Microvascular function relates to insulin sensitivity and blood pressure in normal subjects

Gender-specific regulation of pancreatic islet blood flow, insulin levels and glycaemia in spontaneously diabetic Goto-Kakizaki rats

Patients with diabetes are often treated with a statin for hyperlipidaemia and an ACE (angiotensin-converting enzyme) inhibitor or angiotensin receptor antagonist for hypertension or albuminuria. These drugs may also exert beneficial metabolic effects, causing improved glucose tolerance in patients. Gender-related differences have also been observed in the clinical responsiveness to these drugs, but the mechanism behind this is unclear. In the present study, we have investigated whether these drugs and the fatty acid palmitate influence the pancreatic microcirculation, thereby having an impact on insulin secretion and glycaemia in vivo, in spontaneously diabetic male and female Goto-Kakizaki rats. In male rats, pancreatic IBF (islet blood flow) and total PBF (pancreatic blood flow) were increased significantly by pravastatin, captopril and irbesartan. Serum insulin levels were increased by pravastatin and captopril. Palmitate suppressed pancreatic IBF and increased blood glucose. In female animals, pancreatic IBF was stimulated by captopril, candesartan and irbesartan. Total PBF was increased by captopril, candesartan and irbesartan, and by pravastatin. Palmitate suppressed pancreatic IBF and serum insulin secretion. In conclusion, the present study lends support to the view that a local pancreatic RAS (renin-angiotensin system) and pravastatin may be selectively influencing the pancreatic microcirculation and therefore affecting insulin secretion and glycaemia. NEFAs (non-esterified fatty acids) impaired pancreatic IBF, suppressed insulin secretion and increased blood glucose. Substantial gender-related differences in the vascular and metabolic responses to these drugs prevail in this animal model of diabetes.

Incorporation of the fasting free fatty acid concentration into quantitative insulin sensitivity check index improves its association with insulin sensitivity in adults, but not in children

OBJECTIVE

Based on fasting insulin and glucose, several indices of insulin sensitivity have been developed in adults. Recently, it has been demonstrated that incorporation of the fasting free fatty acid (FFA) concentration improves the association with insulin sensitivity in adults. We investigated the association of clamp-derived insulin sensitivity with indices of insulin sensitivity derived from fasting blood in prepubertal children and adults, with and without incorporation of FFAs.

DESIGN AND METHODS

We studied 59 healthy adults and 29 of them are prepubertal children. We measured insulin sensitivity with the euglycemic-hyperinsulinemic clamp. Based on fasting insulin and glucose, we estimated insulin sensitivity with the homeostasis model assessment (HOMA), the quantitative insulin sensitivity check index (QUICKI), and the revised QUICKI after the incorporation of FFAs.

RESULTS

The associations of HOMA and QUICKI with clamp-derived insulin sensitivity in children (r=-0.55 and 0.54 respectively; P<0.01) were similar to those in adults (r=-0.54 and 0.53 respectively; P<0.01). However, incorporation of FFAs into the QUICKI model resulted in an increase in the association in adults, but not in children (r=0.68 and 0.48 respectively; P<0.01). Adding FFA levels to a regression model with glucose and insulin as independent variables resulted in an increase in the explained variance in clamp-derived insulin sensitivity in adults, but not in children (P value 0.004 in adults and 0.3 in children).

CONCLUSIONS

HOMA and QUICKI are associated with clamp-derived insulin sensitivity in both children and adults. Incorporating fasting levels of FFAs into the QUICKI model improves the association with clamp-derived insulin sensitivity in adults, but not in children.

Abnormal endothelium-dependent microvascular dilator reactivity in pregnancies complicated by normotensive intrauterine growth restriction

OBJECTIVE

Normotensive intrauterine growth restriction and preeclampsia share a similar placenta pathophysiology, whereas maternal clinical manifestations differ. Clinical symptoms of preeclampsia are partly attributed to vascular endothelial dysfunction, but it is unclear whether this phenomenon plays a role in intrauterine growth restriction. Therefore, we investigated microvascular endothelial function in women with intrauterine growth restriction.

STUDY DESIGN

Laser Doppler fluxmetry was used combined with iontophoresis of acetylcholine and sodium nitroprusside, namely, endothelium-dependent and endothelium-independent vasodilators. We studied 12 women with intrauterine growth restriction and 16 controls in the third trimester of pregnancy. All women had prepregnancy body mass indexes < 26.

RESULTS

Acetylcholine-mediated vasodilatation was significantly increased in women with intrauterine growth restriction compared with controls (743% +/- 120% vs 390% +/- 67%, P = .01); sodium nitroprusside-mediated vasodilatation was not different (360% +/- 55% vs 363% +/- 65%, P > .99).

CONCLUSION

Nonobese women with normotensive intrauterine growth restriction show abnormal endothelium-dependent microvascular vasodilatation, suggesting endothelial dysfunction as in preeclampsia. Obviously, for the clinical manifestation of preeclampsia additional factors are required, and a role of metabolic syndrome and obesity has been suggested.

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.

Current concepts in assessment of microvascular endothelial function using laser Doppler imaging and iontophoresis

Effective evaluation of endothelial function is a powerful tool for determining patients at risk of development and progression of cardiovascular disease. As an alternative to invasive tests of endothelial function, several noninvasive methods have been developed, including the use of laser Doppler flowmetry/imaging to measure cutaneous perfusion accompanied by iontophoresis of acetylcholine and sodium nitroprusside. It is clear from previous studies that this technique provides an easy, validated, and reproducible method for investigators to assess and monitor endothelial function in patients with a variety of pathologic conditions, but it may also be used to examine disease progression over time and responsiveness to treatment, thereby facilitating clinical trials. However, a standardization of protocols would help reduce the apparent controversy seen in the literature. With its increasing use by other groups, it is anticipated that further published studies will help to provide a better understanding of the development and progression of cardiovascular disease.

Impaired microvascular perfusion: a consequence of vascular dysfunction and a potential cause of insulin resistance in muscle

Insulin has an exercise-like action to increase microvascular perfusion of skeletal muscle and thereby enhance delivery of hormone and nutrient to the myocytes. With insulin resistance, insulin's action to increase microvascular perfusion is markedly impaired. This review examines the present status of these observations and techniques available to measure such changes as well as the possible underpinning mechanisms. Low physiological doses of insulin and light exercise have been shown to increase microvascular perfusion without increasing bulk blood flow. In these circumstances, blood flow is proposed to be redirected from the nonnutritive route to the nutritive route with flow becoming dominant in the nonnutritive route when insulin resistance has developed. Increased vasomotion controlled by vascular smooth muscle may be part of the explanation by which insulin mediates an increase in microvascular perfusion, as seen from the effects of insulin on both muscle and skin microvascular blood flow. In addition, vascular dysfunction appears to be an early development in the onset of insulin resistance, with the consequence that impaired glucose delivery, more so than insulin delivery, accounts for the diminished glucose uptake by insulin-resistant muscle. Regular exercise may prevent and ameliorate insulin resistance by increasing "vascular fitness" and thereby recovering insulin-mediated capillary recruitment.

Obese but not normal-weight women with polycystic ovary syndrome are characterized by metabolic and microvascular insulin resistance

CONTEXT

Polycystic ovary syndrome (PCOS) and obesity are associated with diabetes and cardiovascular disease, but it is unclear to what extent PCOS contributes independently of obesity.

OBJECTIVE

The objective of the study was to investigate whether insulin sensitivity and insulin's effects on the microcirculation are impaired in normal-weight and obese women with PCOS.

DESIGN AND POPULATION

Thirty-five women with PCOS (19 normal weight and 16 obese) and 27 age- and body mass index-matched controls (14 normal weight and 13 obese) were included. Metabolic Insulin sensitivity (isoglycemic-hyperinsulinemic clamp) and microvascular insulin sensitivity [endothelium dependent (acetylcholine [ACh])] and endothelium-independent [sodium nitroprusside (SNP)] vasodilation with laser Doppler flowmetry was assessed at baseline and during hyperinsulinemia.

