Helmut O. Steinberg

Obesity/insulin resistance is associated with endothelial dysfunction. Implications for the syndrome of insulin resistance.

To test the hypothesis that obesity/insulin resistance impairs both endothelium-dependent vasodilation and insulin-mediated augmentation of endothelium-dependent vasodilation, we studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of methacholine chloride (MCh) or sodium nitroprusside (SNP) during saline infusion and euglycemic hyperinsulinemia in lean insulin-sensitive controls (C), in obese insulin-resistant subjects (OB), and in subjects with non-insulin-dependent diabetes mellitus (NIDDM). MCh induced increments in LBF were approximately 40% and 55% lower in OB and NIDDM, respectively, as compared with C (P < 0.05). Euglycemic hyperinsulinemia augmented the LBF response to MCh by - 50% in C (P < 0.05 vs saline) but not in OB and NIDDM. SNP caused comparable increments in LBF in all groups. Regression analysis revealed a significant inverse correlation between the maximal LBF change in response to MCh and body fat content. Thus, obesity/insulin resistance is associated with (a) blunted endothelium-dependent, but normal endothelium-independent vasodilation and (b) failure of euglycemic hyperinsulinemia to augment endothelium-dependent vasodilation. Therefore, obese/insulin-resistant subjects are characterized by endothelial dysfunction and endothelial resistance to insulins effect on enhancement of endothelium-dependent vasodilation. This endothelial dysfunction could contribute to the increased risk of atherosclerosis in obese insulin-resistant subjects.

Insulin-mediated skeletal muscle vasodilation is nitric oxide dependent. A novel action of insulin to increase nitric oxide release.

The purpose of this study was to examine whether insulins effect to vasodilate skeletal muscle vasculature is mediated by endothelium-derived nitric oxide (EDNO). N-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO synthase, was administered directly into the femoral artery of normal subjects at a dose of 16 mg/min and leg blood flow (LBF) was measured during an infusion of saline (NS) or during a euglycemic hyperinsulinemic clamp (HIC) designed to approximately double LBF. In response to the intrafemoral artery infusion of L-NMMA, LBF decreased from 0.296 +/- 0.032 to 0.235 +/- 0.022 liters/min during NS and from 0.479 +/- 0.118 to 0.266 +/- 0.052 liters/min during HIC, P < 0.03. The proportion of NO-dependent LBF during NS and HIC was approximately 20% and approximately 40%, respectively, P < 0.003 (NS vs. HIC). To elucidate whether insulin increases EDNO synthesis/release or EDNO action, vasodilative responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (MCh) or the endothelium-independent vasodilator sodium nitroprusside (SNP) were studied in normal subjects during either NS or HIC. LBF increments in response to intrafemoral artery infusions of MCh but not SNP were augmented during HIC versus NS, P < 0.03. In summary, insulin-mediated vasodilation is EDNO dependent. Insulin vasodilation of skeletal muscle vasculature most likely occurs via increasing EDNO synthesis/release. Thus, insulin appears to be a novel modulator of the EDNO system.

Elevated circulating free fatty acid levels impair endothelium-dependent vasodilation.

We have recently shown that insulin-resistant obese subjects exhibit impaired endothelial function. Here, we test the hypothesis that elevation of circulating FFA to levels seen in insulin-resistant subjects can impair endothelial function. We studied leg blood flow responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (Mch) or the endothelium-independent vasodilator sodium nitroprusside during the infusion of saline and after raising systemic circulating FFA levels exogenously via a low- or high-dose infusion of Intralipid plus heparin or endogenously by an infusion of somatostatin (SRIF) to produce insulinopenia in groups of lean healthy humans. After 2 h of infusion of Intralipid plus heparin, FFA levels increased from 562+/-95 to 1,303+/-188 micromol, and from 350+/-35 to 3,850+/-371 micromol (P < 0.001) vs. saline for both low- and high-dose groups, respectively. Mch-induced vasodilation relative to baseline was reduced by approximately 20% in response to the raised FFA levels in both groups (P < 0.05, saline vs. FFA, ANOVA). In contrast, similar FFA elevation did not change leg blood flow responses to sodium nitroprusside. During the 2-h SRIF infusion, insulin levels fell, and FFA levels rose from 474+/-22 to 1,042+/-116 micromol (P < 0.01); Mch-induced vasodilation was reduced by approximately 20% (P < 0.02, saline vs. SRIF, ANOVA). Replacement of basal insulin levels during SRIF resulted in a fall of FFA levels from 545+/-47 to 228+/-61 micromol, and prevented the impairment of Mch-induced vasodilation seen with SRIF alone. In conclusion, (a) elevated circulating FFA levels cause endothelial dysfunction, and (b) impaired endothelial function in insulin-resistant humans may be secondary to the elevated FFA concentrations observed in these patients.

