Impact of gain-of-function mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) on glucose and lipid homeostasis

LRP5 loss-of-function mutations have been shown to cause profound osteoporosis and have been associated with impaired insulin sensitivity and dysregulated lipid metabolism. We hypothesized that gain-of-function mutations in LRP5 would also affect these parameters. We therefore studied individuals with LRP5 gain-of-function mutations exhibiting high bone mass (HBM) phenotypes and found that while there was no detected change in insulin sensitivity, there was a significant reduction in serum LDL.

INTRODUCTION

Wnt signaling through LRP5 represents a newly appreciated metabolic pathway, which potentially represents a target for drug discovery in type 2 diabetes and hyperlipidemia. Studies in animal models suggest a physiologic link between LRP5 and glucose and lipid homeostasis; however, whether it plays a similar role in humans is unclear. As current literature links loss-of-function LRP5 to impaired glucose and lipid metabolism, we hypothesized that individuals with an HBM-causing mutation in LRP5 would exhibit improved glucose and lipid homeostasis. Since studies in animal models have suggested that Wnt signaling augments insulin secretion, we also examined the effect of Wnt signaling on glucose-stimulated insulin secretion on human pancreatic islets.

METHODS

This was a matched case-control study. We used several methods to assess glucose and lipid metabolism in 11 individuals with HBM-causing mutations in LRP5. Affected study participants were recruited from previously identified kindreds with HBM-causing LRP5 mutations and included 9 males and 2 females. Two subjects that were being treated with insulin for type 2 diabetes were excluded from our analysis, as this would have obscured our ability to determine the impact of gain-of-function LRP5 mutations on glucose metabolism. The mean age of the evaluated study subjects was 55 ± 7 with a mean BMI of 27.2 ± 2.0. Control subjects were matched and recruited from the general community at an equivalent ratio, with 18 males and 4 females (mean age 56 ± 4; mean BMI 27.2 ± 1.0). Study testing was conducted at an academic medical center.

RESULTS

There were no statistically significant differences between affected and matched control populations for HbA1c (p = 0.06), eAG (p = 0.06), insulin (p = 0.82), HOMA-B (p = 0.34), or HOMA-IR (p = 0.66). The mean Insulin Sensitivity Index (ISI) was also similar between control and affected individuals. Total cholesterol (p = 0.43), triglycerides (TG) (p = 0.56), and HDL (p = 0.32) were not different between the same two groups. In a small subset of studied subjects, intramyocellular and hepatic lipid content were similar in the affected individuals and controls when quantified by proton magnetic resonance spectroscopy (MRS). However, the mean value for serum LDL was significantly lower (p = 0.04) in affected individuals. In primary human islets, there were no differences between control and Wnt treatment groups for insulin secretion measured as area under the curve (AUC) for first phase (p = 0.17) or second phase (p = 0.33) insulin secretion.

CONCLUSIONS

Although our sample size was small, our data do not support the hypothesis that HBM-causing LRP5 mutations, associated with increased Wnt signaling, improve glucose metabolism in humans. However, it does appear that LRP5 variants may affect LDL metabolism, a major risk factor for coronary artery disease. The molecular mechanisms underpinning this effect warrant further study.

High prevalence of diabetes and pre-diabetes in adults with Williams syndrome

A standard oral glucose tolerance test (OGTT) was administered to 28 adults with Williams syndrome (WS). Three quarters of the WS subjects showed abnormal glucose curves, meeting diagnostic criteria for either diabetes or the pre-diabetic state of impaired glucose tolerance. Fasting mean glucose and median insulin levels did not differ significantly in the total WS cohort versus age-gender-BMI matched controls, though the glucose area under the curve was greater in the WS subjects. HbA1c levels were not as reliable as the OGTT in diagnosing the presence of diabetes. Given the high prevalence of impaired glucose regulation, adults with WS should be screened for diabetes, and when present should be treated in accordance with standard medical practice. Hemizygosity for a gene mapping to the Williams syndrome chromosome region (WSCR) is likely the major factor responsible for the high frequency of diabetes in WS. Syntaxin-1A is a prime candidate gene based on its location in the WSCR, its role in insulin release, and the presence of abnormal glucose metabolism in mouse models with aberrantly expressed Stx-1a.

13C/31P NMR studies on the role of glucose transport/phosphorylation in human glycogen supercompensation

This study measured muscle glycogen during a 7-day carbohydrate loading protocol. Twenty healthy subjects (12 male, 8 female) performed 1 hr treadmill/toe-raise exercise immediately before a 3-day low carbohydrate (LoCHO) diet (20 % carbohydrate, 60 % fat, 20 % protein). On day 3 they repeated the exercise and began a 4-day high carbohydrate (HiCHO) diet (90 % carbohydrate, 2 % fat, 8 % protein). The order of administration of the diet was reversed in a subpopulation (n = 3). Interleaved natural abundance 13C/ 31P NMR spectra were obtained before and immediately after exercise, and each day during the controlled diets in order to determine concentrations of glycogen (GLY), glucose-6-phosphate (G6P), and muscle pH. Following exercise, muscle GLY and pH were reduced (p < 0.001) while muscle G6P was elevated (p </= 0.01). During the 3-day LoCHO diet, fasting concentrations of G6P were not different from resting levels. During the first 3 days of the HiCHO diet, resting G6P was elevated (p </= 0.05) compared to before the study. By the final day, muscle glycogen was 1.6-fold greater than before the protocol, while G6P had returned to baseline. Daily GLY and G6P were not affected by the order in which the diet was administered. We conclude that increased glucose transport/phosphorylation plays a role in muscle glycogen supercompensation.

Effects of a novel glucagon receptor antagonist (Bay 27-9955) on glucagon-stimulated glucose production in humans

AIMS/HYPOTHESIS

To study the effects of a specific glucagon receptor antagonist (Bay 27-9955), on plasma glucose concentrations and rates of glucose production in response to hyperglucagonaemia in humans.

METHODS

The study was conducted as a two-dose [Low Dose Bay 27-9955 70 mg, (n = 6), High Dose Bay 27-9955 200 mg, (n = 8)], double blind, placebo controlled, crossover study. Basal glucose production was measured after an overnight fast with [6,6-2H]. At 0 min Bay 27-9955 or placebo was administered and at 120 min an infusion of somatostatin [0.1 microg x (kg x min)(-1)], insulin [24 pmol x (m2 x min)(-1)] and glucagon [3 ng x (kg x min)(-1)] was initiated.

RESULTS

Basal plasma glucose concentrations were about 5 mmol/l and basal rates of glucose production were about 13 micromol x (kg x min)(-1). During the hyperglucagonaemic period, plasma glucagon concentrations doubled to 100 pg/ml, plasma glucose concentration increased by 75 % to a peak of about 10 mmol/l and glucose production doubled to about 23 micromol x (kg x min)(-1) (p < 0.0001 vs basal). In the High Dose Group these effects of glucagon were markedly blunted, plasma glucose concentrations were 7.6 +/- 1.1 mmol/l (p = 0.012 vs placebo) and rates of glucose production increased minimally to 15.3 +/- 1.9 micromol x (kg-min)(-1) (p < 0.0003 vs placebo]. In the Low Dose Group, there was a proportional decrease in the effects of Bay 27-9955 on these parameters.

CONCLUSION/INTERPRETATION

Bay 27-9955 is an effective and safe glucagon antagonist in humans. Given the potentially important role of glucagon in increasing glucose production and gluconeogenesis in patients with Type II (non-insulin-dependent) diabetes mellitus this agent could represent an innovative class of therapeutic agents for the disease.

Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

Stimulating effects of low-dose fructose on insulin-stimulated hepatic glycogen synthesis in humans

Fructose has been shown to have a catalytic effect on glucokinase activity in vitro; however, its effects on hepatic glycogen metabolism in humans is unknown. To address this question, we used (13)C nuclear magnetic resonance (NMR) spectroscopy to noninvasively assess rates of hepatic glycogen synthesis and glycogenolysis under euglycemic (approximately 5 mmol/l) hyperinsulinemic conditions (approximately 400 pmol/l) with and without a low-dose infusion of fructose (approximately 3.5 micromol. kg(-1). min(-1)). Six healthy overnight-fasted subjects were infused for 4 h with somatostatin (0.1 micromol. kg(-1). min(-1)) and insulin (240 pmol. m(-2). min(-1)). During the initial 120 min, [1-(13)C]glucose was infused to assess glycogen synthase flux followed by an approximately 120-min infusion of unlabeled glucose to assess rates of glycogen phosphorylase flux. Acetaminophen was given to assess the percent contribution of the direct and indirect (gluconeogenic) pathways of glycogen synthesis by the (13)C enrichment of plasma UDP-glucuronide and C-1 of glucose. In the control studies, the flux through glycogen synthase and glycogen phosphorylase was 0.31 +/- 0.06 and 0.17 +/- 0.04 mmol/l per min, respectively, and the rate of net hepatic glycogen synthesis was 0.14 +/- 0.05 mmol/l per min. In the fructose studies, the glycogen synthase flux increased 2.5-fold to 0.79 +/- 0.16 mmol/l per min (P = 0.018 vs. control), whereas glycogen phosphorylase flux remained unchanged (0.24 +/- 0.06; P = 0.16 vs. control). The infusion of fructose resulted in a threefold increase in rates of net hepatic glycogen synthesis (0.54 +/- 0.12 mmol/l per min; P = 0.008 vs. control) without affecting the pathways of hepatic glycogen synthesis (direct pathway approximately 60% in both groups). We conclude that during euglycemic hyperinsulinemia, a low-dose fructose infusion causes a threefold increase in net hepatic glycogen synthesis exclusively through stimulation of glycogen synthase flux. Because net hepatic glycogen synthesis has been shown to be diminished in patients with poorly controlled type 1 and type 2 diabetes, stimulation of hepatic glycogen synthesis by this mechanism may be of potential therapeutic value.

Nuclear magnetic resonance studies of hepatic glucose metabolism in humans

Nuclear magnetic resonance (NMR) spectroscopy has made noninvasive and repetitive measurements of human hepatic glycogen concentrations possible. Monitoring of liver glycogen in real-time mode has demonstrated that glycogen concentrations decrease linearly and that net hepatic glycogenolysis contributes only about 50 percent to glucose production during the early period of a fast. Following a mixed meal, hepatic glycogen represents approximately 20 percent of the ingested carbohydrates, while only about 10 percent of an intravenous glucose load is retained by the liver as glycogen. During mixed-meal ingestion, poorly controlled type 1 diabetic patients synthesize only about 30 percent of the glycogen stored in livers of nondiabetic humans studied under similar conditions. Reduced net glycogen synthesis can be improved but not normalized by short-term, intensified insulin treatment. A decreased increment in liver glycogen content following meals was also found in patients with maturity-onset diabetes of the young due to glucokinase mutations (MODY-2). In patients with poorly controlled type 2 diabetes, fasting hyperglycemia can be attributed mainly to increased rates of endogenous glucose production, which was found by 13C NMR to be due to increased rates of gluconeogenesis. Metformin treatment improved fasting hyperglycemia in these patients through a reduction in hepatic glucose production, which could be attributed to a decrease in gluconeogenesis. In conclusion, NMR spectroscopy has provided new insights into the pathogenesis of hyperglycemia in type 1, type 2, and MODY diabetes and offers the potential of providing new insights into the mechanism of action of novel antidabetic therapies.

Contribution of net hepatic glycogen synthesis to disposal of an oral glucose load in humans

The contribution of hepatic glycogen synthesis to whole body glucose disposal after an oral glucose load was examined using (13)C nuclear magnetic resonance (NMR) spectroscopy to measure liver glycogen content in healthy, volunteers after an overnight fast. In group 1 (n = 14), hepatic glycogen synthesis was measured using (13)C-NMR spectroscopy for 240 minutes after ingestion of 98 +/- 1 g glucose. Liver volumes were measured using magnetic resonance imaging (MRI). To assess the direct (glucose --> glucose-6-P --> glucose-1-P --> uridine diphosphate (UDP)-glucose --> glycogen) and indirect (3-carbon units --> --> glycogen) pathways of liver glycogen synthesis, group 2 (n = 6) was studied with an identical glucose load enriched with [1-(13)C]glucose along with acetaminophen to noninvasively assess the (13)C enrichment in hepatic UDP-glucose. The fasting hepatic glycogen content was 305 +/- 17 mmol/L liver, and the liver volume was 1.46 +/- 0.07 L. For the initial 180 minutes after ingestion of glucose, hepatic glycogen concentrations increased linearly (r =.94, P =.0006) achieving a maximum concentration of 390 +/- 7 mmol/L liver and then remained constant until the end of the study. The mean maximum rate of net hepatic glycogen synthesis was 0.48 +/- 0.07 mmol/L liver-minute. Total liver glycogen synthesis could account for 16.7 +/- 3.8 g (17% +/- 4%) of the glucose ingested, and of this, 10.5 +/- 2.4 g (63% +/- 7%) was synthesized by the direct pathway. In conclusion, after ingestion of 98 g of glucose: (1) 16.7 +/- 3.8 g (17% +/- 4%) glucose was stored in the liver as glycogen, and (2) 63% +/- 7% (10.5 +/- 2.4 g) of this glycogen was formed via the direct pathway.

Mechanism by which metformin reduces glucose production in type 2 diabetes

To examine the mechanism by which metformin lowers endogenous glucose production in type 2 diabetic patients, we studied seven type 2 diabetic subjects, with fasting hyperglycemia (15.5 +/- 1.3 mmol/l), before and after 3 months of metformin treatment. Seven healthy subjects, matched for sex, age, and BMI, served as control subjects. Rates of net hepatic glycogenolysis, estimated by 13C nuclear magnetic resonance spectroscopy, were combined with estimates of contributions to glucose production of gluconeogenesis and glycogenolysis, measured by labeling of blood glucose by 2H from ingested 2H2O. Glucose production was measured using [6,6-2H2]glucose. The rate of glucose production was twice as high in the diabetic subjects as in control subjects (0.70 +/- 0.05 vs. 0.36 +/- 0.03 mmol x m(-2) min(-1), P < 0.0001). Metformin reduced that rate by 24% (to 0.53 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0009) and fasting plasma glucose concentration by 30% (to 10.8 +/- 0.9 mmol/l, P = 0.0002). The rate of gluconeogenesis was three times higher in the diabetic subjects than in the control subjects (0.59 +/- 0.03 vs. 0.18 +/- 0.03 mmol x m(-2) min(-1) and metformin reduced that rate by 36% (to 0.38 +/- 0.03 mmol x m(-2) x min(-1), P = 0.01). By the 2H2O method, there was a twofold increase in rates of gluconeogenesis in diabetic subjects (0.42 +/- 0.04 mmol m(-2) x min(-1), which decreased by 33% after metformin treatment (0.28 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0002). There was no glycogen cycling in the control subjects, but in the diabetic subjects, glycogen cycling contributed to 25% of glucose production and explains the differences between the two methods used. In conclusion, patients with poorly controlled type 2 diabetes have increased rates of endogenous glucose production, which can be attributed to increased rates of gluconeogenesis. Metformin lowered the rate of glucose production in these patients through a reduction in gluconeogenesis.

