The presence of two reduced function variants in CYP2C9 influences the acute response to glipizide.

AIMS: To examine whether the presence of two common missense variants in the CYP2C9 gene (rs1799853, encoding Arg144Cys and denoted as *2, and rs1057910, encoding Ile359Leu and denoted as *3) influences the acute physiological response to a single glipizide dose in individuals naïve to diabetes medications. METHODS: In the Study to Understand the Genetics of the Acute Response to Metformin and Glipizide in Humans (SUGAR-MGH), 786 individuals genotyped for rs1799853/rs41291560 (*2) and rs1057910/rs9332214 (*3) were treated with 5 mg glipizide in the fasting state. Glucose and insulin levels were measured at baseline, 30, 60, 90, 120, 180 and 240 min for calculation of phenotypic endpoints of glipizide response. The challenge was aborted as a result of hypoglycaemia, defined as glucose <2.8 mmol/l or hypoglycaemia-related symptoms. RESULTS: Carriers with two reduced function alleles had a 50% larger insulin area under the curve than carriers with zero or one copy (P=0.037), although this finding was primarily driven by an individual with a robust insulin response. In adjusted analyses, the risk of aborting the glipizide challenge was doubled in two-copy carriers (P=0.034). No significant findings were observed in glucose-based endpoints. CONCLUSIONS: Carriers of two reduced function alleles in CYP2C9 may experience an increased insulin response to glipizide and be predisposed to a higher risk of hypoglycaemia, although no effect of genotype was seen in glucose-based measurements. Further studies are needed to clarify the utility of CYP2C9 genotyping to guide sulfonylurea treatment.

Comparative cardiovascular safety of glucagon-like peptide-1 receptor agonists versus other antidiabetic drugs in routine care: a cohort study.

AIMS: To evaluate the comparative cardiovascular disease (CVD) safety of glucagon-like peptide-1 receptor agonists (GLP-1 RAs) in head-to-head comparisons with dipeptidyl peptidase-4 (DPP-4) inhibitors, sulphonylureas or insulin, when added to metformin, as used in 'real-world' patients with type 2 diabetes mellitus (T2DM). METHODS: Within a large US commercial health plan database linked to laboratory test results, we identified three pairwise 1 : 1 propensity-score-matched cohorts of patients with T2DM aged ≥18 years treated with metformin who initiated a GLP-1 RA or a comparator, i.e. DPP-4 inhibitor (n = 35 534), second-generation sulphonylureas (n = 28 138) or insulin (n = 47 068), between 2005 and 2013. We examined the association between drug initiation and a composite CVD endpoint, comprising hospitalizations for acute myocardial infarction, unstable angina, stroke or coronary revascularization. RESULTS: During the course of 1 year, there were 13.9 and 13.7 CVD events per 1000 person-years among propensity-score-matched initiators of GLP-1 RAs versus DPP-4 inhibitors [hazard ratio (HR) 1.02; 95% confidence interval (CI) 0.84-1.24]; and 12.1 versus 14.0 events among initiators of GLP-1 RAs versus sulphonylureas (HR 0.86; 95% CI 0.69-1.08). The effect estimates for GLP-1 RAs versus insulin were sensitive to the adjustment for glycated haemoglobin, after which the HR was 1.01 (95% CI 0.73-1.41). Results were robust across several sensitivity analyses, including an as-treated analysis considering up to 8.7 years of follow-up. CONCLUSIONS: This large study, performing head-to-head comparisons of GLP-1 RAs with other antidiabetic agents in real-world patients, provides estimates of relative safety precise enough to exclude large differences in CVD risk and adds further understanding to results from recent clinical trials.

A randomised trial of salsalate for insulin resistance and cardiovascular risk factors in persons with abnormal glucose tolerance.

