Localization of rate-limiting defect for glucose disposal in skeletal muscle of insulin-resistant type I diabetic patients

Metabolic Transporters in the Peripheral Nerve-What, Where, and Why?

Cellular metabolism is critical not only for cell survival, but also for cell fate, function, and intercellular communication. There are several different metabolic transporters expressed in the peripheral nervous system, and they each play important roles in maintaining cellular energy. The major source of energy in the peripheral nervous system is glucose, and glucose transporters 1 and 3 are expressed and allow blood glucose to be imported and utilized by peripheral nerves. There is also increasing evidence that other sources of energy, particularly monocarboxylates such as lactate that are transported primarily by monocarboxylate transporters 1 and 2 in peripheral nerves, can be efficiently utilized by peripheral nerves. Finally, emerging evidence supports an important role for connexins and possibly pannexins in the supply and regulation of metabolic energy. In this review, we will first define these critical metabolic transporter subtypes and then examine their localization in the peripheral nervous system. We will subsequently discuss the evidence, which comes both from experiments in animal models and observations from human diseases, supporting critical roles played by these metabolic transporters in the peripheral nervous system. Despite progress made in understanding the function of these transporters, many questions and some discrepancies remain, and these will also be addressed throughout this review. Peripheral nerve metabolism is fundamentally important and renewed interest in these pathways should help to answer many of these questions and potentially provide new treatments for neurologic diseases that are partly, or completely, caused by disruption of metabolism.

Origin of the 31 P MR signal at 5.3 ppm in patients with critical limb ischemia

An unknown intense signal (P_un ) with a mean chemical shift of 5.3 ppm was observed in ³¹ P MR spectra from the calf muscles of patients with the diabetic foot syndrome. The aim of the study was to identify the origin of this signal and its potential as a biomarker of muscle injury. Calf muscles of 68 diabetic patients (66.3 ± 8.6 years; body mass index = 28.2 ± 4.3 kg/m² ) and 12 age-matched healthy controls were examined by (dynamic) ³¹ P MRS (3 T system, ³¹ P/¹ H coil). Phantoms (glucose-1-phosphate, P_i and PCr) were measured at pH values of 7.05 and 7.51. At rest, P_un signals with intensities higher than 50% of the P_i intensity were observed in 10 of the 68 examined diabetic subjects. We tested two hypothetical origins of the P_un signal: (1) phosphorus from phosphoesters and (2) phosphorus from extra- and intracellular alkaline phosphate pools. 2,3-diphosphoglycerate and glucose-1-phosphate are the only phosphoesters with signals in the chemical shift region close to 5.3 ppm. Both compounds can be excluded: 2,3-diphosphoglycerate due to the missing second signal component at 6.31 ppm; glucose-1-phosphate because its chemical shifts are about 0.2 ppm downfield from the P_i signal (4.9 ppm). If the P_un signal is from phosphate, it represents a pH value of 7.54 ± 0.05. Therefore, it could correspond to signals of P_i in mitochondria. However, patients with critical limb ischemia have rather few mitochondria and so the P_un signal probably originates from interstitia. Our data suggest that the increased P_un signal observed in patients with the diabetic foot syndrome is a biomarker of severe muscular damage.

Iatrogenic Hyperinsulinemia, Not Hyperglycemia, Drives Insulin Resistance in Type 1 Diabetes as Revealed by Comparison With GCK-MODY (MODY2)

Although insulin resistance consistently occurs with type 1 diabetes, its predominant driver is uncertain. We therefore determined the relative contributions of hyperglycemia and iatrogenic hyperinsulinemia to insulin resistance using hyperinsulinemic-euglycemic clamps in three participant groups (n = 10/group) with differing insulinemia and glycemia: healthy control subjects (euinsulinemia and euglycemia), glucokinase-maturity-onset diabetes of the young (GCK-MODY; euinsulinemia and hyperglycemia), and type 1 diabetes (hyperinsulinemia and hyperglycemia matching GCK-MODY). We assessed the contribution of hyperglycemia by comparing insulin sensitivity in control and GCK-MODY and the contribution of hyperinsulinemia by comparing GCK-MODY and type 1 diabetes. Hemoglobin A_1c was normal in control subjects and similarly elevated for type 1 diabetes and GCK-MODY. Basal insulin levels in control subjects and GCK-MODY were nearly equal but were 2.5-fold higher in type 1 diabetes. Low-dose insulin infusion suppressed endogenous glucose production similarly in all groups and suppressed nonesterified fatty acids similarly between control subjects and GCK-MODY, but to a lesser extent for type 1 diabetes. High-dose insulin infusion stimulated glucose disposal similarly in control subjects and GCK-MODY but was 29% and 22% less effective in type 1 diabetes, respectively. Multivariable linear regression showed that insulinemia-but not glycemia-was significantly associated with muscle insulin sensitivity. These data suggest that iatrogenic hyperinsulinemia predominates in driving insulin resistance in type 1 diabetes.

Differences in muscle metabolism in patients with type I diabetes - influence of gender and nephropathy studied by (31)P MR spectroscopy

Type I diabetes mellitus (DM1) is a complex disease with adverse effects on organs and tissues despite compensation by insulin treatment. The goal of our study was to study how kidney diseases change (31)P MR parameters of muscle metabolism in DM1 patients with respect to gender. 51 DM1 patients (19 m/14 f without and 13 m/5 f with nephropathy) and 26 (14 m/12 f) healthy volunteers were examined using (31)P magnetic resonance spectroscopy at 3T tomograph at rest, and during and after a calf muscle exercise. The exercise consisted of a six-minute plantar flexion using a pedal ergometer followed by a six-minute recovery. It is reflected by reduced relative beta-ATP and increased Pi and phosphodiester signals to phosphocreatine (PCr) at rest and prolongation of the PCr recovery time after the exercise. Measurement on healthy volunteers indicated differences between males and females in pH at the rest and after the exercise only. These differences between patients groups were not significant. We have proven that nephropathy affects the metabolism in diabetic patients and our results confirm significant difference between patients with and without nephropathy. Gender differences in pH were observed only between male and female healthy volunteers.

Increased circulating levels of musclin in newly diagnosed type 2 diabetic patients

BACKGROUND

Musclin is a newly identified skeletal muscle-derived secretory factor, which has been recently characterized as a stimulator that induces insulin resistance in mice. However, the pathophysiological role of musclin in humans remains poorly understood. The aim of this study was to explore the potential correlations between musclin plasma levels and various metabolic parameters in patients with type 2 diabetes mellitus.

MATERIALS AND METHODS

In this hospital-based study, plasma samples were collected from the enrolled individuals, including 38 newly diagnosed, treatment-naive type 2 diabetes mellitus patients and 41 age- and gender-matched control subjects. Plasma musclin levels were examined by radioimmunoassay.

RESULTS

Compared with the control group, musclin plasma levels were significantly higher in untreated type 2 diabetes mellitus patients. Musclin levels in the plasma of newly diagnosed type 2 diabetes mellitus patients were positively correlated with fasting plasma glucose, haemoglobin A_1c, serum insulin, triglycerides and homeostasis model assessment of insulin resistance. Furthermore, multivariate logistic regression analysis showed that the level of musclin was associated with the presence of type 2 diabetes mellitus. Receiver operating characteristic curve analysis yielded an area under the curve for musclin of 0.718 in type 2 diabetes mellitus.

CONCLUSION

The circulating concentration of musclin was significantly increased in type 2 diabetes mellitus patients. Our results suggest that musclin has a strong relationship with insulin resistance in type 2 diabetes mellitus.

