Cutaneous vascular function during acute hyperglycemia in healthy young adults

Acute hyperglycaemia enhances both vascular endothelial function and cardiac and skeletal muscle microvascular function in healthy humans

KEY POINTS

Multiple clinical studies report that acute hyperglycaemia (induced by mixed meal or oral glucose) decreases arterial vascular function in healthy humans. Feeding, however, impacts autonomic output, blood pressure, and insulin and incretin secretion, which may themselves alter vascular function. No prior studies have examined the effect of acute hyperglycaemia on both macro- and microvascular function while controlling plasma insulin concentrations. Macrovascular and microvascular functional responses to euglycaemia and hyperglycaemia were compared. Octreotide was infused throughout both protocols to prevent endogenous insulin release. Acute hyperglycaemia (induced by intravenous glucose) enhanced brachial artery flow-mediated dilatation, increased skeletal muscle microvascular blood volume and flow, and expanded cardiac muscle microvascular blood volume. Compared to other published findings, the results suggest that vascular responses to acute hyperglycaemia differ based on the study population (i.e. normal weight vs. overweight/obese) and/or glucose delivery method (i.e. intravenous vs. oral glucose).

ABSTRACT

High glucose concentrations acutely provoke endothelial cell oxidative stress and are suggested to trigger diabetes-related macro- and microvascular injury in humans. Multiple clinical studies report that acute hyperglycaemia (induced by mixed meal or oral glucose) decreases arterial vascular function in healthy humans. Feeding, however, impacts autonomic output, blood pressure, and insulin and incretin secretion, which may each independently alter vascular function and obscure the effect of acute hyperglycaemia per se. Surprisingly, no studies have examined the acute effects of intravenous glucose-induced hyperglycaemia on both macro- and microvascular function while controlling plasma insulin concentrations. In this randomized study of healthy young adults, we compared macrovascular (i.e. brachial artery flow-mediated dilatation, carotid-femoral pulse wave velocity and post-ischaemic brachial artery flow velocity) and microvascular (heart and skeletal muscle perfusion by contrast-enhanced ultrasound) functional responses to euglycaemia and hyperglycaemia. Octreotide was infused throughout both protocols to prevent endogenous insulin release. Acute intravenous glucose-induced hyperglycaemia enhanced brachial artery flow-mediated dilatation (P = 0.004), increased skeletal muscle microvascular blood volume and flow (P = 0.001), and expanded cardiac muscle microvascular blood volume (P = 0.014). No measure of vascular function changed during octreotide-maintained euglycaemia. Our findings suggest that unlike meal-provoked acute hyperglycaemia, 4 h of intravenous glucose-induced hyperglycaemia enhances brachial artery flow-mediated dilatation, provokes cardiac and skeletal muscle microvascular function, and does not impair aortic stiffness. Previous findings of acute large artery vascular dysfunction during oral glucose or mixed meal ingestion may be due to differences in study populations and meal-induced humoral or neural factors beyond hyperglycaemia per se. (ClinicalTrials.gov number NCT03520569.).

Acute hyperglycaemia does not alter nitric oxide-mediated microvascular function in the skin of adolescents with type 1 diabetes

PURPOSE

We assessed the impact of an acute bout of hyperglycaemia on nitric oxide (NO)-mediated microvascular function in the skin of adolescents with type 1 diabetes (T1DM).

METHODS

Twelve subjects (12-18 years) with T1DM were randomised into a control (n = 6) or hyperglycaemia (n = 6) group. Hyperinsulinaemic clamps were used to manipulate blood glucose level (BGL). Following a baseline period, where all subjects were euglycaemic (20 min), the experimental phase began. During the experimental phase, BGL was elevated to 16.7 ± 0.9 mmol L(-1) in the hyperglyceamic group, while it was maintained at euglycaemia (5.5 ± 0.1 mmol L(-1)) in the control group. Simultaneously, cutaneous microvascular function (% max cutaneous vascular conductance, CVC%) was assessed using laser Doppler fluxometry following stimulation of skin blood flow using localised heating (42 °C). To determine the NO contribution to skin blood flow, two microdialysis sites were assessed, one perfused with Ringers and the other with the NO blocker, NG-monomethyl-L-arginine (L-NMMA).

