Rate Of Gluconeogenesis Research Articles

Regulation of pyruvate metabolism in metabolic-related diseases

Pyruvate is an obligatory intermediate in the oxidative disposal of glucose and a major precursor for the synthesis of glucose, glycerol, fatty acids, and non-essential amino acids. Stringent control of the fate of pyruvate is critically important for cellular homeostasis. The regulatory mechanisms for its metabolism are therefore of great interest. Recent advances include the findings that (a) the mitochondrial pyruvate carrier is sensitive to inhibition by thiazolidinediones; (b) pyruvate dehydrogenase kinases induce the Warburg effect in many disease states; and (c) pyruvate carboxylase is an important determinate of the rates of gluconeogenesis in humans with type 2 diabetes. These enzymes are potential therapeutic targets for several diseases.

A Noncanonical, GSK3-Independent Pathway Controls Postprandial Hepatic Glycogen Deposition

Insulin rapidly suppresses hepatic glucose production and slowly decreases expression of genes encoding gluconeogenic proteins. In this study, we show that an immediate effect of insulin is to redirect newly synthesized glucose-6-phosphate to glycogen without changing the rate of gluconeogenesis. This process requires hepatic Akt2, as revealed by blunted insulin-mediated suppression of glycogenolysis in the perfused mouse liver, elevated hepatic glucose production during a euglycemic-hyperinsulinemic clamp, or diminished glycogen accumulation during clamp or refeeding in mice without hepatic Akt2. Surprisingly, the absence of Akt2 disrupted glycogen metabolism independent of GSK3α and GSK3β phosphorylation, which is thought to be an essential step in the pathway by which insulin regulates glycogen synthesis through Akt. These data show that (1) the immediate action of insulin to suppress hepatic glucose production functions via an Akt2-dependent redirection of glucose-6-phosphate to glycogen, and (2) insulin increases glucose phosphorylation and conversion to glycogen independent of GSK3.

Effects of transaldolase exchange on estimates of gluconeogenesis in type 2 diabetes

Transaldolase (TA) exchange overestimates gluconeogenesis measured with deuterated water (²H₂O). However, it is unknown whether TA differs in people with type 2 diabetes (T2DM). ²H₂O was ingested, and [1-¹³C]acetate and [3-³H]glucose were infused in T2DM (n = 10) and healthy nondiabetic (ND; n = 8) subjects. TA was assessed from the ratio of ¹³C3 to ¹³C4 glucose enrichment (¹³C3/¹³C4) measured by ¹³C NMR. Glucose turnover was measured before (~16-h fast) and during hyperglycemic (~10 mM) moderate-dose insulin (~0.35 mU·kg⁻¹·min⁻¹) clamp. ¹³C3/¹³C4 in T2DM vs. ND was <1.0 and not different at baseline and clamp, indicating equivalent TA. To determine whether incomplete triose phosphate isomerase (TPI) exchange contributed to asymmetric ¹³C3/¹³C4, [U-¹³C]glycerol was infused in lieu of [1-¹³C]acetate during a separate visit in a subset of ND (n = 7) subjects. Ratio of ¹³C3/¹³C4 obtained following either tracer was <1.0 at baseline and during clamp, indicating that TPI exchange was essentially complete and did not contribute to asymmetric glucose enrichment. Uncorrected and corrected rates of gluconeogenesis were no different (P = not significant) in T2DM vs. ND both at baseline and during clamp. TA correction resulted in equivalent estimates of corrected gluconeogenesis in T2DM and ND that were ~25-35% lower than uncorrected gluconeogenesis both at baseline and during the clamp. The asymmetric enrichment of glucose from ¹³C-gluconeogenic tracers is attributable to TA exchange and can be utilized to correct for TA exchange. In conclusion, TA exchange does not differ between T2DM and ND under fasting or hyperglycemic clamp conditions, and the ²H₂O method continues to provide an accurate estimation of gluconeogenesis.

