Defects In Glucose Metabolism Research Articles

Defective glucose and lipid metabolism in human immunodeficiency virus-infected patients with lipodystrophy involve liver, muscle tissue and pancreatic β-cells

Lipodystrophy and insulin resistance are prevalent among human immunodeficiency virus (HIV)-infected patients on combined antiretroviral therapy (HAART). Aiming to provide a detailed description of the metabolic adverse effects of HIV-lipodystrophy, we investigated several aspects of glucose metabolism, lipid metabolism and beta-cell function in lipodystrophic HIV-infected patients. [3-3H]glucose was applied during euglycaemic hyperinsulinaemic clamps in association with indirect calorimetry in 43 normoglycaemic HIV-infected patients (18 lipodystrophic patients on HAART (LIPO), 18 patients without lipodystrophy on HAART (NONLIPO) and seven patients who were naive to antiretroviral therapy (NAIVE) respectively). beta-cell function was evaluated by an intravenous glucose tolerance test. Compared with NONLIPO and NAIVE separately, LIPO displayed markedly reduced ratio of limb to trunk fat (RLF; > 34%, P < 0.001), hepatic insulin sensitivity (> 40%, P < 0.03), incremental glucose disposal (>50%, P < 0.001) and incremental exogenous glucose storage (>50%, P < 0.05). Furthermore, LIPO displayed reduced incremental glucose oxidation (P < 0.01), increased clamp free fatty acids (P < 0.05) and attenuated insulin-mediated suppression of lipid oxidation (P < 0.05) compared with NONLIPO. In combined study groups, RLF correlated with hepatic insulin sensitivity (r = 0.69), incremental glucose disposal (r = 0.71) and incremental exogenous glucose storage (r = 0.40), all P < 0.01. Disposition index (i.e. first-phase insulin response to intravenous glucose multiplied by incremental glucose disposal) was reduced by 46% (P = 0.05) in LIPO compared with the combined groups of NONLIPO and NAIVE, indicating an impaired adaptation of beta-cell function to insulin resistance in LIPO. Our data suggest that normoglycaemic lipodystrophic HIV-infected patients display impaired glucose and lipid metabolism in multiple pathways involving liver, muscle tissue and beta-cell function.

Renal glucosuria due to SGLT2 mutations

Isolated renal glucosuria results from mutations in SGLT2, which codes for an active transporter specific for d-glucose and expressed in the luminal membrane of the renal proximal tubule. In affected individuals, glucosuria leads to pursuit of hyperglycemia to exclude defects in glucose metabolism, and to investigation of renal proximal tubular function to exclude renal Fanconi syndrome. Here we present clinical and molecular data regarding a 19-year-old woman with isolated glucosuria. She was compound heterozygous for two SGLT2 mutations, i.e., a new missense mutation, T200K, and a known missense mutation, N654S.

Impaired insulin signaling in the liver of transgenic rats with low circulating growth hormone levels.

GH is known to regulate glucose and lipid metabolism as well as body growth. Controversy exists as to whether GH-deficient adults are indeed insulin sensitive or insulin resistant. In GH-deficient animal models, however, no clear observation indicating insulin resistance has been made, while increased insulin sensitivity has been reported in those animals. We have produced human GH (hGH) transgenic rats characterized by low circulating hGH levels and virtually no endogenous rat GH secretion. Although the body length of the transgenic rat is normal, they develop massive obesity and insulin resistance, indicating that the transgenic rat is a good model for the analysis of insulin resistance under GH deficiency. In this study, we have examined how GH deficiency affects the early steps of insulin signaling in the liver of the transgenic rat. Circulating glucose and insulin concentrations were significantly higher in the transgenic rats than in their littermates. In addition, impaired glucose tolerance was observed in the transgenic rat. The amount of insulin receptor was smaller in the liver of the transgenic rat, resulting in decreased tyrosine phosphorylation in response to insulin stimulation. The amounts of insulin receptor substrate-1 and -2 (IRS-1 and -2) and insulin-stimulated phosphorylation of IRSs were also smaller in the transgenic rat. Despite the decrease in tyrosine phosphorylation levels of IRSs being mild to moderate (45% for IRS-1 and 16% for IRS-2), associated phosphatidylinositol 3-kinase (PI3-kinase) activity was not increased by insulin stimulation at all in the transgenic rat. To elucidate whether this discrepancy resulted from the alteration in binding of the p85 subunit of PI3-kinase to phosphotyrosine residues of the IRSs, we determined the amount of p85 subunit in the immunocomplexes with anti-phosphotyrosine antibody. Insulin did not affect the amount of p85 subunit associated with phosphotyrosine in the transgenic rats, while it significantly increased in the controls, indicating that alteration may have occurred at the sites of phosphorylated tyrosine residues in IRSs. These results suggest that GH deficiency in the transgenic rat leads to impairment in at least the early steps of insulin signaling in the liver with a resultant defect in glucose metabolism.