Effects of caffeine on muscle glycogen utilization and the neuroendocrine axis during exercise

To examine the effect of caffeine ingestion on muscle glycogen utilization and the neuroendocrine axis during exercise, we studied 20 muscle glycogen-loaded subjects who were given placebo or caffeine (6 mg/kg) in a double blinded fashion 90 min before cycling for 2 h at 65% of their maximal oxygen consumption. Exercise-induced glycogen depletion in the thigh muscle was noninvasively measured by means of 13C nuclear magnetic resonance spectroscopy (NMR) spectroscopy, and plasma concentrations of substrates and neuroendocrine hormones, including beta-endorphins, were also assessed. Muscle glycogen content was increased 140% above normal values on the caffeine trial day (P < 0.001). After cycling for 2 h, caffeine ingestion was associated with a greater increase in plasma lactate (caffeine: +1.0 +/- 0.2 mmol/L; placebo, +0.1 +/- 0.2 mmol/L; P < 0.005), epinephrine (caffeine, +223 +/- 82 pg/mL; placebo, +56 +/- 26 pg/mL; P < 0.05), and cortisol (caffeine, +12 +/- 3 mg/mL; placebo, +2 +/- 2 mg/mL; P < 0.001) levels. However, plasma free fatty acid concentrations increased (caffeine, +814 +/- 133 mmol/L; placebo, +785 +/- 85 mmol/L; P = NS), and muscle glycogen content decreased (caffeine, -57 +/- 6 mmol/L muscle; placebo, -53 +/- 5 mmol/L muscle; P = NS) to the same extent in both groups. At the same time, plasma beta-endorphin levels almost doubled (from 30 +/- 5 to 53 +/- 13 pg/mL; P < 0.05) in the caffeine-treated group, whereas no change occurred in the placebo group. We conclude that caffeine ingestion 90 min before prolonged exercise does not exert a muscle glycogen-sparing effect in athletes with high muscle glycogen content. However, these data suggest that caffeine lowers the threshold for exercise-induced beta-endorphin and cortisol release, which may contribute to the reported benefits of caffeine on exercise endurance.

Mechanism of troglitazone action in type 2 diabetes

To examine the metabolic pathways by which troglitazone improves insulin responsiveness in patients with type 2 diabetes, the rate of muscle glycogen synthesis was measured by 13C-nuclear magnetic resonance (NMR) spectroscopy. The rate-controlling steps of insulin-stimulated muscle glucose metabolism were assessed using 31P-NMR spectroscopic measurement of intramuscular glucose-6-phosphate (G-6-P) combined with a novel 13C-NMR method to assess intracellular glucose concentrations. Seven healthy nonsmoking subjects with type 2 diabetes were studied before and after completion of 3 months of troglitazone (400 mg/day) therapy. After troglitazone treatment, rates of insulin-stimulated whole-body glucose uptake increased by 58+/-11%, from 629+/-82 to 987+/-156 micromol x m(-2) x min(-1) (P = 0.008), which was associated with an approximately 3-fold increase in rates of insulin-stimulated glucose oxidation (from 119+/-41 to 424+/-70 micromol x m(-2) x min(-1); P = 0.018) and muscle glycogen synthesis (26+/-17 vs. 83+/-35 micromol x l(-1) muscle x min(-1); P = 0.025). After treatment, muscle G-6-P concentrations increased by 0.083+/-0.019 mmol/l (P = 0.008 vs. pretreatment) during the hyperglycemic-hyperinsulinemic clamp, compared with no significant changes in intramuscular G-6-P concentrations in the pretreatment study, reflecting an improvement in glucose transport and/or hexokinase activity. The concentrations of intracellular free glucose did not differ between the pre- and posttreatment studies and remained >50-fold lower in concentration (<0.1 mmol/l) than what would be expected if hexokinase activity was rate-controlling. These results indicate that troglitazone improves insulin responsiveness in skeletal muscle of patients with type 2 diabetes by facilitating glucose transport activity, which thereby leads to increased rates of muscle glycogen synthesis and glucose oxidation.

Mechanism of muscle glycogen autoregulation in humans

To examine the mechanism by which muscle glycogen limits its own synthesis, muscle glycogen and glucose 6-phosphate (G-6-P) concentrations were measured in seven healthy volunteers during a euglycemic ( approximately 5.5 mM)-hyperinsulinemic ( approximately 450 pM) clamp using (13)C/(31)P nuclear magnetic resonance spectroscopy before and after a muscle glycogen loading protocol. Rates of glycogen synthase (V(syn)) and phosphorylase (V(phos)) flux were estimated during a [1-(13)C]glucose (pulse)-unlabeled glucose (chase) infusion. The muscle glycogen loading protocol resulted in a 65% increase in muscle glycogen content that was associated with a twofold increase in fasting plasma lactate concentrations (P < 0.05 vs. basal) and an approximately 30% decrease in plasma free fatty acid concentrations (P < 0.001 vs. basal). Muscle glycogen loading resulted in an approximately 30% decrease in the insulin-stimulated rate of net muscle glycogen synthesis (P < 0.05 vs. basal), which was associated with a twofold increase in intramuscular G-6-P concentration (P < 0.05 vs. basal). Muscle glycogen loading also resulted in an approximately 30% increase in whole body glucose oxidation rates (P < 0.05 vs. basal), whereas there was no effect on insulin-stimulated rates of whole body glucose uptake ( approximately 10.5 mg. kg body wt(-1). min(-1) for both clamps) or glycogen turnover (V(syn)/V(phos) was approximately 23% for both clamps). In conclusion, these data are consistent with the hypothesis that glycogen limits its own synthesis through feedback inhibition of glycogen synthase activity, as reflected by an accumulation of intramuscular G-6-P, which is then shunted into aerobic and anaerobic glycolysis.

Intramuscular glycogen and intramyocellular lipid utilization during prolonged exercise and recovery in man: a 13C and 1H nuclear magnetic resonance spectroscopy study

Depletion of muscle glycogen is considered a limiting performance factor during prolonged exercise, whereas the role of the intramyocellular lipid (IMCL) pool is not yet fully understood. We examined 1) intramyocellular glycogen and lipid utilization during prolonged exercise, 2) resynthesis of muscle glycogen and lipids during recovery, and 3) changes in glycogen content between nonexercising and exercising muscles during recovery. Subjects ran on a treadmill at submaximal intensity until exhaustion. Glycogen concentrations were assessed in thigh, calf, and nonexercising forearm muscle, and IMCL content was measured in soleus muscle using magnetic resonance spectroscopy techniques. At the time of exhaustion, glycogen depletion was 2-fold greater in calf than in thigh muscles, but a significant amount of glycogen was left in both leg muscles. The glycogen concentration in nonexercising forearm muscle decreased during the initial 5 h of recovery to 73% of the baseline value. Duringthe exercise, the IMCL content decreased to 67% and subsequently during recovery increased to 83% of the baseline value. In summary, we found during prolonged running 1) significantly greater muscle glycogen utilization in the calf muscle group than in the thigh muscle group, 2) significant utilization of IMCL in the soleus muscle, and 3) a decrease in glycogen content in nonexercising muscle and an increase in glycogen content in recovering muscles during the postexercise phase. These latter data are consistent with the hypothesis that there is transfer of glycogen by the glucose-lactate and the glucose-->alanine cycle from the resting muscle (forearm) to recovering muscles (thigh and calf) after running exercise.

Glycogen loading alters muscle glycogen resynthesis after exercise

This study compared muscle glycogen recovery after depletion of approximately 50 mmol/l (DeltaGly) from normal (Nor) resting levels (63.2 +/- 2.8 mmol/l) with recovery after depletion of approximately 50 mmol/l from a glycogen-loaded (GL) state (99.3 +/- 4.0 mmol/l) in 12 healthy, untrained subjects (5 men, 7 women). To glycogen load, a 7-day carbohydrate-loading protocol increased muscle glycogen 1.6 +/- 0.2-fold (P < or = 0.01). GL subjects then performed plantar flexion (single-leg toe raises) at 50 +/- 3% of maximum voluntary contraction (MVC) to yield DeltaGly = 48.0 +/- 1.3 mmol/l. The Nor trial, performed on a separate occasion, yielded DeltaGly = 47.5 +/- 4.5 mmol/l. Interleaved natural abundance (13)C-(31)P-NMR spectra were acquired and quantified before exercise and during 5 h of recovery immediately after exercise. During the initial 15 min after exercise, glycogen recovery in the GL trial was rapid (32.9 +/- 8.9 mmol. l(-1). h(-1)) compared with the Nor trial (15.9 +/- 6.9 mmol. l(-1). h(-1)). During the next 45 min, GL glycogen synthesis was not as rapid as in the Nor trial (0.9 +/- 2.5 mmol. l(-1). h(-1) for GL; 14.7 +/- 3.0 mmol. l(-1). h(-1) for Nor; P < or = 0.005) despite similar glucose 6-phosphate levels. During extended recovery (60-300 min), reduced GL recovery rates continued (1.3 +/- 0.5 mmol. l(-1). h(-1) for GL; 3.9 +/- 0.3 mmol. l(-1). h(-1) for Nor; P < or = 0.001). We conclude that glycogen recovery from heavy exercise is controlled primarily by the remaining postexercise glycogen concentration, with only a transient synthesis period when glycogen levels are not severely reduced.