Melatonin acts through MT1/MT2 receptors to activate hypothalamic Akt and suppress hepatic gluconeogenesis in rats

Melatonin can contribute to glucose homeostasis either by decreasing gluconeogenesis or by counteracting insulin resistance in distinct models of obesity. However, the precise mechanism through which melatonin controls glucose homeostasis is not completely understood. Male Wistar rats were administered an intracerebroventricular (icv) injection of melatonin and one of following: an icv injection of a phosphatidylinositol 3-kinase (PI3K) inhibitor, an icv injection of a melatonin receptor (MT) antagonist, or an intraperitoneal (ip) injection of a muscarinic receptor antagonist. Anesthetized rats were subjected to pyruvate tolerance test to estimate in vivo glucose clearance after pyruvate load and in situ liver perfusion to assess hepatic gluconeogenesis. The hypothalamus was removed to determine Akt phosphorylation. Melatonin injections in the central nervous system suppressed hepatic gluconeogenesis and increased hypothalamic Akt phosphorylation. These effects of melatonin were suppressed either by icv injections of PI3K inhibitors and MT antagonists and by ip injection of a muscarinic receptor antagonist. We conclude that melatonin activates hypothalamus-liver communication that may contribute to circadian adjustments of gluconeogenesis. These data further suggest a physiopathological relationship between the circadian disruptions in metabolism and reduced levels of melatonin found in type 2 diabetes patients.

The insulin-mimetic effect of Morin: A promising molecule in diabetes treatment

BackgroundType-2 diabetes is a worldwidely diffuse disease characterized by insulin resistance that arises from alterations of receptor and/or post-receptor events of insulin signalling. Studies performed with PTP1B-deficent mice demonstrated that PTP1B is the main negative regulator of insulin signalling. Inhibition or down regulation of this enzyme causes enhanced insulin sensitivity. Hence this enzyme represents the most attractive target for development of innovative anti-diabetic drugs. MethodsSelection of new PTP1B inhibitors among an in house library of polyphenolic compounds was carried out screening their activity. The inhibition mechanism of Morin was determined by kinetic analyses. The cellular action of Morin was assayed on HepG2 cells. Analyses of the insulin signalling pathways was carried out by Western blot methods, glycogen synthesis was estimated by measuring the incorporation of [3H]-glucose, gluconeogenesis rate was assayed by measuring the glucose release in the cell medium. Cell growth was estimated by cell count. Docking analysis was conducted with SwissDock program. ResultsWe demonstrated that Morin: i) is a non-competitive inhibitor of PTP1B displaying a Ki in the μM range; ii) increases the phosphorylation of the insulin receptor and Akt; iii) inhibits gluconeogenesis and enhances glycogen synthesis. Morin does not enhance cell growth. ConclusionsWe have identified Morin as a new small molecular non-competitive inhibitor of PTP1B, which behaves as an activator and sensitizer of the insulin receptor stimulating the metabolic pathways only. General significanceOur study suggests that Morin is a useful lead for development of new low Mr compounds potentially active as antidiabetic drugs.

Use of2H2O for estimating rates of gluconeogenesis: determination and correction of error due to transaldolase exchange

The use of deuterated water as a method to measure gluconeogenesis has previously been well validated and is reflective of normal human physiology. However, there has been concern since the method was first introduced that transaldolase exchange may lead to the overestimation of gluconeogenesis. We examined the impact of transaldolase exchange on the estimation of gluconenogenesis using the deuterated water method under a variety of physiological conditions in humans by using the gluconeogenic tracer [U-(13)C]propionate, (2)H(2)O, and (2)H/(13)C nuclear magnetic resonance (NMR) spectroscopy. When [U-(13)C]propionate was used, (13)C labeling inequality occurred between the top and bottom halves of glucose in individuals fasted for 12-24 h who were weight stable (n = 18) or had lost weight via calorie restriction (n = 7), consistent with transaldolase exchange. Similar analysis of glucose standards revealed no significant difference in the total (13)C enrichment between the top and bottom halves of glucose, indicating that the differences detected were biological, not analytical, in origin. This labeling inequality was attenuated by extending the fasting period to 48 h (n = 12) as well as by dietary carbohydrate restriction (n = 7), both conditions associated with decreased glycogenolysis. These findings were consistent with a transaldolase effect; however, the resultant overestimation of gluconeogenesis in the overnight-fasted state was modest (7-12%), leading to an error of 14-24% that was easily correctable by using either a simultaneous (13)C gluconeogenic tracer or a correction nomogram generated from data in the present study.