MAIN OUTCOME MEASURES

Metabolic insulin sensitivity (M/I value) and the area under the response curves to ACh and SNP curves were measured to assess microcirculatory function at baseline and during insulin infusion (microvascular insulin sensitivity).

RESULTS

Obese women were more insulin resistant than normal-weight women (P < 0.001), and obese PCOS women were more resistant than obese controls (P = 0.02). In contrast, normal-weight women with PCOS had similar insulin sensitivity, compared with normal-weight women without PCOS. Baseline responses to ACh showed no difference in the four groups. ACh responses during insulin infusion were significantly greater in normal-weight PCOS and controls than in obese PCOS and controls. PCOS per se had no significant influence on ACh responses during insulin infusion. During hyperinsulinemia, SNP-dependent vasodilatation did not significantly increase, compared with baseline in the four groups.

CONCLUSION

PCOS per se was not associated with impaired metabolic insulin sensitivity in normal-weight women but aggravates impairment of metabolic insulin sensitivity in obese women. In obese but not normal-weight women, microvascular and metabolic insulin sensitivity are decreased, independent of PCOS. Therefore, obese PCOS women in particular may be at increased risk of metabolic and cardiovascular diseases.

Forearm vascular reactivity and arterial stiffness in asymptomatic adults from the community

Vascular reactivity may affect the stiffness characteristics of the arterial wall. We investigated the association between forearm microcirculatory and conduit artery function and measures of arterial stiffness in 527 asymptomatic non-Hispanic white adults without known cardiovascular disease. High-resolution ultrasonography of the brachial artery (ba) was performed to assess forearm microcirculatory function (ba blood flow velocity, local shear stress, and forearm vascular resistance at rest and during reactive hyperemia) and conduit artery function (ba flow-mediated dilatation [baFMD] and ba nitroglycerin-mediated dilatation [baNMD]). Arterial stiffness was assessed by cuff-derived brachial pulse pressure and aortic pulse wave velocity (aPWV) measured by applanation tonometry. In regression analyses that adjusted for heart rate, mean arterial pressure, height, cardiovascular risk factors, and hypertension medication and statin use, higher baseline ba systolic velocity and systolic shear stress were associated with greater pulse pressure (P=0.0002 and P=0.006, respectively) and higher aPWV (each P<0.0001). During hyperemia, lower ba mean velocity and lower mean shear stress were associated with higher pulse pressure (P=0.045 and P=0.036, respectively), whereas both systolic and mean velocity (P<0.0001 and P=0.002, respectively) and systolic and mean shear stress (P<0.0001 and P=0.003, respectively) were inversely associated with aPWV. baFMD was not associated with pulse pressure but was inversely associated with aPWV (P=0.011). baNMD was inversely associated with pulse pressure (P=0.0002) and aPWV (P=0.008). Our findings demonstrate that impaired forearm microvascular function (in the form of elevated resting blood flow velocity and impaired flow reserve) and impaired brachial artery reactivity are associated with increased arterial stiffness.

A review of methods for assessment of coronary microvascular disease in both clinical and experimental settings

Obstructive disease of the large coronary arteries is the prominent cause for angina pectoris. However, angina may also occur in the absence of significant coronary atherosclerosis or coronary artery spasm, especially in women. Myocardial ischaemia in these patients is often associated with abnormalities of the coronary microcirculation and may thus represent a manifestation of coronary microvascular disease (CMD). Elucidation of the role of the microvasculature in the genesis of myocardial ischaemia and cardiac damage-in the presence or absence of obstructive coronary atherosclerosis-will certainly result in more rational diagnostic and therapeutic interventions for patients with ischaemic heart disease. Specifically targeted research based on improved assessment modalities is needed to improve the diagnosis of CMD and to translate current molecular, cellular, and physiological knowledge into new therapeutic options.

Functional capillary rarefaction in mild blood pressure elevation

Capillary rarefaction is described in patients with moderate to severe hypertension. The study objective was to determine if structural and/or functional capillary rarefaction is detectable and associated with endothelial dysfunction in patients with mild blood pressure elevation (HBP: Systolic blood pressure 130 - 160 mm Hg). Capillary density was quantified by direct capillaroscopy in 110 nondiabetic black and non-black subjects. Endothelial function was quantified by plethysmographic measures of flow-mediated vasodilation. Compared to normotensives (NBP: N = 90), functional capillary rarefaction was detected in HBP (N = 20; P<0.001). Functional capillary density measures correlated with endothelial function (P<0.001). Functional, but not structural, capillary rarefaction is detectable and associated with endothelial dysfunction in both black and non-black individuals with mild blood pressure elevation.

Capillary rarefaction in treated and untreated hypertensive subjects

This study aimed to determine if capillary rarefaction is detectable and associated with endothelial dysfunction in persons with mild systolic blood pressure (SBP) elevation. Capillary density and endothelial function were quantified for 150 nondiabetic participants, grouped by blood pressure (BP) as normotensive, untreated high BP, and treated high BP. Structural capillary rarefaction measures were not different between the three groups. Functional capillary rarefaction measures were significantly lower in both high BP groups compared to normotensives, and correlated inversely with endothelial function. The study findings indicate that the hypertensive vascular pathologic process is already underway at modest levels of blood pressure elevation.

Decreased NO signaling leads to enhanced vasoconstrictor responsiveness in skeletal muscle arterioles of the ZDF rat prior to overt diabetes and hypertension

Approximately 40% of patients with type 2 diabetes present with concurrent hypertension at the time of diabetes diagnosis. Increases in peripheral vascular resistance and correspondingly enhanced vasoconstrictor capacity could have profound implications for the development of hypertension and the progression of insulin resistance to overt diabetes. The purpose of this study was to determine whether skeletal muscle arteriolar vasoconstrictor dysfunction precedes or occurs concurrently with the onset of diabetes and hypertension. Male Zucker diabetic fatty (ZDF) rats were studied at 7, 13, and 20 wk of age to represent prediabetic and short-term and long-term diabetic states, respectively. Conscious mean arterial pressure (MAP), fasted plasma insulin and glucose, vasoconstrictor responses, and passive mechanical properties of isolated skeletal muscle arterioles were measured in prediabetic, diabetic, and age-matched control rats. Elevated MAP was manifest in short-term diabetes (control 117 +/- 1, diabetic 135 +/- 3 mmHg) and persisted with long-term diabetes (control 113 +/- 2, diabetic 135 +/- 3 mmHg). This higher MAP was preceded by augmented arteriolar vasoconstrictor responses to norepinephrine and endothelin-1 and followed by diminished beta-adrenergic vasodilation and enhanced myogenic constriction in long-term diabetes. Furthermore, we demonstrate that diminished nitric oxide (NO) signaling underlies the increases in vasoconstrictor responsiveness in arterioles from prediabetic and diabetic rats. Arteriolar stiffness was not different between control and prediabetic or diabetic rats at any time point studied. Collectively, these results indicate that increases in vasoconstrictor responsiveness resulting from diminished NO signaling in skeletal muscle arterioles precede the development of diabetes and hypertension in ZDF rats.

Severe impairment of endothelial function with the HIV-1 protease inhibitor indinavir is not mediated by insulin resistance in healthy subjects

Endothelial dysfunction may contribute to increased cardiovascular events among HIV-1-infected patients receiving antiretroviral therapy. The HIV-1 protease inhibitor indinavir causes both vascular dysfunction and insulin resistance, but the relationship between the two disturbances is not established. Endothelium-dependent vasodilation (EDV), insulin-mediated vasodilation (IMV), and whole body and leg glucose uptake during a euglycemic hyperinsulinemic clamp (40 mU/m(2)/min) were measured before and after four weeks of indinavir in nine healthy men. EDV fell from 270 +/- 67% above basal to 124 +/- 30% (P = 0.04) and IMV from 56 +/- 14% above basal to 8 +/- 8% (P = 0.001) with indinavir. During the clamp, arteriovenous glucose difference and leg glucose uptake were not significantly different after indinavir and whole-body glucose uptake was only modestly reduced (8.0 +/- 0.8 vs. 7.2 +/- 0.8 mg/kg/min, P = 0.04). The change in EDV did not correlate with the change in whole-body glucose uptake after indinavir (r = 0.21, P = 0.6). Despite marked impairment of endothelial function and IMV with indinavir, only modest, inconsistent reductions in measures of insulin-stimulated glucose uptake occurred. This suggests that indinavir's effects on glucose metabolism are not directly related to indinavir-associated endothelial dysfunction. Studies of the vascular effects of newer protease inhibitors are needed.

