Hepatic glucose disposition during concomitant portal glucose and amino acid infusions in the dog

Metabolite Exchange between Mammalian Organs Quantified in Pigs

Mammalian organs continually exchange metabolites via circulation, but systems-level analysis of this shuttling process is lacking. Here, we compared, in fasted pigs, metabolite concentrations in arterial blood versus draining venous blood from 11 organs. Greater than 90% of metabolites showed arterial-venous differences across at least one organ. Surprisingly, the liver and kidneys released not only glucose but also amino acids, both of which were consumed primarily by the intestine and pancreas. The liver and kidneys exhibited additional unexpected activities: liver preferentially burned unsaturated over more atherogenic saturated fatty acids, whereas the kidneys were unique in burning circulating citrate and net oxidizing lactate to pyruvate, thereby contributing to circulating redox homeostasis. Furthermore, we observed more than 700 other cases of tissue-specific metabolite production or consumption, such as release of nucleotides by the spleen and TCA intermediates by pancreas. These data constitute a high-value resource, providing a quantitative atlas of inter-organ metabolite exchange.

Regulation of hepatic glucose metabolism in health and disease

The liver is crucial for the maintenance of normal glucose homeostasis - it produces glucose during fasting and stores glucose postprandially. However, these hepatic processes are dysregulated in type 1 and type 2 diabetes mellitus, and this imbalance contributes to hyperglycaemia in the fasted and postprandial states. Net hepatic glucose production is the summation of glucose fluxes from gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis and other pathways. In this Review, we discuss the in vivo regulation of these hepatic glucose fluxes. In particular, we highlight the importance of indirect (extrahepatic) control of hepatic gluconeogenesis and direct (hepatic) control of hepatic glycogen metabolism. We also propose a mechanism for the progression of subclinical hepatic insulin resistance to overt fasting hyperglycaemia in type 2 diabetes mellitus. Insights into the control of hepatic gluconeogenesis by metformin and insulin and into the role of lipid-induced hepatic insulin resistance in modifying gluconeogenic and net hepatic glycogen synthetic flux are also discussed. Finally, we consider the therapeutic potential of strategies that target hepatosteatosis, hyperglucagonaemia and adipose lipolysis.

Portal glucose delivery stimulates muscle but not liver protein metabolism

Portal vein glucose delivery (the portal glucose signal) stimulates glucose uptake and glycogen storage by the liver, whereas portal amino acid (AA) delivery (the portal AA signal) induces an increase in protein synthesis by the liver. During a meal, both signals coexist and may interact. In this study, we compared the protein synthesis rates in the liver and muscle in response to portal or peripheral glucose infusion during intraportal infusion of a complete AA mixture. Dogs were surgically prepared with hepatic sampling catheters and flow probes. After a 42-h fast, they underwent a 3-h hyperinsulinemic (4× basal) hyperglucagonemic (3× basal) hyperglycemic (≈160 mg/dl) hyperaminoacidemic (hepatic load 1.5× basal; delivered intraportally) clamp (postprandial conditions). Glucose was infused either via a peripheral (PeG; n = 7) or the portal vein (PoG; n = 8). Protein synthesis was assessed with a primed, continuous [(14)C]leucine infusion. Net hepatic glucose uptake was stimulated by portal glucose infusion (+1 mg·kg(-1)·min(-1), P < 0.05) as expected, but hepatic fractional AA extraction and hepatic protein synthesis did not differ between groups. There was a lower arterial AA concentration in the PoG group (-19%, P < 0.05) and a significant stimulation (+30%) of muscle protein synthesis associated with increased expression of LAT1 and ASCT2 AA transporters and p70S6 phosphorylation. Concomitant portal glucose and AA delivery enhances skeletal muscle protein synthesis compared with peripheral glucose and portal AA delivery. These data suggest that enteral nutrition support may have an advantage over parenteral nutrition in stimulating muscle protein synthesis.