Impaired glucose transport as a cause of decreased insulin-stimulated muscle glycogen synthesis in type 2 diabetes

BACKGROUND

Insulin resistance, a major factor in the pathogenesis of type 2 diabetes mellitus, is due mostly to decreased stimulation of glycogen synthesis in muscle by insulin. The primary rate-controlling step responsible for the decrease in muscle glycogen synthesis is not known, although hexokinase activity and glucose transport have been implicated.

METHODS

We used a novel nuclear magnetic resonance approach with carbon-13 and phosphorus-31 to measure intramuscular glucose, glucose-6-phosphate, and glycogen concentrations under hyperglycemic conditions (plasma glucose concentration, approximately 180 mg per deciliter [10 mmol per liter]) and hyperinsulinemic conditions in six patients with type 2 diabetes and seven normal subjects. In vivo microdialysis of muscle tissue was used to determine the gradient between plasma and interstitial-fluid glucose concentrations, and open-flow microperfusion was used to determine the concentrations of insulin in interstitial fluid.

RESULTS

The time course and concentration of insulin in interstitial fluid were similar in the patients with diabetes and the normal subjects. The rates of whole-body glucose metabolism and muscle glycogen synthesis and the glucose-6-phosphate concentrations in muscle were approximately 80 percent lower in the patients with diabetes than in the normal subjects under conditions of matched plasma insulin concentrations. The mean (+/-SD) intracellular glucose concentration was 2.0+/-8.2 mg per deciliter (0.11+/-0.46 mmol per liter) in the normal subjects. In the patients with diabetes, the intracellular glucose concentration was 4.3+/-4.9 mg per deciliter (0.24+/-0.27 mmol per liter), a value that was 1/25 of what it would be if hexokinase were the rate-controlling enzyme in glucose metabolism.

CONCLUSIONS

Impaired insulin-stimulated glucose transport is responsible for the reduced rate of insulin-stimulated muscle glycogen synthesis in patients with type 2 diabetes mellitus.

Determination of the rate of the glutamate/glutamine cycle in the human brain by in vivo 13C NMR

Recent 13C NMR studies in rat models have shown that the glutamate/glutamine cycle is highly active in the cerebral cortex and is coupled to incremental glucose oxidation in an approximately 1:1 stoichiometry. To determine whether a high level of glutamatergic activity is present in human cortex, the rates of the tricarboxylic acid cycle, glutamine synthesis, and the glutamate/glutamine cycle were determined in the human occipital/parietal lobe at rest. During an infusion of [1-13C]-glucose, in vivo 13C NMR spectra were obtained of the time courses of label incorporation into [4-13C]-glutamate and [4-13C]-glutamine. Using a metabolic model we have validated in the rat, we calculated a total tricarboxylic acid cycle rate of 0.77 +/- 0.07 micromol/min/g (mean +/- SD, n = 6), a glucose oxidation rate of 0.39 +/- 0.04 micromol/min/g, and a glutamate/glutamine cycle rate of 0.32 +/- 0.05 micromol/min/g (mean +/- SD, n = 6). In agreement with studies in rat cerebral cortex, the glutamate/glutamine cycle is a major metabolic flux in the resting human brain with a rate approximately 80% of glucose oxidation.

Preliminary evidence of low cortical GABA levels in localized 1H-MR spectra of alcohol-dependent and hepatic encephalopathy patients

OBJECTIVE

The aim of the study was to compare levels of neuroactive amino acids in the cerebral cortex of healthy subjects, recently detoxified alcohol-dependent patients, and patients with hepatic encephalopathy.

METHOD

Metabolite levels were measured in the occipital cortex by using spatially localized 1H-MRS. Five recently detoxified alcohol-dependent and five hepatic encephalopathy patients with alcohol and non-alcohol-related disease were compared with 10 healthy subjects.

RESULTS

The combined level of gamma-aminobutyric acid (GABA) plus homocarnosine was lower in the alcohol-dependent and hepatic encephalopathy patients than in the healthy subjects.

CONCLUSIONS

The findings suggest that GABA-ergic systems are altered in both alcohol-dependent and hepatic encephalopathy patients.

Contributions of net hepatic glycogenolysis and gluconeogenesis to glucose production in cirrhosis

Net hepatic glycogenolysis and gluconeogenesis were examined in normal (n = 4) and cirrhotic (n = 8) subjects using two independent methods [13C nuclear magnetic resonance spectroscopy (NMR) and a 2H2O method]. Rates of net hepatic glycogenolysis were calculated by the change in hepatic glycogen content before ( approximately 11:00 PM) and after ( approximately 7:00 AM) an overnight fast using 13C NMR and magnetic resonance imaging. Gluconeogenesis was calculated as the difference between the rates of glucose production determined with an infusion of [6,6-2H2]glucose and net hepatic glycogenolysis. In addition, the contribution of gluconeogenesis to glucose production was determined by the 2H enrichment in C-5/C-2 of blood glucose after intake of 2H2O (5 ml/kg body water). Plasma levels of total and free insulin-like growth factor I (IGF-I) and IGF-I binding proteins-1 and -3 were significantly decreased in the cirrhotic subjects (P < 0.01 vs. controls). Postprandial hepatic glycogen concentrations were 34% lower in the cirrhotic subjects (P = 0.007). Rates of glucose production were similar between the cirrhotic and healthy subjects [9.0 +/- 0.9 and 10.0 +/- 0.8 micromol. kg body wt-1. min-1, respectively]. Net hepatic glycogenolysis was 3.5-fold lower in the cirrhotic subjects (P = 0.01) and accounted for only 13 +/- 6% of glucose production compared with 40 +/- 10% (P = 0.03) in the control subjects. Gluconeogenesis was markedly increased in the cirrhotic subjects and accounted for 87 +/- 6% of glucose production vs. controls: 60 +/- 10% (P = 0.03). Gluconeogenesis in the cirrhotic subjects, as determined from the 2H enrichment in glucose C-5/C-2, was also increased and accounted for 68 +/- 3% of glucose production compared with 54 +/- 2% (P = 0.02) in the control subjects. In conclusion, cirrhotic subjects have increased rates of gluconeogenesis and decreased rates of net hepatic glycogenolysis compared with control subjects. These alterations are likely important contributing factors to their altered carbohydrate metabolism.

Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity

To examine the mechanism by which free fatty acids (FFA) induce insulin resistance in human skeletal muscle, glycogen, glucose-6-phosphate, and intracellular glucose concentrations were measured using carbon-13 and phosphorous-31 nuclear magnetic resonance spectroscopy in seven healthy subjects before and after a hyperinsulinemic-euglycemic clamp following a five-hour infusion of either lipid/heparin or glycerol/heparin. IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity was also measured in muscle biopsy samples obtained from seven additional subjects before and after an identical protocol. Rates of insulin stimulated whole-body glucose uptake. Glucose oxidation and muscle glycogen synthesis were 50%-60% lower following the lipid infusion compared with the glycerol infusion and were associated with a approximately 90% decrease in the increment in intramuscular glucose-6-phosphate concentration, implying diminished glucose transport or phosphorylation activity. To distinguish between these two possibilities, intracellular glucose concentration was measured and found to be significantly lower in the lipid infusion studies, implying that glucose transport is the rate-controlling step. Insulin stimulation, during the glycerol infusion, resulted in a fourfold increase in PI 3-kinase activity over basal that was abolished during the lipid infusion. Taken together, these data suggest that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity; this may be a consequence of decreased IRS-1-associated PI 3-kinase activity.

