Control of alanine metabolism in rat liver by transport processes or cellular metabolism

ChREBP downregulates SNAT2 amino acid transporter expression through interactions with SMRT in response to a high-carbohydrate diet

Carbohydrate responsive element-binding protein (ChREBP) has been identified as a primary transcription factor that maintains energy homeostasis through transcriptional regulation of glycolytic, lipogenic, and gluconeogenic enzymes in response to a high-carbohydrate diet. Amino acids are important substrates for gluconeogenesis, but nevertheless, knowledge is lacking about whether this transcription factor regulates genes involved in the transport or use of these metabolites. Here, we demonstrate that ChREBP represses the expression of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) in response to a high-sucrose diet in rats by binding to a carbohydrate response element (ChoRE) site located -160 bp upstream of the transcriptional start site in the SNAT2 promoter region. Additionally, immunoprecipitation assays revealed that ChREBP and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) interact with each other, as part of the complex that repress SNAT2 expression. The interaction between these proteins was confirmed by an in vivo chromatin immunoprecipitation assay. These findings suggest that glucogenic amino acid uptake by the liver is controlled by ChREBP through the repression of SNAT2 expression in rats consuming a high-carbohydrate diet.NEW & NOTEWORTHY This study highlights the key role of carbohydrate responsive element-binding protein (ChREBP) in the fine-tuned regulation between glucose and amino acid metabolism in the liver via regulation of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) expression after the consumption of a high-carbohydrate diet. ChREBP binds to a carbohydrate response element (ChoRE) site in the SNAT2 promoter region and recruits silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor to reduce SNAT2 transcription. This study revealed that ChREBP prevents the uptake of glucogenic amino acids upon the consumption of a high-carbohydrate diet.

Mercury chloride increases hepatic alanine aminotransferase and glucose 6-phosphatase activities in newborn rats in vivo

This work investigated the in vivo and in vitro effects of HgCl2 and ZnCl2 on metabolic enzymes from tissues of young rats to verify whether the physiological and biochemical alterations induced by mercury and prevented by zinc are related to hepatic and renal glucose metabolism. Wistar rats received (subcutaneous) saline or ZnCl2 (27 mg/kg/day) from 3 to 7 days old and saline or HgCl2 (5.0 mg/kg/day) from 8 to 12 days old. Mercury exposure increased the hepatic alanine aminotransferase (∼6-fold) and glucose 6-phosphatase (75%) activity; zinc pre-exposure prevented totally and partially these mercury alterations respectively. In vitro, HgCl2 inhibited the serum (22%, 10 μM) and liver (54%, 100 μM) alanine aminotransferase, serum (53%) and liver (64%) lactate dehydrogenase (10 μM), and liver (53%) and kidney (41%) glucose 6-phosphatase (100 μM) from 10- to 13-day-old rats. The results show that mercury induces distinct alterations in these enzymes when tested in vivo or in vitro as well as when different sources were used. The increase of both hepatic alanine aminotransferase and glucose 6-phosphatase activity suggests that the mercury-exposed rats have increased gluconeogenic activity in the liver. Zinc prevents the in vivo effects on metabolic changes induced by mercury.

Recombinant production of eight human cytosolic aminotransferases and assessment of their potential involvement in glyoxylate metabolism

PH1 (primary hyperoxaluria type 1) is a severe inborn disorder of glyoxylate metabolism caused by a functional deficiency of the peroxisomal enzyme AGXT (alanine-glyoxylate aminotransferase), which converts glyoxylate into glycine using L-alanine as the amino-group donor. Even though pre-genomic studies indicate that other human transaminases can convert glyoxylate into glycine, in PH1 patients these enzymes are apparently unable to compensate for the lack of AGXT, perhaps due to their limited levels of expression, their localization in an inappropriate cell compartment or the scarcity of the required amino-group donor. In the present paper, we describe the cloning of eight human cytosolic aminotransferases, their recombinant expression as His6-tagged proteins and a comparative study on their ability to transaminate glyoxylate, using any standard amino acid as an amino-group donor. To selectively quantify the glycine formed, we have developed and validated an assay based on bacterial GO (glycine oxidase); this assay allows the detection of enzymes that produce glycine by transamination in the presence of mixtures of potential amino-group donors and without separation of the product from the substrates. We show that among the eight enzymes tested, only GPT (alanine transaminase) and PSAT1 (phosphoserine aminotransferase 1) can transaminate glyoxylate with good efficiency, using L-glutamate (and, for GPT, also L-alanine) as the best amino-group donor. These findings confirm that glyoxylate transamination can occur in the cytosol, in direct competition with the conversion of glyoxylate into oxalate. The potential implications for the treatment of primary hyperoxaluria are discussed.

Nutritional control of gene expression: how mammalian cells respond to amino acid limitation

The amino acid response (AAR) pathway in mammalian cells is designed to detect and respond to amino acid deficiency. Limiting any essential amino acid initiates this signaling cascade, which leads to increased translation of a "master regulator," activating transcription factor (ATF) 4, and ultimately, to regulation of many steps along the pathway of DNA to RNA to protein. These regulated events include chromatin remodeling, RNA splicing, nuclear RNA export, mRNA stabilization, and translational control. Proteins that are increased in their expression as targets of the AAR pathway include membrane transporters, transcription factors from the basic region/leucine zipper (bZIP) superfamily, growth factors, and metabolic enzymes. Significant progress has been achieved in understanding the molecular mechanisms by which amino acids control the synthesis and turnover of mRNA and protein. Beyond gaining additional knowledge of these important regulatory pathways, further characterization of how these processes contribute to the pathology of various disease states represents an interesting aspect of future research in molecular nutrition.

The hepatic amino acid system A transport activity, is up-regulated in obese Zucker rats

The utilization of L-alanine by liver is dependent on amino acid uptake from blood. This uptake, mainly mediated by the A transport system, may be regulated by different nutritional and physiologic conditions. The regulation of this transport system by diets with different protein content was tested in lean and obese Zucker rats. High-protein (HP) and low-protein (LP) diets led to changes in the rats' growth patterns, especially in lean animals. However, homeostasis was relatively well maintained, as seen in plasma values, in spite of the increased urea production in the HP groups and increased triacylglycerides in the LP groups. The obese animals took up L-alanine at a higher rate than the lean animals. Obesity led to the emergence of a high-affinity component (K(M) approximately 0.1-0.2 mM) in the transport system, which was not dependent on the protein content of the diet. This component has a 10-fold increase in affinity for L-alanine, but with an approximately 3- to 5-fold reduction in maximal velocity of transport.