Elevated NEFA levels impair glucose effectiveness by increasing net hepatic glycogenolysis.

Acute hyperglycaemia rapidly suppresses endogenous glucose production (EGP) in non-diabetic individuals, mainly by inhibiting glycogenolysis. Loss of this 'glucose effectiveness' contributes to fasting hyperglycaemia in type 2 diabetes. Elevated NEFA levels characteristic of type 2 diabetes impair glucose effectiveness, although the mechanism is not fully understood. Therefore we examined the impact of increasing NEFA levels on the ability of hyperglycaemia to regulate pathways of EGP. We performed 4 h 'pancreatic clamp' studies (somatostatin; basal glucagon/growth hormone/insulin) in seven non-diabetic individuals. Glucose fluxes (D-[6,6-(2)H(2)]glucose) and hepatic glycogen concentrations ((13)C magnetic resonance spectroscopy) were quantified under three conditions: euglycaemia, hyperglycaemia and hyperglycaemia with elevated NEFA (HY-NEFA). EGP was suppressed by hyperglycaemia, but not by HY-NEFA. Hepatic glycogen concentration decreased ~14% with prolonged fasting during euglycaemia and increased by ~12% with hyperglycaemia. In contrast, raising NEFA levels in HY-NEFA caused a substantial ~23% reduction in hepatic glycogen concentration. Moreover, rates of gluconeogenesis were decreased with hyperglycaemia, but increased with HY-NEFA. Increased NEFA appear to profoundly blunt the ability of hyperglycaemia to inhibit net glycogenolysis under basal hormonal conditions.

Metabolic and growth response of mink (Neovison vison) kits until 10 weeks of age when exposed to different dietary protein provision

Growth performance and metabolism were investigated in mink kits (n = 210) exposed to the same dietary treatment as their dams (n = 30), i.e. high (HP; 61% of metabolisable energy, ME), medium (MP; 48% of ME) or low (LP; 30% of ME) protein supply, from birth until 10 weeks of age. The kits were weighed weekly, and were measured by means of balance experiment and indirect calorimetry, in weeks eight and nine post-partum (p.p.). At weaning (seven weeks p.p.) and 10 weeks p.p. one kit per litter was killed and blood, liver and kidneys were collected. Plasma amino acid profiles, and hepatic abundance of mRNA for phosphoenolpyruvate carboxykinase (PEPCK), fructose 1,6-biphosphatase, pyruvate kinase and glucose-6-phosphatase (G-6-Pase) by q-PCR, were determined. There were no differences in live weights among kits the first four weeks of life when kits solely consumed milk, but male LP kits were the heaviest. After transition to solid feed MP kits weighed most at nine weeks of age (p < 0.05). At eight weeks of age, the kits fed the LP diet retained less (p < 0.05) N than HP and MP kits. Heat production did not differ among kits, although protein oxidation was higher (p < 0.001) in HP kits than in LP kits. Kits fed the LP diet had lower (p < 0.05) plasma concentrations of lysine, methionine and leucine than MP kits. Dietary treatment was not reflected in the relative abundance of any of the studied mRNAs, but kits had significantly lower abundance of all studied mRNA than their dams, ranging from 83% less PEPCK abundance to 40% less for G-6-Pase. The kidney mass was smallest (p < 0.01) in kits fed the LP diet, and liver masses were largest (p < 0.001) in HP kits. The results indicate that the LP diet did not meet the protein requirements for mink kits in the transition period from milk to solid feed. The capacity to regulate the rate of gluconeogenesis was even more limited in young mink kits than in adult dams. However, young mink kits can regulate protein oxidation in response to dietary protein supply, probably by adapting the size of the liver and kidneys to the level of protein supply.

