Metabolic Fate Of Glucose Research Articles

AMPK activation is sufficient to increase skeletal muscle glucose uptake and glycogen synthesis but is not required for contraction-mediated increases in glucose metabolism

The AMP-activated protein kinase (AMPK) is a cellular sensor of energetics and when activated in skeletal muscle during contraction can impart changes in skeletal muscle metabolism. Therapeutics that selectively activate AMPK have been developed to lower glucose levels through increased glucose disposal rates as an approach to abrogate the hyperglycemic state of diabetes; however, the metabolic fate of glucose following AMPK activation remains unclear. We have used a combination of in vivo evaluation of glucose homeostasis and ex vivo skeletal muscle incubation to systematically evaluate metabolism following pharmacological activation of AMPK with PF-739, comparing this with AMPK activation through sustained intermittent electrical stimulation of contraction. These methods to activate AMPK result in increased glucose uptake but divergent metabolism of glucose: pharmacological activation results in increased glycogen accumulation while contraction-induced glucose uptake results in increased lactate formation and glucose oxidation. These results provide additional evidence to support a role for AMPK in control of skeletal muscle metabolism and additional insight into the potential for AMPK stimulation with small molecule direct activators.

The intersection of metabolism and inflammation is governed by the intracellular topology of hexokinases and the metabolic fate of glucose.

Hexokinases (HKs) catalyze the first and irreversible step of glucose metabolism. Its product, glucose-6-phosphate (G-6P) serves as a precursor for catabolic processes like glycolysis for adenosine 5'-triphosphate (ATP) production and anabolic pathways including the pentose phosphate pathway (PPP) for the generation of intermediaries like nicotinamide adenine dinucleotide phosphate (NADPH) and ribulose-5-P. Thus, the cellular fate of glucose is important not only for growth and maintenance, but also to determine different cellular activities. Studies in immune cells have demonstrated an intimate linkage between metabolic pathways and inflammation, however the precise molecular mechanisms that determine the cellular fate of glucose during inflammation or aging are not completely understood. Here we discuss a study by De Jesus et al that describes the role of HK1 cytosolic localization as a critical regulator of glucose flux by shunting glucose into the PPP at the expense of glycolysis, exacerbating the inflammatory response of macrophages. The authors convincingly demonstrate a novel mechanism that is independent of its mitochondrial functions, but involve the association to a protein complex that inhibits glycolysis at the level of glyceraldehyde 3-phosphate dehydrogenase. We expand the discussion by comparing previous studies related to the HK2 isoform and how cells have evolved to regulate the mitochondrial association of these two isoforms by non-redundant mechanism.

Exposure of Pancreatic β-Cells to Excess Glucose Results in Bimodal Activation of mTORC1 and mTOR-Dependent Metabolic Acceleration.

SummaryChronic exposure of pancreatic β-cells to excess glucose can lead to metabolic acceleration and loss of stimulus-secretion coupling. Here, we examined how exposure to excess glucose (defined here as concentrations above 5 mM) affects mTORC1 signaling and the metabolism of β-cells. Acute exposure to excess glucose stimulated glycolysis-dependent mTORC1 signaling, without changes in the PI3K or AMPK pathways. Prolonged exposure to excess glucose led to hyperactivation of mTORC1 and metabolic acceleration, characterized by higher basal respiration and maximal respiratory capacity, increased energy demand, and enhanced flux through mitochondrial pyruvate metabolism. Inhibition of pyruvate transport to the mitochondria decelerated the metabolism of β-cells chronically exposed to excess glucose and re-established glucose-dependent mTORC1 signaling, disrupting a positive feedback loop for mTORC1 hyperactivation. mTOR inhibition had positive and negative impacts on various metabolic pathways and insulin secretion, demonstrating a role for mTOR signaling in the long-term metabolic adaptation of β-cells to excess glucose.