Increased dietary protein modifies glucose and insulin homeostasis in adult women during weight loss

Amino acids interact with glucose metabolism both as carbon substrates and by recycling glucose carbon via alanine and glutamine; however, the effect of protein intake on glucose homeostasis during weight loss remains unknown. This study tests the hypothesis that a moderate increase in dietary protein with a corresponding reduction of carbohydrates (CHO) stabilizes fasting and postprandial blood glucose and insulin during weight loss. Adult women (n = 24; >15% above ideal body weight) were assigned to either a Protein Group [protein: 1.6 g/(kg. d); CHO <40% of energy] or CHO Group [protein: 0.8 g/(kg. d); CHO >55%]. Diets were equal in energy (7100 kJ/d) and fat (50 g/d). After 10 wk, the Protein Group lost 7.53 +/- 1.44 kg and the CHO Group lost 6.96 +/- 1.36 kg. Plasma amino acids, glucose and insulin were determined after a 12-h fast and 2 h after a 1.67 MJ test meal containing either 39 g CHO, 33 g protein and 13 g fat (Protein Group) or 57 g CHO, 12 g protein and 14 g fat (CHO Group). After 10 wk, subjects in the CHO Group had lower fasting (4.34 +/- 0.10 vs 4.89 +/- 0.11 mmol/L) and postprandial blood glucose (3.77 +/- 0.14 vs. 4.33 +/- 0.15 mmol/L) and an elevated insulin response to meals (207 +/- 21 vs. 75 +/- 18 pmol/L). This study demonstrates that consumption of a diet with increased protein and a reduced CHO/protein ratio stabilizes blood glucose during nonabsorptive periods and reduces the postprandial insulin response.

The role of leucine in weight loss diets and glucose homeostasis

Debate about the optimum balance of macronutrients for adult weight maintenance or weight loss continues to expand. Often this debate centers on the relative merits or risks of carbohydrates vs. fats; however, there is increasing interest in the optimal level of dietary protein for weight loss. Diets with a reduced ratio of carbohydrates/protein are reported to be beneficial for weight loss, although diet studies appear to lack a fundamental hypothesis to support higher protein intakes. Presently, needs for dietary proteins are established by the recommended daily allowance (RDA) as the minimum level of protein necessary to maintain nitrogen balance. The RDA define the primary use of amino acids as substrates for synthesis of body proteins. There is emerging evidence that additional metabolic roles for some amino acids require plasma and intracellular levels above minimum needs for protein synthesis. The branched-chain amino acid leucine is an example of an amino acid with numerous metabolic roles that function in proportion with cellular concentration. This review provides an overview of the current understanding of metabolic roles of leucine and proposes a metabolic framework to evaluate the merits of a higher protein diet for weight loss.

Selective up-regulation of system a transporter mRNA in diabetic liver

The transport of alanine by system A is an important source of carbons for the synthesis of glucose in the liver. Here, we show that the mRNA encoding the ubiquitously expressed isoform of the rat system A transporter (SAT2) is dramatically increased in liver following streptozotocin-induced diabetes. This increase in SAT2 mRNA is intensified in the gluconeogenic periportal hepatocytes and also in hepatocytes surrounding the central vein. SAT3, the more abundant system A mRNA isoform present in liver, is restricted to perivenous hepatocytes and is also increased following this treatment but to a much lesser extent than SAT2 mRNA. SN1, an abundant system N mRNA isoform expressed in both perivenous and periportal hepatocytes, is not affected by streptozotocin treatment. A pharmacological dose of glucagon also increased both SAT2 and SAT3 mRNA levels in liver while SN1 mRNA levels remained unaffected. These results indicate that the increase in system A activity observed in liver following experimentally induced diabetes or glucagon treatment is due to the selective increase in mRNAs encoding system A transporters.

Effects of VIP and forskolin on alanine metabolism in isolated hepatocytes

The effects of vasoactive intestinal polypeptide (VIP) and of forskolin on alanine metabolism in hepatocytes isolated from fed and fasted rats were examined. VIP and 17 microM forskolin stimulated glucose production, gluconeogenesis from alanine, and ureagenesis, and inhibited glyconeogenesis to comparable degrees. However, combination of 17 microM forskolin with a maximal dose of VIP significantly augmented only the inhibition of glyconeogenesis. At 100 microM, forskolin induced metabolic responses comparable to those induced by glucagon, but similarly, in combination with maximal doses of VIP or glucagon, augmented only inhibition of glycogen synthesis. In addition to demonstrating modulation of alanine metabolism by VIP and forskolin, these results raise questions about the nature of the coupling between VIP receptor occupancy and metabolic response.

The effect of maternal hypoaminoacidaemia on placental uptake and transport of amino acids in pregnant sheep

We developed a model of maternal hyperglycaemia with secondary hyperinsulinaemia and hypoaminoacidaemia in pregnant sheep (H) to determine the effect of these conditions on uterine, uteroplacental and fetal amino-acid uptake rates and fetal amino-acid concentrations [AA]. Results were compared with normal pregnant ewes (C). Plasma glucose concentrations were greater in H versus C animals: 7.7+/-0.3 versus 3.9+/-0.1 mmol/l maternal, P< 0.005; 2.6+/-0.1 versus 1.1+/-0.1 mmol/l fetal, P< 0.005. Maternal insulin concentrations [I] were greater in the H group (132+/-30 H versus 31+/-5 C microU/ml, P< 0.005); fetal [I] were not different (15+/-2 H versus 16+/-2 C microU/mL). Maternal [AA] were lower in H than C groups except for SER (P=ns) and GLY (approx twofold higher, P< 0.01). Uterine, uteroplacental and fetal uptake rates of several AA, particularly the branch chain AA, were lower in H than C animals, producing lower total fetal nitrogen uptake rates (270+/-64 mg N/kg fetus/day H, 696+/-75 mg N/kg fetus/day C, P=0.001) and lower fetal plasma concentrations for the branch chain AA. Most fetal [AA], however, remained at control values, which could occur by relative increase in fetal amino-acid production and/or decrease in utilization, but not by increased uteroplacental transport rates.

