Etto C. Eringa

Endothelial dysfunction and diabetes: roles of hyperglycemia, impaired insulin signaling and obesity

Endothelial dysfunction comprises a number of functional alterations in the vascular endothelium that are associated with diabetes and cardiovascular disease, including changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. Hyperglycemia is a characteristic feature of type 1 and type 2 diabetes and plays a pivotal role in diabetes-associated microvascular complications. Although hyperglycemia also contributes to the occurrence and progression of macrovascular disease (the major cause of death in type 2 diabetes), other factors such as dyslipidemia, hyperinsulinemia, and adipose-tissue-derived factors play a more dominant role. A mutual interaction between these factors and endothelial dysfunction occurs during the progression of the disease. We pay special attention to the possible involvement of endoplasmic reticulum stress (ER stress) and the role of obesity and adipose-derived adipokines as contributors to endothelial dysfunction in type 2 diabetes. The close interaction of adipocytes of perivascular adipose tissue with arteries and arterioles facilitates the exposure of their endothelial cells to adipokines, particularly if inflammation activates the adipose tissue and thus affects vasoregulation and capillary recruitment in skeletal muscle. Hence, an initial dysfunction of endothelial cells underlies metabolic and vascular alterations that contribute to the development of type 2 diabetes.

Microvascular Dysfunction: A Potential Pathophysiological Role in the Metabolic Syndrome

Obesity and a central body fat distribution, hypertension, insulin resistance, glucose intolerance, dyslipidemia, and proinflammatory and prothrombotic factors are all part of the metabolic syndrome. The metabolic syndrome defines a clustering of metabolic risk factors which confers an increased risk for type 2 diabetes and cardiovascular disease.1 In the past years a large amount of research has been aimed at elucidating the pathophysiology underlying this clustering of risk factors, because a better understanding may lead to new therapeutic approaches that specifically target underlying causes of the metabolic syndrome. Recently, it has become clear that microvascular dysfunction, by affecting both pressure and flow patterns, may have consequences not only for peripheral vascular resistance, but also for insulin-mediated changes in muscle perfusion and glucose metabolism, providing a novel pathophysiological framework for understanding the association between hypertension, obesity, and impaired insulin-mediated glucose disposal.2–4 The present article examines recent data concerning the role of microvascular dysfunction as an explanation for the associations among hypertension, obesity, and impaired insulin-mediated glucose disposal. Description of the Microcirculation An exact definition of the microcirculation is elusive. Morphologically, the microcirculation is widely taken to encompass vessels 150 m in diameter. It therefore includes arterioles, capillaries, and venules. Alternatively, a definition based on arterial vessel physiology rather than diameter or structure has been proposed. 3 By this definition, all arterial vessels that respond to increasing pressure by a myogenic reduction in lumen diameter are included in the microcirculation. Such a definition would include the smallest arteries and arterioles in the microcirculation in addition to capillaries and venules. Small arterial and arteriolar components should, therefore, be considered a continuum rather than distinct sites of resistance control. A primary function of the microcirculation is to optimize nutrient and oxygen supply within the tissue in response to variations in demand. A second important function is to avoid large fluctuations in hydrostatic pressure at the level of the capillaries causing disturbances in capillary exchange. Finally, it is at the level of the microcirculation that a substantial proportion of the drop in hydrostatic pressure occurs. The microcirculation is therefore extremely important in determining overall peripheral vascular resistance.

Regulation of Vascular Function and Insulin Sensitivity by Adipose Tissue: Focus on Perivascular Adipose Tissue

ABSTRACT

Physiological concentrations of insulin induce endothelin-mediated vasoconstriction during inhibition of NOS or PI3-kinase in skeletal muscle arterioles

OBJECTIVE To determine the roles of nitric oxide, endothelin-1 and phosphatidylinositol 3-kinase (PI3-kinase) in acute responses of isolated rat skeletal muscle arterioles to insulin. METHODS Rat cremaster first order arterioles were separated from surrounding tissue, cannulated in a pressure myograph and responses to insulin (4 microU/ml-3.4 mU/ml) were studied without intraluminal blood or flow. RESULTS Insulin alone did not significantly affect arteriolar diameter. Non-selective antagonism of endothelin receptors, with PD-142893, uncovered insulin-induced vasodilatation (25+/-8% from baseline at 3.4 mU/ml), which was abolished by inhibition of NO synthesis with N(G)-nitro-L-arginine (L-NA). Inhibition of NO synthesis alone uncovered insulin-induced vasoconstriction at physiological concentrations (21+/-5% from baseline diameter at 34 microU/ml), which was abolished by PD-142893. The NO donor, S-nitroso-N-acetyl-penicillamine (SNAP) inhibited insulin-induced vasoconstriction during NOS inhibition, even at a concentration that did not elicit vasodilatation itself. Inhibition of PI3-kinase, an intracellular mediator of insulin-induced NO production, with wortmannin, also uncovered insulin-induced vasoconstriction (13+/-3% from baseline at 34 microU/ml) that was abolished by PD-142893. CONCLUSIONS Insulin induces both nitric oxide and endothelin-1 activity in rat cremaster first-order arterioles. This study demonstrates for the first time that vasoconstrictive effects of physiological concentrations of insulin during inhibition of NOS activity are mediated by endothelin and that insulin induces endothelin-1-mediated vasoconstriction in isolated skeletal muscle arterioles during inhibition of PI3-kinase. These findings support the hypothesis of altered microvascular reactivity to insulin in conditions of diminished PI3-kinase activity, a prominent feature of insulin resistance.