Lactate production is a prioritized feature of adipocyte metabolism

Adipose tissue is essential for whole-body glucose homeostasis, with a primary role in lipid storage. It has been previously observed that lactate production is also an important metabolic feature of adipocytes, but its relationship to adipose and whole-body glucose disposal remains unclear. Therefore, using a combination of metabolic labeling techniques, here we closely examined lactate production of cultured and primary mammalian adipocytes. Insulin treatment increased glucose uptake and conversion to lactate, with the latter responding more to insulin than did other metabolic fates of glucose. However, lactate production did not just serve as a mechanism to dispose of excess glucose, because we also observed that lactate production in adipocytes did not solely depend on glucose availability and even occurred independently of glucose metabolism. This suggests that lactate production is prioritized in adipocytes. Furthermore, knocking down lactate dehydrogenase specifically in the fat body of Drosophila flies lowered circulating lactate and improved whole-body glucose disposal. These results emphasize that lactate production is an additional metabolic role of adipose tissue beyond lipid storage and release.

Abstract 369: The retinoblastoma protein regulates glucose metabolism in lung cancer

Abstract Lung cancer is among the most frequently diagnosed cancers and is the leading cause of cancer-related deaths in the United States. Of its subtypes, non-small cell lung cancer (NSCLC) accounts for 85% of all lung cancer diagnoses. The retinoblastoma protein (Rb) is a tumor suppressor which functions to regulate cell cycle progression by altering the transcriptional activity of the E2F family of transcription factors. In most NSCLC cases, Rb inactivation occurs via hyperphosphorylation due to loss of upstream regulatory mechanisms including overexpression of cyclin D1 or loss of p16INK4A. Although the canonical fucntions of Rb have been well established, advances in our understanding of Rb have highlighted additional cellular processes regulated by this tumor suppressor in both normal and transformed cells. Increased glucose utilization has become a hallmark of transformed cells and is driven in part by specific genetic alterations in various oncogenes or tumor suppressors. In preliminary studies, we demonstrate a role for Rb in regulating glucose metabolism in both in vitro and in vivo models of lung cancer. Using A549 lung adenocarcinoma cells, which harbor a p16 deletion that allows CDK4/6 inactivation of Rb, we performed in vitro metabolic assays, including glucose uptake and glycolysis as well as measured the expression of key enzymes within glycolysis and downstream metabolic pathways. Each assay was performed following treatment with vehicle control or Palbociclib, a CDK4/6 inhibitor which inhibits the phosphorylation and subsequent inactivation of Rb. Following treatment with Palbociclib, A549 cells show decreased glycolysis and expression of glycolytic genes, without altering glucose uptake. To define the metabolic fates of glucose in vitro, we performed stable isotope resolved metabolomics (SIRM) analysis of 13C-glucose in A549 cells. Consistent with in vitro metabolic assays and gene expression data, SIRM analysis revealed treatment with Palbociclib decreases glucose-derived carbons into glycolytic intermediates and other metabolic pathways. To determine if Rb1 loss results in an altered metabolic phenotype in an in vivo model of lung cancer, we have crossed Rb1lox/lox mice with Kras+/LSLG12D mice to generate Kras+/LSLG12D/Rb1lox/lox and Kras+/LSLG12D/Rb1+/+ mice. Rb1 KO mice exhibit increased tumor burden as well as decreased overall survival compared to Rb1 WT mice. Additionally, SIRM analysis of 13C-glucose incorporation into lung tumors of Kras+/LSLG12D/Rb1+/+ and Kras+/LSLG12D/Rb1lox/lox mice revealed increased glycolysis in lung tumors of Rb1 KO mice compared to Rb1 WT mice. Lastly, immunohistochemistry was performed to determine the metabolic transporter/enzyme expression profile in these tumors. We have observed increased levels of glycolytic genes in Rb1 KO lung tumors compared to Rb1 WT. Taken together, these data indicate that Rb may regulate the metabolic reprogramming of lung cancer. Citation Format: Lindsey Reynolds, Susan Dougherty, Traci Kruer, Brian Clem. The retinoblastoma protein regulates glucose metabolism in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 369.