RESULTS

In the hyperglycaemic group, acute increase in BGL was not associated with changes in skin blood flow (CVC% 82.4 ± 8.7% at 5.5 ± 0.1 mmol L(-1) vs 79.5 ± 9.1% at 16.7 ± 0.9 mmol L(-1), unpaired t tests, P = 0.588) or the contribution of NO to vasodilation.

CONCLUSIONS

These results suggest that, in our group of adolescents with type 1 diabetes, acute hyperglycaemia did not affect skin microvascular NO-mediated function.

Intraluminal hyperglycaemia causes conduit and resistance artery dilatation and inhibits vascular autoregulation in the anaesthetised pig

The effect of intraluminal hyperglycaemia was investigated in the iliac artery of 11 anaesthetised pigs. Following isolation of a test segment, hyperglycaemic blood (40 mmol·L–1) caused a significant dilatation of the artery of 167 ± 208 μm (mean ± SD; n = 6, P = 0.031). Dilatations were reduced by N(G)-nitro-l-arginine methyl esther (250 μg·mL–1) from 145 ± 199 to 38 ± 5 μm), but this was not statistically significant (n = 6, P = 0.18). Intra-arterial infusions of d-glucose (20–40 mmol·L–1·min–1), during graded constrictions, caused statistically significant increases in blood flow (n = 11, P = 0.0013). Vasodilatation was confirmed by measurements of the ratio of immediate pressure steps to flow steps (∂P/∂F) during the graded obstruction experiments, showing a decrease in instantaneous vascular resistance from a control of 0.62 ± 0.30 to 0.33 ± 0.34 mm Hg·mL–1·min–1 (n = 7, P = 0.016). Autoregulation was assessed from the slopes of the plots of steady-state flow versus pressure. There were significant increases in the slope from 2.32 ± 1.03 to 5.88 ± 5.60 mL·min–1·(mm Hg)–1 (n = 7, P = 0.0078), indicating significant impairment of autoregulation. In conclusion, luminal hyperglycaemia relaxes both arterial and resistance vessel smooth muscle.

The interrelationship between air temperature and humidity as applied locally to the skin: the resultant response on skin temperature and blood flow with age differences

BACKGROUND

Most studies of the skin and how it responds to local heat have been conducted with either water, thermodes, or dry heat packs. Very little has been accomplished to look at the interaction between air humidity and temperature on skin temperature and blood flow. With variable air temperatures and humidity's around the world, this, in many ways, is a more realistic assessment of environmental impact than previous water bath studies.

MATERIAL/METHODS

Eight young and 8 older subjects were examined in an extensive series of experiments where on different days, air temperature was 38, 40, or 42°C. and at each temperature, humidity was either 0%, 25%, 50%, 75%, or 100% humidity. Over a 20 minute period of exposure, the response of the skin in terms of its temperature and blood flow was assessed.

RESULTS

For both younger and older subjects, for air temperatures of 38 and 40°C., the humidity of the air had no effect on the blood flow response of the skin, while skin temperature at the highest humidity was elevated slightly. However, for air temperatures of 42°C., at 100% humidity, there was a significant elevation in skin blood flow and skin temperature above the other four air humidity's (p<0.05). In older subjects, the blood flow response was less and the skin temperature was much higher than younger individuals for air at 42°C. and 100% humidity (p<0.05).

CONCLUSIONS

Thus, in older subjects, warm humid air caused a greater rise in skin temperature with less protective effect of blood flow to protect the skin from overheating than is found in younger subjects.

The effect of moist air on skin blood flow and temperature in subjects with and without diabetes

BACKGROUND

Endothelial function is known to be impaired in response to heat in people with diabetes, but little has been done to see how air humidity alters the skin blood flow response to heat.

METHODS

Seventeen male and female subjects were divided in two groups, one with type 2 diabetes and the other the control subjects without diabetes, age-matched to the diabetes group. All subjects participated in a series of experiments to determine the effect of the warming of the skin by air on skin temperature and skin blood flow. On different days, skin temperature was warmed with air that was 38°C, 40°C, or 42°C for 20 min. Also, on different days, at each temperature, the air humidity was adjusted to 0%, 25%, 50%, 75%, or 100% humidity. Skin blood flow and temperature were measured throughout the exposure period. This allowed the interactions between air humidity and temperature to be assessed.

