analysis indicated that LPS reduced both uptake and excretion processes in a balanced manner, so that intrahepatic ICG content was unaffected or even slightly reduced, as confirmed by measurement of ICG contents in the perfused livers. In livers from parallel-treated nonperfused rats, mRNA for the organic anion transporting protein-1 (Oatp1, which is likely to mediate ICG uptake) was unaffected by LPS, whereas the concentration of Oatp1 protein was reduced. Thus LPS induced an acute-phase response that included downregulation of ICG uptake by reduction of Oatp1 protein concentration, possibly at a posttranscriptional level. TNF-alpha appears not to be the mediator because POF did not modify these LPS effects.

Arginine-nitric oxide pathway in plasma membrane of rat hepatocytes during early and late sepsis

OBJECTIVE

To study the transported L-arginine in rat hepatocytes during different stages of sepsis.

DESIGN

A prospective, controlled study.

SUBJECTS

Thirty-six Sprague-Dawley male rats (250 to 300 g) were anesthetized and studied.

INTERVENTIONS

Early sepsis was produced 9 hrs after cecal ligation and puncture (CLP) and late sepsis developed 18 hrs after CLP. The control group underwent sham operation. Plasma membrane of rat hepatocytes was prepared by differential centrifugation. The [3H] L-arginine uptake of plasma membrane vesicles during sepsis was measured and inhibition studies employing omega-nitro-L-arginine methyl ester (L-NAME) and aminoguanidine were performed.

MEASUREMENTS AND MAIN RESULTS

L-arginine transport was saturable, increased linearly with plasma membrane protein concentration, and increased with uptake time up to 5 mins. [3H] L-arginine uptake increased by 77% to 121% (p < .05) during early sepsis, with no significant changes during late sepsis. Comparing inhibitors of nitric oxide synthase, L-NAME was effective in inhibiting L-arginine transport while aminoguanidine was not.

CONCLUSIONS

L-arginine transport was enhanced in rat hepatocytes during the early stage of sepsis. The increased uptake of L-arginine could contribute to the increase production of nitric oxide by hepatocyte during sepsis.

Effects of endotoxin challenge on hepatic amino acid transport during cancer

BACKGROUND

The hepatic uptake of amino acids is increased in both sepsis and cancer, and this response appears to be both global and essential in the catabolic host. Because immunocompromised cancer patients are susceptible to episodes of gram-negative sepsis, we examined the capacity of hepatocytes from normal and tumor-influenced livers to respond to the additional challenge of endotoxemia via increases in the Na+-dependent uptake of glutamine and zwitterionic amino acids by System N and System A, respectively.

MATERIALS AND METHODS

Fischer 344 rats were implanted with methylcholanthrene-induced fibrosarcomas. Control rats were sham-operated and pair-fed. Animal pairs (tumor burden = 8-32% carcass weight) were injected intraperitoneally with either Escherichia coli endotoxin (10 mg/kg) or PBS, and after 4 h, hepatocytes were isolated from the livers of the animals via collagenase perfusion and placed in primary culture. Three hours later, amino acid transport rates were measured using radiolabeled glutamine for System N and alpha-methylaminoisobutyric acid (MeAIB), a nonmetabolizable substrate specific for System A.

RESULTS

Cancer-independent of tumor size-and endotoxin each elicited similar 1.5- to 2-fold inductions of System N activity. When combined, their effects were additive rather than synergistic. In contrast, endotoxin induced an insignificant increase in System A activity, whereas cancer stimulated this carrier 2-fold in either the absence or the presence of endotoxin.

CONCLUSIONS

The primary glutamine and alanine carriers in hepatocytes are differentially influenced during catabolic states, and the tumor-influenced liver is competent to further increase glutamine uptake in response to additional catabolic insults.

