Basal production of nitric oxide (NO) and non-NO vasodilators in the forearm microcirculation in Type 2 diabetes: associations with blood pressure and HDL cholesterol

Vascular nitric oxide resistance in type 2 diabetes

Vascular nitric oxide (NO•) resistance, manifested by an impaired vasodilator function of NO• in both the macro- and microvessels, is a common state in type 2 diabetes (T2D) associated with developing cardiovascular events and death. Here, we summarize experimental and human evidence of vascular NO• resistance in T2D and discuss its underlying mechanisms. Human studies indicate a ~ 13-94% decrease in the endothelium (ET)-dependent vascular smooth muscle (VSM) relaxation and a 6-42% reduced response to NO• donors, i.e., sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), in patients with T2D. A decreased vascular NO• production, NO• inactivation, and impaired responsiveness of VSM to NO• [occurred due to quenching NO• activity, desensitization of its receptor soluble guanylate cyclase (sGC), and/or impairment of its downstream pathway, cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG)] are the known mechanisms underlying the vascular NO• resistance in T2D. Hyperglycemia-induced overproduction of reactive oxygen species (ROS) and vascular insulin resistance are key players in this state. Therefore, upregulating vascular NO• availability, re-sensitizing or bypassing the non-responsive pathways to NO•, and targeting key vascular sources of ROS production may be clinically relevant pharmacological approaches to circumvent T2D-induced vascular NO• resistance.

Impact of acute whole-body cold exposure with concurrent isometric handgrip exercise on aortic pressure waveform characteristics

PURPOSE

Adverse cardiovascular events are more prevalent during winter and in people that exercise/work in cold temperatures. Since pulse wave analysis indices, aortic systolic blood pressure (ASBP), augmentation index (AIx), and wasted left ventricular pressure energy (ΔE w), are stronger predictors of cardiovascular events and myocardial performance than brachial blood pressure (BP), we sought to evaluate the aortic hemodynamic responses during cold exposure with concurrent isometric handgrip exercise (IHG).

METHODS

In a crossover randomized fashion, 20 healthy normotensive men (22.1 ± 2 years) were evaluated, by means of radial applanation tonometry, inside an environmental chamber in the supine position at cold (4 °C) and temperate (24 °C) conditions. Following a 30-min equilibration period, measurements were performed during pre-exercise baseline (REST), in the last 90 s of a 3-min IHG at 30% maximal voluntary contraction, and 3 min immediately after the finalization of IHG bout (recovery, REC).

RESULTS

At REST, brachial systolic BP (BSBP) (12 ± 2 mmHg; P < 0.01), ASBP (14 ± 3 mmHg; P < 0.01), AIx (11 ± 3%; P < 0.05), and ΔE w (737 ± 128 dynes s/cm(2); P < 0.01) were higher in 4 °C compared to 24 °C trial. Compared to REST, IHG significantly increased (P < 0.01) BSBP, ASBP, AIx, and ΔEw, while BSBP and ASBP remained elevated during REC (P < 0.01). Compared to REST and temperate, AIx (11 ± 3%) and ΔE w (793 ± 145 dynes s/cm(2); P < 0.01) were higher during IHG and cold, while BSBP and ASBP were elevated during REC and cold. AIx and ΔE w returned to REST values in both trials, but AIx (11 ± 4%; P < 0.05) and ΔE w (656 ± 132 dynes s/cm(2); P < 0.05) were higher in the cold.

CONCLUSIONS

Cold exposure with concurrent IHG induces a significant increase in aortic hemodynamic markers, which may evoke adverse cardiovascular events.

Implication of low HDL-c levels in patients with average LDL-c levels: a focus on oxidized LDL, large HDL subpopulation, and adiponectin

To evaluate the impact of low levels of high density lipoprotein cholesterol (HDL-c) on patients with LDL-c average levels, focusing on oxidative, lipidic, and inflammatory profiles. Patients with cardiovascular risk factors (n = 169) and control subjects (n = 73) were divided into 2 subgroups, one of normal HDL-c and the other of low HDL-c levels. The following data was analyzed: BP, BMI, waist circumference and serum glucose Total-c, TGs, LDL-c, oxidized LDL, total HDL-c and subpopulations (small, intermediate, and large), paraoxonase-1 (PON1) activity, hsCRP, uric acid, TNF-α, adiponectin, VEGF, and iCAM1. In the control subgroup with low HDL-c levels, significantly higher values of BP and TGs and lower values of PON1 activity and adiponectin were found, versus control normal HDL-c subgroup. However, differences in patients' subgroups were clearly more pronounced. Indeed, low HDL-c subgroup presented increased HbA1c, TGs, non-HDL-c, Ox-LDL, hsCRP, VEGF, and small HDL-c and reduced adiponectin and large HDL. In addition, Ox-LDL, large-HDL-c, and adiponectin presented interesting correlations with classical and nonclassical markers, mainly in the normal HDL-c patients' subgroup. In conclusion, despite LDL-c average levels, low HDL-c concentrations seem to be associated with a poor cardiometabolic profile in a population with cardiovascular risk factors, which is better evidenced by traditional and nontraditional CV biomarkers, including Ox-LDL, large HDL-c, and adiponectin.

