Reproducibility of Glycemic Measures Among Dysglycemic Youth and Adults in the RISE Study.

AIMS: Previous work found poor reproducibility for measures of glycemia in individuals at risk for dysglycemia. Differences between youth and adults have not been assessed. Using youth and adults in the Restoring Insulin Secretion Study, we tested variability and classification concordance for hemoglobin A1C (HbA1c), fasting and 2-hour glucose from oral glucose tolerance tests (OGTTs). METHODS: HbA1c and glucose on repeated samples obtained ∼6 weeks apart were compared in 66 youth (mean age 14.2 years) and 354 adults (52.7 years). Changes, coefficient of variation (CV), and concordance of diagnostic categories between the 2 visits were compared. RESULTS: Mean difference between the 2 visits in HbA1c was higher in youth than adults (P < .001), while fasting glucose was similar and 2-hour glucose was lower in youth (P = .051). CV was smallest for HbA1c compared to fasting and 2-hour glucose. For HbA1c, youth had higher CV (P < .001); whereas CV for 2-hour glucose was lower for youth (P = .041). Classification concordance by HbA1c was lower in youth (P = .004). Using OGTT or HbA1c for classification, intervisit variability produced discordant classification in 20% of youth and 28% of adults. Using both fasting glucose and HbA1c, intervisit variability reduced discordant classification to 16% of adults while not improving classification in youth. CONCLUSIONS: Poor reproducibility and lack of classification concordance highlight the limitations of one-time testing, with important implications for assessing eligibility in clinical trials. Consideration should be given to using more than a single parameter for screening and diagnosis, especially when classification category is important.

Endothelin limits insulin action in obese/insulin-resistant humans.

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.

Interactions between endothelin and nitric oxide in the regulation of vascular tone in obesity and diabetes.

Endothelial dysfunction reflects an imbalance of vasodilators and vasoconstrictors. Endogenous endothelin activity seems to be increased in human obesity and type 2 diabetes, and cellular studies suggest that this factor may itself reduce bioavailable nitric oxide (NO). We studied 20 lean, 20 obese, and 14 type 2 diabetic individuals under three protocols, measuring leg vascular responses to intra-arterial infusions of NG-monomethyl-l-arginine (l-NMMA; an inhibitor of NO synthase) alone or in combination with BQ123 (an antagonist of type A endothelin receptors) or phentolamine (used as a control vasodilator). NO synthase inhibition alone (study 1) produced an approximately 40% increase in leg vascular resistance (LVR) in all three participant groups, which was not statistically different across groups (increase in LVR: lean, 135 +/- 28; obese, 140 +/- 32; type 2 diabetic, 184 +/- 51 units; NS). By design, BQ123 at the infused rate of 3 micromol/min produced equivalent approximately 35% reductions in LVR across groups. The subsequent addition of l-NMMA produced a greater increase in LVR among obese participants than lean or type 2 diabetic participants (study 2: lean, 182 +/- 48; obese, 311 +/- 66; type 2 diabetic, 186 +/- 40; P = 0.07). Compared with study 1, the effect of l-NMMA was magnified by BQ123 in obese participants but not in lean or type 2 diabetic participants (P = 0.005, study 1 vs. 2; P = 0.03 for group effect). Phentolamine (75 mg/min) produced vasodilation in obese participants comparable to that seen with BQ123 but failed to augment the L-NMMA response. Endothelin antagonism unmasks or augments NO synthesis capacity in obese but not type 2 diabetic participants. This suggests that impaired NO bioavailability as a result of endogenous endothelin may contribute to endothelial dysfunction in obesity, in addition to direct vasoconstrictor effects of endothelin. In contrast, endothelin antagonism alone is insufficient to restore impaired NO bioavailability in diabetes.

Endothelin contributes to basal vascular tone and endothelial dysfunction in human obesity and type 2 diabetes.