Blood pressure rise following angiogenesis inhibition by bevacizumab. A crucial role for microcirculation

Arterial hypertension (HT) has been reported in all studies involving bevacizumab, an antiangiogenic agent designed to target vascular endothelial growth factor (VEGF). The mechanism underlying bevacizumab-related HT is not yet clearly understood. As far as endothelial dysfunction and microvascular rarefaction are hallmarks in all forms of HT, we tested the hypothesis that anti-VEGF therapy could alter the microcirculation in nontumor tissues and, thus, result in an increase in blood pressure (BP). We used intravital video microscopy to measure dermal capillary densities in the dorsum of the fingers. Microvascular endothelial function was assessed by laser Doppler flowmetry combined with iontophoresis of pilocarpine (acetylcholine analogue). All measurements were carried out in 18 patients before and after a 6-month treatment with bevacizumab (mean cumulative dose: 3.16 +/- 0.90 g). Mean BP was increased after 6 months of therapy compared with baseline, from 129 +/- 13/75 +/- 7 mmHg to 145 +/- 17/82 +/- 7 mmHg for systolic BP and diastolic BP, respectively (P < 0.0001). Compared with the baseline, mean dermal capillary density at 6 months was significantly lower (75 +/- 12 versus 83 +/- 13/mm(2); P < 0.0001), as well as pilocarpine-induced vasodilation (P < 0.05). Thus, bevacizumab treatment resulted in endothelial dysfunction and capillary rarefaction; both changes are closely associated and could be responsible for the rise in BP observed in most patients.

Selective resistance to vasoactive effects of insulin in muscle resistance arteries of obese Zucker (fa/fa) rats

Obesity is related to insulin resistance and hypertension, but the underlying mechanisms are unclear. Insulin exerts both vasodilator and vasoconstrictor effects on muscle resistance arteries, which may be differentially impaired in obesity.

OBJECTIVES

To investigate whether vasodilator and vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats and the roles of Akt and endothelial NO synthase (eNOS).

METHODS/RESULTS

Effects of insulin were studied in resistance arteries isolated from cremaster muscles of lean and obese Zucker rats. In arteries of lean rats, insulin increased activity of both NO and endothelin (ET-1), resulting in a neutral effect under basal conditions. In arteries of obese rats, insulin induced endothelin-mediated vasoconstriction (-15 +/- 5% at 1 nM, P < 0.05 vs. lean). Insulin induced vasodilatation during endothelin receptor blockade in arteries of lean rats (20 +/- 5% at 1 nM) but not in those of obese rats. Inhibition of NO synthesis increased vascular tone (by 12 +/- 2%) and shifted insulin-mediated vasoreactivity to vasoconstriction (-25 +/- 1% at 1 nM) in lean rats but had no effect in arteries of obese rats, indicating reduced NO activity. Protein analysis of resistance arteries revealed that insulin-mediated activation of Akt was preserved in obese rats, whereas expression of eNOS was markedly decreased.

CONCLUSIONS

Vasodilator but not vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats, and this selective impairment is associated with decreased protein levels of eNOS. These findings provide a new mechanism linking obesity to insulin resistance and hypertension.

Microvascular dysfunction in obesity: a potential mechanism in the pathogenesis of obesity-associated insulin resistance and hypertension

Obesity is an important risk factor for insulin resistance and hypertension and plays a central role in the metabolic syndrome. Insight into the pathophysiology of this syndrome may lead to new treatments. This paper has reviewed the evidence for an important role for the microcirculation as a possible link between obesity, insulin resistance and hypertension.

Microvascular function: a potential link between salt sensitivity, insulin resistance and hypertension

OBJECTIVE

Generalized microvascular dysfunction may contribute to the development of salt sensitivity, insulin resistance and hypertension, and may thus link these cardiovascular risk factors. To test this hypothesis, we examined skin microvascular function, salt sensitivity, insulin sensitivity and blood pressure in 27 normotensive and 26 hypertensive individuals.

METHODS

Capillary density was examined by videomicroscopy during venous congestion and postocclusive reactive hyperaemia. Endothelium-(in)dependent vasodilation was assessed by iontophoresis of acetylcholine and sodium nitroprusside and by laser Doppler flowmetry. Salt sensitivity was determined as the difference in mean arterial pressure (MAP) between a 1-week high-salt diet (approximately 235 mmol NaCl/day) versus low-salt diet (approximately 55 mmol NaCl/day). Insulin sensitivity was measured with the hyperinsulinaemic, euglycaemic clamp, and blood pressure was assessed by 24-h ambulatory blood pressure monitoring.

RESULTS

Salt sensitivity of blood pressure was inversely associated with postocclusive capillary recruitment and endothelium-dependent vasodilation (r=-0.67, P<0.001 and r=-0.60, P<0.01, respectively), but not with capillary density during venous congestion or endothelium-independent vasodilation. Salt sensitivity was negatively associated with insulin sensitivity (r=-0.55, P<0.001) and positively with MAP (r=0.58, P<0.001). Multiple regression analyses suggested that associations between salt sensitivity and both insulin sensitivity and MAP were dependent on microvascular function.

CONCLUSION

Our results suggest a close inverse association between skin microvascular function and salt sensitivity and a role for generalized microvascular defects as a link between salt sensitivity, insulin resistance and hypertension.

Impaired local microvascular vasodilatory effects of insulin and reduced skin microvascular vasomotion in obese women

Our study aim is to investigate whether obesity is characterized by an impairment of insulin-mediated vasodilatory effects and by a modification of basal vasomotion in the skin microvasculature. Forty healthy obese and forty healthy lean women were included. Microvascular effects of insulin as compared to a control substance were measured by cathodal iontophoresis combined with laser Doppler flowmetry. Vasomotion was examined by Fourier transform analyses of skin laser Doppler flow at rest. Locally administered insulin, as compared to the control substance, induced a microvascular vasodilatory response in lean (median (interquartile range): 31.6 (17.1-43.9) vs. 22.9 (16.4-36.7) perfusion units, P=0.04), but not in obese women (28.1 (14.4-47.1) vs. 27.5 (17.5-48.2) perfusion units, P=0.7). The relative insulin-induced increase in blood flow corrected for the control substance was higher in lean than obese women (ANOVA for repeated measures F=3.93, P=0.05). The contribution of the total frequency spectrum 0.01-1.6 Hz and of the frequency intervals 0.01-0.02 Hz and 0.02-0.06 Hz (representative of endothelial and neurogenic activity, respectively) to basal microvascular vasomotion was lower in obese than in lean women (P<0.05 for all). These findings show that obesity is characterized by an impaired direct microvascular vasodilatory effect of insulin and by decreased skin microvascular vasomotion in a way that is suggestive for alterations of endothelial and neurogenic activity.

Impairment of skin capillary recruitment precedes chronic complications in patients with type 1 diabetes

Microvascular function in patients with type 1 diabetes without chronic complications was assessed using skin capillary recruitment during post-occlusive reactive hyperemia (PORH). Structural (maximal) capillary density was evaluated during venous occlusion. The study included 48 consecutive outpatients aged 26.3 +/- 10.8 years with type 1 diabetes (duration of 9.5 years) without chronic complications and 34 control subjects. Intravital capillary video-microscopy was used in the dynamic study of skin capillaries in the dorsum of the fingers and toes. Capillary recruitment during PORH (% increase in mean capillary density, MCD) was significantly higher in the controls than the patients in both the fingers (p < 0.001) and toes (p < 0.001). During venous occlusion, MCD increase was also higher in the controls than the patients in both the fingers (p < 0.05) and toes (p < 0.0001). In patients, no difference was found between MCD at baseline and after venous occlusion in the fingers but a decrease was observed in the toes (p < 0.001). It is concluded that skin capillary function is significantly impaired in both fingers and toes of patients with type 1 diabetes without chronic complications. Moreover, capillary density during venous occlusion did not increase in either extremity in the patients, suggesting that their capillaries at rest are already maximally recruited.