Stimulation of rat hepatic amino acid transport by burn injury

Burn injury accelerates hepatic amino acid metabolism, but the role of transmembrane substrate delivery in this response has not been investigated. We therefore studied the effects of cutaneous scald injury on the Na+-dependent transport of glutamine and alanine in isolated rat liver plasma membrane vesicles. Scald injury resulted in liver damage and a 1.4- to 2.3-fold and 1.5- to 2.8-fold stimulation of hepatic transport rates for glutamine and alanine, respectively, proportional to the total burned surface area (TBSA) after 24 hours. Enhanced uptake of glutamine and alanine was attributable to increases in the maximum velocity (Vmax) of system N and system A activities, respectively. Hepatic amino acid transport activity remained elevated in vesicles from burned animals after 72 hours, but the degree of stimulation (1.3- to 1.7-fold for glutamine and 1.3- to 1.6-fold for alanine) was less than that observed 24 hours after thermal injury. Liver function tests returned to control values after 72 hours as well, indicating rectification of hepatic damage. In contrast to the induction of hepatic system A and system N activity in catabolic states such as cancer and endotoxemia, further studies showed that tumor necrosis factor (TNF) failed to play a significant role in burn-stimulated amino acid transport rates. When combined with plasma liver enzyme profiles, early transient hepatic amino acid transporter stimulation may support amino acid-dependent pathways involved in the repair of burn-dependent hepatic damage.

Inhibition of glycerol metabolism in hepatocytes isolated from endotoxic rats

Sepsis or endotoxaemia inhibits gluconeogenesis from various substrates, the main effect being related to a change in the phosphoenolpyruvate carboxykinase transcription rate. In addition, sepsis has been reported to affect the oxidative phosphorylation pathway. We have studied glycerol metabolism in hepatocytes isolated from rats fasted and injected 16 h previously with lipopolysaccharide from Escherichia coli. Endotoxin inhibited glycerol metabolism and led to a very large accumulation of glycerol 3-phosphate; the cytosolic reducing state was increased. Furthermore glycerol kinase activity was increased by 33% (P<<0.01). The respiratory rate of intact cells was significantly decreased by sepsis, with glycerol or octanoate as exogenous substrates, whereas oxidative phosphorylation (ATP-to-O ratio or respirations in state 4, state 3 and the oligomycin-insensitive state as well as the uncoupled state) was unchanged in permeabilized hepatocytes. Hence the effect on energy metabolism seems to be present only in intact hepatocytes. An additional important feature was the observation of a significant increase in cellular volume in cells from endotoxic animals, which might account for the alterations induced by sepsis.

Administration of Escherichia coli endotoxin to rat increases liver mass and hepatocyte volume in vivo

We have established, in vivo, an increase in liver mass and hepatocyte volume after a single intraperitoneal administration, to fasted rats, of Escherichia coli lipopolysaccharide (0127:B8) at 3 mg/kg. The phenomenon was time- and dose-dependent and could be prevented by treatment with polyclonal antiserum against tumour necrosis factor-alpha (TNF-alpha) before the endotoxin injection. Endotoxin caused an increase of 26% in the hepatic mass compared with fasted controls at 24 h. An increase of 27% in the hepatic water content underlay the altered hepatic mass which could not be accounted for by a change in the volume of hepatic blood and/or interstitial fluid (measured in vivo), suggesting an expansion in the hepatocellular volume. This is supported by an increase of 25% in the K+ content of the endotoxic livers. Morphometric study confirmed a 15% increase in hepatocyte volume after endotoxin administration. The data are discussed in the light of possible metabolic effects of increased hepatocyte volume.