Effect of ghrelin on glucose regulation in mice

Improvement of glucose metabolism after bariatric surgery appears to be from the composite effect of the alterations in multiple circulating gut hormone concentrations. However, their individual effect on glucose metabolism during different conditions is not clear. The objective of this study was to determine whether ghrelin has an impact on glycogenolysis, gluconeogenesis, and insulin sensitivity (using a mice model). Rate of appearance of glucose, glycogenolysis, and gluconeogenesis were measured in wild-type (WT), ghrelin knockout (ghrelin(-/-)), and growth hormone secretagogue receptor knockout (Ghsr(-/-)) mice in the postabsorptive state. The physiological nature of the fasting condition was ascertained by a short-term fast commenced immediately at the end of the dark cycle. Concentrations of glucose and insulin were measured, and insulin resistance and hepatic insulin sensitivity were calculated. Glucose concentrations were not different among the groups during the food-deprived period. However, plasma insulin concentrations were lower in the ghrelin(-/-) and Ghsr(-/-) than WT mice. The rates of gluconeogenesis, glycogenolysis, and indexes of insulin sensitivity were higher in the ghrelin(-/-) and Ghsr(-/-) than WT mice during the postabsorptive state. Insulin receptor substrate 1 and glucose transporter 2 gene expressions in hepatic tissues of the ghrelin(-/-) and Ghsr(-/-) were higher compared with that in WT mice. This study demonstrates that gluconeogenesis and glycogenolysis are increased and insulin sensitivity is improved by the ablation of the ghrelin or growth hormone secretagogue receptor in mice.

Fasting induces ketoacidosis and hypothermia in PDHK2/PDHK4-double-knockout mice.

The importance of PDHK (pyruvate dehydrogenase kinase) 2 and 4 in regulation of the PDH complex (pyruvate dehydrogenase complex) was assessed in single- and double-knockout mice. PDHK2 deficiency caused higher PDH complex activity and lower blood glucose levels in the fed, but not the fasted, state. PDHK4 deficiency caused similar effects, but only after fasting. Double deficiency intensified these effects in both the fed and fasted states. PDHK2 deficiency had no effect on glucose tolerance, PDHK4 deficiency produced only a modest effect, but double deficiency caused a marked improvement and also induced lower insulin levels and increased insulin sensitivity. In spite of these beneficial effects, the double-knockout mice were more sensitive than wild-type and single-knockout mice to long-term fasting, succumbing to hypoglycaemia, ketoacidosis and hypothermia. Stable isotope flux analysis indicated that hypoglycaemia was due to a reduced rate of gluconeogenesis and that slightly more glucose was converted into ketone bodies in the double-knockout mice. The findings establish that PDHK2 is more important in the fed state, PDHK4 is more important in the fasted state, and survival during long-term fasting depends upon regulation of the PDH complex by both PDHK2 and PDHK4.

Late evening snack: Exploiting a period of anabolic opportunity in cirrhosis

Cirrhosis is a state of accelerated starvation with impaired protein synthesis. Increased rate of gluconeogenesis and alterations in skeletal muscle signaling pathways result in anabolic resistance and consequent loss of muscle mass or sarcopenia in cirrhosis. Late evening snack (LES) is an intervention to reduce the postabsorptive (fasting) phase with the potential to improve substrate utilization and reverse sarcopenia. Published reports were evaluated to examine the effect of LES on regulation of substrate utilization (short-term studies) and nutritional outcomes (long-term studies). PubMed, EMBASE, Google scholar and OVID databases were searched. All studies published on LES in cirrhosis were included. Studies that included few (n < 3) subjects and patients with hepatocellular carcinoma were excluded. Late evening snack decreased lipid oxidation and improved nitrogen balance, irrespective of the composition or type of formulation used. Daytime isocaloric isonitrogenous snacks did not have the metabolic or clinical benefit of LES. LES decreased skeletal muscle proteolysis. No studies have examined its effect on muscle protein synthesis. There was inconsistent translation into an increase in lean body or skeletal muscle mass. Improved quality of life occurs but decreased mortality or need for transplantation has not been reported. The optimal composition of LES has not been defined, but based on mechanistic considerations, a branched chain supplemented LES holds most promise. Late evening snack holds the most promise as an intervention to reverse anabolic resistance and sarcopenia of cirrhosis with improved quality of life in patients with cirrhosis. Long term benefit and improved survival need critical evaluation.