Mechanisms of increased gluconeogenesis from alanine in rat isolated hepatocytes after endurance training

This work aimed at further investigating the mechanisms by which liver gluconeogenic capacity from alanine is improved after training in rats, with an isolated hepatocyte model. Compared with controls in hepatocytes from trained rats incubated with gluconeogenic precursors (20 mM), the glucogenic flux (J(glucose)) was increased by 64% from alanine (vs. 21% for glycerol, 18% for lactate-pyruvate 10:1, and 10% for dihydroxyacetone). Maximal intracellular alanine accumulation capacity was also increased by 50%. Further experiments conducted on perifused hepatocytes showed that the putative adaptation at the level of the phosphoenolpyruvate-pyruvate cycle, which could be involved in the increased J(glucose) from lactate-pyruvate, was not involved in the increased J(glucose) from alanine after training. For alanine concentration higher than approximately 1 mM, an increased flux through alanine aminotransferase appeared responsible for the increased J(glucose). This could, in turn, depend on an increased supply of cytosolic 2-oxoglutarate because of the higher mitochondrial respiration observed in hepatocytes from trained rats and the activation of the malate-aspartate shuttle. At lower alanine concentration, the increase in J(glucose) appeared to be entirely due to the improved transport capacity.

Metabolism of [3-13C]alanine in liver of mice infected with cysticerci ofTaenia crassiceps

Carbon-13-decoupled proton spin-echo nuclear magnetic resonance (NMR) spectroscopy, with and without13C population inversion, was used to study carbon flow between the host and the parasite in the mouse - Taenia crassiceps system. This NMR analysis revealed that 2 h after intraduodenal injection of [3-13C]alanine, livers from both uninfected mice and those infected with cysticerci of T. crassiceps contained13C label in glycogen, glucose, succinate, glutamate, alanine, and lactate. Livers of infected animals had a lower percentage of13C in alanine, indicating increased utilization of the substrate. In addition, infected mice had a lower concentration of total hepatic glucose and glutamate. The data are consistent with an increased rate of gluconeogenesis in the liver of infected animals. Cysticerci possessed13C label in glucose, acetate, alanine, and lactate. Since these metacestodes are unable to make glucose de novo from pyruvate, labelled glucose found in cysticerci had to be newly synthesized via the host gluconeogenic pathway and then siphoned off by the parasite.

Human kidney and liver gluconeogenesis: evidence for organ substrate selectivity

To assess the contribution of the human kidney to gluconeogenesis (GN) and its role in conversion of glutamine and alanine to glucose, we used a combination of isotopic and organ balance techniques in nine normal postabsorptive volunteers and measured both overall and renal incorporation of these precursors into glucose before and after infusion of epinephrine. In the postabsorptive basal state, renal incorporation of glutamine (27 +/- 2 mumol/min) and alanine (2.1 +/- 0.5 mumol/min) into glucose accounted for 72.8 +/- 3.3 and 3.9 +/- 0.5% of their overall incorporation into glucose (37 +/- 2 and 51 +/- 6 mumol/min, respectively) and 19.0 +/- 3.5 and 1.4 +/- 0.2%, respectively, of overall renal glucose release. Infusion of epinephrine, which increased systemic and renal glucose release more than twofold (P < 0.001), increased overall glutamine and alanine incorporation into glucose (both P < 0.001) and increased renal GN from glutamine (P < 0.001) but not from alanine (P = 0.15). Renal glutamine GN now accounted for 90.3 +/- 4.0% of overall glutamine GN (P = 0.01 vs. basal), whereas renal alanine GN still accounted for only 4.8 +/- 1.7% of overall alanine GN (P = 0.36 vs. basal). With the assumption that kidney and liver are the only gluconeogenic organs in humans, these results indicate that glutamine GN occurs primarily in kidney, whereas alanine GN occurs almost exclusively in liver. Isotopic studies of glutamine and alanine incorporation into plasma glucose may provide a selective, noninvasive method to assess hepatic and renal GN.

Inhibition of glycogenolysis enhances gluconeogenic precursor uptake by the liver of conscious dogs

We investigated the effect of inhibiting glycogenolysis on gluconeogenesis in 18-h-fasted conscious dogs with the use of intragastric administration of BAY R 3401, a glycogen phosphorylase inhibitor. Isotopic ([3-3H]glucose and [U-14C]alanine) and arteriovenous difference methods were used to assess glucose metabolism. Each study consisted of a 100-min equilibration, a 40-min control, and two 90-min test periods. Endogenous insulin and glucagon secretions were inhibited with somatostatin (0.8 microgram.kg-1.min-1), and the two hormones were replaced intraportally (insulin: 0.25 mU.kg-1.min-1; glucagon: 0.6 ng.kg-1.min-1). Drug (10 mg/kg) or placebo was given after the control period. Insulin and glucagon were kept at basal levels in the first test period, after which glucagon infusion was increased to 2.4 ng.kg-1.min-1; BAY R 3401 decreased tracer-determined endogenous glucose production [rate of glucose production (Ra): 14 +/- 1 to 7 +/- 1 mumol.kg-1.min-1] and net hepatic glucose output (11 +/- 1 to 3 +/- 2 mumol.kg-1.min-1) during test 1. It increased the net hepatic uptake of gluconeogenic substrates from 9.0 +/- 2.0 to 11.6 +/- 0.6 mumol.kg-1.min-1. Basal glycogenolysis was decreased by drug (9.1 +/- 0.7 to 1.5 +/- 0.2 mumol glucosyl U.kg-1.min-1). Placebo had no effect on Ra or the uptake of gluconeogenic precursors by the liver. The rise in glucagon increased Ra by 22 +/- 3 and by 8 +/- 2 mumol.kg-1.min-1 (at 10 min) in placebo and drug, respectively. The rise in glucagon caused little change in the net hepatic uptake (mumol.kg-1.min-1) of gluconeogenic substrates in placebo (8.2 +/- 0.6 to 9.0 +/- 1.0) but increased it markedly (11.6 +/- 0.6 to 15.4 +/- 1.0) in drug. Glucagon increased glycogenolysis by 22.1 +/- 2.5 and by 7.8 +/- 1.6 mumol.kg-1.min-1 in placebo and drug, respectively. The amount of glycogen (mumol glucosyl U/kg) synthesized from gluconeogenic carbon was four times higher in drug (48.6 +/- 9.7) than in placebo (11.3 +/- 1.7). We conclude that BAY R 3401 caused a marked reduction in basal and glucagon-stimulated glycogenolysis. As a result of these changes, there was an increase in the net hepatic uptake of gluconeogenic precursors and in glycogen synthesis.