Insulin Delivery Into the Peripheral Circulation: A Key Contributor to Hypoglycemia in Type 1 Diabetes

Hypoglycemia limits optimal glycemic control in type 1 diabetes mellitus (T1DM), making novel strategies to mitigate it desirable. We hypothesized that portal (Po) vein insulin delivery would lessen hypoglycemia. In the conscious dog, insulin was infused into the hepatic Po vein or a peripheral (Pe) vein at a rate four times of basal. In protocol 1, a full counterregulatory response was allowed, whereas in protocol 2, glucagon was fixed at basal, mimicking the diminished α-cell response to hypoglycemia seen in T1DM. In protocol 1, glucose fell faster with Pe insulin than with Po insulin, reaching 56 ± 3 vs. 70 ± 6 mg/dL (P = 0.04) at 60 min. The change in area under the curve (ΔAUC) for glucagon was similar between Pe and Po, but the peak occurred earlier in Pe. The ΔAUC for epinephrine was greater with Pe than with Po (67 ± 17 vs. 36 ± 14 ng/mL/180 min). In protocol 2, glucose also fell more rapidly than in protocol 1 and fell faster in Pe than in Po, reaching 41 ± 3 vs. 67 ± 2 mg/dL (P < 0.01) by 60 min. Without a rise in glucagon, the epinephrine responses were much larger (ΔAUC of 204 ± 22 for Pe vs. 96 ± 29 ng/mL/180 min for Po). In summary, Pe insulin delivery exacerbates hypoglycemia, particularly in the presence of a diminished glucagon response. Po vein insulin delivery, or strategies that mimic it (i.e., liver-preferential insulin analogs), should therefore lessen hypoglycemia.

New players in high fat diet-induced obesity: LETM1 and CTMP

OBJECTIVE

Obesity contributes to insulin resistance and is a risk factor for diabetes. C-terminal modulator protein (CTMP) and leucine zipper/EF-hand-containing transmembrane protein 1 (LETM1) have been reported to influence the phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB) signaling pathway via the modulation of PKB activity, a key player for insulin signaling. However, it remains unclear whether CTMP and LETM1 are associated with PI3K/PKB signaling in mouse models of obesity.

MATERIALS/METHODS

To address this question, we used two different mouse models of obesity, including high-fat diet (HFD)-induced diabetic mice and genetically modified obese mice (ob/ob mice). The levels of insulin-signaling molecules in these mice were determined by immunohistochemical and Western blot analyses. The involvement of CTMP and LETM1 in PI3K/PKB signaling was investigated in HEK293 cells by transient transfection and adenovirus-mediated infection.

RESULTS

We found that the levels of insulin receptor, phosphorylated PKB, and LETM1 were lower and the level of CTMP was higher in the adipose tissue of obese mice on an HFD compared to lean mice on a chow diet. Similar results were obtained in ob/ob mice. In HEK293 cells, the activation of PKB increased the LETM1 level, and inhibition of PKB increased the CTMP level. The overexpression of CTMP suppressed the insulin-induced increase in PKB phosphorylation, which was abrogated by co-overexpression with LETM1.

CONCLUSION

These results suggest that CTMP and LETM1 may participate in impaired insulin signaling in the adipose tissue of obese mice, raising the possibility that these parameters may serve as new candidate biomarkers or targets in the development of new therapeutic approaches for diabetes.

Features of hepatic and skeletal muscle insulin resistance unique to type 1 diabetes

CONTEXT

Type 1 diabetes is known to be a state of insulin resistance; however, the tissues involved in whole-body insulin resistance are less well known. It is unclear whether insulin resistance is due to glucose toxicity in the post-Diabetes Control and Complications Trial era of tighter glucose control.

OBJECTIVE

We performed this study to determine muscle and liver insulin sensitivity individuals with type 1 diabetes after overnight insulin infusion to lower fasting glucose concentration.

DESIGN, PATIENTS, AND METHODS

Fifty subjects [25 controls without and 25 individuals with type 1 diabetes (diabetes duration 22.9 ± 1.7 yr, without known end organ damage] were frequency matched on age and body mass index by group and studied. After 3 d of dietary control and overnight insulin infusion to normalize glucose, we performed a three-stage hyperinsulinemic/euglycemic clamp infusing insulin at 4, 8, and 40 mU/m(2) · min. Glucose metabolism was quantified using an infusion of [6,6-(2)H(2)]glucose. Hepatic insulin sensitivity was measured using the insulin IC(50) for glucose rate of appearance (Ra), whereas muscle insulin sensitivity was measured using the glucose rate of disappearance during the highest insulin dose.

RESULTS

Throughout the study, glucose Ra was significantly greater in individuals compared with those without type 1 diabetes. The concentration of insulin required for 50% suppression of glucose Ra was 2-fold higher in subjects with type 1 diabetes. Glucose rate of disappearance was significantly lower in individuals with type 1 diabetes during the 8- and 40-mU/m(2) · min stages.

CONCLUSION

Insulin resistance in liver and skeletal muscle was a significant feature in type 1 diabetes. Nevertheless, the etiology of insulin resistance was not explained by body mass index, percentage fat, plasma lipids, visceral fat, and physical activity and was also not fully explained by hyperglycemia.

Molecular mechanism of insulin resistance in obesity and type 2 diabetes

Insulin resistance is a major risk factor for developing type 2 diabetes caused by the inability of insulin-target tissues to respond properly to insulin, and contributes to the morbidity of obesity. Insulin action involves a series of signaling cascades initiated by insulin binding to its receptor, eliciting receptor autophosphorylation and activation of the receptor tyrosine kinase, resulting in tyrosine phosphorylation of insulin receptor substrates (IRSs). Phosphorylation of IRSs leads to activation of phosphatidylinositol 3-kinase (PI3K) and, subsequently, to activation of Akt and its downstream mediator AS160, all of which are important steps for stimulating glucose transport induced by insulin. Although the mechanisms underlying insulin resistance are not completely understood in skeletal muscle, it is thought to result, at least in part, from impaired insulin-dependent PI3K activation and downstream signaling. This review focuses on the molecular basis of skeletal muscle insulin resistance in obesity and type 2 diabetes. In addition, the effects of insulin-sensitizing agent treatment and lifestyle intervention of human insulin-resistant subjects on insulin signaling cascade are discussed. Furthermore, the role of Rho-kinase, a newly identified regulator of insulin action in insulin control of metabolism, is addressed.

Metabolic flexibility and insulin resistance

Metabolic flexibility is the capacity for the organism to adapt fuel oxidation to fuel availability. The inability to modify fuel oxidation in response to changes in nutrient availability has been implicated in the accumulation of intramyocellular lipid and insulin resistance. The metabolic flexibility assessed by the ability to switch from fat to carbohydrate oxidation is usually impaired during a hyperinsulinemic clamp in insulin-resistant subjects; however, this "metabolic inflexibility" is mostly the consequence of impaired cellular glucose uptake. Indeed, after controlling for insulin-stimulated glucose disposal rate (amount of glucose available for oxidation), metabolic flexibility is not altered in obesity regardless of the presence of type 2 diabetes. To understand how intramyocellular lipids accumulate and cause insulin resistance, the assessment of metabolic flexibility to high-fat diets is more relevant than metabolic flexibility during a hyperinsulinemic clamp. An impaired capacity to upregulate muscle lipid oxidation in the face of high lipid supply may lead to increased muscle fat accumulation and insulin resistance. Surprisingly, very few studies have investigated the response to high-fat diets. In this review, we discuss the role of glucose disposal rate, adipose tissue lipid storage, and mitochondrial function on metabolic flexibility. Additionally, we emphasize the bias of using the change in respiratory quotient to calculate metabolic flexibility and propose novel approaches to assess metabolic flexibility. On the basis of current evidence, one cannot conclude that impaired metabolic flexibility is responsible for the accumulation of intramyocellular lipid and insulin resistance. We propose to study metabolic flexibility in response to high-fat diets in individuals having contrasting degree of insulin sensitivity and/or mitochondrial characteristics.

Metabolic flexibility in response to glucose is not impaired in people with type 2 diabetes after controlling for glucose disposal rate

OBJECTIVE

Compared with nondiabetic subjects, type 2 diabetic subjects are metabolically inflexible with impaired fasting fat oxidation and impaired carbohydrate oxidation during a hyperinsulinemic clamp. We hypothesized that impaired insulin-stimulated glucose oxidation is a consequence of the lower cellular glucose uptake rate in type 2 diabetes. Therefore, we compared metabolic flexibility to glucose adjusted for glucose disposal rate in nondiabetic versus type 2 diabetic subjects and in the latter group after 1 year of lifestyle intervention (the Look AHEAD [Action For Health in Diabetes] trial).