In vivo effects of lipopolysaccharide on hepatic free-NAD(P)(+)-linked redox states and cytosolic phosphorylation potential in 48-hour-fasted rats

This study was performed to determine the magnitude and time of onset of in vivo changes in hepatic bioenergetics in response to a sublethal dose of lipopolysaccharide (LPS), a bacterial endotoxin. Male rats (48-hour-fasted) were administered an intraperitoneal injection of LPS (5 mg/kg body weight) or vehicle alone, and the livers were freeze-clamped 5, 30, or 180 minutes or 24 hours later. Liver tissue was extracted with perchloric acid, and the metabolites necessary to calculate NAD(+)- and NADP(+)-linked redox states and the cytosolic phosphorylation potential were measured. There was no significant difference in hepatic cytosolic phosphorylation potential between LPS and control groups at any of the times investigated. This indicated that the ability of the liver to synthesize adenosine triphosphate (ATP) was not compromised under the conditions of the study. No changes in hepatic redox states were observed 5 or 30 minutes after LPS treatment. Three hours after LPS treatment, hepatic cytosolic and mitochondrial free-[NAD+]/[NADH] redox states and the cytosolic free-[NADP+]/[NADPH] redox state were more oxidized. By 24 hours, only NAD(+)-linked redox states were more oxidized than the time-matched controls. Hepatic urea content was elevated at both 3 and 24 hours, compatible with an increased rate of urea synthesis as a consequence of increased amino acid metabolism, whereas hepatic beta-hydroxybutyrate and total ketone bodies were decreased 24 hours after LPS treatment, indicating decreased hepatic ketogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Normalization of tumor-induced increases in hepatic amino acid transport after surgical resection

BACKGROUND

The liver of the host with cancer requires increased amounts of amino acids to support the synthesis of glucose and key defense proteins. To study the effect of the growing tumor on hepatic amino acid uptake, the authors measured hepatic transport activity in tumor-bearing rats and in rats at various times after tumor resection.

METHODS

Fischer-344 rats were implanted subcutaneously with methylcholanthrene-induced fibrosarcoma cells (MCA sarcoma). When the tumors reached 10% of body weight, hepatic amino acid transport activity was assayed or the animals underwent surgical removal of the tumor. In animals that underwent tumor excision, livers were removed at 1, 3, or 5 days post-resection, and hepatic plasma membrane vesicles (HPMVs) were prepared. Nontumor-bearing pair-fed rats undergoing sham implantation or sham resection served as controls. System N (glutamine), System A (MeAIB), and System y+ (arginine) transport activity were assayed, which allowed the authors to compare differences in tumor-induced rates of transport and the influence of resection on transport activity.

RESULTS

System A transport activity was unaltered by tumor growth. In contrast, the presence of the growing tumor increased arginine and glutamine uptake by the liver. Hepatic glutamine transport remained elevated for 5 days after tumor resection, although by postoperative day 5 there was a trend toward normalization. In contrast, arginine transport remained increased by twofold onpost-resection day 1 and had normalized by postoperative day 3. The enhanced arginine transport was a result of an increase in maximal transport velocity (Vmax) rather than a change in carrier affinity.

CONCLUSIONS

Increases in hepatic amino acid transport normalize within several days of tumor resection, indicating a key role for the tumor in the induction of this response. The observation that hepatic glutamine transport activity remains augmented after tumor resection longer than any other transporter studied suggests a key role for this amino acid in overall hepatic nitrogen metabolism and may partially explain the persistent glutamine depletion that is characteristic of the tumor-bearing host.

Dietary regulation of the hepatic system n glutamine transporter in tumor-bearing rats

BACKGROUND

Hepatocytes possess a novel, plasma-membrane, sodium ion (Na+)-independent, glutamine transporter (system n), which functions to transport glutamine out of the cell into the blood. In the tumor-bearing rat, the activity of system n increases but its regulation is unknown. We hypothesized that the increase in system n that occurs in rats with cancer was related to a fall in the circulating glutamine concentration.

METHODS

Ten male rats underwent flank implantation with a cube of methylcholanthrene-induced fibrosarcoma cells and 10 rats underwent a sham operation. After 9 days of standard diet, all rats were randomized to receive either a glutamine-enriched oral diet or an isonitrogenous diet without supplemental glutamine, for 1 week. Tumors and livers were harvested 16 days postimplantation. Arterial blood samples were obtained from all animals. Hepatic plasma membrane vesicles were prepared and the carrier-mediated, Na(+)-independent transport of glutamine was assayed.