Metabolic fate of glucose in the brain of APP/PS1 transgenic mice at 10months of age: a 13C NMR metabolomic study.

Alzheimer's disease (AD) has been associated with the disturbance of brain glucose metabolism. The present study investigates brain glucose metabolism using 13C NMR metabolomics in combination with intravenous [1-13C]-glucose infusion in APP/PS1 transgenic mouse model of amyloid pathology at 10months of age. We found that brain glucose was significantly accumulated in APP/PS1 mice relative to wild-type (WT) mice. Reductions in 13C fluxes into the specific carbon sites of tricarboxylic acid (TCA) intermediate (succinate) as well as neurotransmitters (glutamate, glutamine, γ-aminobutyric acid and aspartate) from [1-13C]-glucose were also detected in the brain of APP/PS1 mice. In addition, our results reveal that the 13C-enrichments of the C3 of alanine were significantly lower and the C3 of lactate have a tendency to be lower in the brain of APP/PS1 mice than WT mice. Taken together, the development of amyloid pathology could cause a reduction in glucose utilization and further result in decreases in energy and neurotransmitter metabolism as well as the lactate-alanine shuttle in the brain.

Metabolic fate of glucose and candidate signaling and excess-fuel detoxification pathways in pancreatic β-cells

Glucose metabolism promotes insulin secretion in β-cells via metabolic coupling factors that are incompletely defined. Moreover, chronically elevated glucose causes β-cell dysfunction, but little is known about how cells handle excess fuels to avoid toxicity. Here we sought to determine which among the candidate pathways and coupling factors best correlates with glucose-stimulated insulin secretion (GSIS), define the fate of glucose in the β-cell, and identify pathways possibly involved in excess-fuel detoxification. We exposed isolated rat islets for 1 h to increasing glucose concentrations and measured various pathways and metabolites. Glucose oxidation, oxygen consumption, and ATP production correlated well with GSIS and saturated at 16 mm glucose. However, glucose utilization, glycerol release, triglyceride and glycogen contents, free fatty acid (FFA) content and release, and cholesterol and cholesterol esters increased linearly up to 25 mm glucose. Besides being oxidized, glucose was mainly metabolized via glycerol production and release and lipid synthesis (particularly FFA, triglycerides, and cholesterol), whereas glycogen production was comparatively low. Using targeted metabolomics in INS-1(832/13) cells, we found that several metabolites correlated well with GSIS, in particular some Krebs cycle intermediates, malonyl-CoA, and lower ADP levels. Glucose dose-dependently increased the dihydroxyacetone phosphate/glycerol 3-phosphate ratio in INS-1(832/13) cells, indicating a more oxidized state of NAD in the cytosol upon glucose stimulation. Overall, the data support a role for accelerated oxidative mitochondrial metabolism, anaplerosis, and malonyl-CoA/lipid signaling in β-cell metabolic signaling and suggest that a decrease in ADP levels is important in GSIS. The results also suggest that excess-fuel detoxification pathways in β-cells possibly comprise glycerol and FFA formation and release extracellularly and the diversion of glucose carbons to triglycerides and cholesterol esters.

Metabolic fate of glucose in rats with traumatic brain injury and pyruvate or glucose treatments: A NMR spectroscopy study