Regulation of hepatic glucose uptake and storage in vivo

In the postprandial state, the liver takes up and stores glucose to minimize the fluctuation of glycemia. Elevated insulin concentrations, an increase in the load of glucose reaching the liver, and the oral/enteral/portal vein route of glucose delivery (compared with the peripheral intravenous route) are factors that increase the rate of net hepatic glucose uptake (NHGU). The entry of glucose into the portal vein stimulates a portal glucose signal that not only enhances NHGU but concomitantly reduces muscle glucose uptake to ensure appropriate partitioning of a glucose load. This coordinated regulation of glucose uptake is likely neurally mediated, at least in part, because it is not observed after total hepatic denervation. Moreover, there is evidence that both the sympathetic and the nitrergic innervation of the liver exert a tonic repression of NHGU that is relieved under feeding conditions. Further, the energy sensor 5'AMP-activated protein kinase appears to be involved in regulation of NHGU and glycogen storage. Consumption of a high-fat and high-fructose diet impairs NHGU and glycogen storage in association with a reduction in glucokinase protein and activity. An understanding of the impact of nutrients themselves and the route of nutrient delivery on liver carbohydrate metabolism is fundamental to the development of therapies for impaired postprandial glucoregulation.

A high-fat, high-fructose diet accelerates nutrient absorption and impairs net hepatic glucose uptake in response to a mixed meal in partially pancreatectomized dogs

The aim of this study was to elucidate the impact of a high-fat, high-fructose diet (HFFD; fat, 52%; fructose, 17%), in the presence of a partial (~65%) pancreatectomy (PPx), on the response of the liver and extrahepatic tissues to an orally administered, liquid mixed meal. Adult male dogs were fed either a nonpurified, canine control diet (CTR; fat, 26%; no fructose; n = 5) or a HFFD (n = 5) for 8 wk. Diets were provided in a quantity to maintain neutral or positive energy balance in CTR or HFFD, respectively. Dogs underwent a sham operation or PPx at wk 0, portal and hepatic vein catheterization at wk 6, and a mixed meal test at wk 8. Postprandial glucose concentrations were significantly greater in the HFFD group (14.5 ± 2.0 mmol/L) than in the CTR group (9.2 ± 0.5 mmol/L). Impaired glucose tolerance in HFFD was due in part to accelerated gastric emptying and glucose absorption, as indicated by a more rapid rise in arterial plasma acetaminophen and the rate of glucose output by the gut, respectively, in HFFD than in CTR. It was also attributable to lower net hepatic glucose uptake (NHGU) in the HFFD group (5.5 ± 3.9 μmol · kg(-1) · min(-1)) compared to the CTR group (26.6 ± 7.0 μmol · kg(-1) · min(-1)), resulting in lower hepatic glycogen synthesis (GSYN) in the HFFD group (10.8 ± 5.4 μmol · kg(-1) · min(-1)) than in the CTR group (30.4 ± 7.0 μmol · kg(-1) · min(-1)). HFFD also displayed aberrant suppression of lipolysis by insulin. In conclusion, HFFD feeding accelerates gastric emptying and diminishes NHGU and GSYN, thereby impairing glucose tolerance following a mixed meal challenge. These data reveal a constellation of deleterious metabolic consequences associated with consumption of a HFFD for 8 wk.

Portal infusion of amino acids is more efficient than peripheral infusion in stimulating liver protein synthesis at the same hepatic amino acid load in dogs

BACKGROUND

Hepatic glucose uptake is enhanced by the portal delivery of glucose, which creates a negative arterioportal substrate gradient. Hepatic amino acid (AA) utilization may be regulated by the same phenomenon, but this has not been proven.

OBJECTIVE

We aimed to assess hepatic AA balance and protein synthesis with or without a negative arterioportal AA gradient.

DESIGN

Somatostatin was infused intravenously, and insulin and glucagon were replaced intraportally at 4- and 3-fold basal rates, respectively, in 3 groups (n = 9 each) of conscious dogs with catheters for hepatic balance measurement. Arterial glucose concentrations were clamped at 9 mmol/L. An AA mixture was infused intravenously to maintain basal concentrations (EuAA), intraportally to mimic the postmeal AA increase (PoAA), or intravenously (PeAA) to match the hepatic AA load in PoAA. Protein synthesis was assessed with a primed, continuous [(14)C]leucine infusion.