Gluconeogenesis in the livers of diet-restricted rats--a 13C nuclear magnetic resonance study

BACKGROUND

Gluconeogenic activity is reduced during starvation. However, it is less clear whether the utilization of gluconeogenic substrates is diminished with mild but prolonged diet restriction and, if so, whether there are intrinsic changes in the gluconeogenic pathway. We examined gluconeogenesis in the livers of diet-restricted rats with 13C nuclear magnetic resonance (NMR) spectroscopy.

METHODS

Fischer 344 rats were given 88% (DR group) of what was consumed by the weight-matched ad libitum-fed normal rats (CL group). At the end of 5 weeks, the removed livers were perfused with [3-13C] alanine while 13C NMR spectroscopy was performed.

RESULTS

The final body and liver weights were the same for the two groups. In DR rats, both intrahepatic [3-13C] alanine and metabolites generated via pyruvate and oxaloacetate, including aspartate and carbamoyl aspartate, appeared in significantly reduced amounts. There was also marked diminution in the production of glucose.

CONCLUSIONS

In the livers of DR rats, alanine uptake through System A transport, the fluxes through pyruvate carboxylase, the biosynthesis of pyrimidine nucleotides, and the production of glucose from alanine were all significantly decreased with mild intake restriction. Attenuated protein synthesis in the liver of diet-restricted animals may be the cause for this decreased utilization of alanine for gluconeogenesis.

Effect of protein malnutrition on neutral amino acid transport by rat hepatocytes during development

Hepatocytes from suckling rats whose mothers were fed a low-protein diet (9% protein) showed a lower capacity for Na(+)-dependent L-alanine uptake [due to a decrease in maximal uptake rate (Vmax) of a low-affinity component of transport] and were not able to respond to insulin or glucagon, whereas those from suckling pups whose mothers were fed the control diet (17% protein) had already developed the ability to upregulate L-alanine transport after hormone treatment. When animals from low-protein-fed mothers were weaned onto a hypoprotein diet, the overall capacity for Na(+)-dependent L-alanine uptake (apparent Vmax) and its responsiveness to pancreatic hormones were restored. Hepatocytes from these animals showed a lower response to glucocorticoid treatment. Amino acid availability was dramatically decreased in suckling and weanling rats fed a low-protein diet. These results support the hypothesis that nutrient supply is an important factor in the proper development of hepatic transport functions during the suckling-weaning transition.

Up-regulation of liver system A for neutral amino acid transport in euglycemic hyperinsulinemic rats

To determine the role of insulin on the in vivo modulation of liver system A activity, we used the euglycemic hyperinsulinemic clamp coupled to the measurement of solute uptakes into plasma membrane vesicles partially purified from livers of hyperinsulinemic rats and their saline-infused controls. The clamp was performed in chronically catheterized rats, either in the fasted state, 24 h after surgery (Group I), or after 3 days of recovery (Group II). System A activity, measured as the MeAIB-inhibitable L-alanine uptake, was selectively induced by hyperinsulinemia, although the effect was much greater in Group II than in Group I rats (137% vs. 24% over the basal values, respectively). This might be explained by the higher basal levels found in those liver plasma membrane vesicles from Group I fasted animals. Hyperinsulinemia also decreased blood amino acids but to a similar extent in both experimental groups. This suggests that amino acid depletion by itself may not cause up-regulation of system A. Other transport activities involved in neutral amino acid transport (Systems ASC, N and L) were not modified by the clamp. The induction of system A cannot be explained by changes in the dissipation rate of the Na+ transmembrane gradient, because the differences between insulin- and saline-infused rats remained even when the electrochemical Na+ gradient was disrupted in the presence of monensin. Thus, hyperinsulinemia might induce an increase in the number of transporters inserted into the plasma membrane.

Regulatory and molecular aspects of mammalian amino acid transport

Images

L-alanine uptake by rat liver parenchymal and haematopoietic cells during the perinatal period

Alanine disposal by liver parenchymal and haematopoietic cells from 21-day fetuses, newborns and adult rats was studied. Preparations selectively enriched in either haematopoietic cells or hepatocytes were obtained by direct perfusion of fetal- and neonatal-rat livers. L-Alanine transport into liver parenchymal cells was best fitted to two Na(+)-dependent saturable systems. The high-affinity system showed a much higher activity (Vmax.) in hepatocytes from fetuses and newborns than in those from adult rats (2.4, 4.3 and 0.3 nmol/8 min per 10(6) cells for fetuses, newborns and adults respectively). Vmax. for the low-affinity component was slightly lower during the perinatal period than in the adult (about 30 nmol/8 min per 10(6) cells for hepatocytes from fetuses and newborns, versus 48 nmol/8 min per 10(6) cells for adult rat parenchymal cells). Haematopoietic cells from fetal-rat livers showed significant Na(+)-dependent L-alanine uptake which was completely abolished after birth. These results show that the transport systems involved in L-alanine uptake by liver parenchymal cells are fully developed before birth. This probably contributes to fulfilling the high requirement for neutral amino acids for protein synthesis during development. Haematopoietic cells may play an important role in liver amino acid metabolism during fetal life.

The effects of lactate loading on alanine and glucose metabolism in the conscious dog

The effect of lactate per se on alanine and glucose metabolism was studied in five overnight-fasted conscious dogs. Somatostatin was infused to inhibit endogenous pancreatic insulin and glucagon release and the hormones were replaced intraportally at basal rates. Saline (n = 5) or lactate (at 25 and 50 mumol.kg-1.min-1 for 90 minutes each) was infused, and blood samples were taken during the last 30 minutes of each 90-minute period. Insulin, epinephrine, norepinephrine, and cortisol levels remained unchanged during saline or lactate infusion. Glucagon level decreased slightly during lactate (94 +/- 7 to 74 +/- 9 and 79 +/- 8 pg/mL) and saline (91 +/- 8 to 90 +/- 4 and 81 +/- 11 pg/mL) infusions. There were no significant changes in lactate or alanine levels or net hepatic balances with saline infusion. Blood lactate level increased from 657 +/- 74 to 1,718 +/- 126 and 3,300 +/- 321 mumol/L (both P < .05) during the low- and high-lactate infusion periods, respectively. The liver produced lactate during the control (5.57 +/- 2.92 mumol.kg-1 x min-1) and low-lactate infusion (1.75 +/- 2.58 mumol.kg-1 x min-1) periods, but consumed lactate (3.89 +/- 3.31 mumol.kg-1 x min -1; P < .05) during the high-lactate infusion period.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