Urinary cortisol is inversely associated with capillary recruitment in women: a potential explanation for the cortisol–blood pressure relationship

The relationships of cortisol with elevated blood pressure and insulin resistance are likely to be the result of a complex interplay of different mechanisms. We hypothesize that cortisol is associated with impaired microvascular function and that this contributes to cortisol-associated high blood pressure and insulin resistance. We examined 24 h urinary free cortisol excretion in 56 healthy adults (26 women). Blood pressure was assessed by 24 h ambulatory measurements. Insulin sensitivity was determined using the hyperinsulinaemic euglycaemic clamp technique. Skin capillary recruitment after arterial occlusion was visualized with videomicroscopy and endothelium-(in)dependent vasodilation was evaluated with iontophoresis of acetylcholine and sodium nitroprusside combined with laser Doppler fluxmetry. Men were characterized by higher urinary cortisol excretion [median (interquartile range), 162 (130–194) compared with 118 (99–156) nmol/24 h, P<0.05]. In women, but not in men, urinary cortisol excretion was associated with impaired capillary recruitment (r=−0.66, P<0.001), higher systolic blood pressure (r=0.64, P<0.001) and lower insulin sensitivity (r=−0.43, P<0.05). Urinary cortisol excretion was not associated with endothelium-(in)dependent vasodilation in men or women. Regression analysis demonstrated that capillary recruitment statistically explained 37% of the association between urinary cortisol and blood pressure in women. Capillary recruitment did not explain part of the association between urinary cortisol and insulin sensitivity. In conclusion, urinary cortisol excretion is inversely associated with capillary recruitment in women, but not in men, and capillary recruitment explains part of the cortisol–blood pressure relationship. These data suggest that, in women, impairment of capillary function mediates some of the adverse effects of cortisol and thus may provide a target to prevent such adverse effects.

Impaired endothelial function in healthy African-American adolescents compared with Caucasians

OBJECTIVE

To determine whether African-American adolescents have endothelial dysfunction compared with Caucasians and whether differences are a result of differences in insulin sensitivity calculated from total glucose (S(I)) or secretion.

STUDY DESIGN

Thirty-three Caucasian (13.6 +/- 2.6 years of age; body mass index [BMI] 21.6 +/- 4.4 kg/m2 mean +/- SD) and 25 African-American (13.3 +/- 2.9 years of age; BMI 24.0 +/- 4.4 kg/m2) adolescents were studied. Forearm blood flow (FBF; plethysmography) was measured before and after 5 minutes of arterial occlusion. S(I) and acute insulin response to glucose (AIRG) were measured using intravenous glucose tolerance tests and minimal modeling.

RESULTS

Baseline FBF did not differ between races. Postocclusion FBF was lower in African-Americans (17.2 +/- 1.2 vs 22.6 +/- 1.2 mL/dL/minute, P = .006). AIRG was higher in African-Americans (6050 +/- 940 vs 2410 +/- 30 microU minute/mL, P = .001). Pubertal stage had no effect. S(I) did not differ by race or pubertal stage. In African-Americans, percent fall in FVR following arterial occlusion correlated (r = 0.67, P = .001) with log AIRG. No relationships were found between percent fall in FVR and S(I) in either race.

CONCLUSION

African-American adolescents have decreased endothelial function. This may be a result of increased insulin secretion. Endothelial dysfunction in African-American adolescents may predispose to cardiovascular and type II diabetes.

Endothelial function in patients with type 1 diabetes evaluated by skin capillary recruitment

The aim of the present study was to evaluate skin capillary density and recruitment of the upper and lower extremities of patients with type 1 diabetes under chronic treatment without clinical manifestations of diabetes-related complications. This cross-sectional observational study included 59 (27.1+/-10.6 years) consecutive outpatients with type 1 diabetes [duration 10 (1; 45) years] and 41 age- and sex-matched healthy controls. We used intravital video-microscopy to measure basal and maximal (during venous congestion) skin capillary densities as well as capillary recruitment using post-occlusive reactive hyperemia (PORH) in the dorsum of the fingers and toes. Mean capillary density (MCD) of the fingers at baseline was not different between controls and patients (123.02+/-22.6 and 132.3+/-28.9 capillaries/mm(2), respectively; P=0.08). In contrast, baseline MCD of the toes was lower in controls, when compared to patients (84.6+/-19.8 and 96.2+/-23.4 capillaries/mm(2), respectively; P=0.01). Capillary recruitment during PORH (% increase of the number of capillaries/mm(2)) was significantly higher in controls compared to patients both in fingers [7 (-8; 33) and -1.0 (-35, 13), respectively; P=0.000] and toes [6 (-20; 46) and 0 (-24; 20), respectively; P=0.000]. During venous occlusion, capillary density increase (% increase of the number of capillaries/mm(2)) was also higher in controls compared to patients both in fingers [3 (-14; 23) and 0.0 (-30; 29.2), respectively; P=0.02] and toes [9.3 (-18; 51) and -7 (-34; 22), respectively; P=0.000]. Our results showed that patients with type 1 diabetes, although not presenting skin capillary rarefaction, display skin microvascular functional alterations in both extremities characterized by an absence of capillary reserve.

Vasoactive drugs enhance pancreatic islet blood flow, augment insulin secretion and improve glucose tolerance in female rats

Pravastatin, irbesartan and captopril are frequently used in the treatment of patients with Type 2 diabetes. These drugs also exert beneficial metabolic effects, causing an improved glucose tolerance in patients, but the precise mechanisms by which this is achieved remain elusive. To this end, we have studied whether these drugs influence insulin secretion in vivo through effects on islet blood perfusion. Captopril (3 mg/kg of body weight), irbesartan (3 mg/kg of body weight) and pravastatin (0.5 mg/kg of body weight) were injected intravenously into anaesthetized female Wistar rats. Blood flow rates were determined by a microsphere technique. Blood glucose concentrations were measured with test reagent strips and serum insulin concentrations were measured by ELISA. Pancreatic blood flow was markedly increased by pravastatin (P<0.001), captopril (P<0.05) and irbesartan (P<0.01). Pancreatic islet blood flow was significantly and preferentially enhanced after the administration of captopril (P<0.01), irbesartan (P<0.01) and pravastatin (P<0.001). Kidney blood flow was enhanced significantly by pravastatin (P<0.01), irbesartan (P<0.05) and captopril (P<0.01). Captopril and pravastatin also enhanced late-phase insulin secretion and positively influenced glycaemia in intraperitoneal glucose tolerance tests. In conclusion, the present study suggests that a local pancreatic renin-angiotensin system and pravastatin treatment may be selectively controlling pancreatic islet blood flow, augmenting insulin secretion and thereby improving glucose tolerance. Our findings indicate significant gender-related differences in the vascular response to these agents. Since statins and renin-angiotensin system inhibitors are frequently used by diabetic patients, the antidiabetic actions of these drugs reported previously might occur, in part, through the beneficial direct islet effects shown in the present study.

BMI and retinal vascular caliber in children

OBJECTIVE

In adult populations, changes in retinal vascular caliber have been linked with obesity and metabolic syndrome. We examined the association of BMI and weight with retinal vascular caliber in children.

RESEARCH METHODS AND PROCEDURES

This was a school-based, cross-sectional study of 768 children, 7 to 9 years old, randomly sampled from the Singapore Cohort Study of the Risk Factors for Myopia. Participants had digital retinal photographs. Retinal vascular caliber was measured using a computer-based program and combined to provide average calibers of arterioles and venules in that eye. Weight and height were measured using standardized protocol. These data were used to calculate BMI.