Administration of sodium pyruvate (SP; 9.08 μmol/kg, i.p.), ethyl pyruvate (EP; 0.34 μmol/kg, i.p.) or glucose (GLC; 11.1 μmol/kg, i.p.) to rats after unilateral controlled cortical impact (CCI) injury has been reported to reduce neuronal loss and improve cerebral metabolism. In the present study these doses of each fuel or 8% saline (SAL; 5.47 nmoles/kg) were administered immediately and at 1, 3, 6 and 23 h post-CCI. At 24 h all CCI groups and non-treated Sham injury controls were infused with [1,2 13C] glucose for 68 min 13C nuclear magnetic resonance (NMR) spectra were obtained from cortex + hippocampus tissues from left (injured) and right (contralateral) hemispheres. All three fuels increased lactate labeling to a similar degree in the injured hemisphere. The amount of lactate labeled via the pentose phosphate and pyruvate recycling (PPP + PR) pathway increased in CCI-SAL and was not improved by SP, EP, and GLC treatments. Oxidative metabolism, as assessed by glutamate labeling, was reduced in CCI-SAL animals. The greatest improvement in oxidative metabolism was observed in animals treated with SP and fewer improvements after EP or GLC treatments. Compared to SAL, all three fuels restored glutamate and glutamine labeling via pyruvate carboxylase (PC), suggesting improved astrocyte metabolism following fuel treatment. Only SP treatments restored the amount of [4 13C] glutamate labeled by the PPP + PR pathway to sham levels. Milder injury effects in the contralateral hemisphere appear normalized by either SP or EP treatments, as increases in the total pool of 13C lactate and labeling of lactate in glycolysis, or decreases in the ratio of PC/PDH labeling of glutamine, were found only for CCI-SAL and CCI-GLC groups compared to Sham. The doses of SP, EP and GLC examined in this study all enhanced lactate labeling and restored astrocyte-specific PC activity but differentially affected neuronal metabolism after CCI injury. The restoration of astrocyte metabolism by all three fuel treatments may partially underlie their abilities to improve cerebral glucose utilization and to reduce neuronal loss following CCI injury.

Increasing intravenous infusions of glucose improve body condition but not lactation performance in midlactation dairy cows

The present study was intended to test whether intravenously applied glucose would elicit dose effects on lactation performance similar to those observed after gastrointestinal glucose application. Six midlactation cows received intravenous glucose infusions (GI), increasing by 1.25% of the calculated net energy for lactation (NEL) requirement per day, whereas control cows received volume-equivalent saline infusions (SI). Measurements and samples were taken at surplus glucose dose levels of 0, 10, 20, and 30% of the NEL requirement, respectively. Body weight and backfat thickness increased linearly with increasing glucose dose for cows on GI compared with SI. No differences were observed in daily feed intake, milk energy output, and energy-corrected milk yield between treatments. However, milk protein percentage and yield increased linearly with the dose of glucose infused in the GI group. Although milk lactose was not affected by treatment during the infusion period, milk lactose percentage and yield decreased for GI, but not SI, once infusions ceased. Based on 5 diurnal blood samples, daily mean and maximum concentrations of plasma glucose and serum insulin showed linear increases with increasing GI, whereas their daily minimum concentrations were unaffected. At GI of 30% of the NEL requirement, marked hyperglycemia and hyperinsulinemia were observed at 1600h (i.e., 1h postprandially), coinciding with glucosuria. The revised quantitative insulin-sensitivity check index indicated linear development of insulin resistance for the GI treatment but no such change in SI cows. Glucose infusion decreased daily mean and maximum serum β-hydroxybutyrate and daily minimum nonesterified fatty acid concentrations relative to SI, whereas serum urea nitrogen was only numerically decreased by GI. No changes were observed in the serum activities of γ-glutamyl transferase and aspartate transaminase and in the serum concentrations of bilirubin and macrominerals. However, serum phosphorus concentration increased after withdrawal of GI, but not SI. Only in GI cows did glycogen content increase or tend to increase linearly in the liver and skeletal muscle. In conclusion, midlactation dairy cows on an energy-balanced diet directed intravenously infused glucose predominantly to body fat reserves rather than increasing lactation performance. This may suggest that the metabolic fate of glucose is modified by metabolic signals, hormonal signals, or both from the portal-drained viscera when absorbed from the intestine.