Early undernutrition induces glucagon resistance and insulin hypersensitivity in the liver of suckling rats

Developing brains are vulnerable to nutritional insults. Early undernutrition alters their structure and neurochemistry, inducing long-term pathological effects whose causal pathways are not well defined. During suckling, the brain uses glucose and ketone bodies as substrates. Milk is a high-fat low-carbohydrate diet, and the liver must maintain high rates of gluconeogenesis and ketogenesis to address the needs of these substrates. Insulin and glucagon play major roles in this adaptation: throughout suckling, their blood concentrations are low and high, respectively, and the liver maintains low insulin sensitivity and increased glucagon responsiveness. We propose that disturbances in the endocrine profile and available plasma substrates along with undernutrition-related changes in brain cortex capacity for ketone utilization may cause further alterations in some brain functions. We explored this hypothesis in 10-day-old suckling rats whose mothers were severely food restricted from the 14th day of gestation. We measured the plasma/serum concentrations of glucose, ketone body, insulin and glucagon, and hepatic insulin and glucagon responses. Undernutrition led to hypoglycemia and hyperketonemia to 84% (P < 0.001) and 144% (P < 0.001) of control values, respectively. Liver responsiveness to insulin and glucagon became increased and reduced, respectively; intraperitoneal glucagon reduced liver glycogen by 90% (P < 0.01) in control and by 35% (P < 0.05) in restricted. Cortical enzymes of ketone utilization remained unchanged, but their carrier proteins were altered: monocarboxylate transporter (MCT) 1 increased: 73 ± 14, controls; 169 ± 20, undernourished (P < 0.01; densitometric units); MCT2 decreased: 103 ± 3, controls; 37 ± 4, undernourished (P < 0.001; densitometric units). All of these changes, coinciding with the brain growth spurt, may cause some harmful effects associated with early undernutrition.

Excessive Hepatic Mitochondrial TCA Cycle and Gluconeogenesis in Humans with Nonalcoholic Fatty Liver Disease

Approximately one-third of the U.S. population has nonalcoholic fatty liver disease (NAFLD), a condition closely associated with insulin resistance and increased risk of liver injury. Dysregulated mitochondrial metabolism is central in these disorders, but the manner and degree of dysregulation are disputed. This study tested whether humans with NAFLD have abnormal in vivo hepatic mitochondrial metabolism. Subjects with low (3.0%) and high (17%) intrahepatic triglyceride (IHTG) were studied using (2)H and (13)C tracers to evaluate systemic lipolysis, hepatic glucose production, and mitochondrial pathways (TCA cycle, anaplerosis, and ketogenesis). Individuals with NAFLD had 50% higher rates of lipolysis and 30% higher rates of gluconeogenesis. There was a positive correlation between IHTG content and both mitochondrial oxidative and anaplerotic fluxes. These data indicate that mitochondrial oxidative metabolism is ~2-fold greater in those with NAFLD, providing a potential link between IHTG content, oxidative stress, and liver damage.

NOX2 activated by α1-adrenoceptors modulates hepatic metabolic routes stimulated by β-adrenoceptors

The NADPH oxidase (NOX) family of enzymes oxidase catalyzes the transport of electrons from NADPH to molecular oxygen and generates O2•−, which is rapidly converted into H2O2. We aimed to identify in hepatocytes the protein NOX complex responsible for H2O2 synthesis after α1-adrenoceptor (α1-AR) stimulation, its activation mechanism, and to explore H2O2 as a potential modulator of hepatic metabolic routes, gluconeogenesis, and ureagenesis, stimulated by the ARs. The dormant NOX2 complex present in hepatocyte plasma membrane (HPM) contains gp91phox, p22phox, p40phox, p47phox, p67phox and Rac 1 proteins. In HPM incubated with NADPH and guanosine triphosphate (GTP), α1-AR-mediated H2O2 synthesis required all of these proteins except for p40phox. A functional link between α1-AR and NOX was identified as the Gα13 protein. Alpha1-AR stimulation in hepatocytes promotes Rac1-GTP generation, a necessary step for H2O2 synthesis. Negative cross talk between α1-/β-ARs for H2O2 synthesis was observed in HPM. In addition, negative cross talk of α1-AR via H2O2 to β-AR-mediated stimulation was recorded in hepatocyte gluconeogenesis and ureagenesis, probably involving aquaporine activity. Based on previous work we suggest that H2O2, generated after NOX2 activation by α1-AR lightening in hepatocytes, reacts with cAMP-dependent protein kinase A (PKA) subunits to form an oxidized PKA, insensitive to cAMP activation that prevented any rise in the rate of gluconeogenesis and ureagenesis.