RESULTS

In this population, the mean retinal arteriolar and venular calibers were 156.40 microm [95% confidence interval (CI), 155.44 to 157.36] and 225.43 microm (95% CI, 224.10 to 226.74) respectively. After controlling for age, gender, race, parental monthly income, axial length, birth weight, and birth length, each 3.1 kg/m2 (standard deviation) increase in BMI was associated with a 2.55-microm (95% CI, 1.21 to 3.89; p < 0.001) larger retinal venular caliber. In multivariable analysis, greater weight was also significantly associated with larger retinal venular caliber. BMI and weight were not associated with retinal arteriolar caliber. Height was not significantly associated with retinal arteriolar or venular caliber.

DISCUSSION

Greater BMI and weight are associated with larger retinal venular caliber in healthy children.

Diffusion model for iontophoresis measured by laser-Doppler perfusion flowmetry, applied to normal and preeclamptic pregnancies

We present a physical model to describe iontophoresis time recordings. The model is a combination of monodimensional material diffusion and decay, probably due to transport by blood flow. It has four adjustable parameters, the diffusion coefficient, the decay constant, the height of the response, and the shot saturation constant, a parameter representing the relative importance of subsequent shots (in case of saturation). We test the model with measurements of blood perfusion in the capillary bed of the fingers of women who recently had preeclampsia and in women with a history of normal pregnancy. From the fits to the measurements, we conclude that the model provides a useful physical description of the iontophoresis process.

Microvascular dysfunction: causative role in the association between hypertension, insulin resistance and the metabolic syndrome?

The metabolic syndrome defines a clustering of metabolic risk factors that confers an increased risk for type 2 diabetes and cardiovascular disease. The metabolic syndrome seems to have multiple etiological factors and microvascular dysfunction may be one potential factor explaining the clustering of multiple metabolic risk factors including hypertension, obesity, insulin resistance and glucose intolerance. Microvascular dysfunction may increase not only peripheral vascular resistance and blood pressure, but may also decrease insulin-mediated glucose uptake in muscle. The present article summarizes some of the data concerning the role of microvascular dysfunction in the metabolic syndrome.

Insulin resistance and impaired myocardial fatty acid metabolism in dialysis patients with normal coronary arteries

We investigated whether insulin resistance is associated with impaired cardiac fatty acid metabolism in maintenance hemodialysis patients without coronary artery disease. We studied 55 nondiabetic (63+/-11 years old) and 51 diabetic (61+/-10 years old) hemodialysis patients with normal coronary arteries, using single-photon emission computed tomography (SPECT) with an iodinated fatty acid analogue, iodine-123-beta-methyl iodophenyl-pentadecanoic acid ((123)I-BMIPP), to evaluate cardiac fatty acid metabolism. SPECT imaging was graded regionally from 0 (normal) to 4 (absence of tracer) to calculate a summed score for 17 left ventricular segments. Insulin resistance was determined using the homeostasis model assessment index of insulin resistance (HOMA-IR). HOMA-IR correlated with summed BMIPP score in nondiabetic and diabetic patients. Stepwise multiple regression analysis showed that HOMA-IR was independently associated with BMIPP summed score in nondiabetic (beta=0.774, t=9.218, P=0.0001) and diabetic patients (beta=0.792, t=9.079, P=0.0001). Left ventricular ejection fraction was lower in nondiabetic subjects with BMIPP summed score of at least 6 plus HOMA-IR of at least 4 than in others with lower values for both assessments (53.1+/-13.8%, n=20 vs 67.7+/-9.1%, n=23, P=0.0002); this was also true in diabetic subjects (50.9+/-15.2%, n=24 vs 71.0+/-13.6%, n=11, P=0.0007). Association between insulin resistance and impaired cardiac fatty acid metabolism may contribute to left ventricular dysfunction in patients with maintenance hemodialysis without coronary diseases.

Accelerated atherosclerosis in patients with systemic autoimmune diseases

Systemic autoimmune diseases such as systemic lupus erythematosus and Wegener's granulomatosis are associated with a significantly increased prevalence of cardiovascular disease (CVD) compared with age- and sex-matched controls. Many risk factors are involved in the pathogenesis of atherosclerosis, the major underlying cause of CVD. In patients with systemic autoimmune diseases, it has been shown that traditional risk factors for CVD cannot completely explain the prevalence of atherosclerosis. Therefore, in addition to these traditional factors, nontraditional risk factors are suggested to contribute to atherogenesis. All risk factors, traditional and nontraditional, contribute to endothelial activation that, followed by endothelial dysfunction, is seen as one of the first steps in this process. This review updates information on the factors that contribute to accelerated atherosclerosis in patients with systemic autoimmune diseases, such as disease-related factors, inflammatory mediators, and advanced glycation end products.

The inflammatory consequences of psychologic stress: relationship to insulin resistance, obesity, atherosclerosis and diabetes mellitus, type II

Inflammation is frequently present in the visceral fat and vasculature in certain patients with cardiovascular disease (CVD) and/or adult onset Diabetes Mellitus Type II (NIDDM). An hypothesis is presented which argues that repeated acute or chronic psychologically stressful states may cause this inflammatory process. The mediators are the major stress hormones norepinephrine (NE) and epinephrine (E) and cortisol together with components of the renin-angiotensin system (RAS), the proinflammatory cytokines (PIC), as well as free fatty acids (ffa), the latter as a result of lipolysis of neutral fat. NE/E commence this process by activation of NF(kappa)B in macrophages, visceral fat, and endothelial cells which induces the production of toll-like receptors which, when engaged, produce a cascade of inflammatory reactions comprising the acute phase response (APR) of the innate immune system (IIS). The inflammatory process is most marked in the visceral fat depot as well as the vasculature, and is involved in the metabolic events which culminate in the insulin resistance/metabolic syndromes (IRS/MS), the components of which precede and comprise the major risk factors for CVD and NIDDM. The visceral fat has both the proclivity and capacity to undergo inflammation. It contains a rich blood and nerve supply as well as proinflammatory molecules such as interleukin 6 (IL-6), tumor necrosis factor alpha (TNFalpha), leptin, and resistin, the adipocytokines, and acute phase proteins (APP) which are activated from adipocytes and/or macrophages by sympathetic signaling. The inflammation is linked to fat accumulation. Cortisol, IL-6, angiotensin II (angio II), the enzyme 11(beta) hydroxysteroid dehydrogenase-1 and positive energy balance, the latter due to increased appetite induced by the major stress hormones, are factors which promote fat accumulation and are linked to obesity. There is also the capacity of the host to limit fat expansion. Sympathetic signaling induces TNF which stimulates the production of IL-6 and leptin from adipocytes; these molecules promote lipolysis and ffa fluxes from adipocytes. Moreover, catecholamines and certain PIC inhibit lipoprotein lipase, a fat synthesizing enzyme. The brain also participates in the regulation of fat cell mass; it is informed of fat depot mass by molecules such as leptin and ffa. Leptin stimulates corticotrophin releasing hormone in the brain which stimulates the SNS and HPA axes, i.e. the stress response. Also, ffa through portal signaling from the liver evoke a similar stress response which, like the response to psychologic stress, evokes an innate immune response (IIR), tending to limit fat expansion, which culminates in inflammatory cascades, the IRS-MS, obesity and disease if prolonged. Thus, the brain also has the capacity to limit fat expansion. A competition apparently exists between fat expansion and fat loss. In "western" cultures, with excessive food ingestion, obesity frequently results. The linkage of inflammation to fat metabolism is apparent since weight loss diminishes the concentration of inflammatory mediators. The linkage of stress to inflammation is all the more apparent since the efferent pathways from the brain in response to fat signals, which results in inflammation to decrease and limit fat cell mass, is the same as the response to psychologic stress, which strengthens the hypothesis presented herein.