Elucidation of the metabolic fate of glucose in Trichoderma ressei using EST analysis and cDNA microarray: a step towards metabolic engineering of the regulatory networks for anaerobic metabolism for glucose

Glycolysis, the tricarboxylic acid cyle (TCA) and the electron transport chain are part of the metabolic systems of the cell required for the production of energy in the form of ATP from glucose and are common to all eukaryotic microorganisms. Different regulatory networks have developed during evolution in different microorganisms to control these networks to produce ATP by different pathways. For example, in the presence of high concentration of glucose the yeast Saccharomyces cerevisiae produces energy by anaerobic metabolism accumulating ethanol as the end product of the process. The cellulolytic fungus Trichoderma reesei, however, produces energy from glucose by aerobic metabolism. We have recently addressed this difference using EST analysis and cDNA microarrays. We show that in T. reesei expression of the networks of genes encoding the enzymes of glycolysis, the TCA cycle and the proteins of the electron transport chain is programmed in a way that favours the oxidation of pyruvate via the TCA cycle rather than its reduction to acetaldehyde and then ethanol by fermentation. In addition, our results indicate that the metabolic step required to regenerate NAD+ is regulated differently in S. cerevisiae and T. reesei, in Trichoderma preventing the regeneration of this pivotal product that is required for anaerobic metabolism. Thus, in T. reesei, in contrast to S. cerevisiae, aerobic metabolism prevails in the presence of high levels of glucose. To turn this preferentially respiratory microorganism into a fermenter, we are replacing the promoters of critical genes whose products control the direction of flow of the primary metabolites toward respiratory metabolism, with tightly glucose-repressible promoter. We expect that these promoter-engineering will channel the metabolites toward anaerobic metabolism of glucose and turn this preferentially respiratory microorganism into a fermenter-a step that will be required if T. reesei is to be used for the efficient production of ethanol fuel from cellulose biomass. from 2009 American University in Cairo Research Conference Cairo, Egypt. 5 April 2009

Caloric intake and liver dysfunction in critically ill patients

Despite increasing evidence that critically ill patients have lower energy requirements than expected, most guidelines continue to recommend elevated caloric requirements in these patients, particularly in septic patients. This practice leads to liver dysfunction when artificial nutrition is employed and worsens the prognosis of these patients. This review is focused on recent developments in the pathogenesis of artificial nutrition associated liver dysfunction in critically ill patients. The liver plays a pivotal role in managing nutritional substrates, and it is involved in the inflammatory response to injury and sepsis. The landmark phenomenon is insulin resistance and changes in the metabolic fates of glucose and fat. Glucose and lipids can act as toxics synergistically with inflammation to induce liver dysfunction. There are experimental evidences that insulin resistance in critically ill patients can share the same biochemical mechanisms and metabolic fates involved in insulin resistance of type 2 diabetes mellitus and metabolic syndrome. Furthermore, steatosis is also a common feature in both clinical pictures The pathogenesis of artificial nutrition associated with liver dysfunction is related to overfeeding and sepsis with a pathophysiology, similar to metabolic syndrome and type 2 diabetes. Changing nutritional strategies and adding new drugs will prevent, in part, liver dysfunction in these patients.

Changes in mammary uptake and metabolic fate of glucose with once-daily milking and feed restriction in dairy cows

The aim of this review is to better understand the regulation of milk yield in response to once-daily milking and feed restriction. Glucose is the principal precursor for the synthesis of lactose (a major osmotic agent in milk), and participates in determining the milk volume produced. When applying these two breeding factors, reductions in milk yield are associated with a reduction in milk lactose yield and in the arterial flow of glucose, due to a decrease in the mammary blood flow. The ability of the udder to extract glucose is altered with once-daily milking but not necessarily with feed restriction. Lactose synthesis is down-regulated in response to once-daily milking and feed restriction but the percentage of the extracted glucose which is converted into lactose is differently affected in response to treatments. No marked change is observed with once daily milking whereas this would be increased with feed restriction and in contrast, depressed with fasting.