Wolff-Parkinson-White Syndrome

The history of the Wolff-Parkinson-White (WPW) syndrome is a 50-year–long tale of speculation and discussion among physiologists, anatomists, and clinicians about how to explain the frequently occurring tachycardias in patients showing a strange ECG.1 That riddle was solved in 1967 in Amsterdam when Durrer and associates2 showed that the WPW syndrome was based on a second connection between the atrium and ventricle apart from the normal AV node–His pathway. The presence of conduction over such an accessory AV pathway was demonstrated by epicardial mapping during sinus rhythm. The crucial role of that structure in the tachycardia mechanism was proven by programmed electrical stimulation of the heart and mapping of cardiac activation by intracardiac catheters.3 It was shown that an impulse circulating in a circuit (consisting of atrium–AV node–His pathway–ventricle–accessory AV pathway) was responsible for the tachycardias usually found in these patients. Essential for initiation of the tachycardia is a difference in the electrophysiological properties of the 2 AV connections, which allows the occurrence of unidirectional block in 1 of the 2 structures. This difference can be exposed not only by critically timed atrial and ventricular premature beats but also by changes in the sinus rate (eg, during exercise). For perpetuation of the tachycardia, the circulation time of the impulse in the tachycardia circuit must be longer than the duration of the refractory period of the different parts of the tachycardia circuit.4 See p 3030 It also became clear that the anterograde refractory period of the accessory AV pathway determines the ventricular rate during atrial fibrillation.5 This helps to identify WPW patients at risk for dying suddenly when atrial fibrillation supervenes. The demonstration that an accessory AV pathway is an essential link both in the reentrant tachycardia mechanism and the ventricular rate during atrial fibrillation has …

Glucose uptake and metabolism by cultured human skeletal muscle cells: rate-limiting steps.

To use primary cultures of human skeletal muscle cells to establish defects in glucose metabolism that underlie clinical insulin resistance, it is necessary to define the rate-determining steps in glucose metabolism and to improve the insulin response attained in previous studies. We modified experimental conditions to achieve an insulin effect on 3-O-methylglucose transport that was more than twofold over basal. Glucose phosphorylation by hexokinase limits glucose metabolism in these cells, because the apparent Michaelis-Menten constant of coupled glucose transport and phosphorylation is intermediate between that of transport and that of the hexokinase and because rates of 2-deoxyglucose uptake and phosphorylation are less than those of glucose. The latter reflects a preference of hexokinase for glucose over 2-deoxyglucose. Cellular NAD(P)H autofluorescence, measured using two-photon excitation microscopy, is both sensitive to insulin and indicative of additional distal control steps in glucose metabolism. Whereas the predominant effect of insulin in human skeletal muscle cells is to enhance glucose transport, phosphorylation, and steps beyond, it also determines the overall rate of glucose metabolism.

Skeletal muscle triglyceride. An aspect of regional adiposity and insulin resistance.

Recent evidence derived from four independent methods indicates that an excess triglyceride storage within skeletal muscle is linked to insulin resistance. Potential mechanisms for this association include apparent defects in fatty acid metabolism that are centered at the mitochondria in obesity and in type 2 diabetes. Specifically, defects in the pathways for fatty acid oxidation during postabsorptive conditions are prominent, leading to diminished use of fatty acids and increased esterification and storage of lipid within skeletal muscle. These impairments in fatty acid metabolism during fasting conditions may be related to a metabolic inflexibility in insulin resistance that is not limited to defects in glucose metabolism during insulin-stimulated conditions. Thus, there is substantial evidence implicating perturbations in fatty acid metabolism during accumulation of skeletal muscle triglyceride and in the pathogenesis of insulin resistance. Weight loss by caloric restriction improves insulin sensitivity, but the effects on fatty acid metabolism are less conspicuous. Nevertheless, weight loss decreases the content of triglyceride within skeletal muscle, perhaps contributing to the improvement in Insulin action with weight loss. Alterations in skeletal muscle substrate metabolism provide insight into the link between skeletal muscle triglyceride accumulation and insulin resistance, and they may lead to more appropriate therapies to improve glucose and fatty acid metabolism in obesity and in type 2 diabetes.