Pancreatic islet blood flow is selectively enhanced by captopril, irbesartan and pravastatin, and suppressed by palmitate

Diabetic patients are often treated with a lipid lowering statin and an ACE inhibitor or angiotensin receptor antagonist against hypertension or albuminuria. These drugs may also improve glucose tolerance, but the mechanism for this remains elusive. We now studied whether these drugs and the fatty acid palmitate influence insulin secretion in vivo in rats through effects on islet blood perfusion. Whole pancreatic blood flow was markedly increased by captopril and irbesartan, and decreased by palmitate. Islet blood flow was significantly and preferentially enhanced by captopril, irbesartan, and pravastatin, and suppressed by palmitate. Both captopril and irbesartan raised serum insulin concentrations significantly. However, glycemia was not affected in any group. In conclusion, the present study suggests that a local pancreatic RAS and pravastatin may be selectively controlling pancreatic islet blood flow and thereby influencing insulin secretion. The antidiabetic actions of statins and RAS inhibitors might in part occur through the beneficial direct islet effects shown here. Conversely, free fatty acids that are elevated in type 2 diabetic patients may contribute to an impaired nutritive islet blood flow and thereby further aggravate the diabetic state by limiting the supply of insulin needed to curb hyperglycemia.

Obstructive sleep apnea and insulin resistance: a role for microcirculation?

Obstructive sleep apnea is an increasingly recognized medical problem. The recent attention to its frequency in the general population and its important role in metabolic, vascular, and behavioral aspects have sharply increased the number and nature of investigations, thereby revealing new aspects that open new approaches in research. Whereas obstructive sleep apnea is a well-known phenomenon accompanying obesity and diabetes, new findings strongly suggest that this close relationship may also operate in the opposite direction. Indeed obstructive sleep apnea may be a primary feature inducing or aggravating a series of vascular and metabolic disturbances closely resembling the metabolic syndrome. This review will discuss established and potential mechanisms responsible for these changes. Obstructive sleep apnea indeed appears to gather all the elements necessary to induce insulin resistance, hypertension, and possibly heart failure. After careful analysis of these modifications and considering how they are intertwined, we propose that microcirculation could represent the common denominator mediating the progression of this pathology, as it is eventually the case in the metabolic syndrome and diabetes domain. This plausible hypothesis is discussed in detail and should be verified by appropriate preclinical and clinical protocols, which are now achievable by using noninvasive techniques in humans.

The insulin sensitiser pioglitazone does not influence skin microcirculatory function in patients with type 2 diabetes treated with insulin

AIMS/HYPOTHESIS

Insulin resistance is associated with abnormal microvascular function. Treatment with insulin sensitisers may provoke oedema, suggesting microvascular effects. The mechanisms underlying the peripheral oedema observed during glucose-lowering treatment with thiazolidinediones are unclear. Therefore we examined the effect of pioglitazone on microvascular variables involved in oedema formation.

METHODS

Subjects (40-80 years) with type 2 diabetes and on insulin were randomised to 9 weeks of pioglitazone therapy (30 mg/day; n=14) or placebo (n=15). The following assessments were performed at baseline and 9 weeks: microvascular filtration capacity; isovolumetric venous pressure; capillary pressure; capillary recruitment following venous or arterial occlusion; postural vasoconstriction; and maximum blood flow. A number of haematological variables were also measured including vascular endothelium growth factor (VEGF), IL-6 and C-reactive protein (CRP).

RESULTS

Pioglitazone did not significantly influence any microcirculatory variable as compared with placebo (analysis of covariance [ANCOVA] for microvascular filtration capacity for the two groups, p=0.26). Mean VEGF increased with pioglitazone (61.1 pg/ml), but not significantly more than placebo (9.76 pg/ml, p=0.94). HbA(1c) levels and the inflammatory markers IL-6 and CRP decreased with pioglitazone compared with placebo (ANCOVA: p=0.009, p=0.001 and p=0.004, respectively).

CONCLUSIONS/INTERPRETATION

Pioglitazone improved glycaemic control and inflammatory markers over 9 weeks but had no effect on microcirculatory variables associated with oedema or insulin resistance in type 2 diabetic patients treated with insulin.

Does microvascular dysfunction link obesity with insulin resistance and hypertension?

Obesity and obesity-associated clinical disorders are becoming an increasing public health burden. In this perspective, we postulate that impairment of microvascular function links obesity with insulin resistance and hypertension. Obesity is characterized by generalized microvascular dysfunction, which is associated with, and may precede, the development of insulin resistance and hypertension. Understanding of mechanisms involved in obesity-associated microvascular dysfunction may reveal new therapeutic targets. In obesity, cellular regulatory mechanisms of vasoreactivity are shifted towards vasoconstriction, with an increased role for endothelin-1 and a decreased role for nitric oxide. In addition, communicative pathways between adipose tissue and the microvasculature comprise increased release of adipokines and increased sympathetic activity. Although one mechanism may dominate, microvascular defects in obesity are probably caused by an integrated response consisting of endocrine, vasocrine and neurogenic mechanisms. This remains a fruitful area for future research.

TNF-α levels are associated with skin capillary recruitment in humans: a potential explanation for the relationship between TNF-α and insulin resistance

The mechanism by which TNF-α (tumour necrosis factor-α) may cause insulin resistance is not clear. On the basis of experiments in rats, TNF-α has been suggested to cause defects in capillary function, with a decreased access of insulin and glucose to tissues. To test this hypothesis in humans, we assessed serum TNF-α concentrations, skin capillary recruitment and insulin sensitivity in a group of 37 healthy adults. In addition, we measured these variables in 21 of their prepubertal children. Serum TNF-α levels were measured by sandwich enzyme immunoassay, and insulin sensitivity was assessed with the hyperinsulinaemic euglycaemic clamp technique. Capillary recruitment during post-occlusive reactive hyperaemia was evaluated by videomicroscopy. In the adults, serum TNF-α levels were associated with both capillary recruitment (r=−0.40, P=0.02) and insulin sensitivity (r=−0.33, P=0.05). In addition, capillary recruitment was associated with insulin sensitivity (r=0.34, P=0.04). Regression analysis showed that the association between TNF-α and insulin sensitivity [−0.527 mg·kg−1 of body weight·min−1 per pmol/l per pg/ml TNF-α (95% confidence interval, −1.066 to 0.011); P=0.05] decreased by 30% after adjustment for capillary recruitment. In the children, neither capillary recruitment (r=0.33, P=0.2) nor insulin sensitivity (r=−0.24, P=0.4) was significantly associated with TNF-α. In conclusion, in adults, but not in children, serum TNF-α levels are associated with capillary recruitment during post-occlusive hyperaemia, which, in part, can explain the relationship between TNF-α and insulin resistance. Our data suggest that these relationships are initiated during growth from childhood to adulthood.

Retinal vessel diameters and obesity: a population-based study in older persons

OBJECTIVE

Obesity is linked with large vessel atherosclerosis and diabetes. Its association with microvascular changes is less clear. We investigated the associations among retinal vessel diameters, vessel wall signs, and BMI in an older population.

RESEARCH METHODS AND PROCEDURES

Retinal photographs were taken on 3654 persons aged 49+ years at baseline of the Blue Mountains Eye Study in Australia. Arteriolar and venular diameters were measured from digitized retinal photographs of the right eyes. BMI was calculated as weight (kilograms)/height (meters2). Incident obesity was defined in persons with BMI < or = 30 at baseline but > 30 after 5 years. A significant weight gain was defined as an increase in BMI of 2+ SDs (4 or more units) over the 5-year period.

RESULTS

At baseline, mean BMI was 26.1 (+/-4.6) in this population. At 5-year examinations, 177 (10.0% of 1773 at risk) developed incident obesity, and 136 (6.4% of 2143 at risk) had significant weight gain. After adjusting for age, sex, smoking, triglyceride levels, and mean arterial blood pressure, persons with wider retinal venular diameters had a higher risk of incident obesity (odds ratio, 1.8; 95% confidence interval, 1.0 to 3.1, comparing the highest with lowest venular diameter quintiles) and significant weight gain (odds ratio, 1.7; 95% confidence interval, 0.9 to 3.2). These associations were attenuated with further adjustment for baseline BMI. Arteriolar diameter was unrelated with baseline or change in BMI.

DISCUSSION

Wider retinal venular diameter is associated with risk of obesity, independent of hypertension, diabetes, lipids, and cigarette smoking. These data may support a role for impaired microvascular function in the course of weight gain.