RESULTS

When compared to nontumor-bearing animals, tumor-bearing rats that were fed a control diet exhibited hypoglutaminemia and a 2.3-fold increase in the activity of system n. Glutamine dietary supplementation produced blood glutamine levels that were similar in both tumor-bearing and nontumor-bearing rats, apparently abrogating the increase in system n activity that was observed in tumor-bearing rats that were not fed supplemental glutamine. Tumor-bearing animals receiving supplemental glutamine had a decreased number of system n carriers (Vmax) in the hepatic plasma membrane compared to that of tumor-bearing animals receiving a control diet; this apparently abrogated the glutamine efflux rate. Glutamine feeding did not alter system n activity in nontumor-bearing controls.

CONCLUSIONS

In the tumor-bearing animal model, system n is modulated by the circulating glutamine concentration. This is the first study that demonstrates the ability of specialized nutrition to "downregulate" transport activity in vivo. Provision of glutamine-enriched diets to the host with cancer may maintain hepatic glutamine levels and prevent host glutamine depletion.

Cyclo-oxygenase blockade abrogates the endotoxin-induced increase in Na(+)-dependent hepatic amino acid transport

BACKGROUND

Endotoxemia is characterized by a marked increase in the uptake of amino acids by the liver, but the regulation of this response has not been fully elucidated. In the current study, we investigated the potential role of prostaglandins as mediators of this response. We examined the in vivo effects of the anti-inflammatory agent ketorolac, a cyclo-oxygenase inhibitor that blocks prostaglandin synthesis, on hepatic amino acid transport activity in endotoxin-treated rats.

METHODS

We assayed the activities of the Na(+)-dependent transport systems A and N in hepatic plasma membrane vesicles prepared from endotoxemic rats that were pretreated with ketorolac or vehicle. Hepatic plasma membrane vesicles were prepared by differential centrifugation, and the transport of [3H]glutamine (system N) and [3H]2-methylamino-isobutyric acid (system A) was assayed. Hepatic plasma membrane vesicles were also prepared from normal rats that received prostaglandin E2, and glutamine and MeAIB transport were measured.

RESULTS

Endotoxin treatment resulted in a twofold to threefold increase in Na(+)-dependent amino acid transport activity in hepatic plasma membrane vesicles secondary to an increase in the transport Vmax, which was consistent with the appearance of increased numbers of corresponding transporter proteins in the hepatocyte plasma membrane. Pretreatment with ketorolac almost completely abrogated the endotoxin-induced increase in hepatic amino acid transport. Administration of prostaglandin E2 to normal rats resulted in a statistically significant increase in glutamine and alanine transport by hepatic plasma membrane vesicles prepared from these animals.

CONCLUSIONS

Prostaglandins play a key role in mediating the accelerated hepatic amino acid transport that occurs during endotoxemia.

Accelerated hepatic arginine transport in the tumor-bearing rat

BACKGROUND

Arginine plays a pivotal role in regulating ureagenesis, polyamine biosynthesis, and nitric oxide production, metabolic pathways that may be stimulated in the liver of the tumor-bearing host. Normally, plasma arginine is excluded from the hepatocyte intracellular space by the low basal activity of its membrane transporter. We hypothesized that progressive malignant disease is associated with an increase in carrier-mediated arginine transport across the hepatocyte plasma membrane.

METHODS

Twenty-four adult Fischer 344 rats were implanted subcutaneously with fibrosarcomas (TBR) and were studied when the tumors were small [10 +/- 1% of body weight (BW)], medium-sized (15 +/- 1% of BW), and large (25 +/- 1% of BW). Groups of control rats (n = 24) were pair-fed to match carcass weights of the three TBR groups. Livers were excised, and hepatocyte plasma membrane vesicles (HPMVs) were prepared by Percoll density gradient centrifugation. Arginine transport by HPMVs was assayed by a rapid mixing/filtration technique. Vesicle purity and functionality were assessed by membrane enzyme marker enrichments and transportability into an osmotically active space.

