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Measuring endothelial function

Apr 22, 2002

The endothelium’s functioning is not yet routinely tested. Most of the practical techniques for directly measuring this activity use ultrasound to measure movement of blood in an artery after the flow has been altered either by injecting drugs that would normally dilate it, or by blocking the flow with a tourniquet. The more convenient and less invasive method usually involves applying a tourniquet to the forearm for one to five minutes to occlude the blood flow through the brachial artery. When the tourniquet is removed, normal endothelial cells respond by opening the vessel wider so that the blood flows faster than before the tourniquet was applied (a hyperemia response). This increase can be measured by ultrasound. 43, 44 If the endothelial cells are impaired, when the tourniquet is released they will not dilate the blood vessel more than before the tourniquet pinched the artery closed, and blood flow will not increase reflexively. The tourniquet creates shear stress (pressure and distortion from the force of the tourniquet and the resulting disrupted blood flow), and the normal endothelium will respond by releasing nitric oxide, causing the underlying smooth muscle to relax, and the artery to dilate.

A number of blood tests report on endothelial function, including concentration of insulin, clotting substances, inflammatory substances such as C-reactive protein, and the cholesterol profile, especially the triglyceride : high-density lipoprotein ratio (TG : HDL). One reasonable criticism aimed at blaming endothelial dysfunction for many of our woes, is that the condition is difficult to measure directly. True, but the evidence, circumstantial though it now is, overwhelmingly points to endothelial dysfunction as a large influence in type 2 diabetes, obesity, and other diseases. In fact, generalized endothelial dysfunction may be the cause behind most diseases.

Endothelial dysfunction precedes diabetes
Women who contract diabetes during pregnancy are said to have gestational diabetes. Although the condition often resolves after the baby is delivered, these women are known to be at greater risk for type-2 diabetes in the future. Anastasiou and colleagues published their observations on otherwise healthy women with a history of gestational diabetes. 45 The researchers studied both obese and nonobese women with that history but no present signs of diabetes, giving them a glucose-tolerance test to see how they tolerated large loads of glucose, and found that they already had abnormal endothelial function or endothelial dysfunction. These women currently showed no overt signs of diabetes but had endothelial dysfunction and were likely to contract diabetes. Individuals with a history of low birthweight demonstrate endothelial dysfunction as children and also are at greater risk for developing insulin resistance, type 2 diabetes, and CHD. 46-50

In perhaps a more straightforward demonstration of endothelial dysfunction preceding the development of diabetes, Caballero et al used Laser Doppler and high resolution ultrasound to directly look for abnormalities in vascular reactivity in micro- and macrocirculation (respectively) in four age and sex comparable groups: 30 healthy normoglycemic subjects with no history of type 2 diabetes in a first-degree relative (controls), 39 healthy normoglycemic subjects with a history of type 2 diabetes in one or both parents (relatives), 32 subjects with impaired glucose tolerance (IGT), and 42 patients with type 2 diabetes without vascular complications. 51 They also measured the following blood tests as indicators for endothelial dysfunction: entothelin-1 (ET-1), von Willebrand factor (vWF), soluble adhesion molecules, and vascular cell adhesion molecules. Significantly less blood flow in both micro- and macrovasculature was observed in the group with type 2 diabetes, followed by the group with IGT and the group with a family history of IGT, than in the healthy control subjects. Patients with IGT and normoglycemic patients with a parental history of diabetes also had increased levels of ET-1 and cellular adhesion molecules consistent with increased activation of the endothelium. These results suggest that abnormalities in vascular reactivity and markers of endothelial cell activation are present early in individuals at risk of developing type 2 diabetes, even at a stage when normal glucose tolerance exists. 51

Many studies suggest that how elevated the blood glucose is matters less than how long the patient has had type-2 diabetes, suggesting a shared underlying pathophysiologic process—endothelial cell dysfunction. Enderle et al suggest that a longer period of undetected diabetes rather than poor glucose control impairs endothelial-dependent vasodilatation in type 2 diabetes. 52 Although severe hyperglycemia can reduce endothelial function even in healthy humans 53 , normal endothelial function has been described in patients with uncomplicated type 1 diabetes. 52, 54, 55 . These observations further suggest endothelial dysfunction’s contribution to insulin resistance rather than a direct toxic effect of hyperglycemia per se. 56

Several studies show that risk factors for heart disease appear even before diabetes is recognizable. A 1998 study observing 17,000 men for twenty years found that those whose blood glucose was elevated but not high enough to meet the criteria for diabetes, had greater risk of dying from heart disease. 11 After following 2,000 nondiabetic, apparently healthy middle-aged men, for twenty-two years, Norwegian researchers found that, by themselves, fasting blood glucose values in the upper normal range accurately predicted death by a cardiac event. 57 An editorial accompanying the article on the smaller study suggests that even moderately elevated blood glucose levels increase the risk, and doctors should more aggressively treat elevated blood glucose in these patients even though they are not yet diabetic. 58 The editorialist’s logic is flawed, failing to appreciate that the moderate rise in blood glucose is not the villain but rather marks or results from a deeper problem — widespread damage to the endothelial cells that line all blood vessels. This damage contributes to insulin resistance, hypertension, dyslipidemia, proteinuria, atherosclerosis, and elevated glucose, which just happens to rise as these other conditions appear.