Matching treatment to pathophysiology in type 2 diabetes

Background: Because type 2 diabetes is a progressive condition, > 50% of all patients whose disease is initially controlled with diet and exercise will eventually need single or multiple pharmacologic agents to maintain adequate glycemic control. Although current treatment standards require that combination therapy be instituted only after the failure of monotherapy, the results of the Diabetes Control and Complications Trial and the United Kingdom Prospective Diabetes Study suggest that aggressive initial treatment is crucial to slowing the evolution of long-term complications associated with this disease. Objectives: This article reviews the diagnosis and classification of type 2 diabetes, describes the multiple defects in glucose metabolism associated with the disease, and discusses the various pharmacologic options for achieving glycemic control in these patients. Methods: The information in this review was compiled through a search of MEDLINE®. Search terms included but were not limited to type 2 diabetes and antihyperglycemic agents. In addition, abstracts were identified using the Web sites of diabetes-related professional organizations. Results: Two pathophysiologic mechanisms, insulin resistance and impaired insulin secretion, are usually present at diagnosis in type 2 diabetes. Several studies have shown that combination therapy with antihyperglycemic agents having different mechanisms of action provides greater efficacy than treatment with single agents. Conclusions: Current research suggests that early aggressive treatment with combination therapy achieves glycemic control at lower doses and with fewer side effects than monotherapy with either component.

GLUT4: a key player regulating glucose homeostasis? Insights from transgenic and knockout mice (review).

Studies in which GLUT4 has been overexpressed in transgenic mice provide definitive evidence that glucose transport is rate limiting for muscle glucose disposal. Transgenic overexpression of GLUT4 selectively in skeletal muscle results in increased whole body glucose uptake and improves glucose homeostasis. These studies strengthen the hypothesis that the level of muscle GLUT4 affects the rate of whole body glucose disposal, and underscore the importance of GLUT4 in skeletal muscle for maintaining whole body glucose homeostasis. Studies in which GLUT4 has been ablated or 'knocked-out' provide proof that GLUT4 is the primary effector for mediating glucose transport in skeletal muscle and adipose tissue. Genetic ablation of GLUT4 results in impaired insulin tolerance and defects in glucose metabolism in skeletal muscle and adipose tissue. Because impaired muscle glucose transport leads to reduced whole body glucose uptake and hyperglycaemia, understanding the molecular regulation of glucose transport in skeletal muscle is important to develop effective strategies to prevent or reduce the incidence of Type II diabetes mellitus. In patients with Type II diabetes mellitus, reduced glucose transport in skeletal muscle is a major factor responsible for reduced whole body glucose uptake. Overexpression of GLUT4 in skeletal muscle improves glucose homeostasis in animal models of diabetes mellitus and protects against the development of diabetes mellitus. Thus, GLUT4 is an attractive target for pharmacological intervention strategies to control glucose homeostasis. This review will focus on the current understanding of the role of GLUT4 in regulating cellular glucose uptake and whole body glucose homeostasis.

Increased insulin sensitivity and decreased insulin secretion in offspring of insulin-sensitive type 2 diabetic patients