Vascular smooth muscle function in type 2 diabetes mellitus: a systematic review and meta-analysis

AIMS/HYPOTHESIS

In type 2 diabetes, in contrast to the well-documented endothelial dysfunction, studies assessing vascular smooth muscle (VSM) function have yielded discrepant results over the last two decades. We therefore sought to determine whether or not VSM function is impaired in individuals with type 2 diabetes.

METHODS

We conducted a systematic search of MEDLINE, Cochrane, Scopus and Web of Science databases, from their respective inceptions until December 2012, for articles evaluating VSM function in individuals with type 2 diabetes. A meta-analysis was performed to compare the standardised mean difference (SMD) in VSM function between individuals with type 2 diabetes and age-matched controls. Subgroup analyses and meta-regression were used to identify sources of heterogeneity.

RESULTS

Twenty-seven articles (1,042 individuals with type 2 diabetes and 601 control subjects) were included in this analysis. VSM function was significantly impaired in diabetic compared with control subjects (SMD -0.68, 95% CI -0.84, -0.52; p < 0.001). Although moderate heterogeneity among studies was found (I (2) = 52%), no significant publication bias was detected. Subgroup analyses showed a further decline in VSM function assessed in the microcirculation compared with the macrocirculation of individuals with type 2 diabetes (p = 0.009). In meta-regression, VSM function in the microcirculation was inversely associated with BMI and triacylglycerols and was positively associated with HDL-cholesterol.

CONCLUSIONS/INTERPRETATION

In addition to the endothelium, the VSM is a source of vascular dysfunction in type 2 diabetes. An exacerbation of VSM function in the microcirculation may be a distinctive feature in type 2 diabetes.

Effect of dietary nitrate on blood pressure, endothelial function, and insulin sensitivity in type 2 diabetes

Diets rich in green, leafy vegetables have been shown to lower blood pressure (BP) and reduce the risk of cardiovascular disease. Green, leafy vegetables and beetroot are particularly rich in inorganic nitrate. Dietary nitrate supplementation, via sequential reduction to nitrite and NO, has previously been shown to lower BP and improve endothelial function in healthy humans. We sought to determine if supplementing dietary nitrate with beetroot juice, a rich source of nitrate, will lower BP and improve endothelial function and insulin sensitivity in individuals with type 2 diabetes (T2DM). Twenty-seven patients, age 67.2±4.9 years (18 male), were recruited for a double-blind, randomized, placebo-controlled crossover trial. Participants were randomized to begin, in either order, a 2-week period of supplementation with 250ml beetroot juice daily (active) or 250ml nitrate-depleted beetroot juice (placebo). At the conclusion of each intervention period 24-h ambulatory blood pressure monitoring, tests of macro- and microvascular endothelial function, and a hyperinsulinemic isoglycemic clamp were performed. After 2 weeks administration of beetroot juice mean ambulatory systolic BP was unchanged: 134.6±8.4mmHg versus 135.1±7.8mmHg (mean±SD), placebo vs active-mean difference of -0.5mmHg (placebo-active), p=0.737 (95% CI -3.9 to 2.8). There were no changes in macrovascular or microvascular endothelial function or insulin sensitivity. Supplementation of the diet with 7.5mmol of nitrate per day for 2 weeks caused an increase in plasma nitrite and nitrate concentration, but did not lower BP, improve endothelial function, or improve insulin sensitivity in individuals with T2DM.

The influence of autonomic dysfunction associated with aging and type 2 diabetes on daily life activities

Type 2 diabetes (T2D) and ageing have well documented effects on every organ in the body. In T2D the autonomic nervous system is impaired due to damage to neurons, sensory receptors, synapses and the blood vessels. This paper will concentrate on how autonomic impairment alters normal daily activities. Impairments include the response of the blood vessels to heat, sweating, heat transfer, whole body heating, orthostatic intolerance, balance, and gait. Because diabetes is more prevalent in older individuals, the effects of ageing will be examined. Beginning with endothelial dysfunction, blood vessels have impairment in their ability to vasodilate. With this and synaptic damage, the autonomic nervous system cannot compensate for effectors such as pressure on and heating of the skin. This and reduced ability of the heart to respond to stress, reduces autonomic orthostatic compensation. Diminished sweating causes the skin and core temperature to be high during whole body heating. Impaired orthostatic tolerance, impaired vision and vestibular sensing, causes poor balance and impaired gait. Overall, people with T2D must be made aware and counseled relative to the potential consequence of these impairments.