RESULTS

Arginine uptake by HPMVs was mediated by both saturable carrier-mediated (System y+) and nonsaturable (diffusion) components. The time course of arginine uptake in HPMVs in the three groups showed similar equilibrium transport rates, indicating similar vesicle sizes. The presence of the growing tumor resulted in a 40-120% increase in System y(+)-mediated arginine transport in HPMVs. This response was dependent on tumor size and was due to a stimulation of carrier Vmax, suggesting an increase in the number of functional System y+ carriers in the hepatocyte plasma membrane. The Na(+)-dependent transport of the System A analog MeAIB was also increased, but only in rats with large tumors.

CONCLUSIONS

Tumor growth results in a progressive increase in hepatic arginine transport, a response mediated primarily by an increase in the activity of System y+. This accelerated transport may amplify the availability of arginine to support key arginine-dependent metabolic pathways in the hepatocyte.

Harry M. Vars Research Award. Influence of fasting on glutamine transport in rat liver

During starvation, the liver switches from an organ of net glutamine uptake to an organ of net glutamine release to help maintain blood glutamine levels. We hypothesized that this shift in hepatic glutamine exchange was regulated at the level of the hepatocyte plasma membrane by adaptive changes in glutamine transport. To test this hypothesis, adult rats (200 g) were allowed to consume regular rat food ad libitum (fed, n = 8) or were fasted for 72 hours (fasted, n = 8, access to water allowed). Livers were excised and hepatocyte plasma membrane vesicles were prepared by differential and Percoll density gradient centrifugation. Vesicle purity and functionality were assessed by marker enzyme measurements, classic "overshoots," and time courses, which showed similar vesicle size. Uptake of 3H-glutamine by hepatocyte plasma membrane vesicles in the presence and absence of sodium was assayed by a rapid mixing/filtration method, which reflects actual transport across the hepatocyte cell membrane in vivo. Fasted rats lost 15 +/- 2% of body weight; fed rats gained weight. Na(+)-dependent glutamine transport (system "N," mediates uptake into the hepatocyte) fell by 22% in the starved group, indicating a diminished rate of glutamine transport into the hepatocyte. In contrast, carrier-mediated Na(+)-independent glutamine transport (system "n," mediates the release of glutamine out of the cell) doubled in the starved animals. Diffusion of glutamine across the vesicle membrane was unchanged by starvation.(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine transport in human liver. Characterization and effects of nitric oxide synthase inhibitors

OBJECTIVE

Arginine transport was characterized and studied in human liver.

SUMMARY BACKGROUND DATA

Plasma arginine uptake may regulate hepatocyte intracellular availability and the subsequent biosynthesis of nitric oxide (NO), but little is known about arginine transport across the human hepatocyte plasma membrane.

METHODS

The authors characterized plasma membrane transport of 3[H]-L-arginine in hepatic plasma membrane vesicles (HPMVs) and in hepatocytes isolated and cultured from human liver biopsy specimens. They also studied the effects of the NO synthase inhibitors omega-nitro-L-arginine methyl ester (L-NAME) and N-methyl-arginine (NMA) on arginine transport in HPMVs and in cultured cells.

RESULTS

Arginine transport was saturable, Na(+)-independent, temperature and pH sensitive, and was inhibited by the naturally occurring amino acids lysine, homoarginine, and ornithine (System y+ substrates). Arginine transport by both vesicles and cultured hepatocytes was significantly attenuated by NO synthase inhibitors, suggesting that the arginine transporter and the NO synthase enzyme may share a structurally similar arginine binding site. Dixon plot analysis showed the blockade to occur by competitive, rather than noncompetitive, inhibition. In vivo treatment of rats with lipopolysaccharide (LPS) resulted in a twofold stimulation of saturable arginine transport in the liver. This LPS-induced hepatic arginine transport activity was also inhibited by L-NAME. These data indicate that arginine transport by human hepatocytes is mediated primarily by the Na(+)-independent transport System y+.