Endothelium-dependent vasodilation is impaired in clinical states of insulin resistance such as obesity and type 2 diabetes. Individuals who have hyperinsulinemic insulin resistance have relatively elevated circulating levels of endothelin (ET)-1, suggesting that ET-1 may be important in the endothelial dysfunction and alterations of vascular tone in these conditions. In 8 lean subjects, 12 nondiabetic obese subjects, and 8 subjects with type 2 diabetes, we measured basal and methacholine-stimulated rates of leg blood flow (LBF) and total serum nitrates (NOx) before and after the intrafemoral arterial administration of BQ123, a specific blocker of ET(A) receptors. BQ123 produced significant vasodilation in the obese and type 2 diabetic subjects (leg vascular resistance = mean arterial pressure/LBF fell by 34 and 36%; P < 0.005) but not in the lean subjects (13%; P = NS, P = 0.018 comparing all groups). ET(A) blockade did not change basal NOx flux (NOx*LBF). This suggests increased basal ET-1 constrictor tone among obese and type 2 diabetic subjects. BQ123 corrected the baseline defect in endothelium-dependent vasodilation seen in obese and type 2 diabetic subjects, suggesting an important contribution of ET-1 to endothelial dysfunction in these subjects. In contrast to basal conditions, stimulated NOx flux was augmented by BQ123 in obese and type 2 diabetic subjects but not in L subjects (P = 0.04), suggesting a combined effect of ET(A) blockade to reduce constrictor tone and augment dilator tone. Endothelin seems to contribute to endothelial dysfunction and the regulation of vascular tone in human obesity and type 2 diabetes.

Insulin resistance as a therapeutic target for improved endothelial function: metformin.

Endothelial dysfunction is a feature of a variety of clinical states of insulin resistance, and increasingly it is recognized that pre-diabetic states of insulin resistance are associated not only with insulin resistance but with increased cardiovascular risk. The metabolic syndrome which typically accompanies insulin resistance brings aberrations in a number of classical cardiovascular risk factors, but it appears that insulin resistance itself represents an additional, non-classical risk factor. Therefore, the approach to treating the endothelium in patients with the metabolic syndrome might include therapies targeting insulin resistance. In this review, we provide a detailed overview of the current state of knowledge regarding the biguanide metformin and its effects on the endothelium. Its mode of action is reviewed, along with the available data from laboratory and experimental studies related to vascular function in animals and in humans. Metformin has beneficial effects on endothelial function which appear to be mediated through its effects to improve insulin resistance. Therapeutically targeting insulin resistance appears to be a viable route to improving endothelial function in clinical states of insulin resistance.

Diabetes and heart disease an evidence-driven guide to risk factors management in diabetes.

Type 2 diabetes is a worldwide epidemic. Cardiovascular diseases remain the major cause of death in patients with diabetes, partly because of the association of diabetes and the metabolic syndrome. In this review, we will discuss the evidence for treatment and prevention of cardiovascular diseases in patients with diabetes. Aggressive treatment of hypertension and dyslipidemia is at the cornerstone in the management of heart disease in those patients. Despite its known benefit on the prevention of the microvascular complications of diabetes, intensive glycemic control may or may not have a significant effect on reducing macrovascular diseases. Finally, lifestyle changes and other cardiovascular therapies aimed at preventing heart disease may also prevent or delay the development of diabetes.

Role of endogenous ET-1 in the regulation of myocardial blood flow in lean and obese humans.

Endothelin is an important determinant of peripheral vascular tone, and increased endogenous endothelin activity contributes to peripheral vascular dysfunction in human obesity. The contributions of endothelin to the regulation of coronary vascular tone in health in humans have not been well studied. We hypothesized that the contribution of endothelin to the regulation of myocardial perfusion would be augmented in human obesity. Using [NH(3)]ammonia positron emission tomography (PET), we measured myocardial perfusion under resting and adenosine-stimulated conditions on two separate days, with and without concurrent exposure to BQ123, an antagonist of type A endothelin receptors (1 micromol/min IV beginning 90 min before measurement). We studied 10 lean and 9 obese subjects without hypertension, hyperlipidemia, or diabetes mellitus. We observed a BQ123-induced increase in resting myocardial perfusion of approximately 40%, not different between lean and obese subjects (BQ123-induced increase in flow: lean 0.12 +/- 0.20, obese 0.32 +/- 0.51 ml/g/min, P = 0.02 BQ123 effect, P = 0.27 comparing response across groups). Although basal flow rates varied by region of the myocardium, the BQ123 effect was seen in all regions. BMI and cholesterol were significantly related to BQ123-induced increases in basal tone in multivariable analysis. There was no baseline difference in the adenosine-stimulated increase in blood flow between lean and obese subjects, and BQ123 failed to augment these responses in either group. These observations suggest that endothelin is an important contributor to the regulation of myocardial perfusion under resting conditions in healthy lean and obese humans, with increased contributions in proportion to increasing obesity.

Endothelin contributes differently to peripheral vascular tone and blood pressure in human obesity and diabetes.