AIMS/HYPOTHESIS: Chronic sub-acute inflammation contributes to the pathogenesis of type 2 diabetes mellitus and cardiovascular disease. High doses of salicylate reduce inflammation, glucose and triacylglycerols, and may improve insulin sensitivity, suggesting therapeutic potential in impaired fasting glucose and/or impaired glucose tolerance. This trial aimed to evaluate the effect of salsalate vs placebo on insulin resistance and glycaemia in impaired fasting glucose and/or impaired glucose tolerance. METHODS: We conducted a 12 week, two-centre, randomised, placebo-controlled study to evaluate the effect of salsalate (up to 4 g/day) vs placebo on systemic glucose disposal. Secondary objectives included treatment effects on glycaemia, inflammation and cardiovascular risk factors. Seventy-eight participants with impaired fasting glucose and/or impaired glucose tolerance from two VA healthcare systems were enrolled. Randomisation assignment was provided by the coordinating center directly to site pharmacists, and participants and research staff were blinded to treatment assignment. RESULTS: Seventy-one individuals were randomised to placebo (n = 36) or salsalate (n = 35). Glucose disposal did not change in either group (salsalate 1% [95% CI -39%, 56%]; placebo 6% [95% CI -20%, 61%], p = 0.3 for placebo vs salsalate). Fasting glucose was reduced by 6% during the study by salsalate (p = 0.006) but did not change with placebo. Declines in glucose were accompanied by declines in fasting C-peptide with salsalate. Insulin clearance was reduced with salsalate. In the salsalate group, triacylglycerol levels were lower by 25% (p = 0.01) and adiponectin increased by 53% (p = 0.02) at the end of the study. Blood pressure, endothelial function and other inflammation markers did not differ between groups. Adipose tissue nuclear factor κB (NF-κB) activity declined in the salsalate group compared with placebo (-16% vs 42%, p = 0.005), but was not correlated with metabolic improvements. The frequency of tinnitus was low but tended to be higher with salsalate therapy (n = 4 vs n = 2). CONCLUSIONS/INTERPRETATION: In summary, salsalate therapy was well tolerated, lowered fasting glucose, increased adiponectin and reduced adipose tissue NF-κB activity. These changes were not related to changes in peripheral insulin sensitivity, suggesting additional mechanisms for metabolic improvement. TRIAL REGISTRATION: ClinicalTrials.gov NCT00330733. FUNDING: Office of Research and Development, Medical Research Service, Department of Veterans Affairs and NIH K24 DK63214.

Modulation of ß-cell function: a translational journey from the bench to the bedside.

Both decreased insulin secretion and action contribute to the pathogenesis of type 2 diabetes (T2D) in humans. The insulin receptor and insulin signalling proteins are present in the rodent and human ß-cell and modulate cell growth and function. Insulin receptors and insulin signalling proteins in ß-cells are critical for compensatory islet growth in response to insulin resistance. Rodents with tissue-specific knockout of the insulin receptor in the ß-cell (ßIRKO) show reduced first-phase glucose-stimulated insulin secretion (GSIS) and with aging develop glucose intolerance and diabetes, phenotypically similar to the process seen in human T2D. Expression of multiple insulin signalling proteins is reduced in islets of patients with T2D. Insulin potentiates GSIS in isolated human ß-cells. Recent studies in humans in vivo show that pre-exposure to insulin increases GSIS, and this effect is diminished in persons with insulin resistance or T2D. ß-Cell function correlates to whole-body insulin sensitivity. Together, these findings suggest that pancreatic ß-cell dysfunction could be caused by a defect in insulin signalling within ß-cell, and ß-cell insulin resistance may lead to a loss of ß-cell function and/or mass, contributing to the pathophysiology of T2D.

Cardiovascular risk factors and menstrual cycle phase in pre-menopausal women.