Vascular complications in diabetes mellitus: the role of endothelial dysfunction

The endothelium is a complex organ with a multitude of properties essential for control of vascular functions. Dysfunction of the vascular endothelium is regarded as an important factor in the pathogenesis of diabetic micro- and macro-angiopathy. Endothelial dysfunction in Type I and II diabetes complicated by micro- or macro-albuminuria is generalized in that it affects many aspects of endothelial function and occurs not only in the kidney. The close linkage between microalbuminuria and endothelial dysfunction in diabetes is an attractive explanation for the fact that microalbuminuria is a risk marker for atherothrombosis. In Type I diabetes, endothelial dysfunction precedes and may cause diabetic microangiopathy, but it is not clear whether endothelial dysfunction is a feature of the diabetic state itself. In Type II diabetes, endothelial function is impaired from the onset of the disease and is strongly related to adverse outcomes. It is not clear whether impaired endothelial function is caused by hyperglycaemia or by other factors. Impaired endothelial function is closely associated with and may contribute to insulin resistance regardless of the presence of diabetes. Endothelial dysfunction in diabetes originates from three main sources. Hyperglycaemia and its immediate biochemical sequelae directly alter endothelial function or influence endothelial cell functioning indirectly by the synthesis of growth factors, cytokines and vasoactive agents in other cells. Finally, the components of the metabolic syndrome can impair endothelial function.

The effect of glucose and mental stress on cutaneous microvascular endothelial function

Glucose and mental stress, independently, have been found to impair arterial endothelial function (an indicator of vascular health). The present study sought to determine whether the combination of glucose and stress would have a greater effect on microvascular endothelial function than each on its own. To assess endothelial function, surges in skin blood flow (reactive hyperemia), following the release of cuff pressure to the upper arm at 200 mmHg for 5 min, were measured with laser Doppler flowmetry in 40 young, healthy females. Endothelial function did not change significantly following a 5-min mathematics stressor or the consumption of 75 g of glucose. However, the combination of glucose and stress impaired endothelium-dependent dilatation 30 min after glucose consumption. These findings suggest that combinations of vascular risk factors may be more threatening to cardiovascular health than singularly occurring factors.

Does aerobic fitness influence microvascular function in healthy adults at risk of developing Type 2 diabetes?

AIM

To investigate whether aerobic fitness is associated with skin microvascular function in healthy adults with an increased risk of developing Type 2 diabetes.

METHODS

Twenty-seven healthy normal glucose-tolerant humans with either a previous diagnosis of gestational diabetes or having two parents with Type 2 diabetes and 27 healthy adults who had no history of diabetes were recruited. Maximal oxygen uptake was assessed using an incremental exercise test to exhaustion. Skin microvascular function was assessed using laser Doppler techniques as the maximum skin hyperaemic response to a thermal stimulus (maximum hyperaemia) and the forearm skin blood flow response to the iontophoretic application of acetylcholine (ACh) and sodium nitroprusside.

RESULTS

Maximal oxygen uptake was not significantly different in the 'at-risk' group compared with healthy controls. Maximum hyperaemia was reduced in those 'at risk' (1.29 +/- 0.30 vs. 1.46 +/- 0.33 V, P = 0.047); however, the peak response to acetylcholine or sodium nitroprusside did not differ in the two groups. A significant positive correlation was demonstrated between maximal oxygen uptake and maximum hyperaemia (r = 0.52, P = 0.006 l/min and r = 0.60, P = 0.001 ml/kg/min) and peak ACh response (r = 0.40, P = 0.04 l/min and r = 0.47, P = 0.013 ml/kg/min) in the 'at-risk' group when expressed in absolute (l/min) or body mass-related (ml/kg/min) terms. No significant correlations were found in the control group.

CONCLUSIONS

In this 'at-risk' group with skin microvascular dysfunction maximal oxygen uptake was not reduced compared with healthy controls. However, in the 'at-risk' group alone, individuals with higher levels of aerobic fitness also had better microvascular and endothelial responsiveness.

Long-term treatment with losartan versus atenolol improves insulin sensitivity in hypertension: ICARUS, a LIFE substudy

OBJECTIVE

Hypertension and insulin resistance might be associated through peripheral vascular hypertrophy/rarefaction which compromises skeletal muscle blood flow and decreases glucose uptake, inducing insulin resistance. We hypothesized that treatment with losartan as compared to atenolol would improve insulin sensitivity through regression of peripheral vascular hypertrophy/rarefaction.

METHODS

In 70 hypertensive patients with electrocardiographic left ventricular hypertrophy, we measured minimal forearm vascular resistance (MFVR) by plethysmography and insulin sensitivity (M/IG) by a 2-h isoglycemic hyperinsulinemic clamp at baseline and after 1, 2 and 3 years of blinded treatment with atenolol- or losartan-based regimens.

RESULTS

Blood pressures were reduced similarly in the two treatment groups. After 3 years, MFVR was increased (3.7 versus 3.2 mmHg x min x 100, P < 0.05) and M/IG decreased (8.6 versus 12.1 l/kg x mmol x min, P < 0.05) in patients treated with atenolol, whereas MFVR and M/IG were unchanged (3.5 versus 3.5 mmHg x min x 100 and 12.6 versus 11.1 l/kg x mmol x min, both P = NS) in patients treated with losartan. As compared to atenolol, losartan treatment was associated with less increase in MFVR (4.3 versus 27%, P < 0.05) and less decrease in M/IG (24 versus -14%, P < 0.01). The relative change in M/IG was inversely associated with the relative change in MFVR (r = -0.16, P < 0.05) independently of the relative change in body mass index (r = -0.29, P < 0.001).

CONCLUSIONS

As compared to atenolol, losartan treatment was associated with less peripheral vascular hypertrophy/rarefaction and higher insulin sensitivity. The relative change in MFVR and M/IG were inversely related, supporting the hypothesis that peripheral vascular changes in hypertension may induce insulin resistance. The ability of losartan to preserve insulin sensitivity may explain the lower incidence of new onset diabetes in patients treated with losartan in the LIFE study.

Abnormal Endothelium-Dependent Microvascular Reactivity in Recently Preeclamptic Women

OBJECTIVE

To assess endothelial function at the level of skin microvasculature, using iontophoretic administration of acetylcholine (endothelium-dependent vasodilator) and sodium nitroprusside (endothelium-independent vasodilator), in women who recently had a preeclamptic pregnancy.

METHODS

Microvascular skin reactivity was assessed by laser Doppler perfusion monitoring and iontophoresis of acetylcholine (ACh) and sodium nitroprusside (SNP) in 25 women with a history of early onset preeclampsia and 23 women with previous uncomplicated pregnancies, all of whom were between 3 and 11 months postpartum.

RESULTS

Mean (+/- standard error of the mean) ACh-mediated vasodilatation, expressed as a percentage increase in flux, was higher in women who recently had a preeclampsia than in controls (535 +/- 46% versus 314 +/- 29%, P < .001). In contrast, SNP-mediated vasodilatation was not significantly different (560 +/- 71% versus 483 +/- 69%, P = .4) in both groups. Linear regression analysis revealed that the difference in ACh-mediated vasodilatation was explained by preeclampsia (P = .004), whereas vascular risk factors such as maternal age, diastolic blood pressure, and family history of premature cardiovascular diseases had no significant effect.

CONCLUSION

The increased ACh-mediated vasodilatation in the microcirculation of recently preeclamptic women indicates abnormal endothelial function. Furthermore, it may represent a compensatory response to an impaired vasodilatory response of the macrocirculation, thereby supporting the hypothesis of an underlying (micro)angiopathy.

Insulin resistance as a contributor to myocardial ischaemia independent of obstructive coronary atheroma: a role for insulin sensitisation?