Metabolic Fate of Glucose on 3T3-L1 Adipocytes Treated with Grape Seed-Derived Procyanidin Extract (GSPE). Comparison with the Effects of Insulin

In this paper we investigate the effects of a grape seed procyanidin extract (GSPE) on the metabolic fate of glucose in adipocytes. Differentiated 3T3-L1 cells were treated with 140 mg/L GSPE or 100 nM insulin for a short period (1 h, acute treatment) or for a long period (15 h, chronic treatment). 2-Deoxy-[1-(3)H]glucose uptake and [1-(14)C]glucose incorporation into cells, glycogen, and lipid were measured. We found that GSPE mimicked the anabolic effects of insulin but there were several important differences. GSPE stimulated glycogen synthesis less than insulin. After chronic exposure, GSPE induced a higher incorporation of glucose into lipid, mainly due to the increase in glucose directed to glycerol synthesis. Our main conclusions, therefore, are that GSPE has insulinomimetic properties and activates glycogen and lipid synthesis. However, the differences between the effects of GSPE and the effects of insulin indicate that GSPE uses mechanisms complementary to those of insulin signaling pathways to bring about these effects.

Effect of Dietary Fish Oil on Insulin Sensitivity and Metabolic Fate of Glucose in the Skeletal Muscle of Normal Rats

The aim of this work was to study the effect of the administration of cod liver oil on the non-oxidative and oxidative fate of glucose metabolism in the skeletal muscle of normal rats. To achieve this goal, the gastrocnemius was examined regarding glucose oxidation, glycogen synthase activity and glycogen storage both at baseline and during euglycemic hyperinsulinemic clamping. The results show that dietary fish oil decreases plasma insulin levels without alteration in glucose homeostasis (at baseline). In addition, the observed enhancement in whole body glucose utilization during clamping suggests an increased peripheral insulin sensitivity. Furthermore, under insulin-stimulated glucose disposal, an enhancement in the glycolytic pathway (increased levels of muscle glucose-6-phosphate and plasma lactate) rather than changes in the oxidation (pyruvate dehydrogenase complex) and storage components of glucose metabolism was observed in the skeletal muscle of rats fed dietary fish oil. These results coupled with the hypolipidemic effects of fish oil may have implications for the prevention and/or management of some pathological states manifested by insulin resistance with or without dyslipidemia.

Changes in major components of tea fungus metabolites during prolonged fermentation

Changes in major components and microbes in tea fungus broth (or kombucha; teakwass) prepared from nine different sources during a prolonged fermentation of up to 60 days were investigated. Cell concentrations of both yeasts and acetic acid bacteria in broth were generally higher than those in the cellulosic pellicles. The residual sucrose concentration decreased linearly with time, although the rate fell after the first month. Metabolic fates of glucose and fructose produced as a result of the hydrolysis of sucrose were different. Glucose was not produced in parallel with fructose (0.085 g 100 ml(-1) d(-1)) but was produced with a lower initial rate (0.041 g 100 ml(-1) d(-1)). Both titratable acidity and gluconic acid increased steadily with time for all samples, although gluconic acid was not generated for 6 days until the fermentation had begun. Acetic acid increased slowly to a maximum value of 1.1 g 100 ml(-1) after 30 days; thereafter, it decreased gradually. Gluconic acid contributed to the titratable acidity and thus, the taste of tea fungus broth, during the final stage of fermentation. It is concluded that the desired quality or composition of kombucha can be obtained through the proper control of fermentation time.

The effect of insulin on the uptake and metabolic fate of glucose in isolated perfused hearts of dyslipemic rats