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

The mechanisms by which hepatic alanine consumption is increased during endotoxemia were investigated to gain further insight into the altered amino acid metabolism which characterizes critical illness. Rats were studied 12 hr after receiving endotoxin (ENDO) or saline. Hepatic alanine delivery was determined in vivo and hepatic alanine content was measured. Hepatocyte transport activity was studied by evaluation of [3H]-alanine accumulation in hepatocyte plasma membrane vesicles (HPMVs). Vesicle integrity was demonstrated by electron microscopy and a 14-fold enrichment in 5'-nucleotidase. Endotoxin treatment resulted in a state of hyperalaninemia and a threefold increase in hepatic alanine delivery (2.79 +/- 0.17 mu mole/100 g body weight/min in controls vs 8.13 +/- 0.98 in ENDO animals; P less than 0.001). Data from HPMVs revealed the presence of a high- and low-affinity component of alanine transport. Endotoxin treatment resulted in a 30% decrease in the Vmax of the high-affinity transport component (3355 +/- 177 pmole/mg protein/10 sec in controls vs 2338 +/- 270 in the ENDO group; P less than 0.05). Concomitant with the observed changes in alanine delivery and transport activity, endotoxin treatment resulted in a 56% rise in hepatic alanine content (2.53 +/- 0.29 mu mole/g liver in controls vs 3.95 +/- 0.23 in ENDO; P less than 0.005). These data indicate that the accelerated hepatic alanine consumption which occurs during endotoxemia is primarily the result of increased hepatic substrate delivery. Despite the resultant repression of transport activity, delivery begins to outdistance the metabolic capacity of the hepatocyte to utilize alanine and intracellular alanine levels rise.

Differences in L-alanine uptake by livers of Wistar and lean Zucker rats

The L-alanine uptake by livers of Wistar and lean Zucker rats has been studied. The hepatic uptake and fractional extraction rates of alanine were estimated in 50-55 day old rats. No significant differences in amino acid concentrations and blood flows in afferent and efferent liver vessels were seen in lean Zucker rats when compared with Wistar rats. However, the hepatic uptake (1.6 +/- 0.1 and 0.7 +/- 0.1 mumol/min/100 g bw, p less than 0.01) and the fractional extraction (26.8 +/- 2.1 and 15.2 +/- 3.1%, p less than 0.05) were much lower in Zucker than in Wistar rats. The hepatic active transport of L-alanine was determined in vitro using isolated plasma membrane vesicles. Vesicles isolated from livers of lean Zucker rats showed similar values of Km (2.5 +/- 0.7 vs. 2.0 +/- 0.5 mM for Wistar and Zucker respectively, N.S.), but lower values of Vmax when compared with Wistar rats (1.1 +/- 0.1 vs 0.6 +/- 0.005 nmol/mg prot 5 s, p less than 0.01, for Wistar and lean Zucker rats respectively). These results indicate that, the liver of lean Zucker rats concentrates alanine less efficiently than the liver of Wistar rats. This fact correlates well with a lower capacity of the Na(+)-dependent L-alanine transport in liver plasma membrane vesicles from lean Zucker rats.

Interactions between propionate and amino acid metabolism in isolated sheep hepatocytes

The purpose of the present study was to evaluate the contribution of various substrates to glucose synthesis in isolated sheep hepatocytes, and more specifically to quantify the contribution of propionate to gluconeogenesis. Liver cells from fed sheep have a very high capacity for propionate utilization and conversion into glucose. The gluogenicity of lactate or amino acids was very low in hepatocytes from fed sheep, but was significantly increased in hepatocytes from starved animals. Amino acids such as alanine or glutamine were characterized by a substantial utilization towards ureogenesis; whereas their conversion to glucose was very low. Propionate utilization and conversion into glucose was inhibited by butyrate, ammonia and especially ethanol (by up to 80%). Ethanol promoted a striking accumulation of intracellular malate in hepatocytes incubated with propionate (reaching 14.9 mumol/g cell) and led to a depletion of phosphoenolpyruvate; ethanol inhibition could be counteracted by pyruvate. Propionate and butyrate enhanced ureogenesis from ammonia in ruminant liver cells but their effects were not additive. Propionate also elicited a marked increase in cellular concentrations of phosphoserine and serine, particularly in the presence of ammonia; such effects could influence phospholipid metabolism in the liver. These findings emphasize the contribution of propionate, compared with the other glucogenic substrates, to glucose synthesis in ruminants and point to the possibilities of modulation of the glucogenicity of propionate by various substrates which may be present in portal blood.

Effect of simultaneous ingestion of L-amino acids and whole protein on plasma amino acid and urea nitrogen concentrations in humans

The effect of whole protein and L-amino acid ingestion on plasma amino acid (PAA) and urea nitrogen (UN) concentrations was investigated. Ten males ingested equivalent amounts of nitrogen as (trial 1) cottage cheese, (trial 2) an L-amino acid mixture, (trial 3) cottage cheese and L-amino acids. Mean changes in total PAA between trials 1 (342 mumol/liter) and 2 (719 mumol/liter) and trials 1 (342 mumol/liter) and 3 (981 mumol/liter) at 30 min and trials 1 (547 mumol/liter) and 3 (143 mumol/liter) at 150 min differed significantly. Mean changes in essential PAA between trials 1 (180 mumol/liter) and 2 (420 mumol/liter) and trials 1 (180 mumol/liter) and 3 (500 mumol/liter) at 30 min differed significantly. Mean changes in essential PAA between trials 1 (247 mumol/liter) and 3 (334 mumol/liter) at 60 min and between trials 1 (252 mumol/liter) and 3 (80 mumol/liter) at 150 min differed significantly. Mean increments in total and essential PAA were higher and peaked faster but decreased more quickly after trials 2 and 3 than after trial 1. Mean changes in plasma UN did not differ between trials. Ingestion of either L-amino acids, whole protein or the mixture of L-amino acids and whole protein was equally effective in increasing total PAA over 4 hr.