BACKGROUND: Exogenous estrogens have been shown to affect markers of cardiovascular risk in women. AIM: The objective of this study was to determine the effect of menstrual cycle phase on markers of cardiovascular risk in young, healthy women with regular menstrual cycles. SUBJECTS AND METHODS: This prospective cohort study examined 20 healthy pre-menopausal women at 2 time-points in the menstrual cycle, in early follicular phase and early luteal phase. RESULTS: In the early luteal phase, levels of estrogen, progesterone, LH, total cholesterol, and HDL were significantly higher, compared with the early follicular phase. In contrast, there were no significant differences in LDL or triglyceride levels between the 2 phases. Furthermore, there were no significant effects of menstrual cycle phase on glycemic indices (fasting blood glucose, glycohemoglobin or homeostatic model assessment of insulin resistance), markers of inflammation (C-reactive protein, soluble CD40 ligand, intercellular adhesion molecule-1, vascular cell adhesion molecule-1, or adiponectin), or vascular function, as measured by brachial artery reactivity. CONCLUSIONS: Although menstrual cycle phase affects total cholesterol and HDL levels, it does not affect other markers of cardiovascular risk in young women with regular menstrual cycles.

Glucagon-like peptide-1 (GLP-1) receptors are not overexpressed in pancreatic islets from patients with severe hyperinsulinaemic hypoglycaemia following gastric bypass.

AIMS/HYPOTHESIS: Glucagon-like peptide-1 (GLP-1) receptors are highly overexpressed in benign insulinomas, permitting in vivo tumour visualisation with GLP-1 receptor scanning. The present study sought to evaluate the GLP-1 receptor status in vitro in other pancreatic disorders leading to hyperinsulinaemic hypoglycaemia, specifically after gastric bypass surgery. METHODS: Fresh frozen pancreatic tissue samples (n=7) from six gastric bypass surgery patients suffering from hyperinsulinaemic hypoglycaemia were evaluated for GLP-1 receptor content using in vitro receptor autoradiography, and compared with normal pancreas and with pancreatic insulinoma tissues. RESULTS: GLP-1 receptor analysis of the pancreatic tissues, which histopathologically were compatible with nesidioblastosis and originated from post-bypass hypoglycaemic patients, revealed a mean density value of GLP-1 receptors in the islets of 1,483 ± 183 dpm/mg tissue. Pharmacological characterisation indicated the presence of specific GLP-1 receptors. The density of islet GLP-1 receptor in post-gastric bypass patients did not differ from that of normal pancreas (1,563 ± 104 dpm/mg tissue, n = 10). Receptor density in pancreatic acini was low in post-bypass and control conditions. In contrast, benign insulinomas showed a high density of GLP-1 receptors, with a mean value of 8,302 ± 1,073 dpm/mg tissue (n = 6). CONCLUSIONS/INTERPRETATION: In contrast to insulinoma, hyperinsulinaemic hypoglycaemia after gastric bypass surgery is not accompanied by overexpression of GLP-1 receptor in individual islets. Thus, patients with post-gastric bypass hyperinsulinaemic hypoglycaemia are not candidates for GLP-1 receptor imaging in vivo using radiolabelled exendin. These GLP-1 receptor data support the notion that the islet pathobiology of post-gastric bypass hypoglycaemia is distinctly different from that of benign insulinomas.

Hypoglycaemia following gastric bypass surgery--diabetes remission in the extreme?

Postprandial hypoglycaemia is increasingly recognised as a complication of gastric bypass surgery. While post-bypass hypoglycaemia is often responsive to dietary modification, a subset of individuals develop life-threatening neuroglycopenia, with loss of consciousness, seizures and motor vehicle accidents. Such patients require complex nutritional and medical management strategies to reduce postprandial insulin secretion and stabilise glucose excursions, using medications including acarbose, octreotide and diazoxide, and frequent monitoring of glucose values. In an article in this issue of Diabetologia, nationwide registry data from Sweden were used to assess the frequency of severe hypoglycaemia and potentially related diagnoses (e.g. confusion, syncope, seizures, accidental death) following obesity surgery. Relative risk of hypoglycaemia and related diagnoses were two- to sevenfold higher in the post-gastric bypass population, but absolute risk was small. While these data underscore that hypoglycaemia is an important complication of gastric bypass, many questions regarding frequency, pathogenesis and optimal therapy remain unanswered. Given that hypoglycaemia is usually evaluated in the outpatient setting, more precise assessments of hypoglycaemia frequency will require prospective longitudinal studies in post-bypass cohorts. Until such data are available, practitioners should have a higher awareness of symptoms consistent with neuroglycopenia in patients with a history of bariatric surgery. Understanding the beneficial and challenging metabolic consequences of bariatric surgery is a key imperative for the diabetes community, as such data may yield novel insights into mechanisms by which bariatric surgery can lead to diabetes remission.