Polycystic Ovary Syndrome Is Associated With Endothelial Dysfunction

Background —We recently reported endothelial dysfunction as a novel cardiovascular risk factor associated with insulin resistance/obesity. Here, we tested whether hyperandrogenic insulin-resistant women with polycystic ovary syndrome (PCOS) who are at increased risk of macrovascular disease display impaired endothelium-dependent vasodilation and whether endothelial function in PCOS is associated with particular metabolic and/or hormonal characteristics. Methods and Results —We studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (MCh) and to euglycemic hyperinsulinemia in 12 obese women with PCOS and in 13 healthy age- and weight-matched control subjects (OBW). LBF increments in response to MCh were 50% lower in the PCOS group than in the OBW group (P <0.01). Euglycemic hyperinsulinemia increased LBF above baseline by 30% in the PCOS and 60% in OBW group (P <0.05 between groups). Across all subjects, the maximal LBF response to MCh exhibited a strong inverse correlation with free testosterone levels (r =−0.52, P <0.007). This relationship was stronger than with any other parameter, including insulin sensitivity. Conclusions —PCOS is characterized by (1) endothelial dysfunction and (2) resistance to the vasodilating action of insulin. This endothelial dysfunction appears to be associated with both elevated androgen levels and insulin resistance. Given the central vasoprotective role of endothelium, these findings could explain, at least in part, the increased risk for macrovascular disease in women with PCOS.

Insulin-mediated skeletal muscle vasodilation contributes to both insulin sensitivity and responsiveness in lean humans.

Whether insulin-mediated vasodilation is important in determining insulins overall action to stimulate glucose uptake is unknown. To this end, we measured leg glucose uptake during euglycemic hyperinsulinemic clamps performed at two insulin doses (40 mU/m2 per min, n = 6 and 120 mU/m2 per min, n = 15) alone and during a superimposed intrafemoral artery infusion of GN-monomethyl-L-arginine (L-NMMA) designed to blunt insulin-mediated vasodilation. During the higher dose study, hyperinsulinemia resulted in about a twofold rise in basal leg blood flow from 0.24 +/- 0.02 to 0.45 +/- 0.05 liter/min, P < 0.0001. L-NMMA infusion resulted in a net 21% reduction in leg glucose uptake from 114 +/- 18 mg/min to 85 +/- 13 mg/min, P < 0.001. We also found a significant relationship between the rate of insulin-stimulated whole body glucose uptake and the magnitude of flow dependent glucose uptake (r = 0.57, P = 0.02). Data obtained during the lower dose insulin infusion resulted in similar findings. In conclusion, in healthy lean subjects, insulin-stimulated muscle blood flow contributes to both insulin responsiveness and insulin sensitivity. The most insulin-sensitive subjects appear to be the most reliant on muscle perfusion for insulin action. Insulin-mediated vasodilation is an important physiological determinant of insulin action.