CrbpI Modulates Glucose Homeostasis and Pancreas 9-cis-Retinoic Acid Concentrations

Cellular retinol-binding protein type I (CrbpI), encoded by Rpb1, serves as a chaperone of retinol homeostasis, but its physiological effects remain incompletely understood. We show here that the Rbp1(-/-) mouse has disrupted retinoid homeostasis in multiple tissues, with abnormally high 9-cis-retinoic acid (9cRA), a pancreas autacoid that attenuates glucose-stimulated insulin secretion. The Rbp1(-/-) pancreas has increased retinol and intense ectopic expression of Rpb2 mRNA, which encodes CrbpII: both would contribute to increased β-cell 9cRA biosynthesis. 9cRA in Rbp1(-/-) pancreas resists postprandial and glucose-induced decreases. Rbp1(-/-) mice have defective islet expression of genes involved in glucose sensing and insulin secretion, as well as islet α-cell infiltration, which contribute to reduced glucose-stimulated insulin secretion, high glucagon secretion, an abnormally high rate of gluconeogenesis, and hyperglycemia. A diet rich in vitamin A (as in a standard chow diet) increases pancreas 9cRA and impairs glucose tolerance. Crbp1 attenuates the negative impact of vitamin A (retinol) on glucose tolerance, regardless of the dietary retinol content. Rbp1(-/-) mice have an increased rate of fatty acid oxidation and resist obesity when fed a high-fat diet. Thus, glucose homeostasis and energy metabolism rely on Rbp1 expression and its moderation of pancreas retinol and of the autacoid 9cRA.

Lack of plasma albumin enhances glucose tolerance and insulin secretion

Congenital analbuminemic individuals and rats (NAR) present several metabolic abnormalities, such as hypertriglyceridemia and plasma free fatty acids (FFA) deficiency. We aimed at studying glucose homeostasis and insulin secretion in these dyslipidemic NAR. Measurements of plasma biochemical variables, insulin levels, isolated islet insulin secretion and glucose homeostasis tests were performed in female NAR and age‐matched controls (SDR, Sprague‐Dawley rats). Plasma glucose levels were similar in fed and fasted NAR and SDR. However, insulinemia was higher in fed NAR than in SDR (P ≤ 0.05). NAR showed increased glucose tolerance compared to SDR (P ≤ 0.05). This enhanced glucose tolerance was associated with higher insulinemia after the glucose load, and with similar insulin sensitivity between groups. Compared to SDR, isolated islets from NAR secreted more insulin in response to glucose and leucine. Despite similar liver glycogen content in fully fed condition, NAR had lower glycogen content (40% of control) after 6‐h fasting. The injection of pyruvate (gluconeogenic substrate) elicited a faster rise in glycemia in NAR than in SDR. In conclusion, NAR display enhanced glucose tolerance, insulin secretion, and gluconeogenesis rate. The higher glucose tolerance is attributed to enhanced islet responsiveness to secretagogues, while peripheral insulin sensitivity is not altered.

Blood glucose regulation during fasting in rats under food restriction since birth

The effect of severe food restriction since birth o n regulation of fasting glycemia in male Wistar rat s was investigated. The control group (CG) had free suppl y of chow, while the restriction group (RG) receive d 50% of the amount ingested by the CG. The experiments were don e in adult (60 days) overnight fasted rats in which glycemia, liver free glucose levels and hepatic glycogen conc entration were measured. In part of the experiments in situ liver perfusion was done. The results showed that livers from the RG had higher glycogenolysis rates but low er gluconeogenesis rates from L-alanine (10 mM). Since RG showed maintained glycemia during fasting, it c ould be concluded that livers from RG produced glucose pref erentially from glycogenolysis in detriment of gluc oneogenesis. These findings demonstrated that in spite of severe caloric restriction, the metabolic adaptations of the liver did exist to assure the maintenance of blood glucose fo r brain supply during fasting.