RESEARCH DESIGN AND METHODS

Macronutrient oxidation rates under fasting and hyperinsulinemic conditions (clamp at 80 mU/m(2) per min), body composition (dual-energy X-ray absorptiometry), and relevant hormonal/metabolic blood variables were assessed in 59 type 2 diabetic and 42 nondiabetic individuals matched for obesity, sex, and race. Measures were repeated in diabetic participants after weight loss.

RESULTS

Metabolic flexibility to glucose (change in respiratory quotient [RQ]) was mainly related to insulin-stimulated glucose disposal rate (R(2) = 0.46, P < 0.0001) with an additional 3% of variance accounted for by plasma free fatty acid concentration at the end of the clamp (P = 0.03). The impaired metabolic flexibility to glucose observed in type 2 diabetic versus nondiabetic subjects (Delta RQ 0.06 +/- 0.01 vs. 0.10 +/- 0.01, respectively, P < 0.0001) was no longer observed after adjusting for glucose disposal rate (P = 0.19). Additionally, the increase in metabolic flexibility to glucose after weight loss was accounted for by the concomitant increase in insulin-stimulated glucose disposal rate.

CONCLUSIONS

This study suggests that metabolic inflexibility to glucose in type 2 diabetic subjects is mostly related to defective glucose transport.

Type B lactic acidosis and insulin-resistant hyperglycemia in an adolescent following cardiac surgery

OBJECTIVE

To report the presence of type B lactic acidosis and insulin-resistant hyperglycemia following cardiopulmonary bypass in a pediatric patient.

DESIGN

Case report.

SETTING

Tertiary referral children's hospital pediatric intensive care unit.

PATIENT

Fourteen-year-old child with hyperlactatemia and hyperglycemia following cardiac surgery.

INTERVENTIONS AND RESULTS

We report a patient who following cardiopulmonary bypass for repair of his congenital heart disease developed type B lactic acidosis and hyperglycemia resistant to insulin therapy. Resolution of his hyperlactatemia and hyperglycemia occurred approximately 24 hrs postoperatively without apparent ill effect.

CONCLUSIONS

Type B lactic acidosis is a phenomenon that may occur in the pediatric population in conjunction with insulin-resistant hyperglycemia. We observed that its resolution corresponded to improvement in the patient's hyperglycemia.

Substrate source utilization during moderate intensity exercise with glucose ingestion in Type 1 diabetic patients

Substrate oxidation and the respective contributions of exogenous glucose, glucose released from the liver, and muscle glycogen oxidation were measured by indirect respiratory calorimetry combined with tracer technique in eight control subjects and eight diabetic patients (5 men and 3 women in both groups) of similar age, height, body mass, and maximal oxygen uptake, over a 60-min exercise period on cycle ergometer at 50.8% (SD 4.0) maximal oxygen uptake [131.0 W (SD 38.2)]. The subjects and patients ingested a breakfast (containing ∼80 g of carbohydrates) 3 h before and 30 g of glucose (labeled with 13C) 15 min before the beginning of exercise. The diabetic patients also received their usual insulin dose [Humalog = 9.1 U (SD 0.9); Humulin N = 13.9 U (SD 4.4)] immediately before the breakfast. Over the last 30 min of exercise, the oxidation of carbohydrate [1.32 g/min (SD 0.48) and 1.42 g/min (SD 0.63)] and fat [0.33 g/min (SD 0.10) and 0.30 g/min (SD 0.10)] and their contribution to the energy yield were not significantly different in the control subjects and diabetic patients. Exogenous glucose oxidation was also not significantly different in the control subjects and diabetic patients [6.3 g/30 min (SD 1.3) and 5.2 g/30 min (SD 1.6), respectively]. In contrast, the oxidation of plasma glucose and oxidation of glucose released from the liver were significantly lower in the diabetic patients than in control subjects [14.5 g/30 min (SD 4.3) and 9.3 g/30 min (SD 2.8) vs. 27.9 g/30 min (SD 13.3) and 21.6 g/30 min (SD 12.8), respectively], whereas that of muscle glycogen was significantly higher [28.1 g/30 min (SD 15.5) vs. 11.6 g/30 min (SD 8.1)]. These data indicate that, compared with control subjects, in diabetic patients fed glucose before exercise, substrate oxidation and exogenous glucose oxidation overall are similar but plasma glucose oxidation is lower; this is associated with a compensatory higher utilization of muscle glycogen.

The relationship between peripheral glucose utilisation and insulin sensitivity in the regulation of hepatic glucose production: studies in normal and alloxan-diabetic dogs

BACKGROUND

Hepatic glucose overproduction (HGP) of diabetes could be primary or could occur in response to the metabolic needs of peripheral (skeletal muscle (SkM)) tissues. This question was tested in normal and diabetic dogs.

METHODS

HGP, SkM glucose uptake (Rd(tissue)), metabolic clearance of glucose (MCRg) and glycolytic flux (GF(exog)), and SkM biopsies were measured in the same dogs before and after alloxan-induced diabetes. Normal dogs were exposed to (1) an extended 20-h fast, (2) low- and high-dose glucose infusions (GINF) at basal insulinaemia, and chronic diabetic dogs were exposed to (3) hyperglycaemia, (4) phlorizin-induced normoglycaemia, and (5) poor and good diabetic control.

RESULTS

(1) Prolonged fast: HGP, Rd(tissue), and GF(exog) fell in parallel (p < 0.05). (2) Low-dose GINF: plasma glucose, insulin, Rd(tissue), MCRg, and GF(exog) were unchanged, but HGP fell by approximately 40%, paralleling the supplemental GINF. (3) High-dose GINF at basal insulin: plasma glucose doubled and synchronous changes in HGP, Rd(tissue), MCRg, and GF(exog) occurred; IC(glucose), G6P, and glycogen were unchanged. (4) Hyperglycaemic diabetes: HGP was raised (p < 0.05), matching urinary glucose loss (UGL) and decreased MCR(g), and maintaining normal basal Rd(tissue) and GF(exog). SkM IC(glucose) was increased and glycogen decreased (both p < 0.05). (5) Phlorizin-induced normoglycaemia in diabetic dogs: HGP rose, matching the increased UGL, while maintaining normal Rd(tissue) and GF(exog). Intramuscular substrates normalised. (6) Whole body and SkM metabolism normalised with correction of the insulin resistance and good diabetic control.

CONCLUSION

HGP reflects whether SkM is in a state of relative glucose 'excess' or absolute/relative glucose 'deprivation'.

Prevention of obesity and insulin resistance by glucokinase expression in skeletal muscle of transgenic mice

In type 2 diabetes, glucose phosphorylation, a regulatory step in glucose utilization by skeletal muscle, is impaired. Since glucokinase expression in skeletal muscle of transgenic mice increases glucose phosphorylation, we examined whether such mice counteract the obesity and insulin resistance induced by 12 wk of a high-fat diet. When fed this diet, control mice became obese, whereas transgenic mice remained lean. Furthermore, high-fat fed control mice developed hyperglycemia and hyperinsulinemia (a 3-fold increase), indicating that they were insulin resistant. In contrast, transgenic mice were normoglycemic and showed only a mild increase in insulinemia (1.5-fold). They also showed improved whole body glucose tolerance and insulin sensitivity and increased intramuscular concentrations of glucose 6-phosphate and glycogen. A parallel increase in uncoupling protein 3 mRNA levels in skeletal muscle of glucokinase-expressing transgenic mice was also observed. These results suggest that the rise in glucose phosphorylation by glucokinase expression in skeletal muscle leads to increased glucose utilization and energy expenditure that counteracts weight gain and maintains insulin sensitivity.