RESULTS

Net hepatic glucose uptake in the PoAA condition was < or =50% of that in the EuAA and PeAA conditions (P < 0.05). In the PoAA and PeAA conditions, hepatic intracellular leucine concentrations were 2- to 2.5-fold those in the EuAA condition (P < 0.05); net hepatic leucine uptake and [(14)C]leucine utilization were approximately 2-fold greater (P < 0.05) and albumin synthesis was 30% greater (P < 0.05) in the PoAA condition than in the EuAA and PeAA conditions. Phosphorylation of ribosomal protein S6 [downstream of the mammalian target of Rapamycin complex 1 (mTORC1)] was significantly higher in the PoAA, but not PeAA, condition than in the EuAA condition.

CONCLUSIONS

Portal, but not peripheral, AA delivery significantly enhanced hepatic protein synthesis under conditions in which AAs, glucose, insulin, and glucagon did not differ at the liver, an effect apparently mediated by mTORC1 signaling.

Hepatic adaptations to sucrose and fructose

The liver is an important site of postprandial glucose disposal, accounting for the removal of up to 30% of an oral glucose load. The liver is also centrally involved in dietary lipid and amino acid uptake, and the presence of either or both of these nutrients can influence hepatic glucose uptake. The composition of ingested carbohydrate also influences hepatic glucose metabolism. For example, fructose can increase hepatic glucose uptake. In addition, fructose extraction by the liver is exceedingly high, approaching 50% to 70% of fructose delivery. The selective hepatic metabolism of fructose, and the ability of fructose to increase hepatic glucose uptake can, under appropriate conditions (eg, diets enriched in sucrose or fructose, high fructose concentrations), provoke major adaptations in hepatic metabolism. Potential adaptations that can arise in response to these conditions and putative mechanisms driving these adaptations are the subject of this review.

Interaction of a selective serotonin reuptake inhibitor with insulin in the control of hepatic glucose uptake in conscious dogs

Whether hyperinsulinemia is required for stimulation of net hepatic glucose uptake (NHGU) by a selective serotonin reuptake inhibitor (SSRI) was examined in four groups of conscious 42-h-fasted dogs, using arteriovenous difference and tracer ([3-3H]glucose) techniques. Experiments consisted of equilibration (-120 to -30 min), basal (-30 to 0 min), and experimental periods (Exp; 0-240 min). During Exp, somatostatin, intraportal insulin [at basal (Ins groups) or 4-fold basal rates (INS groups)], basal intraportal glucagon, and peripheral glucose (to double hepatic glucose load) were infused. In the Fluv-Ins (n = 7) and Fluv-INS groups (n = 6), saline was infused intraportally from 0 to 90 min (P1), and fluvoxamine was infused intraportally at 2 microg x kg(-1) x min(-1) from 90 to 240 min (P2). Sal-Ins (n = 9) and Sal-INS (n = 8) received intraportal saline in P1 and P2. NHGU during P2 was 8.4 +/- 1.4 and 6.9 +/- 2.3 micromol x kg(-1) x min(-1) in Sal-Ins and Fluv-Ins, respectively (not significant), and 13.3 +/- 2.2 and 20.9 +/- 3.1 micromol x kg(-1) x min(-1) (P < 0.05) in Sal-INS and Fluv-INS. Unidirectional (tracer-determined) hepatic glucose uptake was twofold greater (P < 0.05) in Fluv-INS than Sal-INS. Net hepatic carbon retention during P2 was significantly greater in Fluv-INS than Sal-INS (18.5 +/- 2.7 vs. 12.2 +/- 1.9 micromol x kg(-1) x min(-1)). Nonhepatic glucose uptake was reduced in Fluv-INS vs. Sal-INS (20.0 +/- 1.3 vs. 38.4 +/- 5.4 micromol x kg(-1) x min(-1), P < 0.05). Intraportal fluvoxamine enhanced NHGU and net hepatic carbon retention in the presence of hyperinsulinemia but not euinsulinemia, suggesting that hepatocyte-targeted SSRIs may reduce postprandial hyperglycemia.