Microvascular Dysfunction: A Potential Mechanism in the Pathogenesis of Obesity‐associated Insulin Resistance and Hypertension

Please cite this paper as: de Boer, Meijer, Wijnstok, Jonk, Houben, Stehouwer, Smulders, Eringa and Serné (2012). Microvascular Dysfunction: A Potential Mechanism in the Pathogenesis of Obesity‐associated Insulin Resistance and Hypertension. Microcirculation 19(1), 5–18.

Physiological Concentrations of Insulin Induce Endothelin-Dependent Vasoconstriction of Skeletal Muscle Resistance Arteries in the Presence of Tumor Necrosis Factor-α Dependence on c-Jun N-Terminal Kinase

Objective—Tumor necrosis factor-α (TNF-α) has been linked to obesity-related insulin resistance and impaired endothelium-dependent vasodilatation, but the mechanisms have not been elucidated. To investigate whether TNF-α directly impairs insulin-mediated vasoreactivity in skeletal muscle resistance arteries and the role of c-Jun N-terminal kinase (JNK) in this interference. Methods and Results—Insulin-mediated vasoreactivity of isolated resistance arteries of the rat cremaster muscle to insulin (4 to 3400 &mgr;U/mL) was studied in the absence and presence of TNF-α (10 ng/mL). Although insulin or TNF-α alone did not affect arterial diameter, insulin induced dose-dependent vasoconstriction of cremaster resistance arteries in the presence of TNF-α, (−12±1% at 272 &mgr;U/mL). Blocking endothelin receptors in the absence of TNF-α uncovered insulin-mediated vasodilatation (18±6% at 272 &mgr;U/mL) but not in the presence of TNF-α (2±2% at 272 &mgr;U/mL), showing that TNF-α inhibits vasodilator effects of insulin. Using digital imaging microscopy, we discovered that TNF-α activates JNK in arterial endothelium, visible as an increase in phosphorylated JNK. Moreover, inhibition of JNK with the cell-permeable peptide inhibitor L-JNKI abolished insulin-mediated vasoconstriction in the presence of TNF-α, showing that JNK is required for interaction between TNF-α and insulin. Conclusions—TNF-α inhibits vasodilator but not vasoconstrictor effects of insulin in skeletal muscle resistance arteries, resulting in insulin-mediated vasoconstriction in the presence of TNF-α. This effect of TNF-α is critically dependent on TNF-α–mediated activation of JNK.

Reactive oxygen species-induced stimulation of 5 ' AMP-activated protein kinase mediates sevoflurane-induced cardioprotection

Background— 5′AMP-activated protein kinase (AMPK), a well-known regulator of cellular energy status, is also implicated in ischemic preconditioning leading to cardioprotection. We hypothesized that AMPK is involved in anesthetic-induced cardioprotection and that this activation is mediated by reactive oxygen species (ROS). Methods and Results— Isolated Langendorff-perfused rat hearts were subjected to 35 minutes of global ischemia (I) followed by 120 minutes of reperfusion (I/R). Hearts were assigned to a control group (Con) or a sevoflurane (Sevo) group receiving 3 times 5-minute episodes of sevoflurane (2.5vol%) before I/R. Phosphorylation of both AMPK and endothelial nitric oxide synthase (eNOS) were determined by Western blot analysis. Cardioprotection was assessed after I/R from recovery of left ventricular pressure and from infarct size (triphenyltetrazolium chloride staining). In the control group, ischemia resulted in a 2-fold increase in phosphorylation levels of AMPK (Con 0.13±0.01 versus Con-I 0.28±0.05, P<0.05), which was sustained after 120 minutes of reperfusion (Con-I/R 0.26±0.02, P<0.05). Sevoflurane preconditioning had no affect on AMPK phosphorylation before ischemia (Sevo 0.12±0.03, P>0.05), but almost doubled the increase in AMPK phosphorylation relative to control after ischemia (Sevo-I 0.48±0.09, P<0.05), an effect that was sustained after reperfusion (Sevo-I/R 0.49±0.12, P<0.05). The AMPK-inhibitor compound C (10 &mgr;mol/L) reduced the sevoflurane-mediated increase in phosphorylation of AMPK and its target eNOS and abolished cardioprotection. The ROS-scavenger n-(2-mercaptopropionyl)-glycine (1 mmol/L) blunted the sevoflurane-mediated increase in AMPK and eNOS phosphorylation and prevented cardioprotection. Conclusions— Sevoflurane-induced AMPK activation protects the heart against ischemia and reperfusion injury and relies on upstream production of ROS.