There is good evidence to suggest that insulin resistance and its surrogate markers are at least modest independent cardiovascular risk factors. However, as well as long term effects on atheromatous coronary disease, there is a well described correlation between markers of insulin resistance and endothelial dysfunction. In this review, the evidence for a relation between endothelial dysfunction and myocardial ischaemia is summarised. The evidence for a correlation between insulin resistance and endothelial dysfunction and the proposed cellular mechanisms are also examined. Finally, the potential role for insulin sensitising strategies is looked at and recent data examining their effects on both endothelial function and clinical symptoms is examined. In conclusion, it was found that insulin sensitising modalities have a potential role in the amelioration of angina and that randomised controlled studies are therefore warranted.

Ethnic differences in skin microvascular function and their relation to cardiac target-organ damage

BACKGROUND

People of Black African descent have increased risks of vascular target-organ damage not explained by greater blood pressures.

OBJECTIVE

To study ethnic differences in the microvasculature.

DESIGN AND METHODS

Flow (flux) in microcirculatory skin vessels was assessed using laser Doppler fluximetry in 181 Afro-Caribbean and European men and women aged 40-65 years from the general population in London, UK. Flux in response to maximal heating (maximal hyperaemic response) was measured and minimum vascular resistance calculated. Peak flux and time to peak after an ischaemic stimulus were also measured. Target-organ damage was assessed using echocardiographic interventricular septal thickness (IVST).

RESULTS

In men, maximum hyperaemic response was attenuated in Afro-Caribbeans [109 arbitrary units (au), 25th and 75th percentiles 101, 117] compared with Europeans [165 (155, 179) au; P = 0.008]. Minimum vascular resistance was greater in Afro-Caribbeans, significantly so in men [(1.22 (1.18, 1.28) au/mmHg compared with 0.80 (0.77, 0.83) au/mmHg; P = 0.006]. Peak ischaemic response was attenuated in Afro-Caribbean men and women compared with Europeans (35.6 au compared with 49.5 au; P < 0.001) and time to peak was prolonged (14.1 s compared with 12.5 s; P = 0.07). These ethnic differences could not be accounted for by standard cardiovascular risk factors. IVST was greater in Afro-Caribbeans than in Europeans. Minimum vascular resistance and peak response accounted for a small proportion of this ethnic difference, in addition to conventional factors.

CONCLUSIONS

Afro-Caribbeans have poorer microvascular structure and function, unexplained by conventional risk factors, which may contribute to greater rates of vascular target-organ damage.

Effects of prior moderate exercise on postprandial metabolism and vascular function in lean and centrally obese men

OBJECTIVES

We investigated whether a session of prior exercise could ameliorate postprandial endothelial dysfunction.

BACKGROUND

Endothelial function is impaired after fat ingestion, and this may be related to rises in triglyceride concentrations. Exercise reduces postprandial triglyceride concentrations.

METHODS

Ten lean (waist <90 cm) and 10 centrally obese (waist >100 cm) middle-aged men each underwent two oral fat tolerance tests (blood taken fasting and for 8 h after a high-fat meal containing 80 g fat and 70 g carbohydrate). On the afternoon before one test, subjects performed a 90-min treadmill walk (exercise); no exercise was performed before the control test. Endothelium-dependent and -independent microvascular function was assessed using laser Doppler imaging in the fasted state and at two hourly intervals during the 8-h postprandial period.

RESULTS

Exercise reduced both fasting and postprandial triglyceride concentrations by 25% in both the lean and centrally obese groups (p < 0.0005). For all subjects taken together, exercise improved fasting endothelium-dependent function by 25% (p < 0.05), and, although there was a significant postprandial decrease in both endothelium-dependent and -independent function in both the control and exercise trials (p < 0.01), postprandial endothelium-dependent and -independent function were 15% and 20% higher, respectively, in the exercise trial than the control trial (both p < 0.05).

CONCLUSIONS

A session of prior exercise improves fasting and postprandial vascular function in middle-aged men. This may be one mechanism by which exercise influences cardiovascular risk.

Vasoconstrictor effects of insulin in skeletal muscle arterioles are mediated by ERK1/2 activation in endothelium

Insulin exerts both NO-dependent vasodilator and endothelin-dependent vasoconstrictor effects on skeletal muscle arterioles. The intracellular enzymes 1-phosphatidylinositol 3-kinase (PI3-kinase) and Akt have been shown to mediate the vasodilator effects of insulin, but the signaling molecules involved in the vasoconstrictor effects of insulin in these arterioles are unknown. Our objective was to identify intracellular mediators of acute vasoconstrictor effects of insulin on skeletal muscle arterioles. Rat cremaster first-order arterioles ( n = 40) were isolated, and vasoreactivity to insulin was studied using a pressure myograph. Insulin induced dose-dependent vasoconstriction of skeletal muscle arterioles (up to −22 ± 3% of basal diameter; P < 0.05) during PI3-kinase inhibition with wortmannin (50 nmol/l). Insulin-induced vasoconstriction was abolished by inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) with PD-98059 (40 μmol/l). In addition, inhibition of ERK1/2 without PI3-kinase inhibition uncovered insulin-mediated vasodilatation in skeletal muscle arterioles (up to 37 ± 10% of baseline diameter; P < 0.05). Effects of insulin on ERK1/2 activation in arterioles were then investigated by Western blot analysis. Insulin induced a transient 2.4-fold increase in ERK1/2 phosphorylation (maximal at ∼15 min) in skeletal muscle arterioles ( P < 0.05). Removal of the arteriolar endothelium abolished insulin-induced vasoconstriction, which suggests that activation of ERK1/2 in endothelial cells is involved in acute insulin-mediated vasoconstriction. To investigate this, acute effects of insulin on ERK1/2 phosphorylation were studied in human microvascular endothelial cells. In support of the findings in skeletal muscle arterioles, insulin induced a 1.9-fold increase in ERK1/2 phosphorylation (maximal at ∼15 min) in microvascular endothelial cells ( P < 0.05). We conclude that acute vasoconstrictor effects of insulin in skeletal muscle arterioles are mediated by activation of ERK1/2 in endothelium. This ERK1/2-mediated vasoconstrictor effect antagonizes insulin-induced, PI3-kinase-dependent vasodilatation in skeletal muscle arterioles. These findings provide a novel mechanism by which insulin may determine blood flow and glucose disposal in skeletal muscle.

Free fatty acid levels modulate microvascular function: relevance for obesity-associated insulin resistance, hypertension, and microangiopathy

To test the hypothesis that free fatty acids (FFAs) modulate microvascular function and that this contributes to obesity-associated insulin resistance, hypertension, and microangiopathy, we examined the effects of both FFA elevation in lean women and FFA lowering in obese women on skin microvascular function. A total of 16 lean and 12 obese women underwent, respectively, Intralipid plus heparin (or saline) infusion and overnight acipimox (or placebo) treatment. We measured capillary recruitment with capillaroscopy and endothelium-(in)dependent vasodilation by iontophoresis of acetylcholine and sodium nitroprusside before and during hyperinsulinemia (40 mU . m(-2) . min(-1)). FFA elevation impaired capillary recruitment and acetylcholine-mediated vasodilation before (44.6 +/- 16.8 vs. 56.9 +/- 18.9%, P < 0.05; and 338 +/- 131 vs. 557 +/- 162%, P < 0.01, respectively) and during (54.0 +/- 21.3 vs. 72.4 +/- 25.4%, P < 0.01; and 264 +/- 186 vs. 685 +/- 199%, P < 0.01, respectively) hyperinsulinemia. FFA lowering improved capillary recruitment before (50.9 +/- 14.6 vs. 37.4 +/- 9.3%, P < 0.01) and during (66.8 +/- 20.6 vs. 54.8 +/- 15.4%, P < 0.05) hyperinsulinemia. Changes in FFA levels were inversely associated with changes in capillary recruitment and insulin sensitivity in lean (r = -0.46, P = 0.08; and r = -0.56, P = 0.03) and in obese (r = -0.70, P = 0.02; and r = -0.62, P = 0.04) women. Regression analyses showed that changes in capillary recruitment statistically explained approximately 29% of the association between changes in FFA levels and insulin sensitivity. In conclusion, FFA levels modulate microvascular function and may contribute to obesity-associated insulin resistance, hypertension, and microangiopathy.