Long-term Safety and Tolerability of Colesevelam HCl in Subjects with Type 2 Diabetes.

The bile acid sequestrant, colesevelam hydrochloride, is approved for glycemic control in adults with type 2 diabetes. In three double-masked, placebo-controlled studies, colesevelam hydrochloride 3.75 g/day demonstrated its glycemic-lowering properties when added to existing metformin-, insulin-, or sulfonylurea-based therapy in adults with inadequately controlled type 2 diabetes. This was a 52-week open-label extension study conducted at 63 sites in the United States and one site in Mexico to further evaluate the safety and tolerability of colesevelam hydrochloride in subjects with type 2 diabetes. All subjects who completed the three double-masked, placebo-controlled studies were eligible to enroll in this open-label extension. In total, 509 subjects enrolled and received open-label colesevelam hydrochloride 3.75 g/day for 52 weeks. Safety and tolerability of colesevelam hydrochloride was evaluated by the incidence and severity of adverse events. In total, 360 subjects (70.7%) completed the extension. Of the safety population, 361 subjects (70.9%) experienced an adverse event, most (88.1%) being mild or moderate in severity. Fifty-six adverse events (11.0%) were drug-related; the most frequent drug-related adverse events were constipation and dyspepsia. Thirty-five subjects (6.9%) discontinued due to an adverse event. Fifty-four subjects (10.6%) experienced a serious adverse event; only one was considered drug-related (diverticulitis). Seventeen subjects (3.3%) experienced hypoglycemia; most episodes were mild or moderate in severity. Glycemic improvements with colesevelam hydrochloride were seen without change in weight over 52 weeks (0.2 kg mean reduction from baseline). Colesevelam hydrochloride was safe and well-tolerated as long-term therapy for patients with type 2 diabetes.

Family history of diabetes impacts on interactions between minimal model estimates of insulin sensitivity and glucose effectiveness.

AIMS/HYPOTHESIS: Insulin resistance and glucose effectiveness (S(G)) are major determinants of glucose tolerance and independently predict the development of type 2 diabetes in individuals with a family history of disease. We examined the inter-relationship between insulin sensitivity (S(I)) and S(G) in offspring of two parents with type 2 diabetes and in individuals with no family history of diabetes. METHODS: Fifty non-diabetic individuals, including 26 offspring of two type 2 diabetic parents (family history, FH+) and 24 with no family history of diabetes (FH-) similar in gender, age, ethnicity and body mass index (BMI) were studied. Each subject underwent a 100-g oral glucose tolerance test (OGTT) and insulin modified frequently sampled intravenous glucose tolerance, analysed using the Bergman's minimal model (MINMOD). RESULTS: Thirteen subjects of the FH+ group and nine of the FH- group had impaired glucose tolerance (IGT). S(I) and S(G) were independent variables in the FH+ group, while they correlated highly with each other in the FH- group (r = 0.69, p = 0.0002). The relationship between S(I) and S(G) persisted when analysing the IGT and normal glucose tolerance subgroups separately, demonstrating that these associations were not because of differences in glycaemia. Consistently, S(G) strongly correlated with additional measures of insulin resistance only in the FH- group, including fasting insulin (r = 0.56, p = 0.004), homeostasis model assessment of insulin resistance (r = 0.57 p = 0.003) and BMI (r = 0.66, p = 0.0004). CONCLUSIONS: These results demonstrate that familial factors impart important physiological differences in the inter-relationship between insulin-dependent and insulin-independent glucose disposal, which may be important in modulating risk for development of disease.

Patients with neuroglycopenia after gastric bypass surgery have exaggerated incretin and insulin secretory responses to a mixed meal.