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

Involuntary leg movements affect interstitial nutrient gradients and blood flow in rat skeletal muscle

To evaluate the effect of passive muscle shortening and lengthening (PSL) on the transcapillary exchange of glucose, lactate, and insulin in the insulin-stimulated state, microdialysis was performed in rat quadriceps muscle. Electrical pulsatile stimulation (0.1 ms, 0.3-0.6 V, 1 Hz) was performed on the sciatic nerve in one leg to induce passive tension on the quadriceps during a hyperinsulinemic-euglycemic clamp (10 mU x kg(-1) x min(-1)). In the non-insulin-stimulated (basal) state, the muscle arterial-interstitial (A-I) concentration difference of glucose was 1.6 +/- 0.3 mM (P < 0.01). During insulin infusion, it remained unaltered in resting muscle (1.3 +/- 0.3 mM) but diminished during PSL. In the basal state there was no A-I concentration difference of lactate, whereas in the insulin infusion state it increased significantly and was significantly greater in moving (2.8 +/- 0.5 mM, P < 0.01) than in resting muscle (0.7 +/- 0.4 mM). The A-I concentration difference of insulin was equal in resting and moving muscle: 86 +/- 7 and 100 +/- 8 microU/ml, respectively. Muscle blood flow estimated by use of radiolabeled microspheres increased during PSL from 17 +/- 4 to 34 +/- 6 ml x 100 g(-1) x min(-1) (P < 0.05). These results confirm that diffusion over the capillary wall is partly rate limiting for the exchange of insulin and glucose and lactate in resting muscle. PSL, in addition to insulin stimulation, increases blood flow and capillary permeability and, as a result, diminishes the A-I concentration gradient of glucose but not that of insulin or lactate.

In vivo effects of insulin and bis(maltolato)oxovanadium (IV) on PKB activity in the skeletal muscle and liver of diabetic rats

In this study, the in vivo effects of insulin and chronic treatment with bis(maltolato)oxovanadium (IV) (BMOV) on protein kinase B (PKB) activity were examined in the liver and skeletal muscle from two animal models of diabetes, the STZ-diabetic Wistar rat and the fatty Zucker rat. Animals were treated with BMOV in the drinking water (0.75-1 mg/ml) for 3 (or 8) weeks and sacrificed with or without insulin injection. Insulin (5 U/kg, i.v.) increased PKBalpha activity more than 10-fold and PKBbeta activity more than 3-fold in both animal models. Despite the development of insulin resistance, insulin-induced activation of PKBalpha was not impaired in the STZ-diabetic rats up to 9 weeks of diabetes, excluding a role for PKBalpha in the development of insulin resistance in type 1 diabetes. Insulin-induced PKBalpha activity was markedly reduced in the skeletal muscle of fatty Zucker rats as compared to lean littermates (fatty: 7-fold vs. lean: 14-fold). In contrast, a significant increase in insulin-stimulated PKBalpha activity was observed in the liver of fatty Zucker rats (fatty: 15.7-fold vs. lean: 7.6-fold). Chronic treatment with BMOV normalized plasma glucose levels in STZ-diabetic rats and decreased plasma insulin levels in fatty Zucker rats but did not have any effect on basal or insulin-induced PKBalpha and PKBbeta activities. In conclusion (i) in STZ-diabetic rats PKB activity was normal up to 9 weeks of diabetes; (ii) in fatty Zucker rats insulin-induced activation of PKBalpha (but not PKBbeta) was markedly altered in both tissues; (iii) changes in PKBalpha activity were tissue specific; (iv) the glucoregulatory effects of BMOV were independent of PKB activity.

Short-term effects of growth hormone on myocardial glucose uptake in healthy humans

Cardiac muscle is characterized by insulin resistance in specific heart diseases such as coronary artery disease and congestive heart failure, but not in generalized disorders like diabetes mellitus and essential hypertension when cardiac manifestations are absent. To examine whether the insulin antagonistic effect of growth hormone (GH) acts upon the heart, we compared insulin-stimulated whole body and myocardial glucose uptake with and without GH administration during a 3.5-h euglycemic-hyperinsulinemic clamp in eight healthy males. Myocardial 2-deoxy-2-[(18)F]fluoro-D-glucose uptake was measured with positron emission tomography. The data were converted to myocardial glucose uptake by tracer kinetic analysis. GH did not change the rate-pressure product. GH decreased whole body insulin-stimulated glucose disposal by 26% (48.0 +/- 12.1 vs. control 62.8 +/- 6.1 micromol. kg(-1). min(-1), P < 0.02). Free fatty acids were suppressed to a similar extent with and without GH during the insulin clamp. Insulin-stimulated myocardial glucose uptake was similar in the presence and in the absence of GH (0.34 +/- 0.05 and 0.31 +/- 0.03 micromol. g(-1). min(-1), P = 0.18). In conclusion, GH does not impair insulin-stimulated myocardial glucose uptake despite a considerable whole body insulin antagonistic effect. Myocardial insulin resistance is not an inherent consequence of whole body insulin resistance.

Normal insulin-dependent activation of Akt/protein kinase B, with diminished activation of phosphoinositide 3-kinase, in muscle in type 2 diabetes

To determine whether the serine/threonine kinase Akt (also known as protein kinase B) is activated in vivo by insulin administration in humans, and whether impaired activation of Akt could play a role in insulin resistance, we measured the activity and phosphorylation of Akt isoforms in skeletal muscle from 3 groups of subjects: lean, obese nondiabetic, and obese type 2 diabetic. Vastus lateralis biopsies were taken in the basal (overnight fast) and insulin-stimulated (euglycemic clamp) states. Insulin-stimulated glucose disposal was reduced 31% in obese subjects and 63% in diabetic subjects, compared with lean subjects. Glycogen synthase (GS) activity in the basal state was reduced 28% in obese subjects and 49% in diabetic subjects, compared with lean subjects. Insulin-stimulated GS activity was reduced 30% in diabetic subjects. Insulin treatment activated the insulin receptor substrate-1-associated (IRS-1-associated) phosphoinositide 3-kinase (PI 3-kinase) 6.1-fold in lean, 3.7-fold in obese, and 2.4-fold in diabetic subjects. Insulin also stimulated IRS-2-associated PI 3-kinase activity 2.2-fold in lean subjects, but only 1.4-fold in diabetic subjects. Basal activity of Akt1/Akt2 (Akt1/2) and Akt3 was similar in all groups. Insulin increased Akt1/2 activity 1.7- to 2. 0-fold, and tended to activate Akt3, in all groups. Insulin-stimulated phosphorylation of Akt1/2 was normal in obese and diabetic subjects. In lean subjects only, insulin-stimulated Akt1/2 activity correlated with glucose disposal rate. Thus, insulin activation of Akt isoforms is normal in muscle of obese nondiabetic and obese diabetic subjects, despite decreases of approximately 50% and 39% in IRS-1- and IRS-2-associated PI 3-kinase activity, respectively, in obese diabetic subjects. It is therefore unlikely that Akt plays a major role in the resistance to insulin action on glucose disposal or GS activation that is observed in muscle of obese type 2 diabetic subjects.

Surgery-induced insulin resistance in human patients: relation to glucose transport and utilization

To investigate the underlying molecular mechanisms for surgery-induced insulin resistance in skeletal muscle, six otherwise healthy patients undergoing total hip replacement were studied before, during, and after surgery. Patients were studied under basal conditions and during physiological hyperinsulinemia (60 microU/ml). Biopsies of vastus lateralis muscle were used to measure GLUT-4 translocation, glucose transport, and glycogen synthase activities. Surgery reduced insulin-stimulated glucose disposal (P < 0.05) without altering the insulin-stimulated increase in glucose oxidation or suppression of endogenous glucose production. Preoperatively, insulin infusion increased plasma membrane GLUT-4 in all six subjects (P < 0.05), whereas insulin-stimulated GLUT-4 translocation only occurred in three patients postoperatively (not significant). Moreover, nonoxidative glucose disposal rates and basal levels of glycogen synthase activities in muscle were reduced postoperatively (P < 0.05). These findings demonstrate that peripheral insulin resistance develops immediately postoperatively and that this condition might be associated with perturbations in insulin-stimulated GLUT-4 translocation as well as nonoxidative glucose disposal, presumably at the level of glycogen synthesis.