Time course of the hepatic adaptation to TPN: interaction with glycogen depletion

In response to chronic (5 days) TPN, the liver becomes a major site of glucose disposal, removing approximately 45% (4.5 mg.kg(-1).min(-1)) of exogenous glucose. Moreover, approximately 70% of glucose is not stored but released as lactate. We aimed to determine in chronically catheterized conscious dogs the time course of adaptation to TPN and the glycogen depletion impact on early time course. After an 18-h (n = 5) fast, TPN was infused into the inferior vena cava for 8 (n = 5) or 24 h (n = 6). A third group, of 42-h-fasted animals (n = 6), was infused with TPN for 8 h. TPN was infused at a rate designed to match the dog's calculated basal energy and nitrogen requirements. NHGU (-2.3 +/- 0.1 to 2.2 +/- 0.7 to 3.9 +/- 0.6 vs. -1.7 +/- 0.3 to 1.1 +/- 0.5 to 2.9 +/- 0.4 mg.kg(-1).min(-1), basal to 4 to 8 h, 18 vs. 42 h) and net hepatic lactate release (0.7 +/- 0.3 to 0.6 +/- 0.1 to 1.4 +/- 0.2 vs. -0.6 +/- 0.1 to 0.1 +/- 0.1 to 0.8 +/- 0.1 mg.kg(-1).min(-1), basal to 4 to 8 h) increased progressively. Net hepatic glycogen repletion and tracer determined that glycogen syntheses were similar. After 24 h of TPN, NHGU (5.4 +/- 0.6 mg.kg(-1).min(-1)) and net hepatic lactate release (2.6 +/- 0.4 mg.kg(-1).min(-1)) increased further. In summary, 1) most hepatic adaptation to TPN occurs within 24 h after initiation of TPN, and 2) prior glycogen depletion does not augment hepatic adaptation rate.

Nonesterified fatty acids and hepatic glucose metabolism in the conscious dog

We used tracer and arteriovenous difference techniques in conscious dogs to determine the effect of nonesterified fatty acids (NEFAs) on net hepatic glucose uptake (NHGU). The protocol included equilibration ([3-(3)H]glucose), basal, and two experimental periods (-120 to -30, -30 to 0, 0-120 [period 1], and 120-240 min [period 2], respectively). During periods 1 and 2, somatostatin, basal intraportal insulin and glucagon, portal glucose (21.3 micromol.kg(-1).min(-1)), peripheral glucose (to double the hepatic glucose load), and peripheral nicotinic acid (1.5 mg.kg(-1).min(-1)) were infused. During period 2, saline (nicotinic acid [NA], n = 7), lipid emulsion (NA plus lipid emulsion [NAL], n = 8), or glycerol (NA plus glycerol [NAG], n = 3) was infused peripherally. During period 2, the NA and NAL groups differed (P < 0.05) in rates of NHGU (10.5 +/- 2.08 and 4.7 +/- 1.9 micromol.g(-1).min(-1)), respectively, endogenous glucose R(a) (2.3 +/- 1.4 and 10.6 +/- 1.0 micromol.kg(-1).min(-1)), net hepatic NEFA uptakes (0.1 +/- 0.1 and 1.8 +/- 0.2 micromol.kg(-1).min(-1)), net hepatic beta-hydroxybutyrate output (0.1 +/- 0.0 and 0.4 +/- 0.1 micromol.kg(-1).min(-1)), and net hepatic lactate output (6.5 +/- 1.7 vs. -2.3 +/- 1.2 micromol.kg(-1).min(-1)). Hepatic glucose uptake and release were 2.6 micro mol. kg(-1). min(-1) less and 3.5 micro mol. kg(-1). min(-1) greater, respectively, in the NAL than NA group (NS). The NAG group did not differ significantly from the NA group in any of the parameters listed above. In the presence of hyperglycemia and relative insulin deficiency, elevated NEFAs reduce NHGU by stimulating hepatic glucose release and suppressing hepatic glucose uptake.