Mechanism by which glucose and insulin inhibit net hepatic glycogenolysis in humans

13C NMR spectroscopy was used to assess flux rates of hepatic glycogen synthase and phosphorylase in overnight-fasted subjects under one of four hypoglucagonemic conditions: protocol I, hyperglycemic (approximately 10 mM) -hypoinsulinemia (approximately 40 pM); protocol II, euglycemic (approximately 5 mM) -hyperinsulinemia (approximately 400 pM); protocol III, hyperglycemic (approximately 10 mM) -hyperinsulinemia (approximately 400 pM); and protocol IV; euglycemic (approximately 5 mM) -hypoinsulinemia (approximately 40 pM). Inhibition of net hepatic glycogenolysis occurred in both protocols I and II compared to protocol IV but via a different mechanism. Inhibition of net hepatic glycogenolysis occurred in protocol I mostly due to decreased glycogen phosphorylase flux, whereas in protocol II inhibition of net hepatic glycogenolysis occurred exclusively through the activation of glycogen synthase flux. Phosphorylase flux was unaltered, resulting in extensive glycogen cycling. Relatively high rates of net hepatic glycogen synthesis were observed in protocol III due to combined stimulation of glycogen synthase flux and inhibition of glycogen phosphorylase flux. In conclusion, under hypoglucagonemic conditions: (a) hyperglycemia, per se, inhibits net hepatic glycogenolysis primarily through inhibition of glycogen phosphorylase flux; (b) hyperinsulinemia, per se, inhibits net hepatic glycogenolysis primarily through stimulation of glycogen synthase flux; (c) inhibition of glycogen phosphorylase and the activation of glycogen synthase are not necessarily coupled and coordinated in a reciprocal fashion; and (d) promotion of hepatic glycogen cycling may be the principal mechanism by which insulin inhibits net hepatic glycogenolysis and endogenous glucose production in humans under euglycemic conditions.

13C/31P NMR studies on the mechanism of insulin resistance in obesity

The mechanism of insulin resistance in obesity was examined in ten obese (BMI 33 +/- 1 kg/m2) and nine lean (BMI 22 +/- 1 kg/m2) Caucasian women during a hyperglycemic-hyperinsulinemic clamp using 13C and 31P nuclear magnetic resonance (NMR) spectroscopy to measure rates of muscle glycogen synthesis and intramuscular glucose-6-phosphate (G-6-P) concentrations. Under similar steady-state plasma concentrations of glucose (approximately 11 mmol/l) and insulin (approximately 340 pmol/l), rates of muscle glycogen synthesis were reduced approximately 70% in the obese subjects (52 +/- 8 micromol/[l muscle-min]) as compared with the rates in the lean subjects (176 +/- 22 micromol/[l muscle-min]; P < 0.0001). Basal concentrations of intramuscular G-6-P were similar in the obese and lean subjects; but during the clamp, G-6-P failed to increase in the obese group (deltaG-6-P obese 0.044 +/- 0.011 vs. lean 0.117 +/- 0.011 mmol/l muscle; P < 0.001), reflecting decreased muscle glucose transport and/or phosphorylation activity. We conclude that insulin resistance in obesity can be mostly attributed to impairment of insulin-stimulated muscle glycogen synthesis due to a defect in glucose transport and/or phosphorylation activity.

Effect of epinephrine on muscle glycogenolysis and insulin-stimulated muscle glycogen synthesis in humans

To examine the effects of a physiological increase in plasma epinephrine concentration (approximately 800 pg/ml) on muscle glycogenolysis and insulin-stimulated glycogenesis, we infused epinephrine [1.2 micrograms.(m2 body surface)-1.min-1] for 2 h and monitored muscle glycogen and glucose 6-phosphate (G-6-P) concentrations with 13C/31P nuclear magnetic resonance (NMR) spectroscopy. Epinephrine caused an increase in plasma glucose (delta approximately 50 mg/dl), lactate (delta approximately 1.4 mM), free fatty acids (delta approximately 1,200 microM at peak), and whole body glucose oxidation (delta approximately 0.85 mg.kg-1.min-1) compared with levels in a group of control subjects (n = 4) in the presence of slight hyperinsulinemia (approximately 13 microU/ml, n = 8) or basal insulin (approximately 7 microU/ml, n = 7). However, epinephrine did not induce any detectable changes in glycogen or G-6-P concentrations, whereas muscle inorganic phosphate (Pi) decreased by 35%. Epinephrine infusion during a euglycemic-hyperinsulinemic clamp (n = 8) caused a 45% decrease in the glucose infusion rate that could be mostly attributed to a 73% decrease in muscle glycogen synthesis rate. After an initial increase to approximately 160% of basal values, G-6-P levels decreased by approximately 30% with initiation of the epinephrine infusion. We conclude that a physiological increase in plasma epinephrine concentration 1) has a negligible effect on muscle glycogenolysis at rest, 2) decreases muscle Pi, which may maintain phosphorylase activity at a low level, and 3) causes a major impairment in insulin-stimulated muscle glycogen synthesis, possibly due to inhibition of glucose transport-phosphorylation activity.

Effects of insulin-like growth factor I on glucose metabolism in rats with liver cirrhosis

To determine the effect of insulin-like growth factor I (IGF-I) on glucose metabolism in cirrhosis, a 2-h euglycemic clamp with IGF-I (0.65 nmol.kg-1.min-1) or insulin (12 pmol.kg-1.min-1) was performed in awake rats with carbon tetrachloride-induced liver cirrhosis. Rates of [3-3H]glucose-determined whole body glucose turnover were similar in the fasting state in cirrhotic and control rats (36.4 +/- 2.6 and 37.7 +/- 2.8 mumol.kg-1.min-1, respectively). In the control group, IGF-I and insulin had similar effects on turnover (81.6 +/- 27.0 and 76.1 +/- 9.9 mumol.kg-1.min-1), muscle glycogen synthesis (47.5 +/- 12.3 and 37.5 +/- 2.5 nmol.g muscle-1.min-1), and suppression of endogenous glucose production (EGP; -54 +/- 14 and -60 +/- 12%). Cirrhotic rats were markedly insulin resistant, reflected by a 43% reduction of turnover (43.8 +/- 9.4 mumol.g muscle-1.min-1; P = 0.03), a 73% reduction in muscle glycogen synthesis (10.2 +/- 3.4 nmol.g muscle-1.min-1; P < 0.0001), and a diminished suppression of EGP (-32 +/- 17% vs. control: -56 +/- 14%; P < 0.05). In contrast, during the IGF-I clamps, turnover increased threefold in the cirrhotic rats (P = 0.001), rates of muscle glycogen synthesis were 7.4 times higher than during the insulin stimulation (P < 0.0001), and EGP was suppressed by 80 +/- 12% (P < 0.05). In conclusion, insulin resistance in cirrhotic rats is mostly due to defects in insulin-stimulated muscle glycogen synthesis, and the ability of IGF-I to stimulate muscle glycogen synthesis as well as suppress EGP is maintained in cirrhotic rats. These findings suggest that alterations in both hepatic and peripheral glucose metabolism in patients with cirrhosis might be amenable to IGF-I therapy.

In vivo NMR investigation of intramuscular glucose metabolism in conscious rats

In vivo 13C nuclear magnetic resonance (NMR) spectroscopy was used to determine quantitatively the flux of muscle glycolysis, glycogen synthesis, pyruvate dehydrogenase, and pyruvate carboxylation in the hindlimb of conscious rats. 13C NMR spectroscopy was used to observe [1-13C]glucose label precursor incorporation into intramuscular [1-13C]glycogen, [3-13C]lactate, and [3-13C]alanine during a hyperglycemic (approximately 11 mM)-hyperinsulinemic (10 mU.kg-1.min-1) clamp. The glycogen synthesis rate was calculated to be 224 +/- 23 nmol.g-1.min-1. The kinetic data obtained from the label turnover in the intramuscular C-3 lactate and C-3 alanine metabolite pools, as well as in plasma C-3 lactate and C-3 alanine, were combined with a steady-state rate analysis to determine the glycolytic flux (67.4 +/- 10.1 nmol.g-1.min-1). Steady-state isotopomer analysis of glutamate and pyruvate in skeletal muscle tissue extracts was used to determine the anaplerotic contribution of substrate via pyruvate carboxylation (Vpc). The pyruvate dehydrogenase flux (Vpdh) was calculated after a steady-state flux correction for Vpc. Calculated values of Vpc and Vpdh were 24.8 +/- 4.3 and 110.0 +/- 18.7 nmol.g-1.min-1, respectively. In addition, [2-13C]acetate was used in a separate study to determine that pyruvate carboxylation was the major pathway for anaplerosis in skeletal muscle under conditions of hyperglycemia-hyperinsulinemia.