L-lactate uptake by rat liver. Effect of food deprivation and substrate availability

We have studied the role of substrate availability on net L-lactate uptake by liver of anaesthetized fed and 24 h-fasted rats. L-Lactate was infused through a mesenteric vein at infusion rates equivalent to 0, 0.125, 0.25 and 0.5 times the basal turnover rate (Rt). By these means we were able to increase L-lactate portal concentrations up to 5.5 mM, without significant changes in portal pH. In the basal state (0 Rt), a net L-lactate uptake by liver was found in 24 h-fasted animals. No net balance was observed in fed rats. Infusion of L-lactate in fed animals failed to induce a net hepatic uptake, except when L-lactate levels in portal vein were raised above 5 mM. In fasted animals, net L-lactate uptake by liver increased linearly (r = 0.99) as a function of L-lactate concentration in the portal vein, even beyond the saturation of its specific carrier. It is concluded that, first, the L-lactate carrier does not limit net L-lactate uptake, and second, that substrate availability is an important factor modulating net L-lactate uptake by liver.

Rat splanchnic net oxygen consumption, energy implications

1. The blood flow, PO2, pH and PCO2 have been estimated in portal and suprahepatic veins as well as in hepatic artery of fed and overnight starved rats given an oral glucose load. From these data the net intestinal, hepatic and splanchnic balances for oxygen and bicarbonate were calculated. The oxygen consumption of the intact animal has also been measured under comparable conditions. 2. The direct utilization of oxygen balances as energy equivalents when establishing the contribution of energy metabolism of liver and intestine to the overall energy expenses of the rat, has been found to be incorrect, since it incorporates the intrinsic error of interorgan proton transfer through bicarbonate. Liver and intestine produced high net bicarbonate balances in all situations tested, implying the elimination (by means of oxidative pathways, i.e. consuming additional oxygen) of high amounts of H+ generated with bicarbonate. The equivalence in energy output of the oxygen balances was then corrected for bicarbonate production to 11-54% lower values. 3. Intestine and liver consume a high proportion of available oxygen, about one-half in basal (fed or starved) conditions and about one-third after gavage, the intestine consumption being about 15% in all situations tested and the liver decreasing its oxygen consumption with gavage.

Fluxes and membrane transport of amino acids in rat liver under different protein diets

The aim of the present work was to evaluate in vivo the role of the transport step in hepatic amino acid metabolism. To vary hepatic utilization of amino acids, rats were adapted to diets containing various concentrations of casein (5, 15, and 60%). In rats fed 5 or 15% casein diets, Gln and Glu were released by the liver, and there was a significant uptake of Ala. Hepatic fluxes of amino acids increased considerably after adaptation to high-casein diet (up to 1.55 mumol.min-1.g liver-1 for Ala), because of the rise in afferent concentrations as well as enhanced uptake percentage (peaking at 60-75% for most glucogenic amino acids). Adaptation to a high-protein diet led to induction of not only system A but also of most of the other transport systems (Gly, anionic, T, y+, and to a lesser extent system N); only systems ASC and L were unchanged. The study of amino acid repartition between liver and plasma with different diets indicates that transport could modulate utilization of Ala, Ser, Thr, Gly, Gln, and Asp. For Arg and Asn, present in very low concentrations in liver under any condition, the transport step should be the major locus of control of their metabolism. For amino acids chiefly transported by nonconcentrative systems, such as aromatic amino acids, cellular metabolism could also be limited by the transport process. In conclusion, during adaptation to a high-protein diet, there is apparently a coordinated adaptation of amino acid transport and of their intracellular metabolism. For some amino acids, induction of catabolic enzymes seems greater than that of transport, so that the transport step may play an important role in control of metabolic fluxes. For example, concentration of amino acids such as Thr may be markedly depressed in rats adapted to a high-protein diet.

Na(+)-dependent transport of alanine and serine by liver plasma-membrane vesicles from rats fed a low-protein or a high-protein diet

Plasma-membrane vesicles prepared from the liver of rats fed either a low-(LP) or a high-protein (HP) diet exhibited Na(+)-dependent active transport of alanine and serine. The process gave apparent kinetic parameters compatible with a single saturable component for both amino acids. Na,K-ATPase (EC 3.6.1.37), marker of the basolateral domain of the hepatocyte plasma-membrane, was chosen as reference for the expression of amino acid transport in vesicle preparations. The high-protein diet induced a significant increase in liver Na,K-ATPase activity also found in corresponding plasma-membrane preparations, in parallel with an increase in the capacity towards amino acid transport. This suggests that in rats fed the high protein diet, transcellular Na+ exchange, although increased, remains well balanced. N-Methylaminoisobutyric acid (MeAIB), due to its poor velocity, proved unsuitable to distinguish between systems A and ASC in the experimental model. Comparing Na(+)- and Li(+)-driven transport, a family of carriers with strict Na(+)-dependency (A-like) was evidenced in LP vesicles but not in HP vesicles. The sensitivity to the lowering of the pH from 7.5 to 6.5 in the external medium was similar in both type of vesicles when Na+ was the driving ion. In the HP vesicles the Li(+)-tolerant, pH-insensitive component (ASC-like) was increased in parallel with overall Na(+)-dependent transport. These functional properties suggest that the carriers involved in the stimulation of transport in HP vesicles are composite in nature. Increasing concentrations of an amino acid mixture mimicking the changes of portal aminoacidemia inhibited the transport of alanine and of serine. The degree of inhibition was correlated with the relative concentration of substrate and was independent of the nutritional treatment.

Epidermal growth factor and cyclic AMP stimulate Na+/H+ exchange in isolated rat hepatocytes

Na+/H+ exchange in acid-loaded isolated hepatocytes was measured using the intracellular pH indicator biscarboxyethyl-carboxyfluorescein (BCECF) to follow intracellular pH (pHi). The rate of amiloride-sensitive Na(+)-dependent recovery from cytoplasmic-acid-loading was found to be increased in cells treated with epidermal growth factor (EGF), 8-(4-chlorophenylthio)adenosine 3',5'-monophosphate (ClPhScAMP) or phorbol 12-myristate 13-acetate (PMA). These three agents increased the rate of Na+/H+ exchange to similar extents and their effects were not additive. The stimulation was shown in all three cases to be due an alkaline shift of 0.1 in the set point pH of the Na+/H+ exchanger. Experiments measuring the uptake of 22Na+ into acid-loaded primary hepatocyte monolayer cultures confirmed these results. EGF, ClPhScAMP and PMA significantly increased the amiloride-inhibitable accumulation of 22Na+, thus providing further evidence that Na+/H+ exchange is stimulated by these effectors.