To investigate the early defects of glucose metabolism in insulin-sensitive type 2 diabetes, we performed oral and frequently sampled intravenous glucose tolerance tests (OGTT and FSIGT) with minimal model analysis in 15 offspring of Japanese type 2 diabetics with normal insulin sensitivity (insulin resistance index of homeostasis model assessment [HOMA-R] < 2.0) and in 20 healthy control subjects without a family history of type 2 diabetes. The frequency of impaired glucose tolerance (IGT) was 40% (6 of 15) in the offspring and 0% (0 of 20) in the controls. Fasting plasma glucose (4.8 ± 0.1 v 4.6 ± 0.1 mmol/L, P = .18) and immunoreactive insulin ([IRI] 29.9 ± 2.5 v 28.3 ± 2.5 pmol/L, P ± = .64) were comparable between the offspring and the controls. The rate of glucose disappearance (KG) was significantly lower in the offspring versus the control group (2.00 ± 0.22 v 2.60 ± 0.17 min−1, P = .03). The insulin sensitivity index (Si) was significantly greater in the offspring versus the controls (2.68 ± 0.41 v 1.71 ± 0.17 × 10−4 · min−1 · pmol/L, P = .02). First-phase insulin secretion (FPI) to intravenous glucose was significantly lower in the offspring versus the control group (886 ± 110 v 2,296 ± 267 min · pmol/L, P < .01). Glucose effectiveness (SG) was comparable between the offspring and control groups. The disposition index (Si × FPI) was significantly lower in the offspring versus the controls (2,106 ± 256 v 3,652 ± 490 × 10−4, P = .02). When the offspring were subdivided into 2 groups by glucose tolerance status, both normal glucose tolerance (NGT) offspring and IGT offspring showed a significant decrease in FPI and increase in Si. Thus, although the offspring of insulin-sensitive type 2 diabetics had increased insulin sensitivity, the impairment in insulin secretion was more dominant. Our results suggest that the early metabolic abnormality in insulin-sensitive type 2 diabetes is an insulin secretory dysfunction despite increased insulin sensitivity.

A mutation in PRKAG3 associated with excess glycogen content in pig skeletal muscle.

A high proportion of purebred Hampshire pigs carries the dominant RN- mutation, which causes high glycogen content in skeletal muscle. The mutation has beneficial effects on meat content but detrimental effects on processing yield. Here, it is shown that the mutation is a nonconservative substitution (R200Q) in the PRKAG3 gene, which encodes a muscle-specific isoform of the regulatory gamma subunit of adenosine monophosphate-activated protein kinase (AMPK). Loss-of-function mutations in the homologous gene in yeast (SNF4) cause defects in glucose metabolism, including glycogen storage. Further analysis of the PRKAG3 signaling pathway may provide insights into muscle physiology as well as the pathogenesis of noninsulin-dependent diabetes mellitus in humans, a metabolic disorder associated with impaired glycogen synthesis.

The importance of the β-cell in the pathogenesis of type 2 diabetes mellitus

β-cell dysfunction and insulin resistance are two central, interrelated defects in the pathophysiology of type 2 diabetes. By the time a patient’s hyperglycemia is recognized, disruption of the normal relationship between β-cell function and insulin sensitivity is already well established. The pathophysiology and progression of defects in glucose metabolism from normal glucose tolerance to impaired glucose tolerance to frank type 2 diabetes have been studied extensively. Insulin sensitivity has wide intersubject variability, and many individuals at risk for type 2 diabetes are insulin resistant. β-cell changes in patients with type 2 diabetes include defects in insulin secretion, proinsulin conversion to insulin, and amyloid deposition in the islet. Studies in several ethnic groups have established that the progression from normal glucose tolerance to frank type 2 diabetes results from a gradual deterioration in β-cell function in the presence of insulin resistance. Furthermore, the recently completed landmark United Kingdom Prospective Diabetes Study demonstrated that type 2 diabetes is a progressive disease and that this progression is due to declining β-cell function.

Peripheral tissue glucose uptake is not reduced after an oral glucose load in Southern Italian subjects at risk of developing non-insulin-dependent diabetes mellitus