Mechanism of impaired insulin-stimulated muscle glucose metabolism in subjects with insulin-dependent diabetes mellitus

To determine the mechanism of impaired insulin-stimulated muscle glycogen metabolism in patients with poorly controlled insulin-dependent diabetes mellitus (IDDM), we used 13C-NMR spectroscopy to monitor the peak intensity of the C1 resonance of the glucosyl units in muscle glycogen during a 6-h hyperglycemic-hyperinsulinemic clamp using [1-(13)C]glucose-enriched infusate followed by nonenriched glucose. Under similar steady state (t = 3-6 h) plasma glucose (approximately 9.0 mM) and insulin concentrations (approximately 400 pM), nonoxidative glucose metabolism was significantly less in the IDDM subjects compared with age-weight-matched control subjects (37+/-6 vs. 73+/-11 micromol/kg of body wt per minute, P < 0.05), which could be attributed to an approximately 45% reduction in the net rate of muscle glycogen synthesis in the IDDM subjects compared with the control subjects (108+/-16 vs. 195+/-6 micromol/liter of muscle per minute, P < 0.001). Muscle glycogen turnover in the IDDM subjects was significantly less than that of the controls (16+/-4 vs. 33+/-5%, P < 0.05), indicating that a marked reduction in flux through glycogen synthase was responsible for the reduced rate of net glycogen synthesis in the IDDM subjects. 31P-NMR spectroscopy was used to determine the intramuscular concentration of glucose-6-phosphate (G-6-P) under the same hyperglycemic-hyperinsulinemic conditions. Basal G-6-P concentration was similar between the two groups (approximately 0.10 mmol/kg of muscle) but the increment in G-6-P concentration in response to the glucose-insulin infusion was approximately 50% less in the IDDM subjects compared with the control subjects (0.07+/-0.02 vs. 0.13+/-0.02 mmol/kg of muscle, P < 0.05). When nonoxidative glucose metabolic rates in the control subjects were matched to the IDDM subjects, the increment in the G-6-P concentration (0.06+/-0.02 mmol/kg of muscle) was no different than that in the IDDM subjects. Together, these data indicate that defective glucose transport/phosphorylation is the major factor responsible for the lower rate of muscle glycogen synthesis in the poorly controlled insulin-dependent diabetic subjects.

Increased glucose transport-phosphorylation and muscle glycogen synthesis after exercise training in insulin-resistant subjects

BACKGROUND

Insulin resistance in the offspring of parents with non-insulin-dependent diabetes mellitus (NIDDM) is the best predictor of development of the disease and probably plays an important part in its pathogenesis. We studied the mechanism and degree to which exercise training improves insulin sensitivity in these subjects.

METHODS

Ten adult children of parents with NIDDM and eight normal subjects were studied before starting an aerobic exercise-training program, after one session of exercise, and after six weeks of exercise. Insulin sensitivity was measured by the hyperglycemic-hyperinsulinemic clamp technique combined with indirect calorimetry, and the rate of glycogen synthesis in muscle and the intramuscular glucose-6-phosphate concentration were measured by carbon-13 and phosphorus-31 nuclear magnetic resonance spectroscopy, respectively.

RESULTS

During the base-line study, the mean (+/-SE) rate of muscle glycogen synthesis was 63 +/- 9 percent lower in the offspring of diabetic parents than in the normal subjects (P < 0.001). The mean value increased 69 +/- 10 percent (P = 0.04) and 62 +/- 11 percent (P = 0.04) after the first exercise session and 102 +/- 11 percent (P = 0.02) and 97 +/- 9 percent (P = 0.008) after six weeks of exercise training in the offspring and the normal subjects, respectively. The increment in glucose-6-phosphate during hyperglycemic-hyperinsulinemic clamping was lower in the offspring than in the normal subjects (0.039 +/- 0.013 vs. 0.089 +/- 0.009 mmol per liter, P = 0.005), reflecting reduced glucose transport-phosphorylation, but this increment was normal in the offspring after the first exercise session and after exercise training. Basal and stimulated insulin secretion was higher in the offspring than the normal subjects and was not altered by the exercise training program.

CONCLUSIONS

Exercise increases insulin sensitivity in both normal subjects and the insulin-resistant offspring of diabetic parents because of a twofold increase in insulin-stimulated glycogen synthesis in muscle, due to an increase in insulin-stimulated glucose transport-phosphorylation.

Impaired hepatic glycogen synthesis in glucokinase-deficient (MODY-2) subjects

All glucokinase gene mutations identified to date have been localized to exons that are common to the pancreatic and hepatic isoforms of the enzyme. While impaired insulin secretion has been observed in glucokinase-deficient subjects the consequences of this mutation on hepatic glucose metabolism remain unknown. To examine this question hepatic glycogen concentration was measured in seven glucokinase-deficient subjects with normal glycosylated hemoglobin and 12 control subjects using 13C nuclear magnetic spectroscopy during a day in which three isocaloric mixed meals were ingested. The relative fluxes of the direct and indirect pathways of hepatic glycogen synthesis were also assessed using [1-13C]glucose in combination with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. Average fasting hepatic glycogen content was similar in glucokinase-deficient and control subjects (279+/-20 vs 284+/-14 mM; mean+/-SEM), and increased in both groups after the meals with a continuous pattern throughout the day. However, the net increment in hepatic glycogen content after each meal was 30-60% lower in glucokinase-deficient than in the control subjects (breakfast, 46% lower, P < 0.02; lunch, 62% lower, P = 0.002; dinner; 30% lower, P = 0.04). The net increment over basal values 4 h after dinner was 105 +/-18 mM in glucokinase-deficient and 148+/-11 mM in control subjects (P = 0.04). In the 4 h after breakfast, flux through the gluconeogenic pathway relative to the direct pathway of hepatic glycogen synthesis was higher in glucokinase-deficient than in control subjects (50+/-2% vs 34+/-5%; P = 0.038). In conclusion glucokinase-deficient subjects have decreased net accumulation of hepatic glycogen and relatively augmented hepatic gluconeogenesis after meals. These results suggest that in addition to the altered beta cell function, abnormalities in liver glycogen metabolism play an important role in the pathogenesis of hyperglycemia in patients with glucokinase-deficient maturity onset diabetes of young.

Noninvasive assessment of hepatic triglyceride content in humans with 13C nuclear magnetic resonance spectroscopy

Hepatic lipid content was assessed noninvasively in 15 patients with hepatic steatosis by 13C nuclear magnetic resonance (NMR) spectroscopy, and compared in a double-blind fashion with histological grading and morphometric quantitation of fat in liver biopsies taken within 2 weeks of the study. The lipid content in the liver biopsies was expressed as the volume fraction of total parenchyma occupied by fat. Hepatic triglyceride content was determined by comparing the 13C NMR signal intensity in vivo with the signal intensity obtained from a lipid phantom of known concentrations. There was an approximately 30-fold increase in the 13C NMR signals of the saturated carbons (methyl/methylene [CH2]n) region of hepatic triglycerides from patients with grade 4 steatosis compared with those with grade 0, yielding a good dynamic range for measuring hepatic triglyceride content. The correlation coefficient between the morphometric and 13C NMR techniques was 0.89 (P < .01). These studies demonstrate that 13C NMR spectroscopy can be used to noninvasively assess hepatic triglyceride content in humans. This method may be clinically useful for diagnosis and follow-up of patients with hepatic steatosis.

Mechanism of free fatty acid-induced insulin resistance in humans

To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13C and 31P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 +/- 0.02 mM [mean +/- SEM]; control) or high (1.93 +/- 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic (approximately 5.2 mM) hyperinsulinemic (approximately 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be approximately 46% of control values after 6 h (P < 0.00001). Augmented lipid oxidation was accompanied by a approximately 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion (P < 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to approximately 50% of control values (4.0 +/- 1.0 vs. 9.3 +/- 1.6 mumol/[kg.min], P < 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at approximately 1.5 h (195 +/- 25 vs. control: 237 +/- 26 mM; P < 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an approximately 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation.