CONCLUSIONS

Besides inhibition of the NO synthase enzyme, the ability of arginine derivatives to block NO production may also be due to their ability to competitively inhibit arginine transport across the hepatocyte plasma membrane. The use of selective arginine derivatives that compete with arginine at the plasma membrane level may be a metabolic strategy that can be used to modulate the septic response.

Tumor necrosis factor stimulates amino acid transport in plasma membrane vesicles from rat liver

Severe infection is characterized by a translocation of amino acids from the periphery to the liver, an event that is mediated in part by cytokines such as tumor necrosis factor-alpha (TNF). We investigated the activities of Na(+)-dependent transport systems A, ASC, and N in hepatic plasma membrane vesicles (HPMVs) prepared from rats treated with TNF in vivo. TNF did not alter sodium uptake but resulted in time- and dose-dependent fivefold and 50% maximal increases in system A and system N activity, respectively, in HPMVs secondary to an increase in the transport Vmax. Maximal increases in transport were observed 4 h after exposure to TNF and had returned to basal levels within 24 h. Similarly, system ASC activity was stimulated 80% in HPMVs from rats treated with TNF. Incubation of HPMVs from normal rats in vitro with TNF did not alter transport activity. Pretreatment of animals with the glucocorticoid receptor antagonist RU 38486 attenuated the TNF-induced enhancement in transport activity by 50%. The marked increase in Na(+)-dependent amino acid transport activity by TNF is mediated in part by the glucocorticoid hormones and represents an important mechanism underlying the accelerated hepatic amino acid uptake that occurs during critical illness.

Na(+)-dependent glutamine transport in the liver of tumour-bearing rats

In rats with advanced malignant disease, the liver extracted circulating glutamine at a ratio three times faster than the liver of control non-tumour-bearing animals. This augmented uptake occurred in spite of a fall in circulating glutamine levels, implying an increase in hepatocyte plasma membrane transport. Na(+)-dependent glutamine transport activity (System N) was increased nearly two-fold in hepatocyte plasma membrane vesicles from tumour-bearing rats; this increase in System N activity was proportional to tumour size and was due to an increase in carrier Vmax with no change in carrier affinity. Measurement of System N activity in isolated hepatocytes incubated with serum from tumour-bearing rats demonstrated a significant increase in glutamine transport compared with cells incubated with serum from control rats. These data indicate that the liver of rats with advanced malignant disease displays accelerated glutamine consumption. This increased uptake is due, in part, to enhanced carrier-mediated transport activity, and is mediated by a circulating factor(s) that is not present (or inactive) in non-tumour-bearing controls.

Metabolism and nutritional frontiers in pediatric surgical patients

The successful treatment of pediatric surgical disease requires an understanding of the acute metabolic stress response. Poor clinical outcome can result when the metabolic demands of acute injury exceed the ability of endogenous host mechanisms to compensate. Appropriate exogenous supplementation may provide the metabolic and nutritional support crucial to recovery. As knowledge in this area grows, more effective treatment strategies are evolving. The potential for further advances, especially in the infant critical care population, offers the hope for substantial progress in the near future.