Male Wistar rats chronically (15 weeks) fed a sucrose-rich diet (SRD; 63% w/w) developed hypertriglyceridemia and impaired glucose homeostasis. Hearts from these animals were isolated and perfused using the Langendorff recirculating method. Glucose at levels similar to those found in the animal in vivo was used as the only exogenous substrate. The hearts were perfused for 30 minutes in the presence or absence of insulin (30 mU/mL) in the perfusion medium. In the absence of the hormone, glucose uptake was impaired and the glucose utilization was reduced, with a significant increase of lactate release. Glucose oxidation, which was estimated from the activation state of the enzyme pyruvate dehydrogenase complex (PDHc), was depressed mainly due to both an increase of PDH kinase and a decrease of PDHa (active form of PDHc) activities. Although the addition of insulin in the perfusion medium improved the above parameters, it was unable to normalize them. The present results suggest that at least two different mechanisms might contribute to insulin resistance and to the impaired glucose metabolism in the perfused hearts of the dyslipemic SRD-fed animals: (1) reduced basal and insulin-stimulated glucose uptake and its utilization or (2) increased availability and oxidation of lipids (low PDHa and high PDH kinase activities), which in turn decrease glucose uptake and utilization. Thus, this nutritional experimental model may be useful to study how impaired glucose homeostasis, increases plasma free fatty acid levels and hypertriglyceridemia could contribute to heart tissue malfunction.

Metabolic Fate of Glucose in Vascular Smooth Muscle During Contraction Induced by Norepinephrine

The metabolism of glucose in porcine carotid artery was tracked by isotopic methods during sustained isometric contraction induced by 100 microM norepinephrine (NE). In resting muscles, 74 and 18% of glucose taken up was accounted for by glycolysis and glycogen synthesis, respectively. Lactate production accounted for 69%, pyruvate production for 12%, and glucose oxidation accounted for 14% of glycolytic flux. The oxidation of glucose accounted for 57% of the consumption of O2 and thus constituted the primary oxidative substrate. During contraction by NE, glucose-uptake declined modestly below the resting basal rate. Glycolysis of external glucose and lactate production decreased and then increased with sustained contraction. Norepinephrine stimulated simultaneous glycogenolysis and glycogen synthesis, with net glycogen synthesis prevailing over 90 min of isometric contraction. Furthermore, NE modified the distribution of glucosyl units throughout the glycogen pool. The steady state rate of oxidation of glucose did not increase during NE contraction, even though O2-consumption increased. In contrast, increased glucose oxidation was demonstrable during contraction induced by 80 mm KCl. Furthermore, oxidation of exogenous fatty acid could be demonstrated during NE-induced contraction. Thus, NE exerts multiple effects on glucose and glycogen metabolism in smooth muscle, but it does not stimulate glucose oxidation.

13C and 31P NMR Studies on the Effects of Increased Plasma Free Fatty Acids on Intramuscular Glucose Metabolism in the Awake Rat

The effects of increased plasma free fatty acids (FFA) on insulin-dependent whole body glucose disposal, skeletal muscle glycolysis, glycogen synthesis, pyruvate versus FFA/ketone oxidation, and glucose 6-phosphate (Glu-6-P) were investigated in the awake rat. A control group (glycerol-infused) and high plasma FFA group (Liposyn-infused) were clamped at euglycemia (approximately 6 mM)-hyperinsulinemia (10 milliunits/kg/min) throughout the experiment (180-240 min). In the initial experiment, 13C NMR was used to observe [1-13C]glucose incorporation into [1-13C]glycogen in the rat hindlimb for glycogen synthesis calculations and into [3-13C]lactate and [3-13C]alanine for glycolytic flux calculations. These experiments were followed by 31P NMR measurements of Glu-6-P changes under identical conditions of the initial experiment. Plasma FFA concentrations were 2.25 +/- 0.36 and 0.20 +/- 0.03 mM in the high plasma FFA and control groups respectively (p < 0.0005). Glucose infusion rates (Ginf) decreased significantly in the Liposyn-infused rats (29.5 +/- 0.7 and 27.2 +/- 1.2 mg/kg/min for control and high plasma FFA group, respectively, at 15 min to 30.7 +/- 2.3 and 17.7 +/- 1.3 mg/kg/min, respectively, at the end of the experiment, p < 0.002). Glycogen synthesis rates were 163 +/- 32 and 104 +/- 17 nmol/g/min, and glycolytic rates were 57.9 +/- 8.0 and 19. 5 +/- 3.6 nmol/g/min (p < 0.002) in the control and high plasma FFA groups, respectively. The relative flux of pyruvate versus free fatty acids and ketones entering the tricarboxylic acid cycle was greater in the control (57 +/- 9%) versus high plasma FFA group (25 +/- 4%) (p < 0.005) as assessed by [4-13C]glutamate/[3-13C]lactate steady state isotopic enrichment measurements. Finally, Glu-6-P concentrations increased by 29.8 +/- 7.0 and 52.8 +/- 12.3% (p < 0. 05) in the control and high plasma FFA groups, respectively, above their basal concentrations by 180 min. In conclusion, we have demonstrated the ability to use in vivo NMR to elucidate the metabolic fate of glucose within skeletal muscle of an awake rat during a euglycemic-hyperinsulinemic clamp and increased levels of plasma FFA. These data suggest that increased concentrations of plasma FFA inhibit insulin-stimulated muscle glucose metabolism in the rat through inhibition of glycolysis.