Vascular function, insulin resistance and fatty acids

Over the past 10 years it has become clear that intact vascular function, especially at the level of the endothelium, is paramount in the prevention or delay of cardiovascular disease. It has also become clear that insulin itself, in addition to its metabolic actions, directly effects vascular endothelium and smooth muscle. Insulin, at normal physiologic concentrations, causes changes in skeletal muscle blood flow in healthy, insulin-sensitive subjects. Insulins effect on the endothelium is mediated through its own receptor and insulin signalling pathways, resulting in the increased release of nitric oxide. Insulins vascular actions are impaired in insulin-resistant conditions such as obesity, Type II (non-insulin-dependent) diabetes mellitus and hypertension, which could contribute to the excessive rates of cardiovascular disease in these groups. Insulin-resistant states of obesity and Type II diabetes show a multitude of metabolic abnormalities that could cause vascular dysfunction. Non-esterified fatty acid levels increase long before hyperglycaemia becomes present. Raised non-esterified fatty acids impair insulins effect on glucose uptake in skeletal muscle and the vascular endothelium and thus could have detrimental effects on the vasculature, leading to premature cardiovascular disease.

Type II Diabetes Abrogates Sex Differences in Endothelial Function in Premenopausal Women

BACKGROUND Obesity is a more potent cardiovascular risk factor (CVRF) in men than in women. Because traditional CVRFs cannot fully account for this sex difference, we tested the hypothesis that compared with men, women exhibit more robust endothelial function independent of obesity and that this sex difference is abrogated by diabetes. METHODS AND RESULTS We studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (Mch) and the endothelium-independent vasodilator sodium nitroprusside (SNP) in groups of lean, obese (OB), and type II diabetic (DM) premenopausal women and age- and body mass index-matched men. LBF response to intrafemoral administration of L-NMMA, an inhibitor of nitric oxide synthase, was also assessed in normal men and women. Maximum LBF increments in response to Mch were 347+/-57% versus 231+/-22% in lean women versus men (P<0.05) and 203+/-25% versus 111+/-17% in OB women versus men (P<0.01), respectively. In DM, maximum LBF increments in response to Mch were 104+/-24% and 138+/-33% in women and men, respectively, (P=NS). LBF decrements in response to L-NMMA were 34.9+/-4.1% and 17.1+/-4.2% in women and men, respectively (P<0.01). The response to SNP was not different between sexes and groups. CONCLUSIONS Premenopausal nondiabetic women exhibit more robust endothelium-dependent vasodilation owing to higher rates of nitric oxide release than men. Given the protective vascular action of nitric oxide, this difference may partially explain the lower incidence of macrovascular disease in women. In premenopausal women, DM causes impairment of endothelial function beyond that observed with obesity alone and leads to endothelial dysfunction similar to that observed in DM men. These findings may help explain the similar rates of coronary artery disease and mortality in diabetic men and women.

Screening for Coronary Artery Disease in Patients With Diabetes

Coronary artery disease (CAD) accounts for a large fraction of the morbidity, mortality, and cost of diabetes. Recognizing this, nearly 10 years ago the American Diabetes Association published a consensus recommendation that clinicians consider a risk factor–guided screening approach to early diagnosis of CAD in both symptomatic and asymptomatic patients. Subsequent clinical trial results have not supported those recommendations. Since the prior consensus statement, newer imaging methods, such as coronary artery calcium scoring and noninvasive angiography with computed tomography (CT) techniques, have come into use. These technologies, which allow quantitation of atherosclerotic burden and can predict risk of cardiac events, might provide an approach to more widespread coronary atherosclerosis screening. However, over this same time interval, there has been recognition of diabetes as a cardiovascular disease (CVD) equivalent, clear demonstration that medical interventions should provide primary and secondary CVD risk reduction in diabetic populations, and suggestive evidence that percutaneous coronary revascularization may not provide additive survival benefit to intensive medical management in patients with stable CAD. This additional evidence raises the question of whether documenting asymptomatic atherosclerosis or ischemia in people with diabetes is warranted. More data addressing this issue will be forthcoming from the BARI 2-D (Bypass Angioplasty Revascularization Investigation 2 Diabetes) trial. Until then, for patients with type 2 diabetes who are asymptomatic for CAD, we recommend that testing for atherosclerosis or ischemia, perhaps with cardiac CT as the initial test, be reserved for those in whom medical treatment goals cannot be met and for selected individuals in whom there is strong clinical suspicion of very-high-risk CAD. Better approaches to identify such individuals based on readily obtained clinical variables are sorely needed.