Direct and indirect effects of amino acids on hepatic glucose metabolism in humans

AIM/HYPOTHESIS

The study was designed to examine the contribution of direct (substrate-mediated) and indirect (hormone-mediated) effects of amino acids on hepatic glucose metabolism in healthy men.

METHODS

The protocols were: (i) CON+S (n=7): control conditions with somatostatin to inhibit endogenous hormone release resulting in fasting plasma concentrations of amino acids, insulin (approximately 28 pmol/l) and glucagon (approximately 65 ng/l), (ii) AA+S ( n=7): amino acid infusion-fasting insulinaemia-fasting glucagonaemia, (iii) GLUC+S ( n=6): fasting amino acids-fasting insulinaemia-hyperglucagonaemia (approximately 99 ng/l) and (iv) AA-S (n=5): amino acid infusion without somatostatin resulting in amino acid-induced hyperinsulinaemia (approximately 61 pmol/l)-hyperglucagonaemia (approximately 147 ng/l). Net glycogenolysis was calculated from liver glycogen concentrations using (13)C nuclear magnetic resonance spectroscopy. Total gluconeogenesis (GNG) was calculated by subtracting net glycogenolysis from endogenous glucose production (EGP) which was measured with [6,6-(2)H(2)]glucose. Net GNG was assessed with the (2)H(2)O method.

RESULTS

During AA+S and GLUC+S, plasma glucose increased by about 50% (p<0.01) due to a comparable rise in EGP. This was associated with a 53-% (p<0.05) and a 65% increase (p<0.01) of total and net GNG during AA+S, whereas net glycogenolysis rose by 70% (p<0.001) during GLUC+S. During AA-S, plasma glucose remained unchanged despite nearly-doubled (p<0.01) total GNG.

CONCLUSION/INTERPRETATION

Conditions of postprandial amino acid elevation stimulate secretion of insulin and glucagon without affecting glycaemia despite markedly increased gluconeogenesis. Impaired insulin secretion unmasks the direct gluconeogenic effect of amino acids and increases plasma glucose.

Nonhepatic response to portal glucose delivery in conscious dogs

The glycemic and hormonal responses and net hepatic and nonhepatic glucose uptakes were quantified in conscious 42-h-fasted dogs during a 180-min infusion of glucose at 10 mg. kg(-1). min(-1) via a peripheral (Pe10, n = 5) or the portal (Po10, n = 6) vein. Arterial plasma insulin concentrations were not different during the glucose infusion in Pe10 and Po10 (37 +/- 6 and 43 +/- 12 microU/ml, respectively), and glucagon concentrations declined similarly throughout the two studies. Arterial blood glucose concentrations during glucose infusion were not different between groups (125 +/- 13 and 120 +/- 6 mg/dl in Pe10 and Po10, respectively). Portal glucose delivery made the hepatic glucose load significantly greater (36 +/- 3 vs. 46 +/- 5 mg. kg(-1). min(-1) in Pe10 vs. Po10, respectively, P < 0.05). Net hepatic glucose uptake (NHGU; 1.1 +/- 0. 4 vs. 3.1 +/- 0.4 mg. kg(-1). min(-1)) and fractional extraction (0. 03 +/- 0.01 vs. 0.07 +/- 0.01) were smaller (P < 0.05) in Pe10 than in Po10. Nonhepatic (primarily muscle) glucose uptake was correspondingly increased in Pe10 compared with Po10 (8.9 +/- 0.4 vs. 6.9 +/- 0.4 mg. kg(-1). min(-1), P < 0.05). Approximately one-half of the difference in NHGU between groups could be accounted for by the difference in hepatic glucose load, with the remainder attributable to the effect of the portal signal itself. Even in the absence of somatostatin and fixed hormone concentrations, the portal signal acts to alter partitioning of a glucose load among the tissues, stimulating NHGU and reducing peripheral glucose uptake.