CONTEXT AND OBJECTIVE: Hyperinsulinemic hypoglycemia is newly recognized as a rare but important complication after Roux-en-Y gastric bypass (GB). The etiology of the syndrome and metabolic characteristics remain incompletely understood. Recent studies suggest that levels of incretin hormones are increased after GB and may promote excessive beta-cell function and/or growth. PATIENTS AND METHODS: We performed a cross-sectional analysis of metabolic variables, in both the fasting state and after a liquid mixed-meal challenge, in four subject groups: 1) with clinically significant hypoglycemia [neuroglycopenia (NG)] after GB surgery, 2) with no symptoms of hypoglycemia at similar duration after GB surgery, 3) without GB similar to preoperative body mass index of the surgical cohorts, and 4) without GB similar to current body mass index of the surgical cohorts. RESULTS: Insulin and C-peptide after the liquid mixed meal were both higher relative to the glucose level achieved in persons after GB with NG compared with asymptomatic individuals. Glucagon, glucagon-like peptide 1, and glucose-dependent insulinotropic peptide levels were higher in both post-GB surgical groups compared with both overweight and morbidly obese persons, and glucagon-like peptide 1 was markedly higher in the group with NG. Insulin resistance, assessed by homeostasis model assessment of insulin resistance, the composite insulin sensitivity index, or adiponectin, was similar in both post-GB groups. Dumping score was also higher in both GB groups but did not discriminate between asymptomatic and symptomatic patients. Notably, the frequency of asymptomatic hypoglycemia after a liquid mixed meal was high in post-GB patients. CONCLUSION: A robust insulin secretory response was associated with postprandial hypoglycemia in patients after GB presenting with NG. Increased incretin levels may contribute to the increased insulin secretory response.

Severe hypoglycaemia post-gastric bypass requiring partial pancreatectomy: evidence for inappropriate insulin secretion and pancreatic islet hyperplasia.

AIMS/HYPOTHESIS: Postprandial hypoglycaemia following gastric bypass for obesity is considered a late manifestation of the dumping syndrome and can usually be managed with dietary modification. We investigated three patients with severe postprandial hypoglycaemia and hyperinsulinaemia unresponsive to diet, octreotide and diazoxide with the aim of elucidating the pathological mechanisms involved. METHODS: Glucose, insulin, and C-peptide were measured in the fasting and postprandial state, and insulin secretion was assessed following selective intra-arterial calcium injection. Pancreas histopathology was assessed in all three patients. RESULTS: All three patients had evidence of severe postprandial hyperinsulinaemia and hypoglycaemia. In one patient, reversal of gastric bypass was ineffective in reversing hypoglycaemia. All three patients ultimately required partial pancreatectomy for control of neuroglycopenia; pancreas pathology of all patients revealed diffuse islet hyperplasia and expansion of beta cell mass. CONCLUSIONS/INTERPRETATION: These findings suggest that gastric bypass-induced weight loss may unmask an underlying beta cell defect or contribute to pathological islet hyperplasia, perhaps via glucagon-like peptide 1-mediated pathways.

The role of total and high-molecular-weight complex of adiponectin in vascular function in offspring whose parents both had type 2 diabetes.

AIMS/HYPOTHESIS: Adiponectin is an adipokine with insulin-sensitising and anti-atherogenic properties. We studied the role played by total adiponectin and by the bioactive high-molecular-weight (HMW) oligomeric complexes of adiponectin in vascular function in offspring whose parents both had type 2 diabetes, a population at high risk of diabetes and atherosclerosis. METHODS: Total and %HMW adiponectin, the cytokines C-reactive protein, interleukin-6 and plasminogen activator inhibitor-1 (PAI-1), as well as lipid profiles were assayed in 19 offspring, each with two type 2 diabetic parents. Subjects underwent OGTTs and IVGTTs. Endothelium-dependent vasodilation (EDV) was assessed by brachial artery ultrasonography. RESULTS: There was a significant relationship between %HMW and total adiponectin levels (r=0.72, p=0.001). Despite an expected strong positive correlation between HDL-cholesterol and adiponectin levels (r=0.52, p=0.04), as well as HDL-cholesterol and EDV (r=0.56, p<0.02), there was no significant relationship between either total adiponectin or % HMW adiponectin and EDV. Adiponectin was inversely associated with PAI-1 (r=0.50, p=0.05), but did not correlate with the inflammatory markers C-reactive protein or interleukin-6. CONCLUSIONS/INTERPRETATION: In offspring of diabetic parents, a population at high risk of diabetes and atherosclerotic disease, there is no relationship between total or %HMW adiponectin and endothelium-dependent vasodilation. However, low adiponectin was associated with impaired fibrinolysis as manifested by increased levels of plasminogen activator inhibitor-1.