RESULTS

For the control subjects, the moisture in the air had no different effect on skin blood flow at air temperatures of 38°C and 40°C (analysis of variance, P>0.05), although skin blood flow progressively increased at each air temperature that was applied. But for the warmest air temperature, 42°C, although the four lower humidities had the same effect on skin blood flow, air at 100% humidity caused the largest increase in skin blood flow. In contrast, in the subjects with diabetes, blood flow was always significantly less at any air temperature applied to the skin than was observed in the control subjects (P<0.05), and skin blood flow was significantly higher for the two higher humidities for the two higher air temperatures. Skin temperature paralleled these findings.

CONCLUSION

These data show that individuals with diabetes do not tolerate moist, warm air above 50% humidity as well as controls without diabetes.

A device to measure heat flow through the skin in people with diabetes

INTRODUCTION

As people age, and especially for older people with diabetes, there is increased susceptibility to burns. However, this is not true for all older people or all people with diabetes. The factors that predict burn susceptibility in specific members of the population with diabetes have not been elucidated. To understand the heat transfer properties of the skin in different parts of the body and how it is altered by skin blood flow, age, and glycemic control, a new device was developed.

METHODS

The device was a Plexiglas (Arkema, Colombes, France) capsule (thermode) that has a footprint on the skin of 20 cm(2), with the side contacting the skin made of thin brass. The thermode was machined to allow the free flow of water through the interior with the exception of a small hole for a laser Doppler flow meter to assess blood flow under the capsule. Flow directors kept the water flow even on the under surface of the capsule and minimized turbulent flow until high water flow rates are forced through the capsule.

RESULTS

When tested, the device provided even heat on the brass surface and could show the movement of heat into the skin and the corresponding changes in skin blood flow and temperature. In limited testing, clear differences were seen in heat flux in people with diabetes versus controls.

CONCLUSION

This device might be very useful in determining the early onset of diabetes-related skin damage. Future studies should include examining different regions of the body and variables such as hemoglobin A1c.

Effects of oral glucose load on endothelial function and on insulin and glucose fluctuations in healthy individuals

Background/aims. Postprandial hyperglycemia, an independent risk factor for cardiovascular disease, is accompanied by endothelial dysfunction. We studied the effect of oral glucose load on insulin and glucose fluctuations, and on postprandial endothelial function in healthy individuals in order to better understand and cope with the postprandial state in insulin resistant individuals. Methods. We assessed post-oral glucose load endothelial function (flow mediated dilation), plasma insulin, and blood glucose in 9 healthy subjects. Results. The largest increases in delta FMD values (fasting FMD value subtracted from postprandial FMD value) occurred at 3 hours after both glucose or placebo load, respectively: 4.80 ± 1.41 (P = .009) and 2.34 ± 1.47 (P = .15). Glucose and insulin concentrations achieved maximum peaks at one hour post-glucose load. Conclusion. Oral glucose load does not induce endothelial dysfunction in healthy individuals with mean insulin and glucose values of 5.6 mmol/L and 27.2 mmol/L, respectively, 2 hours after glucose load.

Differential effects of glucose on agonist-induced relaxations in human mesenteric and subcutaneous arteries

BACKGROUND AND PURPOSE

Acute periods of hyperglycaemia are strongly associated with vascular disorder, yet the specific effects of high glucose on human blood vessel function are not fully understood. In this study we (1) characterized the endothelial-dependent relaxation of two similarly sized but anatomically distinct human arteries to two different agonists and (2) determined how these responses are modified by acute exposure to high glucose.

EXPERIMENTAL APPROACH

Ring segments of human mesenteric and subcutaneous arteries were mounted in a wire myograph. Relaxations to acetylcholine and bradykinin were determined in a control (5 mM) and high glucose (20 mM) environment over a 2 and 6 h incubation period.