IGF-I increases forearm blood flow without increasing forearm glucose uptake

Decreased insulin-mediated muscle glucose uptake is a characteristic feature of non-insulin-dependent diabetes mellitus and other insulin-resistant states. It has been suggested that an impairment in the ability of insulin to augment limb blood flow, resulting in diminished glucose delivery to muscle, may contribute to this abnormality. In this study, we used human insulin-like growth factor (IGF) I in conjunction with the forearm balance technique to determine whether forearm glucose uptake could be stimulated by increasing blood flow without directly stimulating the intrinsic ability of the muscle to extract glucose. IGF-I was infused intra-arterially in healthy controls at a rate of either 0.4 microg . kg-1 . min-1 (high IGF) or 0.04 microg . kg-1 . min-1 (low IGF) for 140 min. With high IGF, forearm blood flow increased approximately twofold (34 +/- 3 vs. 64 +/- 8 ml . min-1 . l forearm volume-1, P < 0.01), and arteriovenous glucose concentration difference (a-v difference) increased modestly (0.19 +/- 0.05 vs. 0.31 +/- 0.08 mM, P = 0.32), resulting in an increased forearm glucose uptake (6.4 +/- 1.7 vs. 21.7 +/- 7.4 micromol . min-1 . l forearm volume-1, P = 0.09 vs. basal). With low IGF, forearm blood flow increased by 59% (29 +/- 4 vs. 46 +/- 9 ml . min-1 . l forearm volume-1, P < 0.05) and was associated with a proportional decrease in the a-v difference (0. 29 +/- 0.04 vs. 0.18 +/- 0.05 mM, P < 0.05). Forearm glucose uptake therefore was not significantly different from basal values (7.6 +/- 0.6 vs. 6.9 +/- 1.8 micromol . min-1 . kg-1). These data demonstrate that increasing blood flow without increasing the intrinsic ability of the muscle to extract glucose does not increase forearm muscle glucose uptake.

13C/31P NMR studies of glucose transport in human skeletal muscle

The muscle intracellular (IC) free glucose concentration and the rate of muscle glycogen synthesis were measured by using in vivo 13C and 31P NMR spectroscopy in normal volunteers under hyperinsulinemic ( approximately 300 pM) clamp conditions at the following three plasma glucose levels: euglycemia ( approximately 6 mM), mild ( approximately 10 mM), and high ( approximately 16 mM) hyperglycemia. In keeping with biopsy studies, muscle IC free glucose concentration at euglycemia (-0.03 +/- 0.03 mmol/kg of muscle, mean +/- SEM, n = 10) was not statistically different from zero. A small but statistically significant amount of IC free glucose was observed during mild and high hyperglycemia: 0.15 +/- 0.08 (n = 5) and 0.43 +/- 0.20 mmol/kg of muscle (n = 5), respectively. Muscle glycogen synthesis rate, in mmol per kg of muscle per min, was 111 +/- 11 at euglycemia (n = 10), 263 +/- 29 during mild hyperglycemia (n = 5), and 338 +/- 42 during high hyperglycemia (n = 5), these three rates being significantly different from each other. As previous in vitro and in vivo studies, these rates suggest a Km (concentration at which unidirectional glucose transport reaches half-maximal rate) of the muscle glucose transport system in the 15-25 mM range under hyperinsulinemic conditions. The low concentrations of muscle IC free glucose observed under hyperinsulinemic conditions were interpreted, with this estimate and in the framework of metabolic control theory, as glucose transport being the predominant step controlling muscle glucose flux not only at euglycemia but also during hyperglycemia.

Myocardial insulin resistance in patients with syndrome X

Insulin resistance is common in patients with angina pectoris, a positive exercise electrocardiogram, and normal coronary angiograms (syndrome X). It is still not known whether insulin resistance affects the cardiac muscle itself and, if so, whether insulin resistance involves myocardial hemodynamics and energy metabolism. We investigated hemodynamics as well as metabolite exchanges across the heart and the forearm in eight patients with syndrome X and eight control subjects during a baseline period after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Myocardial hemodynamics and metabolism were studied at rest, during pace stress, and in the recovery period after pacing. Neither coronary sinus blood flow nor forearm blood flow differed between the groups before and during the clamp. Whole body insulin-stimulated glucose uptake was decreased in the patients (15.6+/-2.1 vs. 23.1+/-2.0 micromol x kg-1 x min-1). Insulin-stimulated glucose uptake in the forearm and the cardiac muscle was equally reduced in the patients (46+/-5 and 48+/-5%). Myocardial glucose uptake correlated with total arterial delivery in the control subjects (r = 0.63, P < 0.01), but not in patients (r = 0.22, P = 0.13). Carbohydrate and lipid oxidation was similar in the two groups at rest, and changes during the clamp were not different in control subjects and patients either at rest, during pacing, or in the recovery period. Patients with syndrome X exhibit myocardial insulin resistance, but cardiac energy metabolism remains unaffected. In patients with syndrome X, insulin-stimulated glucose uptake is independent from myocardial blood flow.

Evidence for 100% 13C NMR visibility of glucose in human skeletal muscle

The accuracy of the measurement of total muscle glucose by in vivo 13C NMR spectroscopy was tested in five normal volunteers during a euglycemic [1-13C]glucose infusion. The NMR visible concentration calibrated using an external reference was compared with that calculated from plasma glucose concentration, assuming that glucose remained extracellular. The NMR measurement always provided higher values than the calculation from plasma glucose: 0.51 +/- 0.035 (mean +/- SE) versus 0.38 +/- 0.005 mmol/liter of muscle on average. This systematic difference was interpreted as reflecting the presence of muscle glucose-6-phosphate, co-resonating with free glucose. Thus, glucose appeared to be virtually 100% NMR visible in human skeletal muscle.

Chronic hyperglycemia impairs endothelial function and insulin sensitivity via different mechanisms in insulin-dependent diabetes mellitus

BACKGROUND

We explored whether chronic hyperglycemia is associated with defects in endothelium-dependent vasodilatation in vivo and whether defects in the hemodynamic effects of insulin explain insulin resistance.

METHODS AND RESULTS

Vasodilator responses to brachial artery infusions of acetylcholine, sodium nitroprusside, and NG-monomethyl-L-arginine and, on another occasion, in vivo insulin sensitivity (euglycemic insulin clamp combined with the forearm catheterization technique) were determined in 18 patients with insulin-dependent diabetes mellitus (IDDM) and 9 normal subjects. At identical glucose and insulin levels, insulin stimulation of whole-body and forearm glucose uptake was 57% reduced in the IDDM patients compared with normal subjects (P < .001). The defect in forearm glucose uptake was attributable to a defect in glucose extraction (glucose AV difference, 1.1 +/- 0.2 versus 1.9 +/- 0.2 mmol/L, P < .001, IDDM versus normal subjects), not blood flow. Within the group of IDDM patients, hemoglobin A1c was inversely correlated with forearm blood flow during administration of acetylcholine (r = -.50, P < .02) but not sodium nitroprusside (r = .07). The ratio of endothelium-dependent to endothelium-independent blood flow was approximately 40% lower in patients with poor glycemic control than in normal subjects or patients with good or moderate glycemic control.

CONCLUSIONS

We conclude that chronic hyperglycemia is associated with impaired endothelium-dependent vasodilatation in vivo and with a glucose extraction defect during insulin stimulation. These data imply that chronic hyperglycemia impairs vascular function and insulin action via distinct mechanisms. The defect in endothelium-dependent vasodilatation could contribute to the increased cardiovascular risk in diabetes.