Portal and peripheral cortisol levels in obese humans.

AIMS/HYPOTHESIS: Excess total body and visceral fat has been associated with insulin resistance, diabetes and the metabolic syndrome. Excess glucocorticoids produce both central obesity and diabetes. However, systemic glucocorticoid levels are normal in typical Type 2 diabetes and persons with idiopathic obesity. Glucocorticoids can be produced locally through the enzyme 11 beta hydroxysteroid dehydrogenase type 1 (11 beta HSD-1). Transgenic mice with selective overexpression in adipose tissue of 11 beta HSD-1 to levels seen in humans develop visceral obesity, hyperlipidaemia and insulin-resistant diabetes associated with a 2.7-fold increase in corticosterone levels in portal compared to peripheral circulation. To examine whether the liver is exposed to higher levels of glucocorticoids, which may undergo metabolic degradation prior to measurement in the systemic circulation, we assessed concentrations of cortisol in the portal and peripheral circulation in morbidly obese humans. METHODS: Portal and peripheral blood samples were obtained simultaneously from six morbidly obese humans with and without diabetes during bariatric abdominal surgery. The samples were assessed for serum cortisol to determine whether an increase in the portal to peripheral circulation is found in obese humans. Insulin, which undergoes metabolic clearance in the liver, and thyroxin (free T(4)), which does not, were also assessed. RESULTS: Levels of serum cortisol (698.8+/-200.4 vs 696.3+/-232.4 nmol/l, portal vs peripheral, p=0.9) and free T(4) (22.0+/-7.8 vs 20.6+/-8.1 pmol/l, portal vs peripheral, p=0.3) were not significantly different in portal compared to peripheral circulation. Portal insulins were significantly higher than peripheral levels (466.7+/-302.9 vs 78.5+/-50.9 pmol/l, portal vs peripheral, p=0.03). CONCLUSIONS/INTERPRETATION: These observations suggest that in morbidly obese humans the liver is not exposed to excess glucocorticoids.

Prevention of type 2 diabetes: are we ready?

Type 2 diabetes is the most common metabolic disease. The cost of diabetes to the individual and to society, and the pandemic prevalence makes disease prevention of extreme importance. Persons with impaired glucose tolerance, modest elevations in blood glucose that remain below levels diagnostic for diabetes, are at increased risk of progression to overt diabetes. New studies evaluate the role of lifestyle interventions including diet and exercise, and the potential role of multiple classes of pharmaceutical agents including the insulin sensitizers, biguanides and thiazolidendiones, and carbohydrase and lipase inhibitors in disease prevention of such high-risk individuals. Many of these strategies appear to be effective to delay, and perhaps prevent, the development of type 2 diabetes and thus should be considered for broader clinical application. Awareness of the extent of the problem and the potential benefits of prevention needs to be raised in both physicians and in the at high risk population.

Type 2 diabetes: new drugs, new perspectives.

Detection at younger ages is rapidly increasing. Although diet and exercise remain the mainstays of therapy, physicians can now choose from among a variety of medications targeting different pathophysiologic facets of the disease. The newest drugs are the thiazolidinediones, a totally novel class of insulin sensitizers.

Effect of vanadium(IV) compounds in the treatment of diabetes: in vivo and in vitro studies with vanadyl sulfate and bis(maltolato)oxovandium(IV).