The roles of insulin and glucagon in the regulation of hepatic glycogen synthesis and turnover in humans

To determine the respective roles of insulin and glucagon for hepatic glycogen synthesis and turnover, hyperglycemic clamps were performed with somatostatin [0.1 micrograms/(kg.min)] in healthy young men under conditions of: (I) basal fasting) portal vein insulinemia-hypoglucagonemia, (II) basal portal vein insulinemia-basal glucagonemia, and (III) basal peripheral insulinemia-hypoglucagonemia. Synthetic rates, pathway (direct versus indirect) contributions, and percent turnover of hepatic glycogen were assessed by in vivo 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by a natural abundance glucose chase in conjunction with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. In the presence of hyperglycemia (10.4 +/- 0.1 mM) and basal portal vein insulinemia (192 +/- 6 pM), suppression of glucagon secretion (plasma glucagon, I:31 +/- 4, II: 63 +/- 8 pg/ml) doubled the hepatic accumulation of glycogen (Vsyn) compared with conditions of basal glucagonemia [I: 0.40 +/- 0.06, II: 0.19 +/- 0.03 mumol/(liter.min): P < 0.0025]. Glycogen turnover was markedly reduced (I: 19 +/- 7%, II: 69 +/- 12%; P < 0.005), so that net rate of glycogen synthesis increased approximately fivefold (P < 0.001) by inhibition of glucagon secretion. The relative contribution of gluconeogenesis (indirect pathway) to glycogen synthesis was lower during hypoglucagonemia (42 +/- 6%) than during basal glucagonemia (54 +/- 5%; P < 0.005). Under conditions of basal peripheral insulinemia (54 +/- 2 pM) and hypoglucagonemia (III) there was negligible hepatic glycogen synthesis and turnover. In conclusion, small changes in portal vein concentrations of insulin and glucagon independently affect hepatic glycogen synthesis and turnover. Inhibition of glucagon secretion under conditions of hyperglycemia and basal concentrations of insulin results in: (a) twofold increase in rate of hepatic glycogen synthesis, (b) reduction of glycogen turnover by approximately 73%, and (c) augmented percent contribution of the direct pathway to glycogen synthesis compared with conditions of basal glucagonemia.

Contribution of net hepatic glycogenolysis to glucose production during the early postprandial period

Relative contributions of net hepatic glycogenolysis and gluconeogenesis to glucose production during the first 12 h of a fast were studied in 13 healthy volunteers by noninvasively measuring hepatic glycogen content using 13C nuclear magnetic resonance spectroscopy. Rates of net hepatic glycogenolysis were calculated by multiplying the change in liver glycogen content with liver volume determined by magnetic resonance imaging. Rates of gluconeogenesis were calculated as the difference between rates of glucose production determined with an infusion of [6,6-2H]-glucose and net hepatic glycogenolysis. At 6 P.M. a liquid mixed meal (1,000 kcal; 60% as glucose) was given, to which [2-2H]glucose was added to trace glucose absorption. Hepatic glycogen content was measured between 11 P.M. and 1 A.M. and between 3 and 6 A.M. At 11 P.M. the concentration was 470 mM and it decreased linearly during the night. The mean liver volume was 1.47 +/- 0.06 liters. Net hepatic glycogenolysis (5.8 +/- 0.8 mumol.kg body wt-1.min-1) accounted for, on average, 45 +/- 6% and gluconeogenesis for 55 +/- 6% of the rate of whole body glucose production (12.6 +/- 0.6 mumol.kg body wt-1.min-1). In conclusion, this study shows that, even early in the phase of the postabsorptive period when liver glycogen stores are maximal, gluconeogenesis contributes approximately 50% to hepatic glucose production.

Triiodothyronine treatment increases substrate cycling between pyruvate carboxylase and malic enzyme in perfused rat liver

The relative roles of pyruvate kinase and malic enzyme in substrate cycling between pyruvate and oxaloacetate were examined in perfused livers of 24-hour-fasted normal and triiodothyronine (T3)-treated rats using an inhibitor of malic enzyme (hydroxymalonate). Livers were perfused for 60 minutes in a recirculating system with [3-13C]alanine (10 mmol/L, 99% 13C-enriched). The combined flux through pyruvate kinase plus malic enzyme relative to pyruvate carboxylase flux was assessed by the 13C-enrichment ratio of alanine C2 to glucose C5 in the perfusate, determined with 13C and 1H nuclear magnetic resonance (NMR) spectroscopy. In normal rat livers, the relative carbon flux through pyruvate kinase plus malic enzyme to pyruvate carboxylase was 0.18 +/- 0.04, and increased to 0.44 +/- 0.08 (P < .05) in the T3-treated group. After addition of hydroxymalonate, this relative carbon flux was unchanged in normal rat livers, but decreased to 0.15 +/- 0.04 (P < .01) in the T3-treated group, suggesting that the increased carbon flux in T3-treated livers was caused by increased flux through malic enzyme. Malic enzyme activity increased from 0.36 +/- 0.05 U/g liver in normal livers to 2.51 +/- 0.50 U/g liver (P < .05) in the T3-treated group, whereas there was no effect of T3 treatment on pyruvate kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired net hepatic glycogen synthesis in insulin-dependent diabetic subjects during mixed meal ingestion. A 13C nuclear magnetic resonance spectroscopy study

Hepatic glycogen concentration was measured in six subjects with insulin-dependent diabetes mellitus (IDDM) and nine weight-matched control subjects using 13C nuclear magnetic resonance spectroscopy during a day in which three isocaloric mixed meals were ingested. The relative fluxes of the direct and indirect (3 carbon units-->-->glycogen) pathways of hepatic glycogen synthesis were also assessed using [1-13C]glucose in combination with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. Mean fasting hepatic glycogen content was similar in the two groups. After each meal, hepatic glycogen content increased, peaking 4-5 h after the meal in both groups. By 11:00 p.m. the IDDM subjects had synthesized only 30% of the glycogen that was synthesized by the control group [IDDM subjects, net increment = 44 +/- 20 (mean +/- SE) mM; control subjects, net increment = 144 +/- 14 mM; P < 0.05]. After breakfast the flux through the gluconeogenic pathway relative to the direct pathway of hepatic glycogen synthesis was 1.7-fold greater in the IDDM subjects (59 +/- 4%) than in the control subjects (35 +/- 4%, P < 0.0003). In conclusion, under mixed meal conditions, subjects with poorly controlled IDDM have a major defect in net hepatic glycogen synthesis and augmented hepatic gluconeogenesis. The former abnormality may result in an impaired glycemic response to counterregulatory hormones, whereas both abnormalities may contribute to postprandial hyperglycemia.

Substrate cycling between pyruvate and oxaloacetate in awake normal and 3,3'-5-triiodo-L-thyronine-treated rats

Substrate cycling between pyruvate and oxaloacetate was assessed in awake 24-h fasted normal and triiodothyronine (T3)-treated rats. After a 20- or 60-min infusion of [3-13C]alanine (99% enriched, 12 mg/min) the 13C enrichments of liver glucose and alanine carbons were analyzed by 13C and 1H nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. Substrate cycling from phosphoenolpyruvate to pyruvate [via pyruvate kinase (PK)] and from oxaloacetate to pyruvate [via malic enzyme (ME)] relative to the pyruvate carboxylase (PC) flux [i.e., (PK+ME)/PC] was assessed by the ratio of the 13C enrichment of C-2 alanine relative to that in C-5 glucose. In the normal rats (PK+ME)/PC was 0.26 +/- 0.07 (n = 7, t = 20 min) and 0.37 +/- 0.08 (n = 4, t = 60 min). In the T3-treated rats the (PK+ME)/PC increased four- to fivefold to 1.03 +/- 0.19 (n = 8, t = 20 min) and to 1.83 +/- 0.19 (n = 3, t = 60 min) (P < 0.05 vs. normal rats). The liver enzyme activity of PK did not change with T3 treatment (normal 14.22 +/- 5.25 U/g liver vs. T3 treated 13.40 +/- 1.10 U/g liver), whereas both the enzyme activity ratio of PK (normal 0.47 +/- 0.15 vs. T3 treated 0.77 +/- 0.03, P < 0.05) and the activity of ME (normal 0.89 +/- 0.30 U/g liver vs. T3 treated 4.25 +/- 0.60 U/g liver, P < 0.05) increased with T3 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