Effects of L-alanine on membrane potential, potassium (86Rb) permeability and cell volume in hepatocytes from Raja erinacea

Isolated hepatocytes from the elasmobranch Raja erinacea were examined for their regulatory responses to a solute load following electrogenic uptake of L-alanine. The transmembrane potential (Vm) was measured with glass microelectrodes filled with 0.5 M KCl (75 to 208 M omega in elasmobranch Ringer's solution) and averaged -61 +/- 16 mV (S.D.; n = 68). L-Alanine decreased (depolarized) Vm by 7 +/- 3 and 18 +/- 2 mV at concentrations of 1 and 10 mM, respectively. Vm did not repolarize to control values during the 5-10 min impalements, unless the amino acid was washed away from the hepatocytes. The depolarizing effect of L-alanine was dependent on external Na+, and was specific for the L-isomer of alanine, as D- and beta-alanine had no effect. Hepatocyte Vm also depolarized on addition of KCN or ouabain, or when external K+ was increased. Rates of 86Rb+ uptake and efflux were measured to assess the effects of L-alanine on Na+/K+-ATPase activity and K+ permeability, respectively. Greater than 80% of the 86Rb+ uptake was inhibited by 2 mM ouabain, or by substitution of choline+ for Na+ in the incubation media. L-Alanine (10 mM) increased 86Rb+ uptake by 18-49%, consistent with an increase in Na+/K+ pump activity, but had no effect on rubidium efflux. L-Alanine, at concentrations up to 20 mM, also had no measurable effect on cell volume as determined by 3H2O and [14C]inulin distribution. These results indicate that Na+-coupled uptake of L-alanine by skate hepatocytes is rheogenic, as previously observed in other cell systems. However, in contrast to mammalian hepatocytes, Vm does not repolarize for at least 10 min after the administration of L-alanine, and changes in cell volume and potassium permeability are also not observed.

Significance of the alanine aminotransferase reaction in the formation of alpha-ketoglutarate in rat liver mitochondria

The total production of alpha-ketoglutarate from glutamate and isocitrate was estimated in isolated rat liver mitochondria. Mitochondrial alanine aminotransferase converts glutamate to alpha-ketoglutarate [A.K. Groen et al. (1982) Eur. J. Biochem. 122, 87-93], thus participating in the net formation of the tricarboxylic acid cycle intermediates from glutamate. The present investigation indicates a significant contribution of the alanine aminotransferase reaction to glutamate oxidation by isolated rat liver mitochondria in the presence of bicarbonate. It amounted to 41-74 and 7-31% of the total utilization of glutamate in States 4 and 3, respectively, in various conditions in vitro, at pyruvate concentrations in the range of 0.1-10 mM. The participation of glutamate in the total production of alpha-ketoglutarate at physiological concentrations of glutamate, citrate, and isocitrate varied in the range of 72-82%. It was calculated that alpha-ketoglutarate formation by the reaction of alanine aminotransferase amounted to 30 and 5% of the total mitochondrial alpha-ketoglutarate production in States 4 and 3, respectively, at physiological concentrations of its precursors and in the presence of 0.5 mM malate and 0.1 mM pyruvate. It constituted 77-97% of the net production of the tricarboxylic acid cycle intermediates from glutamate in rat liver mitochondria. The importance of alpha-ketoglutarate production via the alanine aminotransferase reaction under various physiological conditions is discussed.

Hepatic uptake of amino acids in late-pregnant rats. Effect of food deprivation

Hepatic availability, uptake and fractional extraction of amino acids were estimated in anaesthetized 21-day-pregnant and age-matched virgin rats, either fed or after 24 h starvation. Amino acid availability was unaltered in fed pregnant rats as compared with fed virgin controls. However, the hepatic uptake of these compounds was higher in the former than in the latter. These adaptations were mediated by an increase in the hepatic capability to take up amino acids in late-pregnant rats, as reflected by the changes found for the fractional extraction rates. The decrease in amino acid availability found after starvation was more pronounced in pregnant than in virgin rats. Nevertheless, the hepatic uptake was similar in both groups. These results indicate that amino acids are not limiting for ureagenesis during late pregnancy, strongly suggesting that the mechanism(s) which modulate urea synthesis may be intracellular in origin.

Hepatic uptake of amino acids at mid-lactation in the rat

Hepatic availability and uptake of amino acids were measured in fed virgin and 15-day-lactating rats. Lactation did not induce any change in total amino acid availability (expressed per 100 g body wt.). Virgin rats showed a nil hepatic balance, and lactation induced a high net uptake. The high drainage of amino acids by mammary gland does not affect hepatic availability.

Alanine uptake by liver at midpregnancy in rats

The participation of the liver to the increase in alanine utilization seen at midpregnancy was studied in 9- and 12-day pregnant rats. Liver fractional extraction of alanine was assessed in vivo from the changes in concentration in afferent and efferent vessels. Hepatic active transport of alanine was determined in vitro using isolated plasma-membrane vesicles. Compared with nonpregnant controls, alanine fractional extraction was significantly increased on day 12 but not on day 9 of pregnancy. Vesicles isolated from 9- and 12-day pregnant animals had a greater capacity for Na+-dependent transport than those from controls. Eadie-Hofstee plotting showed that this increase was due to an increase in Vmax with no change in Km. Both A and ASC systems contributed to the Vmax increase. These results indicate that, although by day 9 the liver has developed an increased capacity for alanine uptake, the actual extraction is seen only by day 12 of pregnancy. At this stage the liver participates actively in the turnover of alanine and the development of hypoalaninemia.

Short-term stimulation of Na+-dependent amino acid transport by dibutyryl cyclic AMP in hepatocytes. Characteristics and partial mechanism

The short-term protein-synthesis-independent stimulation of alanine transport in hepatocytes was further investigated. Cyclic AMP increased the Vmax. of alanine transport. Amino acid transport via systems A, ASC and N was stimulated. A good correlation was found between the initial rate of transport and the cell membrane potential as calculated from the distribution of Cl-. Cyclic AMP increased the rate of alanine transport, stimulated Na+/K+ ATPase (Na+/K+-transporting ATPase) activity and caused membrane hyperpolarization. The time courses and cyclic AMP dose-dependencies of all three effects were similar. Ouabain abolished the effect of cyclic AMP on Cl- distribution and on transport of alanine. The effect of cyclic AMP on alanine transport and Cl- distribution was mimicked by the antibiotic nigericin; the effect of nigericin was also abolished by ouabain. It is concluded that the effect of cyclic AMP on transport is mediated via membrane hyperpolarization. It is suggested that the primary action of cyclic AMP is to increase the activity of an electroneutral Na+/K+-exchange system in the liver cell plasma membrane, thus hyperpolarizing the membrane by stimulating the electrogenic Na+/K+ ATPase.