Transaldolase exchange and its effects on measurements of gluconeogenesis in humans

The deuterated water method is used extensively to measure gluconeogenesis in humans. This method assumes negligible exchange of the lower three carbons of fructose 6-phsophate via transaldolase exchange since this exchange will result in enrichment of carbon 5 of glucose in the absence of net gluconeogenesis. The present studies tested this assumption. ²H₂O and acetaminophen were ingested and [1-¹³C]acetate infused in 11 nondiabetic subjects after a 16-h fast. Plasma and urinary glucuronide enrichments were measured using nuclear magnetic resonance spectroscopy before and during a 0.35 mU·kg FFM⁻¹·min⁻¹ insulin infusion. Rates of endogenous glucose production measured with [3-³H]- and [6,6-²H₂]glucose did not differ either before (14.0 ± 0.7 vs. 13.8 ± 0.7 μmol·kg⁻¹·min⁻¹) or during the clamp (10.4 ± 0.9 vs. 10.9 ± 0.7 μmol·kg⁻¹·min⁻¹), consistent with equilibration and quantitative removal of tritium during triose isomerase exchange. Plasma [3-¹³C] glucose-to-[4-¹³C]glucose and urinary [3-¹³C] glucuronide-to-[4-¹³C]glucuronide ratios were <1.0 (P < 0.001) in all subjects both before (0.66 ± 0.04 and 0.60 ± 0.04) and during (059 ± 0.05 and 0.56 ± 0.06) the insulin infusion, respectively, indicating that ∼35-45% of the labeling of the 5th carbon of glucose by deuterium was due to transaldolase exchange rather than gluconeogenesis. When corrected for transaldolase exchange, rates of gluconeogenesis were lower (P < 0.001) and glycogenolysis higher (P < 0.001) than uncorrected rates both before and during the insulin infusion. In conclusion, assuming negligible dilution by glycerol and near-complete triose isomerase equilibration, these data provide strong experimental evidence that transaldolase exchange occurs in humans, resulting in an overestimate of gluconeogenesis and an underestimate of glycogenolysis when measured with the ²H₂O method. Use of appropriate ¹³C tracers provides a means of correcting for transaldolase exchange.

Pathogenesis of Fasting and Postprandial Hyperglycemia in Type 2 Diabetes: Implications for Therapy

The objective of this research is to gain a greater understanding of the cause of fasting and postprandial hyperglycemia in people with type 2 diabetes. Endogenous glucose production is excessive before eating and fails to appropriately suppress after eating in people with type 2 diabetes. This is due in part to impaired insulin-induced suppression of endogenous glucose production, which is observed early in the evolution of type 2 diabetes. Increased rates of gluconeogenesis and perhaps glycogenolysis contribute to hepatic insulin resistance. Insulin-induced stimulation of hepatic glucose uptake and hepatic glycogen synthesis are reduced in people with type 2 diabetes primarily due to decreased uptake of extracellular glucose presumably because of inadequate activation of hepatic glucokinase. Delayed insulin secretion results in higher peak glucose concentrations particularly when suppression of glucagon is impaired, whereas insulin resistance prolongs the duration of hyperglycemia, which can be marked when both hepatic and extra-hepatic insulin resistance are present.The premise of these studies, as well as those performed by many other investigators, is that an understanding of the pathogenesis of type 2 diabetes will enable the development of targeted therapies that are directed toward correcting specific metabolic defects in a given individual. I, as well as many other investigators, believe that such therapies are likely to be more effective and to have a lower risk than would occur if everyone were treated the same regardless of the underlying cause of their hyperglycemia. While we do not yet have sufficient knowledge to truly individualize therapy, in my opinion this approach will be the norm in the not too distant future.

Gluconeogenesis continues in premature infants receiving total parenteral nutrition

ObjectiveTo determine the contribution of total gluconeogenesis to glucose production in preterm infants receiving total parenteral nutrition (TPN) providing glucose exceeding normal infant glucose turnover rates.Study DesignEight infants (0.955±0.066 kg,...