My fear is that the “experts” are missing the boat: instead of recognizing that elevated glucose signals an underlying general problem, they will lower the criteria for diagnosing diabetes. Millions more will then be treated with drugs that elevate insulin, worsen endothelial-cell damage, make patients fatter and sicker, and cause more deaths. In other words, let’s focus on the problem, insulin resistance, not the result, which is the elevated blood glucose level.

Diabetes: an inflammatory condition

An accumulating body of evidence suggests that an inflammatory process plays a key role in developing the insulin resistance syndrome, 59, 60 type 2 diabetes 61, 62 and cardiovascular disease 63-65 In a recent prospective study, Pradhan et al followed over four years 27,628 women free of diagnosed diabetes, cardiovascular disease and cancer at baseline. 62 Elevated markers of systemic inflammation (C-reactive protein and interleukin 6) were able to powerfully predict the development of type 2 diabetes in these women. 62 Similar results were reported by Barzilay et al. 61 Consistent with this diabetes-inflammation connection is additional evidence showing elevated CRP associated with higher insulin and HbA1c among men and women 66 , and elevated leukocyte counts were significantly associated with diabetes incidence over a period of approximately 20 years. Participants’ risk of developing diabetes increased proportionally with leukocyte counts. 67

C-reactive protein (CRP) Injured endothelial cells are linked to increased C-reactive protein in the blood, which can indicate generalized inflammation, and that can mean risk of death from stroke, MI, and other diseases. Elevated CRP is also associated with endothelial vasodilator dysfunction which could contribute to ischemic events. 68 Made in the liver (in response to interleukin 6 during the acute phase of inflammation), CRP is a marker or indicator for both chronic inflammation and acute injury. The protein binds to damaged tissue and resembles an antibody in that it helps activate inflammation.

Low-grade inflammation and an elevated CRP have been associated with at least doubled or tripled risk for MI, stroke, and atherosclerosis. Led by Paul Ridker, of Harvard Medical School and director, Center for Cardiovascular Disease Prevention at Brigham & Women’s Hospital, researchers found that even tiny elevations in CRP powerfully predict who among groups of men and women are likely to have an MI. Of the twelve measures they looked at, including total cholesterol, LDL cholesterol, and the TG : HDL ratio, the CRP reading most effectively predicted risk. 64 Subsequently, Ridker and his colleagues have shown that elevated levels of CRP in previously healthy women predict the development of type 2 diabetes. 62

A research group led by John Yudkin established that CRP concentrations were related to insulin resistance as well as to such markers of endothelial dysfunction as low HDL, high TGs, and two substances produced by the endothelial cells that help increase blood’s tendency to clot. 69 These substances, von Willebrand factor and tissue plasminogen activator, are released into the blood by the damaged endothelium.

In patients with uncontrolled diabetes, CRP rises, a reaction significantly related to the increase in blood glucose. One possibility is that the increased blood glucose increases viscosity and shear stress, and this now-damaged and dysfunctional endothelium contributes to the inflammation in several ways, including secretion of sticky adhesion molecules, which can be induced by CRP. 70 These molecules act like flypaper, attracting inflammatory white blood cells and causing them to accumulate and stick to the endothelium’s surface This action can trigger release of a host of other substances that influence inflammation and blood-vessel leakage. That CRP is correlated with markers of endothelial dysfunction further suggests that endothelial triggering or activation is related to chronic inflammation.

Inflammation, diabetes, and obesity
One study found that eighteen obese, premenopausal women with no other medical problems had impaired blood flow or impaired endothelial-dependent vasodilation, suggesting that by itself uncomplicated obesity influences endothelial dysfunction or damage to blood vessels. The more fat around the abdomen and internal organs the greater the endothelial dysfunction, and also the higher the concentration of insulin. In fact, insulin resistance and endothelial dysfunction increase as girth at the waist increases. 71 Compared to people of normal weight, even those who are young and overweight or obese (ages seventeen to thirty-nine) have been found to have elevated CRP or higher prevalence of generalized low-grade inflammation. In a 1999 study, Marjolein Visser and her colleagues showed that waist size and CRP were directly related — the bigger the belly, the greater the inflammation. 72 Visser et al’s subsequent research has shown that in children 8 to 16 years of age, being overweight is associated with higher CRP concentrations and higher white blood cell counts. 73 Analyzed results from thousands of adult 74 and children 75 participants in NHANES III provided clear evidence of a relationship between BMI and plasma CRP levels. Together these studies suggest that insulin resistance, inflammation, and endothelial dysfunction increase as body fat, and especially girth at the waist, increases. 72

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