Role of the availability of substrates on hepatic and renal gluconeogenesis in the fasted late pregnant rat

Studies were conducted to examine the role of gluconeogenetic substrate availability on glucose production in the fasted late pregnant rat. Virgin and 21-day pregnant rats were studied after 24 hours' food deprivation. Pregnant animals showed decreased circulating glucose and gluconeogenic amino acid and increased plasma glycerol concentration. Glucose formation was studied in vivo two, five, and ten minutes after the intravenous administration of two concentrations of 14C-alanine, 14C-pyruvate, or 14C-glycerol. Concentrations of 0.2 mmols of 14C-glycerol or 14C-pyruvate, but not of 14C-alanine, enhanced 14C-glucose production in pregnant rats, whereas 1 mmol of any of the three 14C-substrates always enhanced 14C-glucose production in these rats. Both 1 mmol/L and 5 mmol/L 14C-alanine increased 14C-glucose formation in 90-minute-incubated liver slices of fasted pregnant rats, in spite of decreased cytosolic activity of alanine aminotransferase. The three substrates enhanced "in vitro" renal gluconeogenesis in pregnant rats. Under all experimental conditions studied, labeled glycerol was converted more efficiently into glucose than equivalent amounts of any other substrate used, and this difference was greater in pregnant, than in virgin animals. Results indicate that, in spite of enhanced gluconeogenetic activity, maternal glucose production in the fasted state at late gestation is limited by the deficiency of certain substrates, such as amino acids. It is proposed that glycerol derived from enhanced maternal adipose tissue lipolysis constitutes a preferential gluconeogenetic substrate in comparison with others, such as alanine, that are more efficiently transferred through the placenta to the fetus.

Control by amino acids of the activity of system A-mediated amino acid transport in isolated rat hepatocytes

The effect of amino acids, in concentrations corresponding to those found in the portal vein of rats given a high-protein diet, was investigated on the activity of system A amino acid transport in hepatocytes from fed rats. Amino acids counteracted the induction of system A by insulin or glucagon. This effect was observed at all concentrations of hormones tested, up to 1 microM. Amino acids did not affect the basal cyclic AMP concentration in hepatocytes, or the large rise in cyclic AMP elicited by glucagon. The reversal of system-A induction was observed at relatively low concentration of amino acids, corresponding to plasma values reported in rats given a basal diet. Amino acids were separately tested: substrates of system A were particularly efficient, but so were glutamine and histidine. Non-metabolizable substrates of system A, such as 2-aminoisobutyrate, were also inhibitory, suggesting that a part of the effect of amino acids is independent of their cellular metabolism. Provision of additional energy substrates such as lactate and oleate did not affect induction of system A or the inhibitory effects of amino acids. Thus amino acids do not act by serving as an energy source and by maintaining the integrity of hepatocytes. Inhibition of mRNA synthesis by actinomycin practically abolished the effect of amino acids on the induction of system A by glucagon. The results suggest that amino acids may promote the synthesis of protein(s) affecting the activity of system A either directly at the carrier unit or at an intermediate stage of its emergence.

Propionate transport in rat liver cells

Propionate extraction by liver is generally in the range of 95%, which could depend on a transport process across the cell membrane. The study reports conditions in which [14C]propionate uptake can be measured with minimal interferences from metabolism. Propionate uptake by isolated hepatocytes was mediated by two components: a low-affinity component of limited physiological interest and a high-affinity (apparent Km about 0.15 mM) component. This last component displayed a high capacity but was not Na+-dependent nor concentrative. Propionate transport was not markedly affected by acetate, butyrate or other C3 glucogenic compounds; it was inhibited by halogenated monocarboxylates, monochloroacetate and 2-chloropropionate being the most potent. Classical inhibitors of anion transport and of functional-SH groups were ineffective. Propionate uptake was responsive to external pH: stimulated by acidic and depressed by alkaline pH. Hepatic uptake of propionate in vivo was practically quantitative up to 0.8-1.0 mM in afferent plasma, in keeping with the measured capacity of the high-affinity component. It is suggested that propionate uptake is essentially carrier mediated but this process should not be rate limiting for hepatic utilization in physiological conditions.

Quantitative analysis of intermediary metabolism in hepatocytes incubated in the presence and absence of glucagon with a substrate mixture containing glucose, ribose, fructose, alanine and acetate

Hepatocytes were isolated from the livers of fed rats and incubated, in the presence and absence of 100 nM-glucagon, with a substrate mixture containing glucose (10 mM), fructose (4 mM), alanine (3.5 mM), acetate (1.25 mM), and ribose (1 mM). In any given incubation one substrate was labelled with 14C. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, glutamate, lipid glycerol and fatty acids was measured after 20 and 40 min of incubation under quasi-steady-state conditions [Borowitz, Stein & Blum (1977) J. Biol. Chem. 252, 1589-1605]. These data and the measured O2 consumption were analysed with the aid of a structural metabolic model incorporating all reactions of the glycolytic, gluconeogenic, and pentose phosphate pathways, and associated mitochondrial and cytosolic reactions. A considerable excess of experimental measurements over independent flux parameters and a number of independent measurements of changes in metabolite concentrations allowed for a stringent test of the model. A satisfactory fit to the data was obtained for each condition. Significant findings included: control cells were glycogenic and glucagon-treated cells glycogenolytic during the second interval; an ordered (last in, first out) model of glycogen degradation [Devos & Hers (1979) Eur. J. Biochem. 99, 161-167] was required in order to fit the experimental data; the pentose shunt contributed approx. 15% of the carbon for gluconeogenesis in both control and glucagon-treated cells; net flux through the lower Embden-Meyerhof pathway was in the glycolytic direction except during the 20-40 min interval in glucagon-treated cells; the increased gluconeogenesis in response to glucagon was correlated with a decreased pyruvate kinase flux and lactate output; fluxes through pyruvate kinase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase were not coordinately controlled; Krebs cycle activity did not change with glucagon treatment; flux through the malic enzyme was towards pyruvate formation except for control cells during interval II; and 'futile' cycling at each of the five substrate cycles examined (including a previously undescribed cycle at acetate/acetyl-CoA) consumed about 26% of cellular ATP production in control hepatocytes and 21% in glucagon-treated cells.