Studies searching for the inherited defects that cause non—insulin-dependent diabetes mellitus (NIDDM) have been performed mostly in Northern European subjects using the hyperinsulinemic clamp technique. The conclusion drawn from most of these studies is that peripheral insulin resistance is likely a primary inherited defect. Our aim was to examine early defects in glucose metabolism using a more physiological technique in a different ethnic group. For this, a double-label oral glucose tolerance test (OGTT) was performed in young diabetes-prone Southern Italian subjects who had both parents with NIDDM (relatives) and in subjects with no family history of NIDDM (matched for age, weight, and ethnicity). Fasting plasma glucose and insulin in the relatives were normal; however, they had impaired glucose tolerance during the OGTT. This was due to reduced hepatic glucose uptake (17.9 ± 2.8 v 28.1 ± 2.3 g, P < .02). No defects were found in the metabolic clearance rate (MCR) of glucose or endogenous glucose production. During an intravenous glucose tolerance test (IVGTT), insulin sensitivity was again found to be normal (3.04 ± 0.65 in relatives v 2.33 ± 0.38 min −1 per μmol · L −1 · min in controls), with a marked reduction in first-phase insulin secretion in the relatives (110 ± 12 v 211 ± 18 pmol · L −1 · min per mmol · L −1, P < .001). A strong correlation was found between hepatic glucose uptake and insulin secretion ( r = .81, P < .001), which may suggest that the same abnormality operates in both the liver and pancreas. Therefore, the metabolic defect that causes hyperglycemia in diabetes-prone subjects is not always a reduced peripheral insulin sensitivity. The genetic basis of NIDDM may differ between different ethnic groups.

Identification of Cd36 (Fat) as an insulin-resistance gene causing defective fatty acid and glucose metabolism in hypertensive rats.

The human insulin-resistance syndromes, type 2 diabetes, obesity, combined hyperlipidaemia and essential hypertension, are complex disorders whose genetic basis is unknown. The spontaneously hypertensive rat (SHR) is insulin resistant and a model of these human syndromes. Quantitative trait loci (QTLs) for SHR defects in glucose and fatty acid metabolism, hypertriglyceridaemia and hypertension map to a single locus on rat chromosome 4. Here we combine use of cDNA microarrays, congenic mapping and radiation hybrid (RH) mapping to identify a defective SHR gene, Cd36 (also known as Fat, as it encodes fatty acid translocase), at the peak of linkage to these QTLs. SHR Cd36 cDNA contains multiple sequence variants, caused by unequal genomic recombination of a duplicated ancestral gene. The encoded protein product is undetectable in SHR adipocyte plasma membrane. Transgenic mice overexpressing Cd36 have reduced blood lipids. We conclude that Cd36 deficiency underlies insulin resistance, defective fatty acid metabolism and hypertriglyceridaemia in SHR and may be important in the pathogenesis of human insulin-resistance syndromes.

Defects in insulin secretion and insulin action in non-insulin-dependent diabetes mellitus are inherited. Metabolic studies on offspring of diabetic probands.

No studies are available that have compared early defects in glucose metabolism in the offspring of insulin-deficient and insulin-resistant probands with non-insulin-dependent diabetes mellitus (NIDDM). To investigate this issue, we evaluated insulin secretion capacity with oral and intravenous glucose tolerance tests and with the hyperglycemic clamp, and insulin action with the euglycemic insulin clamp in 20 offspring of NIDDM patients with low fasting C-peptide (+/-450 pmol/liter), reflecting deficient insulin secretion (IS-group), 18 offspring of NIDDM patients with high fasting C-peptide (>/= 880 pmol/liter), reflecting insulin resistance (IR-group), and 14 healthy control subjects without a family history of NIDDM. The frequency of impaired glucose tolerance was 45.0% in the IS-group and 50% in the IR-group. The IS-group had lower insulin-glucose response at 30 min in the oral glucose tolerance test (85.2+/-10.0 pmol insulin per mmol glucose) than the control group (136.4+/-23.1 pmol insulin per mmol glucose; P < 0.05) and the IR-group (115.6+/-11.8 pmol insulin per mmol glucose; P = 0.05). Furthermore, the acute insulin response during the first 10 min of an intravenous glucose tolerance test was lower in the IS-group than in the IR-group. Maximal insulin secretion capacity evaluated by C-peptide levels during the hyperglycemic clamp did not differ between the groups. The IR-group had lower rates of whole body glucose uptake (60.1+/-4.6 micromol per lean body mass per minute) than did the control group (84.2+/-5.0 micromol per lean body mass per minute; P < 0.001) or the IS-group (82.6+/-5.9 micromol per lean body mass per minute; P < 0.01) and this was due to reduced glucose nonoxidation. To conclude, both impaired insulin secretion and insulin action seem to be inherited and could represent the primary defects in glucose metabolism in the offspring of NIDDM probands.

Insulin signaling and glucose transport in insulin resistant skeletal muscle. Special reference to GLUT4 transgenic and GLUT4 knockout mice.