Enzymatic phosphorylation of muscle glycogen synthase: a mechanism for maintenance of metabolic homeostasis

We recently analyzed experimental studies of mammalian muscle glycogen synthesis using metabolic control analysis and concluded that glycogen synthase (GSase) does not control the glycogenic flux but rather adapts to the flux which is controlled bv the activity of the proximal glucose transport and hexokinase steps. This model did not provide a role for the well established relationship between GSase fractional activity, determined by covalent phosphorylation, and the rate of glycogen synthesis. Here we propose that the phosphorylation of GSase, which alters the sensitivity to allosteric activation by glucose 6-phosphate (G6P), is a mechanism for controlling the concentration of G6P instead of controlling the flux. When the muscle cell is exposed to conditions which favor glycogen synthesis such as high plasma insulin and glucose concentrations the fractional activity of GSase is increased in coordination with increases in the activity of glucose transport and hexokinase. This increase in GSase fractional activity helps to maintain G6P homeostasis by reducing the G6P concentration required to activate GSase allosterically to match the flux determined by the proximal reactions. This role for covalent phosphorylation also provides a novel solution to the Kacser and Acarenza paradigm which requires coordinated activity changes of the enzymes proximal and distal to a shared intermediate, to avoid unwanted flux changes.

Glucose processing during the intravenous glucose tolerance test

The impact of the dynamic changes in plasma glucose and insulin levels observed during a frequently sampled intravenous (IV) glucose tolerance test (FSIGT) on whole-body glucose processing and muscle glycogen metabolism is not known. Paired randomized FSIGTs were performed in eight healthy subjects (age, 31 years; range, 28 to 35; BMI, 25.4 kg/m2; range, 22.3 to 32.1), one with muscle biopsy samples and one without. The mean time average (0- to 40- and 0- to 120-minute) insulin levels during the test were 26.6 and 11.4 mU/1, respectively. Glucose oxidation increased following the IV glucose bolus (basal 1.34 +/- 0.21 v mean value at 0 to 120 minutes 2.09 +/- 0.22 mg/kg fat-free mass [FFM]/min, P < .02). In contrast, fractional glucose-6-phosphate [G-6-P]) (0.1/10 mmol/L) skeletal muscle glycogen synthase activity in muscle biopsies obtained before and following the IV glucose bolus (-30, 30,60, and 120 minutes, respectively) were unchanged (38.1% +/- 2.3%, 38.3% +/- 2.9%, 38.1% +/- 2.3%, 35.4% +/- 2.3%, NS). Skeletal muscle glycogen concentration decreased slightly (449 +/- 54, 439 +/- 55, and 383 +/- 29, and 438 +/- 48 mmol/kg dry weight, P =.05), indicating no net storage of glucose into glycogen during the FSIGT. G-6-P decreased (0.77 +/- 0.08, 0.64 +/- 0.07, 0.66 +/- 0.07, and 0.54 +/- 0.04 mmol/kg dry weight, P < .05). Levels of the insulin-regulatable glucose transporter, GLUT-4, were unchanged. Insulin sensitivity (Si), glucose effectiveness, and insulin secretion parameters (01 and 02) were not affected by the muscle biopsy procedure. In conclusion, the FSIGT is associated predominantly with increased whole-body glucose oxidation with no apparent activation of muscle glucose storage as glycogen. Thus, the Si measured by the FSIGT, although similar in magnitude to the clamp-derived parameter, represents primarily glucose oxidation, in contrast to the euglycemic clamp, which involves glucose oxidation and storage.

Islet transplantation under the kidney capsule fully corrects the impaired skeletal muscle glucose transport system of streptozocin diabetic rats

Chronic insulin therapy improves but does not restore impaired insulin-mediated muscle glucose uptake in human diabetes or muscle glucose uptake, transport, and transporter translocation in streptozocin diabetic rats. To determine whether this inability is due to inadequate insulin replacement, we studied fasted streptozocin-induced diabetic Lewis rats either untreated or after islet transplantation under the kidney capsule. Plasma glucose was increased in untreated diabetics and normalized by the islet transplantation (110 +/- 5, 452 +/- 9, and 102 +/- 3 mg/dl in controls, untreated diabetics, and transplanted diabetics, respectively). Plasma membrane and intracellular microsomal membrane vesicles were prepared from hindlimb skeletal muscle of basal and maximally insulin-stimulated rats. Islet transplantation normalized plasma membrane carrier-mediated glucose transport Vmax, plasma membrane glucose transporter content, and insulin-induced transporter translocation. There were no differences in transporter intrinsic activity (Vmax/Ro) among the three groups. Microsomal membrane GLUT4 content was reduced by 30% in untreated diabetic rats and normal in transplanted diabetics, whereas the insulin-induced changes in microsomal membrane GLUT4 content were quantitatively similar in the three groups. There were no differences in plasma membrane GLUT1 among the groups and between basal and insulin stimulated states. Microsomal membrane GLUT1 content was increased 60% in untreated diabetics and normalized by the transplantation. In conclusion, an adequate insulin delivery in the peripheral circulation, obtained by islet transplantation, fully restores the muscle glucose transport system to normal in streptozocin diabetic rats.

Over-expression of GLUT4 selectively in adipose tissue in transgenic mice: implications for nutrient partitioning

In summary, over-expression of GLUT4 selectively in fat causes increased flux of glucose into adipocytes and leads to increases in either the replication of immature pre-adipocytes or their differentiation into mature adipocytes resulting in an increase in fat cell number. This is the first model in which obesity is accounted for entirely by adipocyte hyperplasia and, therefore, is useful for studying the mechanisms involved in controlling fat cell number in vivo. GLUT4 over-expression in adipocytes of transgenic animals also increased whole- body insulin sensitivity. However, GLUT4 over-expression exclusively in adipocytes did not protect them from insulin resistance in vivo induced by high-fat feeding, in spite of the fact that insulin resistance was prevented at the level of the adipocyte. Interestingly, GLUT4 over-expression in fat protected the animals from developing further obesity when fed on a high-fat diet. It is possible that this failure to increase adiposity further is due to enhanced partitioning of glucose into fat, which may result in decreased glucose supply to muscle. This in turn may cause diversion of lipid to muscle to be oxidized as fatty acid. This diversion of lipid could result in protection against increased fat deposition in adipocytes. Further studies will be required in order to understand the molecular mechanisms by which GLUT4 over-expression in adipose tissues affects nutrient partitioning between muscle and adipose tissue and what the consequences of this are for whole-body fuel metabolism.

Role of insulin resistance in the pathogenesis of NIDDM

The above discussion illustrating the multitude of variables which influence insulin sensitivity in normal subjects challenges the prevailing view that insulin sensitivity is genetically determined in patients with NIDDM. The lack of accurate quantitation of all determinants of insulin sensitivity in the cross-sectional studies, and the difficulty in distinguishing between insulin secretion and sensitivity in prospective studies implies that the inherited metabolic abnormality in NIDDM still remains to be defined. The methodological difficulties in assessing the fate of glucose in many insulin-resistant states raise the possibility that defects in glycogen synthesis may not be rate-limiting for insulin action. It seems more likely that defects in glucose transport or phosphorylation are rate-limiting for glucose disposal, and thus represent either the primary regulatory steps or the steps via which distal defects signal their influence on glucose uptake. The above considerations should not be interpreted to suggest that insulin resistance is unimportant in the pathogenesis of NIDDM. It clearly increases the risk of developing NIDDM, and more importantly, its early amelioration by lifestyle modification seems sufficient to prevent NIDDM.