KEY RESULTS

Bradykinin-induced relaxation in both sets of vessels was mediated entirely by EDHF whilst that generated by acetylcholine, though principally generated by EDHF, also had contribution from prostacyclin and possibly nitric oxide in mesenteric and subcutaneous vessels, respectively. A 2-h incubation of high glucose impaired bradykinin-induced relaxation of subcutaneous vessels whilst, in contrast, the relaxation generated by bradykinin in mesenteric vessels was enhanced at the same time point. High glucose significantly augmented the relaxation generated by acetylcholine in mesenteric and subcutaneous vessels at a 2 and 6 h incubation point, respectively.

CONCLUSIONS AND IMPLICATIONS

Short periods of high glucose exert a variable influence on endothelial function in human isolated blood vessels that is dependent on factors of time, agonist-used and vessel studied. This has implications for how we view the effects of acute hyperglycaemia found in patients with diabetes mellitus as well as other conditions.

Manifestations of cutaneous diabetic microangiopathy

The etiologies of a variety of skin conditions associated with diabetes have not been fully explained. One possible etiological factor is diabetic microangiopathy, which is known to affect the eyes and kidneys in patients with diabetes. There are many mechanisms by which diabetes may cause microangiopathy. These include excess sorbitol formation, increased glycation end products, oxidative damage, and protein kinase C overactivity. All of these processes occur in the skin, and the existence of a cutaneous diabetic microangiopathy has been well demonstrated. These microangiopathic changes are associated with abnormalities of skin perfusion. Because the skin plays a thermoregulatory role, there is significant capillary redundancy in normal skin. In diabetic patients, loss of capillaries is associated with a decrease in perfusion reserve. This lost reserve is demonstrable under stressed conditions, such as thermal stimulation. The associated failure of microvascular perfusion to meet the requirements of skin metabolism may result in diverse skin lesions in patients with diabetes. Many skin conditions peculiar to diabetes are fairly rare. Necrobiosis lipoidica diabeticorum (NLD) and diabetic bullae occur very infrequently as compared with diabetic retinopathy and nephropathy. Conversely, there is a correlation between diabetic microvascular disease and NLD. This correlation also exists with more common skin conditions, such as diabetic dermopathy. This relationship suggests that diabetic microangiopathy may contribute to these conditions even if it is not primarily causal. Clinically, the major significance of diabetic cutaneous microangiopathy is seen in skin ulceration which is very common and has a major impact on diabetic patients. Many factors contribute to the development of diabetic foot ulcers. Neuropathy, decreased large vessel perfusion, increased susceptibility to infection, and altered biomechanics all play a role, but there is no doubt that inadequate small blood vessel perfusion is a major cause of the inability to heal small wounds that eventually results in ulcer formation. The accessibility of skin capillaries makes cutaneous diabetic microangiopathy an attractive model for research on the evolution of microvascular disease in diabetic patients.

Acute effects of glucose and insulin on vascular endothelium

AIMS/HYPOTHESIS

Chronic exposure to high concentrations of glucose has consistently been demonstrated to impair endothelium-dependent, nitric oxide (NO)-mediated vasodilation. In contrast, several clinical investigations have reported that acute exposure to high glucose, alone or in combination with insulin, triggers vasodilation. The aim of this study was to examine whether elevated glucose itself stimulates endothelial NO formation or enhances insulin-mediated endothelial NO release.

METHODS

We measured NO release and vessel tone ex vivo in porcine coronary conduit arteries (PCAs). Intracellular Ca(2+) was monitored in porcine aortic endothelial cells (PAECs) by fura-2 fluorescence. Expression of the Na(+)/glucose cotransporter-1 (SGLT-1) was assayed in PAECs and PCA endothelium by RT-PCR.

RESULTS

Stimulation of PCAs with D: -glucose, but not the osmotic control L: -glucose, induced a transient increase in NO release (EC(50) approximately 10 mmol/l), mediated by a rise in intracellular Ca(2+) levels due to an influx from the extracellular space. This effect was abolished by inhibitors of the plasmalemmal Na(+)/Ca(2+) exchanger (dichlorobenzamil) and the SGLT-1 (phlorizin), which was found to be expressed in aortic and coronary endothelium. Alone, D: -glucose did not relax PCA, but did augment the effect of insulin on NO release and vasodilation.