Endothelial Dysfunction Is Associated With Cholesterol Levels in the High Normal Range in Humans

BACKGROUND The purpose of this study was to test the hypothesis that cholesterol levels in the high normal range are associated with impaired endothelium-dependent vasodilation. METHODS AND RESULTS We studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of the endothelium-dependent vasodilator methacholine chloride (MCh) or the endothelium-independent vasodilator sodium nitroprusside (SNP) in normal volunteers exhibiting a wide range of total cholesterol levels within the normal range (<75th percentile). LBF increased in a dose-dependent fashion in response to the femoral artery infusions of MCh and SNP (P<.001). LBF responses to MCh were significantly blunted (P<.001) in subjects with high normal cholesterol (195+/-6 mg/dL, n=13) compared with subjects with low normal cholesterol (146+/-5 mg/dL, n=20). Maximal endothelium-dependent vasodilation in the high normal group was decreased by nearly 50% compared with the low normal group (146+/-13% versus 268+/-34%, P<.01). There was a negative correlation between total cholesterol levels and maximal endothelium-dependent vasodilation (total cholesterol, r=-.41, P<.02; LDL cholesterol, r=-.42, P<.02). On the other hand, LBF responses to the endothelium-independent vasodilator SNP did not differ between groups. CONCLUSIONS These data suggest that an inverse and continuous relationship exists between the prevailing cholesterol level and endothelium-dependent vasodilation. Moreover, cholesterol levels even in the normal range may be associated with endothelial dysfunction, thus potentially contributing to the increased risk of macrovascular disease conferred by cholesterol elevations.

Efficacy and tolerability of sitagliptin monotherapy in elderly patients with type 2 diabetes: a randomized, double-blind, placebo-controlled trial

Abstract Objective: Type 2 diabetes in the elderly is an important and insufficiently studied public health problem. This study evaluated sitagliptin monotherapy in patients with type 2 diabetes aged ≥65 years. Research design and methods: This was a randomized, double-blind, placebo-controlled, parallel-group study conducted at 52 sites in the United States. Patients were treated with once-daily sitagliptin (100 or 50 mg, depending on renal function) or placebo for 24 weeks. Key endpoints included change from baseline in glycated hemoglobin (HbA1c), 2-hour post-meal glucose (2-h PMG) and fasting plasma glucose (FPG) at week 24, and average blood glucose on treatment days 3 and 7. Clinical trial registration: NCT00305604. Results: Among randomized patients (N = 206), mean age was 72 years and mean baseline HbA1c was 7.8%. At week 24, HbA1c decreased by 0.7%, 2-h PMG by 61 mg/dL, and FPG by 27 mg/dL in sitagliptin-treated patients compared with placebo (all p < 0.001). On day 3 of treatment, mean average blood glucose was decreased from baseline by 20.4 mg/dL in sitagliptin-treated patients compared with placebo (p < 0.001). In subgroups defined by baseline HbA1c <8.0% (n = 132), ≥8.0% to <9.0% (n = 42), and ≥9.0% (n = 18), the placebo-adjusted reductions in HbA1c with sitagliptin treatment were 0.5%, 0.9%, and 1.6%, respectively. Patients in the sitagliptin and placebo groups had similar rates of adverse events overall (46.1% and 52.9%, respectively); serious adverse events were reported in 6.9% and 13.5%, respectively. No adverse events of hypoglycemia were reported. Potential study limitations include a relatively small number of patients with more severe hyperglycemia (HbA1c ≥9.0%) and the exclusion of patients with severe renal insufficiency. Conclusion: In this study, sitagliptin treatment significantly and rapidly improved glycemic measures and was well tolerated in patients aged ≥65 years with type 2 diabetes.