Impaired activity and gene expression of hexokinase II in muscle from non-insulin-dependent diabetes mellitus patients

After entering the muscle cell, glucose is immediately and irreversibly phosphorylated to glucose-6-phosphate by hexokinases (HK) I and II. Previous studies in rodents have shown that HKII may be the dominant HK in skeletal muscle. Reduced insulin-stimulated glucose uptake and reduced glucose-6-phosphate concentrations in muscle have been found in non-insulin-dependent diabetes mellitus (NIDDM) patients when examined during a hyperglycemic hyperinsulinemic clamp. These findings [correction of finding] are consistent with a defect in glucose transport and/or phosphorylation. In the present study comprising 29 NIDDM patients and 25 matched controls, we tested the hypothesis that HKII activity and gene expression are impaired in vastus lateralis muscle of NIDDM patients when examined in the fasting state. HKII activity in a supernatant of muscle extract accounted for 28 +/- 5% in NIDDM patients and 40 +/- 5% in controls (P = 0.08) of total muscle HK activity when measured at a glucose media of 0.11 mmol/liter and 31 +/- 4 and 47 +/- 7% (P = 0.02) when measured at 0.11 mmol/liter of glucose. HKII mRNA, HKII immunoreactive protein level, and HKII activity were significantly decreased in NIDDM patients (P < 0.0001, P = 0.03, and P = 0.02, respectively) together with significantly decreased glycogen synthase mRNA level and total glycogen synthase activity (P = 0.02 and P = 0.02, respectively). In the entire study population HKII activity estimated at 0.11 and 11.0 mM glucose was inversely correlated with fasting plasma glucose concentrations (r = -0.45, P = 0.004; r = -0.54, P < 0.0001, respectively) and fasting plasma nonesterified fatty acid concentrations (r = -0.46, P = 0.003; r = -0.37, P = 0.02, respectively). In conclusion, NIDDM patients are characterized by a reduced activity and a reduced gene expression of HKII in muscle which may be secondary to the metabolic peturbations. HKII contributes with about one-third of total HK activity in a supernatant of human vastus lateralis muscle.

In vivo regulation of muscle glycogen synthase and the control of glycogen synthesis

The activity of glycogen synthase (GSase; EC 2.4.1.11) is regulated by covalent phosphorylation. Because of this regulation, GSase has generally been considered to control the rate of glycogen synthesis. This hypothesis is examined in light of recent in vivo NMR experiments on rat and human muscle and is found to be quantitatively inconsistent with the data under conditions of glycogen synthesis. Our first experiments showed that muscle glycogen synthesis was slower in non-insulin-dependent diabetics compared to normals and that their defect was in the glucose transporter/hexokinase (GT/HK) part of the pathway. From these and other in vivo NMR results a quantitative model is proposed in which the GT/HK steps control the rate of glycogen synthesis in normal humans and rat muscle. The flux through GSase is regulated to match the proximal steps by "feed forward" to glucose 6-phosphate, which is a positive allosteric effector of all forms of GSase. Recent in vivo NMR experiments specifically designed to test the model are analyzed by metabolic control theory and it is shown quantitatively that the GT/HK step controls the rate of glycogen synthesis. Preliminary evidence favors the transporter step. Several conclusions are significant: (i) glucose transport/hexokinase controls the glycogen synthesis flux; (ii) the role of covalent phosphorylation of GSase is to adapt the activity of the enzyme to the flux and to control the metabolite levels not the flux; (iii) the quantitative data needed for inferring and testing the present model of flux control depended upon advances of in vivo NMR methods that accurately measured the concentration of glucose 6-phosphate and the rate of glycogen synthesis.

Mechanisms and time course of impaired skeletal muscle glucose transport activity in streptozocin diabetic rats

Skeletal muscle glucose transport is altered in diabetes in humans, as well as in rats. To investigate the mechanisms of this abnormality, we measured glucose transport Vmax, the total transporter number, their average intrinsic activity, GLUT4 and GLUT1 contents in skeletal muscle plasma membrane vesicles from basal or insulin-stimulated streptozocin diabetic rats with different duration of diabetes, treated or not with phlorizin. The glucose transport Vmax progressively decreased with the duration of diabetes. In the basal state, this decrease was primarily associated with the reduction of transporter intrinsic activity, which appeared earlier than any change in transporter number or GLUT4 and GLUT1 content. In the insulin-stimulated state, the decrease of transport was mainly associated with severe defects in transporter translocation. Phlorizin treatment partially increased the insulin-stimulated glucose transport by improving the transporter translocation defects. In conclusion, in streptozocin diabetes (a) reduction of intrinsic activity plays a major and early role in the impairment of basal glucose transport; (b) a defect in transporter translocation is the mechanism responsible for the decrease in insulin-stimulated glucose transport; and (c) hyperglycemia per se affects the insulin-stimulated glucose transport by altering the transporter translocation.

Methodological aspects, dose-response characteristics and causes of interindividual variation in insulin stimulation of limb blood flow in normal subjects

To resolve some of the controversy regarding insulin regulation of blood flow, we performed in 20 normal subjects a) a reproducibility study of plethysmographic, Doppler ultrasound and laser Doppler blood flow measurements (n = 7), b) a sequential insulin dose-response study with measurement of forearm (plethysmography), leg (Doppler ultrasound) and skin (laser Doppler) blood flow (n = 12), and c) a sequential insulin dose-response study with comparison of forearm (plethysmography) and calf (plethysmography) blood flow (n = 8). We also searched for factors which might explain the interindividual variation in the blood flow response to insulin. During sequential insulin infusions (2 h each, 61 +/- 2, 139 +/- 6, 462 +/- 15 mU/l), forearm blood flow increased by 17 +/- 6, 50 +/- 14 and 113 +/- 17% (p < 0.05 or less between steps), respectively. The increase at the 61 +/- 2 mU/l insulin concentration barely exceeded methodological variation (13 +/- 2%). In contrast to the continuous increase in blood flow, the glucose arterio venous difference reached its maximum (1.7 +/- 0.2 mmol/l) at the lowest 61 +/- 2 mU/l insulin concentration and remained constant thereafter. Forearm and calf blood flow responses to insulin were virtually identical when determined with plethysmography. In contrast, only a 27% increase was detected in femoral flow index as determined by Doppler ultrasound. Forearm blood flow (per forearm volume) was highly correlated with the relative forearm muscle content (mean 59 +/- 5%, range 24-81%) both basally (r = 0.86, p < 0.001, n = 12) and at all insulin concentrations (r = 0.85-0.92, p < 0.001) indicating that the percent of forearm that is muscle explains 70-85% of interindividual variation in blood flow. In conclusion 1) physiological insulin concentrations stimulate glucose uptake mainly by increasing glucose extraction while supraphysiological insulin concentrations increase forearm glucose uptake predominantly via increases in blood flow. 2) The dose-response characteristics of insulin stimulation of forearm and calf blood flow are similar when determined with strain-gauge plethysmography. 3) Relative forearm muscle content is a key factor in determining both basal forearm blood flow and the interindividual variation in its response to insulin in normal subjects.

Decreased muscle glucose transport/phosphorylation is an early defect in the pathogenesis of non-insulin-dependent diabetes mellitus

Recent studies have demonstrated that reduced insulin-stimulated muscle glycogen synthesis is the major cause of insulin resistance in patients with non-insulin-dependent diabetes mellitus (NIDDM). This reduced rate has been assigned to a defect in either glucose transport or hexokinase activity. However it is unknown whether this is a primary or acquired defect in the pathogenesis of NIDDM. To examine this question, we measured the rate of muscle glycogen synthesis and the muscle glucose 6-phosphate (G6P) concentration using 13C and 31P NMR spectroscopy as well as oxidative and nonoxidative glucose metabolism in six lean, normoglycemic offspring of parents with NIDDM and seven age/weight-matched control subjects under hyperglycemic (approximately 11 mM)-hyperinsulinemic (approximately 480 pM) clamp conditions. The offspring of parents with NIDDM had a 50% reduction in total glucose metabolism, primarily due to a decrease in the nonoxidative component. The rate of muscle glycogen synthesis was reduced by 70% (P < 0.005) and muscle G6P concentration was reduced by 40% (P < 0.003), which suggests impaired muscle glucose transport/hexokinase activity. These changes were similar to those previously observed in subjects with fully developed NIDDM. When the control subjects were studied at similar insulin levels (approximately 440 pM) but euglycemic plasma glucose concentration (approximately 5 mM), both the rate of glycogen synthesis and the G6P concentration were reduced to values similar to the offspring of parents with NIDDM. We conclude that insulin-resistant offspring of parents with NIDDM have reduced nonoxidative glucose metabolism and muscle glycogen synthesis secondary to a defect in muscle glucose transport/hexokinase activity prior to the onset of overt hyperglycemia. The presence of this defect in these subjects suggests that it may be the primary factor in the pathogenesis of NIDDM.