Vanadyl sulfate (VOSO(4)) was given orally to 16 subjects with type 2 diabetes mellitus for 6 weeks at a dose of 25, 50, or 100 mg vanadium (V) daily [Goldfine et al., Metabolism 49 (2000) 1-12]. Elemental V was determined by graphite furnace atomic absorption spectrometry (GFAAS). There was no correlation of V in serum with clinical response, determined by reduction of mean fasting blood glucose or increased insulin sensitivity during euglycemic clamp. To investigate the effect of administering a coordinated V, plasma glucose levels were determined in streptozotocin (STZ)-induced diabetic rats treated with the salt (VOSO(4)) or the coordinated V compound bis(maltolato)oxovandium(IV) (abbreviated as VO(malto)(2)) administered by intraperitoneal (i.p.) injection. There was no relationship of blood V concentration with plasma glucose levels in the animals treated with VOSO(4), similar to our human diabetic patients. However, with VO(malto)(2) treatment, animals with low plasma glucose tended to have high blood V. To determine if V binding to serum proteins could diminish biologically active serum V, binding of both VOSO(4) and VO(malto)(2) to human serum albumin (HSA), human apoTransferrin (apoHTf) and pig immunoglobulin (IgG) was studied with EPR spectroscopy. Both VOSO(4) and VO(malto)(2) bound to HSA and apoHTf forming different V-protein complexes, while neither V compound bound to the IgG. VOSO(4) and VO(malto)(2) showed differences when levels of plasma glucose and blood V in diabetic rodents were compared, and in the formation of V-protein complexes with abundant serum proteins. These data suggest that binding of V compounds to ligands in blood, such as proteins, may affect the available pool of V for biological effects.

Ascorbate restores endothelium-dependent vasodilation impaired by acute hyperglycemia in humans.

BACKGROUND: Endothelium-dependent vasodilation is impaired in patients with insulin-dependent and non-insulin-dependent diabetes mellitus and restored by vitamin C administration, implicating a causative role for oxidant stress. Hyperglycemia per se attenuates endothelium-dependent vasodilation in healthy subjects. Accordingly, this study investigated whether impaired endothelium-dependent vasodilation caused by hyperglycemia in nondiabetic humans is restored by administration of the antioxidant vitamin C. METHODS AND RESULTS: Endothelium-dependent vasodilation was measured by incremental brachial artery administration of methacholine chloride (0.3 to 10 microg/min) during euglycemia, after 6 hours of hyperglycemia (300 mg/dL) created by dextrose (50%) intra-arterial infusion, and with coadministration of vitamin C (24 mg/min) during hyperglycemia. Endothelium-dependent vasodilation was significantly diminished by hyperglycemia (P:=0.02 by ANOVA) and restored by vitamin C (P:=0.04). In contrast, endothelium-dependent vasodilation was not affected by equimolar infusions of mannitol, with and without vitamin C coinfusion (P:=NS). Endothelium-independent vasodilation was measured by incremental infusion of verapamil chloride (10 to 300 microg/min) without and with coadministration of N:(G)-monomethyl-L-arginine (L-NMMA). In the absence of L-NMMA, endothelium-independent vasodilation was not significantly altered during hyperglycemia (P:=NS) but was augmented by vitamin C (P:=0.04). The coadministration of L-NMMA eliminated the vitamin C-related augmentation in verapamil-mediated vasodilation. CONCLUSIONS: Vitamin C administration restores endothelium-dependent vasodilation impaired by acute hyperglycemia in healthy humans in vivo. These findings suggest that hyperglycemia may contribute in part to impaired vascular function through production of superoxide anion.

Metabolic effects of vanadyl sulfate in humans with non-insulin-dependent diabetes mellitus: in vivo and in vitro studies.