CONCLUSIONS/INTERPRETATION

An increased supply of extracellular D: -glucose appears to enhance the activity of the endothelial isoform of nitric oxide synthase by increasing intracellular Na(+) concentrations via SGLT-1, which in turn stimulates an extracellular Ca(2+) influx through the Na(+)/Ca(2+) exchanger. This mechanism may be responsible for glucose-enhanced, insulin-dependent increases in tissue perfusion (including coronary blood-flow), thus accelerating glucose extraction from the blood circulation to limit the adverse vascular effects of prolonged hyperglycaemia.

Nicotine increases initial blood flow responses to local heating of human non-glabrous skin

Nicotine affects the regulation of skin blood flow (SkBF), but the mechanisms involved are not well understood. We tested the hypothesis that acute exposure to nicotine inhibits both the initial neurally mediated component and the later sustained component of SkBF responses to local heating of non-glabrous skin in humans. SkBF (measured by laser-Doppler) responses to local heating of forearm skin from 32 to 42 degrees C were measured in 11 chronic smokers. Heating occurred at one site over 15 min (RAMP) and over 90 s (STEP) at another site, and was maintained for an additional 30 min. STEP heating was also applied to a site pretreated with bretylium via iontophoresis to inhibit noradrenergic neurotransmission. Responses were measured before and after acute administration of nicotine via cigarettes or nasal spray in two experimental sessions. Nicotine decreased resting skin blood flow (P < 0.05); this response was inhibited by bretylium. During RAMP, nicotine increased the initial SkBF at 42 degrees C (by approximately 12%, P < 0.05). For STEP, nicotine increased the initial peak response (by approximately 25%, P < 0.05), and decreased the sustained plateau value (by approximately 10%, P < 0.05). In skin pretreated with bretylium, the increase caused by nicotine in the initial peak value persisted, but the plateau value was not different from pre-nicotine. These data suggest that in abstinent cigarette smokers, nicotine augments initial responses to both gradual and rapid non-painful heating of non-glabrous skin by sensitizing the sensory nerves that mediate the axon reflex associated with rapid vasodilatation. In contrast, nicotine decreases SkBF responses to prolonged heating by activating noradrenergic nerves.

Forearm vascular control during acute hyperglycemia in healthy humans

The vascular endothelium is a site of pathological changes in patients with diabetes mellitus that may be related to severe chronic hyperglycemia. However, it is unclear whether transient hyperglycemia alters vascular function in an otherwise healthy human forearm. To test the hypothesis that acute, moderate hyperglycemia impairs endothelium-dependent forearm vasodilation, we measured vasodilator responses in 25 healthy volunteers (11 F, 14 M) assigned to one of three protocols. In protocol 1, glucose was varied to mimic a postprandial pattern (i.e., peak glucose approximately 11.1 mmol/l) commonly observed in individuals with impaired glucose tolerance. Protocol 2 involved 6 h of mild hyperglycemia (approximately 7 mmol/l). Protocol 3 involved 6 h of euglycemia. Glucose concentration was maintained with a variable systemic glucose infusion. Insulin concentrations were maintained at approximately 65 pmol/l by means of a somatostatin and "basal" insulin infusion. Glucagon and growth hormone were replaced at basal concentrations. Forearm blood flow (FBF) was calculated from Doppler ultrasound measurements at the brachial artery. In each protocol, FBF dose responses to intrabrachial acetylcholine (ACh) and sodium nitroprusside (NTP) were assessed at baseline and at 60, 180, and 360 min of glucose infusion. Peak endothelium-dependent vasodilator responses to ACh were not diminished by hyperglycemia in any trial. For example, peak responses to ACh during protocol 2 were 307 +/- 47 ml/min at euglycemic baseline and 325 +/- 52, 353 +/- 65, and 370 +/- 70 ml/min during three subsequent hyperglycemic trials (P = 0.46). Peak endothelium-independent responses to NTP infusion were also unaffected. We conclude that acute, moderate hyperglycemia does not cause short-term impairment of endothelial function in the healthy human forearm.