Growth hormone regulates amino acid transport in human and rat liver

Human growth hormone (GH) has been shown to improve nitrogen balance in surgical patients and to decrease urea production. This has been thought to be due primarily to an increase in protein synthesis in skeletal muscle. Little attention has focused on the liver as a possible site where GH may modulate amino acid uptake and thereby divert nitrogen away from urea-genesis. The authors hypothesized that GH regulates amino acid transport in hepatocytes at the plasma membrane level. They studied hepatic amino acid transport in 20 healthy surgical patients that received saline, low-dose GH (0.1 mg/kg/day), or high-dose (0.2 mg/kg/day) GH for 3 days before operation. At operation, a 5- to 10-g wedge biopsy of the liver was obtained, and hepatocyte plasma membrane vesicles were prepared by Percoll density gradient centrifugation. Vesicle transport of [3H]-MeAIB, a highly selective system A substrate, and [3H]-glutamine, a selective system N substrate, was measured, employing a rapid mixing/filtration technique. Hepatocyte plasma membrane vesicles were also prepared from 14 rats treated with saline or one of three different GH treatment regimens: (A) 12 hours after chronic GH treatment (6 mg/kg every 12 hours x 4 doses); (B) 4 hours after acute (1 dose) GH treatment; and (C) 4 hours after chronic GH treatment. In human liver vesicles, low-dose GH resulted in a 13% decrease in system A activity (p = not significant), whereas high-dose GH caused a marked 79% decrease (6.7 +/- 1.7 pmol/mg protein/10 seconds in control patients versus 1.4 +/- 0.7 in GH, p less than 0.05). System N was unaffected. Kinetic analysis of MeAIB transport by vesicles from high-dose GH patients showed the reduction in transport to be due to a 63% decrease in the Vmax (maximal transport velocity) with no alteration in the transport Km (carrier affinity). Vesicles from rats treated chronically with GH using a protocol similar to that used for human subjects exhibited decreased system A transport activity (10.4 +/- 0.4 pmol/mg pro/10 seconds in controls versus 7.5 +/- 0.2 in GH, p less than 0.05) secondary to a 59% reduction in the transport Vmax. Chronic growth hormone treatment decreases the activity of system A in both human and rat hepatocytes. This may be one mechanism by which GH diminishes hepatic urea-genesis and spares amino acids for peripheral protein synthesis.

Mechanisms of increased hepatic glutamine uptake in the endotoxin-treated rat

The mechanisms underlying the accelerated hepatic consumption of glutamine that occurs during endotoxemia were investigated in rats 12 hr after treatment with Escherichia coli lipopolysaccharide. Hepatic glutamine delivery and consumption were calculated from measurements of hepatic blood flow and blood glutamine levels. Hepatic glutaminase activity and glutamine and glutamate content were determined. Hepatocyte plasma membrane transport activity was evaluated employing isolated hepatic plasma membrane vesicles (HPMVs). Endotoxin treatment resulted in an 11-fold increase in hepatic glutamine consumption and a 2-fold increase in the delivered load of glutamine to the liver. Hepatic glutamate content doubled while glutamine content was unaffected, not withstanding a decrease in the specific activity of glutaminase. Studies employing HPMVs demonstrated that hepatic plasma membrane transport activity was unaffected by endotoxin treatment. The enhanced hepatic consumption of glutamine secondary to endotoxemia appears to be the result of both a mass-action effect and the concurrent activation of intracellular metabolism. Responses at the level of plasma membrane transport do not appear to play an active role in mediating this enhanced hepatic uptake.

Mechanisms of accelerated hepatic glutamine efflux in the tumour-bearing rat

The mechanisms potentially controlling the net release of glutamine by the liver that occurs in tumour-bearing rats were investigated. Studies were undertaken when the tumour comprised approximately 7% (15 +/- 2 g) of total body weight. Hepatic glutamine gradient ratios were calculated by dividing hepatic glutamine content by arterial blood glutamine concentration. Both sodium-dependent and sodium-independent hepatocyte carrier-mediated glutamine transport were evaluated employing hepatic plasma membrane vesicles (HPMVs). In TBR the hepatic glutamine gradient ratio doubled (P < 0.001) secondary to a 52% increase in hepatic content (P < 0.005) and a 16% decrease in circulating glutamine (P < 0.001). Sodium-dependent glutamine transport was increased in HPMVs from TBR secondary to a 24 +/- 4% increase in the maximal velocity of transport (Vmax; P < 0.01) without alteration in apparent transporter affinity (Km). Saturable sodium-independent carrier-mediated glutamine transport was increased in HPMVs from TBR over CONT to a much greater relative degree owing to a 2.7-fold increase in transport Vmax (P < 0.05) without a change in transport Km. The accelerated hepatic efflux of glutamine which characterizes malignant growth appears to be the result of both mass-action gradient phenomena and alterations at the level of hepatocyte membrane transport activity.