Overexpression of Glut4 protein in muscle increases basal and insulin-stimulated whole body glucose disposal in conscious mice

The effect of increased Glut4 protein expression in muscle and fat on the whole body glucose metabolism has been evaluated by the euglycemic hyperinsulinemic clamp technique in conscious mice. Fed and fasting plasma glucose concentrations were 172 +/- 7 and 78 +/- 7 mg/dl, respectively, in transgenic mice, and were significantly lower than that of nontransgenic littermates (208 +/- 5 mg/dl in fed; 102 +/- 5 mg/dl in fasting state). Plasma lactate concentrations were higher in transgenic mice, (6.5 +/- 0.7 mM in the fed and 5.8 +/- 1.0 mM in fasting state) compared with that of non-transgenic littermates (4.7 +/- 0.3 mM in the fed and 4.2 +/- 0.5 mM in fasting state). In the fed state, the rate of whole body glucose disposal was 70% higher in transgenic mice in the basal state, 81 and 54% higher during submaximal and maximal insulin stimulation. In the fasting state, insulin-stimulated whole body glucose disposal was also higher in the transgenic mice. Hepatic glucose production after an overnight fast was 24.8 +/- 0.7 mg/kg per min in transgenic mice, and 25.4 +/- 2.7 mg/kg per min in nontransgenic mice. Our data demonstrate that overexpression of Glut4 protein in muscle increases basal as well as insulin-stimulated whole body glucose disposal. These results suggest that skeletal muscle glucose transport is rate-limiting for whole body glucose disposal and that the Glut4 protein is a potential target for pharmacological or genetic manipulation for treatment of patients with non-insulin-dependent diabetes mellitus.

Alterations in carbohydrate metabolism during stress: a review of the literature

Patients with sepsis, burn, or trauma commonly enter a hypermetabolic stress state that is associated with a number of alterations in carbohydrate metabolism. These alterations include enhanced peripheral glucose uptake and utilization, hyperlactatemia, increased glucose production, depressed glycogenesis, glucose intolerance, and insulin resistance. The hypermetabolic state is induced by the area of infection or injury as well as by organs involved in the immunologic response to stress; it generates a glycemic milieu that is directed toward satisfying an obligatory requirement for glucose as an energy substrate. This article reviews experimental and clinical data that indicate potential mechanisms for these alterations and emphasizes aspects that have relevance for the clinician.

Impaired glucose transport in skeletal muscle but normal GLUT-4 tissue distribution in glucose-infused rats

This study was undertaken to determine if glucose toxicity in normal rats caused decreased whole body insulin-stimulated glucose disposal and in vivo impaired muscle glucose transport and, if so, whether it was mediated by changes in GLUT-4 content or tissue distribution. Rats were infused with 50% dextrose for 48 h after which they were clamped and injected with 2-deoxy-D-[3H]glucose. Hindlimb muscles were removed for measurement of uptake of radioactivity (glucose transport) and GLUT-4 levels in total, plasma and internal membrane fractions. Dextrose infusions caused significant hyperglycemia [15.5 +/- 1.4 vs. 6.7 +/- 0.3 (SE) mM], hyperinsulinemia [678 +/- 108 vs. 168 +/- 42 (SE) pM], and depressed insulin-mediated whole body glucose disposal [12.8 +/- 2.0 vs. 47.0 +/- 10.6 (SE) mg glucose.kg-1.min-1.pmol insulin-1.1(-1) x 10(3)]. Muscle glucose transport (ng.min-1.mg tissue-1) was significantly decreased in biceps (4.0 +/- 0.6 vs. 13.4 +/- 2.5), gastrocnemius (4.6 +/- 1.1 vs. 12.9 +/- 2.2), and plantaris (5.5 +/- 0.7 vs. 17.5 +/- 3.6) muscles compared with saline-infused rats. The difference in the soleus muscle (13.2 +/- 1.6 vs. 19.4 +/- 2.7) did not quite reach statistical significance. There were no differences in total, plasma, or internal membrane GLUT-4 content between the two groups. It is concluded that glucose toxicity causes impaired insulin-stimulated glucose transport, probably due to decreased activity of GLUT-4.

Regulation of glucose transport by pioglitazone in cultured muscle cells

Recent evidence suggests that pioglitazone, a thiazolidinedione hypoglycemic agent, acts by increasing insulin responsiveness at the peripheral level. We studied the effect of pioglitazone (1 to 50 micrograms/mL) on the glucose transporter and glucose transport in BC3H-1 cells, a continuously cultured skeletal muscle cell line lacking the myoD transcription factor required for cell fusion. Glucose-fed cells (25 mmol/L) responded to insulin with a more than twofold increase in 2-deoxyglucose (2-DOG) uptake as compared with baseline. Treating these cells with pioglitazone alone for 24 hours resulted in a dose-dependent increase in hexose uptake, reaching twofold at 50 micrograms/mL. Combining long-term pioglitazone (10 micrograms/mL for 24 hours) and short-term insulin treatment resulted in an additive effect on 2-DOG uptake over a wide range of insulin concentrations (0.1 to 100 nmol/L) without the desensitization to 2-DOG uptake seen in other systems following long-term insulin administration. To determine the basis of the increased glucose uptake response, the level of specific mRNA and immunoreactive glucose transporter protein was determined. Northern and Western blot studies on glucose-treated cells (25 mmol/L) showed that glucose transporter mRNA and protein increased in parallel following treatment with either pioglitazone or insulin alone. The combination of insulin with pioglitazone resulted in an additive stimulation of glucose transporter mRNA and protein. In summary, pioglitazone stimulates hexose uptake both independently and in combination with insulin in BC3H-1 myocytes. These effects are largely accounted for by increases in glucose transporter mRNA and protein, indicating its potential efficacy in the treatment of non-insulin-dependent diabetes mellitus (NIDDM).(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased concentrations of GLUT1 and GLUT3 glucose transporters in the brains of patients with Alzheimer's disease

Glucose metabolism is depressed in the temporal and parietal regions of the cortex in patients with Alzheimer's disease. We measured the concentrations of two glucose transporters, GLUT1 and GLUT3, in six regions of brains from both control subjects and patients with Alzheimer's disease. The concentrations of both transporters were reduced in the cerebral cortex, with larger and highly significant reductions observed for GLUT3, the putative neuronal glucose transporter. The reductions in GLUT3 were greater than the loss of synapses, and should be considered as a potential cause of the deficits in glucose metabolism.

Effects of glipizide on glucose metabolism and muscle content of the insulin-regulatable glucose transporter (GLUT 4) and glycogen synthase activity during hyperglycaemia in type 2 diabetic patients

To examine whether sulphonylureas influence hyperglycaemia-induced glucose disposal and suppression of hepatic glucose production (HGP) in type 2 diabetes mellitus, a 150-min hyperglycaemic (plasma glucose 14 mmol/l) clamp with concomitant somatostatin infusion was used in eight type 2 diabetic patients before and after 6 weeks of glipizide (GZ) therapy. During the clamp a small replacement dose of insulin was given (0.15 mU/kg per min). Isotopically determined glucose-induced glucose uptake was similar before and after GZ administration which led to improved glycaemic control (basal plasma glucose 12.2 +/- 1.3 vs 8.9 +/- 0.7 mmol/l; P < 0.01). Glucose-induced suppression of HGP was, however, more pronounced during GZ treatment (0.96 +/- 0.14 vs 1.44 +/- 0.20 mg/kg per min; P < 0.02). Following GZ treatment hyperglycaemia failed to stimulate glycogen synthase activity. Moreover, GZ resulted in a significant increase in the immunoreactive abundance of the insulin-regulatable glucose transport protein (GLUT 4) (P < 0.02). In conclusion, these results suggest that GZ therapy in type 2 diabetic patients enhances hepatic sensitivity to hyperglycaemia, while glucose-induced glucose uptake remains unaffected. In addition, GZ tends to normalize the activity of glycogen synthase and increases the content of GLUT 4 protein in skeletal muscle.