Glucose homeostasis is impaired in patients with non-insulin dependent diabetes mellitus (NIDDM) and this defect in due in part, to defects in glucose transport in skeletal muscle. Intense interest is now focused on whether reduced insulin-mediated glucose transport in muscle from NIDDM patients results from alterations in the insulin signal transduction pathway or from alterations in traffic and/or translocation of GLUT4 to the plasma membrane. Recently, potential targets for impaired traffic/translocation of GLUT4 have been reported to include defective phosphorylation of IRS-1 and reduced PI-3 kinase activity. In addition to insulin signaling defects, impaired glucose transport may result from a defect(s) in the activation or functional capacity of GLUT4. Because GLUT4 is dysregulated in skeletal muscle from NIDDM patients, it is an attractive target for gene therapy. Overexpression of GLUT4 in muscle results in increased glucose uptake and metabolism, and protects against the development of insulin resistance in transgenic mice. Genetic ablation of GLUT4 results in impaired insulin tolerance and defects in glucose metabolism in skeletal muscle. Because impaired muscle glucose transport leads to reduced whole body glucose uptake and hyperglycemia, understanding the molecular regulation of glucose transport in skeletal muscle is necessary to develop effective strategies to prevent or reduce the incidence of NIDDM.

Multicopy FZF1 (SUL1) suppresses the sulfite sensitivity but not the glucose derepression or aberrant cell morphology of a grr1 mutant of Saccharomyces cerevisiae.

An ssu2 mutation in Saccharomyces cerevisiae, previously shown to cause sulfite sensitivity, was found to be allelic to GRR1, a gene previously implicated in glucose repression. The suppressor rgt1, which suppresses the growth defects of grr1 strains on glucose, did not fully suppress the sensitivity on glucose or nonglucose carbon sources, indicating that it is not strictly linked to a defect in glucose metabolism. Because the Cln1 protein was previously shown to be elevated in grr1 mutants, the effect of CLN1 overexpression on sulfite sensitivity was investigated. Overexpression in GRR1 cells resulted in sulfite sensitivity, suggesting a connection between CLN1 and sulfite metabolism. Multicopy FZF1, a putative transcription factor, was found to suppress the sulfite sensitive phenotype of grr1 strains, but not the glucose derepression or aberrant cell morphology. Multicopy FZF1 was also found to suppress the sensitivity of a number of other unrelated sulfite-sensitive mutants, but not that of ssu1 or met20, implying that FZF1 may act through Ssu1p and Met20p. Disruption of FZF1 resulted in sulfite sensitivity when the construct was introduced in single copy at the FZF1 locus in a GRR1 strain, providing evidence that FZF1 is involved in sulfite metabolism.

Dexamethasone-induced impairment in skeletal muscle glucose transport is not reversed by inhibition of free fatty acid oxidation

Our previous studies suggested a possible role for the glucose-free fatty acid (FFA) cycle, ie, preferential utilization of FFA by muscle at the expense of glucose, in dexamethasone (DEX)-induced insulin resistance. To determine whether this resistance could be reversed by inhibiting FFA utilization, we used etomoxir, a potent inhibitor of mitochondrial FFA oxidation. Male Sprague-Dawley rats were injected subcutaneously with 1 mg/kg DEX or the vehicle every other day for 10 days, and half of each group was administered 10 mg/kg etomoxir by gavage once per day and 1 hour before the experiment. As expected, etomoxir treatment increased serum FFA levels and inhibited FFA oxidation by diaphragm in vitro. Administration of etomoxir decreased serum glucose and insulin concentrations under basal conditions in both control and DEX-treated animals, implying enhanced insulin sensitivity. DEX treatment significantly increased endogenous glucose production and decreased whole-body glucose disposal, as well as 2-deoxyglucose (2-DG) uptake by skeletal muscle during euglycemic-hyperinsulinemic clamps. Administration of etomoxir led to small but significant increases in glucose disposal rates of both control (14%) and DEX (23%) groups, but had no effect on residual endogenous glucose production. Thus, DEX-induced insulin resistance was marginally ameliorated but not completely reversed by etomoxir. Depressed 2-DG uptake by individual muscle tissues observed in the present study in conjunction with the absence of free intracellular glucose in muscle tissue following glucose-insulin infusion strongly suggests that the primary defect in glucose metabolism is at the level of transport. Neither overall abundance of the insulin-sensitive glucose transporter (GLUT-4) in skeletal muscle nor its distribution between intracellular stores and plasma membrane were modified by DEX treatment, either under basal conditions or in response to acute insulin stimulus. These results suggest a defect(s) in the inherent activity of plasma membrane-bound GLUT-4 as the likely mechanism for DEX-induced insulin resistance.