A liver factor increasing glucose uptake in rat hindquarters

The effect of the liver on glucose uptake in muscle was studied in the isolated rat hindquarter, perfused with and without an isolated rat liver included in a recirculating system. Glucose uptake in the hindquarter was 0.75 +/- 0.10 mumol.min-1 x 100 g bw-1 (mean and SEM), and increased to 1.30 +/- 0.12 during the period when the liver was included in the perfusion (N = 17; p = 0.0001). In experiments where tissue extracts were added to the perfusate during the second period, raw liver extracts increased the glucose uptake from 0.87 +/- 0.11 to 1.25 +/- 0.10 (N = 6; p = 0.003). Deproteinized liver and kidney extracts increased glucose uptake similarly. When the liver and hindquarter were perfused together during the first period, the glucose uptake in the hindquarter was 1.55 +/- 0.16 mumol.min-1 x 100 g bw-1 and decreased gradually during the period of isolated perfusion to 1.27 +/- 0.16 (N = 4; p = 0.15). In control experiments where tissue free extract fluid was added to the perfusate during the second period, or where the hindquarter was perfused either alone or with the liver during both periods, glucose uptake decreased slightly from 0.87 +/- 0.18 to 0.80 +/- 0.13 (N = 9; p = 0.2). In conclusion, a factor from the liver may increase the glucose uptake in muscle tissues. This factor is present in extracts from the liver and kidney, it is probably not a protein, but its nature is not known. It is speculated that glucose intolerance in patients with liver disease may be related to a lack of this factor.

Gluconeogenesis in hepatocytes determined with [2-13C]acetate and quantitative 13C NMR spectroscopy

1. In the present study the major metabolic pathways of glucose metabolism were determined in isolated liver cells using [2-13C]acetate and 13C magnetic resonance spectroscopy. 2. The relative reaction rates of glucose synthesis to the TCA cycle were determined from the 13C distribution in glucose where the overall 13C enrichment of glucose was 6.41 +/- 1.94% (mean +/- SD; n = 6) and the mean 13C enrichment of C1, C2, C5, C6 to C3, C4 was 2.63 +/- 0.30. 3. Since the distribution of tracer in glucose is a function of the relative entry rates of pyruvate to acetyl-CoA into the oxaloacetate pool this was calculated to be 0.32 +/- 0.15 and the factor for carbon exchange (1/P) between the gluconeogenic pathway and the TCA cycle was calculated to be 1.03 +/- 0.20. 4. With this carbon exchange factor and the approximated 13C enrichment of acetyl-CoA the intramitochondrial 13C enrichment of phosphoenolpyruvate was calculated and the "true" rate of hepatic gluconeogenesis from phosphoenolpyruvate estimated. 5. Since acetate was metabolized solely in liver cells the 13C enrichment of acetyl-CoA could be approximated from that of 3-hydroxybutyrate. 6. The carbon 13 enrichment of 3-hydroxybutyrate and phosphoenolpyruvate was 5.89 +/- 0.90% and 5.96 +/- 1.67%, respectively. 7. The per cent gluconeogenesis from phosphoenolpyruvate calculated as the ratio of the 13C enrichment of glucose to that of 3-hydroxybutyrate times 1/P was 107 +/- 8%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dietary protein on the capacity of urea synthesis in rats

The in vivo capacity of urea nitrogen synthesis (CUNS) during alanine stimulation was measured within the blood amino acid concentration interval 7.3-11.6 mmol/l, where urea synthesis is at maximum and independent of substrate concentration. Three groups of rats were fed for 14 days, either a low protein diet (8%), a normal diet (17%), or a high protein diet (53%). Diet protein modified both CUNS and plasma glucagon concentration. CUNS was 5.86 +/- 2.93, 7.43 +/- 2.16, and 19.31 +/- 4.32 mumol/(min.100 g BW) (mean +/- SD, N = 6), respectively. The corresponding plasma glucagon concentrations after alanine stimulation were 222 +/- 400, 633 +/- 229, and 1700 +/- 627 ng/l, respectively. The in vivo kinetics of urea production is regulated by dietary protein, possibly via glucagon. This implies that the liver plays an active part in adaptation of whole body nitrogen homeostasis to dietary changes.

Hepatic regeneration following glucagon treatment

Liver regeneration was studied as a function of time after suppression of the normal glucagon response. Rats were given a daily subcutaneous injection of 20 micrograms zincglucagon for 14 days, whereafter a 70% hepatectomy was performed. In the glucagon treated rats the rise in plasma glucagon concentration was diminished after hepatectomy. At intervals from 12 to 384 hours after hepatectomy, the gain in liver weight, the hepatic DNA content, and the antipyrine clearance were measured. All 3 variables were found to be significantly higher in animals with diminished glucagon response. The results indicate that prevention of the normal increase in glucagon concentration leads to signs of increased liver regeneration after 70% hepatectomy.

Time dependent stimulating effect of glucagon on the capacity of urea-N synthesis in rats

The effect of glucagon on the capacity of urea-N synthesis was examined in 24 rats as a function of time. First, the conditions for saturation of urea synthesis under glucagon influence were studied by the kinetics of urea-N synthesis rate in relation to arterial blood alpha-amino-N concentration between 5 and 17 mmol/l in 21 nephrectomized rats given zinc-glucagon (20 micrograms s.c. per day) for 14 days. Alanine was infused so that steady state concentrations of total alpha-amino-N was attained in each rat. The urea-N synthesis rate was calculated as accumulation in total body water corrected for intestinal hydrolysis. The relationship suggested a barrier limited substrate inhibition kinetics, as earlier found in control rats, and data were examined accordingly by non-linear regression analysis. The estimated kinetic constants were: Vmax = 71 mumol/(min X 100 g body wt), Km = 5.4 mmol/l, Ki = 2.4 mmol/l, and the barrier = 4.4 mmol/l. Vmax was increased three times compared with controls. The capacity of urea-N synthesis, i.e. the zenith of the relation, was attained in the concentration interval 7.5 to 12.0 mmol/l, as in controls. The capacity of urea-N synthesis was determined during i.v. infusion of zinc-glucagon (0.15 microgram per min) and after 2, 8, and 14 days of daily s.c. injections of 20 micrograms zinc-glucagon. Rats given zinc-protamine solution were controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased capacity of urea synthesis in streptozotocin diabetes in rats

Diabetes was induced in Wistar rats by intravenous streptozotocin, 75 mg/kg. Four and 14 days after streptozotocin, fasting insulin decreased to about one-third, and fasting glucagon increased three-fold. The urea-N synthesis rate, stimulated by infusion of alanine, was measured at different amino acid concentrations 14 days after streptozotocin in 24 rats. The relationship was compatible with a barrier limited substrate inhibition kinetics. Data were examined accordingly by non-linear regression analysis. Among the estimated kinetic constants, only the 70% increase in Vmax was different from control values. In control rats the capacity of urea nitrogen synthesis, as measured within the amino acid concentration interval 7.3-11.6 mmol/l, was 10.2 +/- 1.1 mumol . (min 100 g BW)-1 (mean +/- SEM). The capacity was not different in 4 day diabetic rats, whereas it doubled in 14 day diabetic rats, 20.9 +/- 1.7 mumol (min 100 g BW)-1. The alanine elimination rate was 35% higher in the 14 day diabetic rats compared both to 4 day diabetic and control rats. The increase of urea synthesis is suggested to be due to enzyme induction by glucagon. The net nitrogen balance was negative at amino acid concentrations up to 25 mmol/l, indicating that the urea synthesis was increased at the expense of amino nitrogen.

Methods for internal quality control in the mycobacteriology laboratory

A proposal is given for internal quality control in the mycobacteriology laboratory by continuously monitoring the yields of positive results in microscopy and culture as well as the contamination rates of the culture media. The figures thus obtained can be processed as is customary done in clinical chemistry in order not only to detect deviations, but also to discern whether they are due to chance or whether they indicate a change in the quality of performance. Some examples are given to illustrate the possibilities offered by this control system.