Perivascular Adipose Tissue Control of Insulin-Induced Vasoreactivity in Muscle Is Impaired in db/db Mice

Microvascular recruitment in muscle is a determinant of insulin sensitivity. Whether perivascular adipose tissue (PVAT) is involved in disturbed insulin-induced vasoreactivity is unknown, as are the underlying mechanisms. This study investigates whether PVAT regulates insulin-induced vasodilation in muscle, the underlying mechanisms, and how obesity disturbs this vasodilation. Insulin-induced vasoreactivity of resistance arteries was studied with PVAT from C57BL/6 or db/db mice. PVAT weight in muscle was higher in db/db mice compared with C57BL/6 mice. PVAT from C57BL/6 mice uncovered insulin-induced vasodilation; this vasodilation was abrogated with PVAT from db/db mice. Blocking adiponectin abolished the vasodilator effect of insulin in the presence of C57BL/6 PVAT, and adiponectin secretion was lower in db/db PVAT. To investigate this interaction further, resistance arteries of AMPKα2+/+ and AMPKα2−/− were studied. In AMPKα2−/− resistance arteries, insulin caused vasoconstriction in the presence of PVAT, and AMPKα2+/+ resistance arteries showed a neutral response. On the other hand, inhibition of the inflammatory kinase Jun NH2-terminal kinase (JNK) in db/db PVAT restored insulin-induced vasodilation in an adiponectin-dependent manner. In conclusion, PVAT controls insulin-induced vasoreactivity in the muscle microcirculation through secretion of adiponectin and subsequent AMPKα2 signaling. PVAT from obese mice inhibits insulin-induced vasodilation, which can be restored by inhibition of JNK.

Birth Weight Relates to Salt Sensitivity of Blood Pressure in Healthy Adults

The association between birth weight and blood pressure is well established but at present unexplained. According to the Borst-Guyton concept, chronic hypertension can occur only with a shift in the renal pressure–natriuresis relationship resulting in increased salt sensitivity of blood pressure. We assessed salt sensitivity of blood pressure in a group of 27 healthy adults whose birth weight was available. Birth weight was ascertained from birth certificates or announcements. Salt sensitivity of blood pressure was determined as difference in mean arterial pressure (MAP) between a 1-week high-salt (≈235 mmol NaCl/d) versus low-salt diet (≈55 mmol NaCl/d). Creatinine clearance was estimated according to the formula of Cockcroft and Gault. Birth weight was negatively associated with salt sensitivity of blood pressure (r=−0.60, P=0.002). The creatinine clearance was positively associated with birth weight (r=0.53; P=0.008) but did not influence the association between birth weight and salt sensitivity of blood pressure. Birth weight is associated with salt sensitivity of blood pressure, and this may play a role in the maintenance of elevated blood pressure in individuals with a low birth weight.

Endothelial dysfunction in (pre)diabetes: Characteristics, causative mechanisms and pathogenic role in type 2 diabetes

Endothelial dysfunction associated with diabetes and cardiovascular disease is characterized by changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. These endothelial alterations contribute to excess cardiovascular disease in diabetes, but may also play a role in the pathogenesis of diabetes, especially type 2. The mechanisms underlying endothelial dysfunction in diabetes differ between type 1 (T1D) and type 2 diabetes (T2D): hyperglycemia contributes to endothelial dysfunction in all individuals with diabetes, whereas the causative mechanisms in T2D also include impaired insulin signaling in endothelial cells, dyslipidemia and altered secretion of bioactive substances (adipokines) by adipose tissue. The close association of so-called perivascular adipose tissue with arteries and arterioles facilitates the exposure of vascular endothelium to adipokines, particularly if inflammation activates the adipose tissue. Glucose and adipokines activate specific intracellular signaling pathways in endothelium, which in concert result in endothelial dysfunction in diabetes. Here, we review the characteristics of endothelial dysfunction in diabetes, the causative mechanisms involved and the role of endothelial dysfunction(s) in the pathogenesis of T2D. Finally, we will discuss the therapeutic potential of endothelial dysfunction in T2D.