Metabolic fate of glucose in reversible low-flow ischemia of the isolated working rat heart.

The acute adaptation of myocardial glucose metabolism in response to low-flow ischemia and reperfusion was investigated in isolated working rat hearts perfused with bicarbonate saline containing glucose (10 mM) and insulin (40 microU/ml). Reversible low-flow ischemia was induced by reducing coronary perfusion pressure from 100 to 35 cmH2O. Tritiated glucose was used to assess rates of glucose transport and phosphorylation, flux from glucose to pyruvate, and oxidation of exogenous glucose. Rates of glycogen synthesis and glycolysis were also assessed. With ischemia, cardiac power decreased by more than two-thirds. Rates of glucose uptake and flux from glucose to pyruvate remained unchanged, while glucose oxidation declined by 61%. Rates of lactate release more than doubled, and fractional enrichment of glycogen remained the same. During reperfusion, glucose oxidation returned to the preischemic values. When isoproterenol was added during ischemia, glucose uptake increased, glycogen decreased, and lactate release increased. No effect was seen with pacing. We conclude that during low-flow ischemia and with glucose as the only exogenous substrate, net glucose uptake remains unchanged. There is a reversible redirection between glycolysis and glucose oxidation, while glycogen synthesis continues during ischemia and is enhanced with reperfusion.

Basal plasma insulin levels exert a qualitative but not quantitative effect on glucose-mediated glucose uptake

We assessed the effect of hyperglycemia on glucose uptake in the presence of normal basal insulin levels or somatostatin-induced hypoinsulinemia in seven normal volunteers during a 200-min hyperglycemic clamp (+ 9 mmol/l) carried out with [3-3H]glucose and indirect calorimetry. Hyperglycemia increased glucose uptake to 22.4 +/- 2.6 and 21.3 +/- 1.6 mumol.kg-1.min-1 with and without insulin replacement, respectively. Normoinsulinemia increased glucose oxidation (delta = + 4.5 +/- 0.6 mumol.kg-1.min-1) and nonoxidative glucose metabolism (delta = + 5.2 +/- 1.7 mumol.kg-1.min-1), whereas with insulinopenia, glucose oxidation did not change (delta = -0.3 +/- 0.6 mumol.kg-1.min-1), and nonoxidative glucose metabolism increased (delta = + 48.7 +/- 0.8 mumol.kg-1.min-1). Nonoxidative glucose metabolism was higher during insulinopenic (13.5 +/- 1.8 mumol.kg-1.min-1) than normoinsulinemic hyperglycemia (9.8 +/- 2.7 mumol.kg-1.min-1; P < 0.01). Plasma FFA concentration and lipid oxidation were higher with insulinopenia. Blood lactate and alanine concentrations were greater with normoinsulinemia. 1) hyperglycemia promotes glucose uptake by stimulating both nonoxidative and oxidative glucose disposal; 2) the ability of hyperglycemia to enhance total body glucose uptake is similar with and without normoinsulinemia; 3) although acute insulinopenia does not impair the ability of hyperglycemia to stimulate glucose uptake, it plays a critical role in determining the intracellular metabolic fate of glucose taken up in response to hyperglycemia.