Compensatory hyperinsulinemia and the forearm vasodilator response during an oral glucose-tolerance test in obese hypertensives

To determine whether obese hypertensive patients manifest a diminished regional vasodilator response to an oral glucose challenge. Nineteen obese hypertensives (body mass index 34.1 +/- 1.3 kg/m2, blood pressure 144 +/- 3/96 +/- 2 mmHg) and 13 lean controls (23.6 +/- 0.3 kg/m2, 127 +/- 4/83 +/- 3 mmHg) were studied. After 1 week on a standard diet, forearm blood flow (plethysmography), systemic hemodynamics (impedance cardiography), glucose and insulin levels were measured at baseline and again at 15, 30, 60, 90 and 120 min during an oral glucose-tolerance test (OGTT). Forearm blood flow increased after the oral glucose ingestion in obese hypertensives and lean controls. The rise in forearm blood flow was greater in obese hypertensives than in lean controls during the 2-h OGTT. Insulin levels also increased more in obese patients after the glucose load. During the second hour of the OGTT the ratio of changes in forearm blood flow and insulin level, an index of vascular insulin resistance, was similar in both groups. In contrast to the regional hemodynamic findings, total systemic vascular resistance decreased in lean controls but did not change in obese hypertensives during the second hour of the OGTT. The forearm vasodilator response to oral glucose in overweight, non-diabetic hypertensives is not impaired. If dynamic increases in flow are indeed important to insulin-mediated glucose disposal, then these observations raise the possibility that the greater increase of regional blood flow during an OGTT in obese hypertensives represents a component of the compensatory response for their defect in glucose metabolism.

Human hexokinase II: localization of the polymorphic gene to chromosome 2

Type 2 (non-insulin-dependent) diabetes mellitus is characterized by decreased levels of glucose 6-phosphate in skeletal muscle. It has been suggested that the lower concentrations of glucose 6-phosphate contribute to the defect in glucose metabolism noted in muscle tissue of subjects with Type 2 diabetes or subjects at increased risk of developing Type 2 diabetes. Lower levels of glucose 6-phosphate could be due to a defect in glucose uptake, or phosphorylation, or both. Hexokinase II is the isozyme of hexokinase that is expressed in skeletal muscle and is responsible for catalysing the phosphorylation of glucose in this tissue. The recent demonstration that mutations in another member of this family of glucose phosphorylating enzymes, glucokinase, can lead to the development of Type 2 diabetes prompted us to begin to examine the possible role of hexokinase II in the development of this genetically heterogeneous disorder. As a first step, we have cloned the human hexokinase II gene (HK2) and mapped it to human chromosome 2, band p13.1, by fluorescence in situ hybridization to metaphase chromosomes. In addition, we have identified and characterized a simple tandem repeat DNA polymorphism in HK2 and used this DNA polymorphism to localize this gene within the genetic linkage map of chromosome 2.

Trehalose-6-phosphate, a new regulator of yeast glycolysis that inhibits hexokinases

Trehalose-6-phosphate (P) competitively inhibited the hexokinases from Saccharomyces cerevisiae. The strongest inhibition was observed upon hexokinase II, with a K i, of 40 μM, while in the case of hexokinase I the K i was 200 μM. Glucokinase was not inhibited by trehalose-6-P up to 5 mM. This inhibition appears to have physiological significance, since the intracellular levels of trehalose-6-P were about 0.2 mM. Hexokinases from other organisms were also inhibited, while glucokinases were unaffected. The hexokinase from the yeast, Yarrowia lipolytica, was particularly sensitive to the inhibition by trehalose-6-P: when assayed with 2 mM fructose an apparent K i, of 5 μM was calculated. Two S. cerevisiae mutants with abnormal levels of trehalose-6-P exhibited defects in glucose metabolism. It is concluded that trehalose-6-P plays an important role in the regulation of the first steps of yeast glycolysis, mainly through the inhibition of hexokinase II.