Endothelin contributes to abnormalities in peripheral blood vessel function of subjects with obesity, with or without concurrent type 2 diabetes mellitus, but it is unknown if endothelin contributes specifically to obesity and diabetes-associated changes in blood pressure. We evaluated the effect of systemic endothelin antagonism on peripheral and central hemodynamics and peripheral vascular tone in lean, obese, and type 2 diabetic subjects without overt hypertension by cuff plethysmography. We measured the effects of acute systemic infusions of BQ123 (an antagonist of type A endothelin receptors) in seven lean (body mass index [BMI] 22.7 +/- 3.2 kg/m(2)), seven obese (BMI 35.8 +/- 4.6), and six diabetic subjects (BMI 38.2 +/- 5.0, glycosylated hemoglobin 8.1 +/- 2.2%). BQ123 was infused via antecubital vein sequentially at infusion rates from 0.1 to 1.0 mumol/min. Diastolic blood pressure was significantly lower than baseline across this dose range, but without a clear dose dependence and without differences in the dose response across groups. Obese and diabetic subjects exhibited progressive dilation of peripheral blood vessels (P

Hyperinsulinemia fails to augment ET-1 action in the skeletal muscle vascular bed in vivo in humans.

Endogenous endothelin action is augmented in human obesity and type 2 diabetes and contributes to endothelial dysfunction and impairs insulin-mediated vasodilation in humans. We hypothesized that insulin resistance-associated hyperinsulinemia could preferentially drive endothelin-mediated vasoconstriction. We applied hyperinsulinemic-euglycemic clamps with higher insulin dosing in obese subjects than lean subjects (30 vs. 10 mU.m(-2).min(-1), respectively), with the goal of matching insulin's nitric oxide (NO)-mediated vascular effects. We predicted that, under these circumstances, insulin-stimulated endothelin-1 (ET-1) action (assessed with the type A endothelin receptor antagonist BQ-123) would be augmented in proportion to hyperinsulinemia. NO bioactivity was assessed using the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. Insulin-mediated vasodilation and insulin-stimulated NO bioavailability were well matched across groups by this approach. As expected, steady-state insulin levels were approximately threefold higher in obese than lean subjects (109.2 +/- 10.2 pmol/l vs. 518.4 +/- 84.0, P = 0.03). Despite this, the augmentation of insulin-mediated vasodilation by BQ-123 was not different between groups. ET-1 flux across the leg was not augmented by insulin alone but was increased with the addition of BQ-123 to insulin (P = 0.01 BQ-123 effect, P = not significant comparing groups). Endothelin antagonism augmented insulin-stimulated NO bioavailability and NOx flux, but not differently between groups and not proportional to hyperinsulinemia. These findings do not support the hypothesis that insulin resistance-associated hyperinsulinemia preferentially drives endothelin-mediated vasoconstriction.

Insulin resistance, metabolic syndrome and vascular diseases: update on mechanistic linkages.

Epidemiological associations are now well-established between insulin resistance, the metabolic syndrome and worsened cardiovascular outcomes. A direct role of insulin in vascular biology is also now broadly recognized. Specifically, insulin can directly stimulate the action of nitric oxide synthase, an effect that can be demonstrated both in vitro and in vivo. Insulin resistance, whether present endogenously or produced experimentally through exposure to fatty acids, glucosamine or tumour necrosis factor alpha, is associated with impaired endothelium-dependent vasodilation and, specifically, with impaired insulin-stimulated vasodilation. A number of potential molecular explanations for these observations are being pursued, with evidence to support a number of concurrent pathogenic mechanisms. These include insulin resistance-associated reductions in nitric oxide availability due to increases in oxidative stress (not requiring the presence of hyperglycemia), reduced availability of tetrahydrobiopterin and excess levels of asymmetrical dimethylarginine. A strong body of evidence also supports an excess of the vasoconstrictor endothelin, which may result directly from hyperinsulinemia and/or indirectly due to a loss of the suppressive effects of nitric oxide on endothelin production and action. The current leading edge of investigations into the association between insulin-resistant states and vascular dysfunction involves the expanding repertoire of adipocyte-derived hormones. Of these, particular interest has been focused on adiponectin, which has both vascular and metabolic actions, and may contribute importantly to the connection between metabolism and vascular function. Progress along these novel lines of investigation will continue to expand the understanding of the mechanisms linking insulin resistance, the metabolic syndrome and vascular disease.