To investigate the efficacy and mechanism of action of vanadium salts as oral hypoglycemic agents, 16 type 2 diabetic patients were studied before and after 6 weeks of vanadyl sulfate (VOSO4) treatment at three doses. Glucose metabolism during a euglycemic insulin clamp did not increase at 75 mg/d, but improved in 3 of 5 subjects receiving 150 mg VOSO4 and 4 of 8 subjects receiving 300 mg VOSO4. Basal hepatic glucose production (HGP) and suppression of HGP by insulin were unchanged at all doses. Fasting glucose and hemoglobin A1c (HbA1c) decreased significantly in the 150- and 300-mg VOSO4 groups. At the highest dose, total cholesterol decreased, associated with a decrease in high-density lipoprotein (HDL). There was no change in systolic, diastolic, or mean arterial blood pressure on 24-hour ambulatory monitors at any dose. There was no apparent correlation between the clinical response and peak serum level of vanadium. The 150- and 300-mg vanadyl doses caused some gastrointestinal intolerance but did not increase tissue oxidative stress as assessed by thiobarbituric acid-reactive substances (TBARS). In muscle obtained during clamp studies prior to vanadium therapy, insulin stimulated the tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1 (IRS-1), and Shc proteins by 2- to 3-fold, while phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with IRS-1 increased 4.7-fold during insulin stimulation (P = .02). Following vanadium, there was a consistent trend for increased basal levels of insulin receptor, Shc, and IRS-1 protein tyrosine phosphorylation and IRS-1-associated PI 3-kinase, but no further increase with insulin. There was no discernible correlation between tyrosine phosphorylation patterns and glucose disposal responses to vanadyl. While glycogen synthase fractional activity increased 1.5-fold following insulin infusion, there was no change in basal or insulin-stimulated activity after vanadyl. There was no increase in the protein phosphatase activity of muscle homogenates to exogenous substrate after vanadyl. Vanadyl sulfate appears safe at these doses for 6 weeks, but at the tolerated doses, it does not dramatically improve insulin sensitivity or glycemic control. Vanadyl modifies proteins in human skeletal muscle involved in early insulin signaling, including basal insulin receptor and substrate tyrosine phosphorylation and activation of PI 3-kinase, and is not additive or synergistic with insulin at these steps. Vanadyl sulfate does not modify the action of insulin to stimulate glycogen synthesis. Since glucose utilization is improved in some patients, vanadyl must also act at other steps of insulin action.

Acute hyperglycemia attenuates endothelium-dependent vasodilation in humans in vivo.

BACKGROUND: Endothelial function is impaired in patients with diabetes mellitus. However, the factors contributing to this defect are currently unknown. Hyperglycemia attenuates endothelium-dependent relaxation in normal rabbit arteries in vitro and rat arterioles in vivo. Accordingly, this study examined the effect of acute hyperglycemia on endothelium-dependent vasodilation in nondiabetic humans in vivo. METHODS AND RESULTS: Endothelium-dependent vasodilation was assessed through brachial artery infusion of methacholine chloride both before and during 6 hours of local hyperglycemia (300 mg/dL) achieved by intra-arterial infusion of 50% dextrose. Forearm blood flow was determined by plethysmography. In a group of 10 subjects, there was a trend toward attenuated methacholine-mediated vasodilation during hyperglycemia compared with euglycemia (P=.07 by ANOVA; maximal response, 13.3+/-2.8 versus 14.7+/-1.5 mL x min(-1) x 100 mL(-1), respectively). In these subjects, the systemic serum insulin levels increased significantly during the dextrose infusion (P<.001). To eliminate the confounding vasoactive effects of insulin, the protocol was repeated during systemic infusion of octreotide (30 ng x kg(-1) x min(-1)) to inhibit pancreatic secretion of insulin. In these subjects (n=10), hyperglycemia significantly attenuated the forearm blood flow response to methacholine (P<.01 by ANOVA; maximal response, 16.9+/-2.5 before versus 12.7+/-1.8 mL x min(-1) x 100 mL(-1) during hyperglycemia). Methacholine-mediated vasodilation was not attenuated by an equimolar infusion of mannitol (P>.40), nor did hyperglycemia reduce endothelium-independent vasodilation to verapamil (P>.50). CONCLUSIONS: Acute hyperglycemia impairs endothelium-dependent vasodilation in healthy humans in vivo. This finding suggests that elevated glucose may contribute to the endothelial dysfunction observed in patients with diabetes mellitus.