Simultaneous renal hypertension and type 2 diabetes exacerbate vascular endothelial dysfunction in rats

Despite the high rate of occurrence of both diabetes and hypertension in humans, the cardiovascular effects of the two conditions have not been investigated when they occur simultaneously. Thus this study examined the vascular effects of simultaneous type 2 diabetes and renal hypertension on endothelial function. Serum malondialdehyde and systolic blood pressure (SBP) were measured, glucose tolerance test (GTT) was performed, and concentration-response to phenylephrine (PE) in the absence and presence of nitro-l-arginine methyl ester (l-NAME), acetylcholine and sodium nitroprusside were conducted on aortic rings from diabetic control, type 2 diabetes, sham-operated, renal hypertensive, and simultaneous type 2 diabetes plus hypertension rats respectively. Hypertension, diabetes, and simultaneous diabetes and hypertension were associated with either increased or decreased maximal responses (E(max)) of PE dependent on in the presence or absence of l-NAME. There was also increased serum malondialdehyde and decreased E(max) of acetylcholine. Thus simultaneous hypertension and diabetes caused a greater decrease in E(max) of acetylcholine compared to that seen with either diabetes or hypertension alone higher than that seen in hypertension. The blood glucose during GTT was lower than that seen in diabetes groups. Thus simultaneous type 2 diabetes and the SBP was renal hypertension is associated with improved glucose tolerance, but with further deterioration of endothelial dysfunction compared with either condition alone.

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.

Reduction of large-conductance Ca²(+) -activated K(+) channel with compensatory increase of nitric oxide in insulin resistant rats

Cardiovascular disease prevalence and mortality are both increased by insulin resistance, hypertension, and atherosclerosis. The large-conductance Ca(2+)-activated K(+) channel (BK(Ca)) plays a pivotal role in the diastolic function of vascular smooth muscle cells. However, the role of this channel in insulin resistance remains unknown. Male Sprague-Dawley rats were randomly divided into an insulin resistant group and control group. We investigated the BK(Ca) current and subunit expression in myocytes from aortas and mesenteric arteries by Western blot, real-time PCR and the whole-cell patch-clamp methods. BK(Ca) current was decreased in smooth muscle cells in insulin resistant rats, compared with that in control group. Peak BK(Ca) current at + 60 mV was significantly decreased after iberiotoxin (IBTX) perfusion at 100 nmol/L (64.2 ± 4.7 versus 20.3 ± 3.5% in thoracic aortas and 65.6 ± 6.2 versus 29.3 ± 3.9% in mesenteric arteries, both p < 0.01). However, there was no significant difference in BK(Ca) alpha subunit between the two groups, both at the level of mRNA and protein. BK(Ca) beta 1 subunit expression in aortas and mesenteric arteries from the insulin resistant group was lower than in those from control group. The plasma level of nitric oxide was higher in the insulin resistant group than in the control group. Our results demonstrated that the BK(Ca) channel is decreased both in macrovessels and microvessels in insulin resistant rats. These impairments may be related to the down-regulation of β1 subunit expression and compensatory increase in plasma nitric oxide levels.

The effect of type-2-diabetes-related vascular endothelial dysfunction on skin physiology and activities of daily living

A common factor contributing to organ damage in type 2 diabetes mellitus (T2DM) is impaired tissue blood flow caused by damage to vascular endothelial cells (VECs). Damage can occur even before the clinical diagnosis of diabetes. It can be caused by both a high average blood glucose concentration and/or large daily spikes in blood glucose. While much of the present literature focuses on the damage to VECs and organs from these large glucose excursions, this review will focus on the consequence of this damage, that is, how endothelial cell damage in diabetes affects normal daily activities (e.g., exercise, reaction to typical stimuli) and various treatment modalities (e.g.. contrast baths and electrical stimulation therapy). It is important to understand the effects of VEC damage such as poor skin blood flow, compromised thermoregulation, and altered response to skin pressure in designing diabetes technologies as simple as heating pads and as complex as continuous glucose monitors. At the simplest level, people with diabetes have poor circulation to the skin and other organs. In the skin, even the blood flow response to locally applied pressure, such as during standing, is different than for people who do not have T2DM. Simple weight bearing on the foot can occlude the skin circulation. This makes the skin more susceptible to damage. In addition, endothelial damage has far-reaching effects on the whole body during normal activities of daily living, including an impaired response to local heat, such as hot packs and contrast baths, and higher body temperatures during whole body heating due to impaired blood flow and a reduced ability to sweat. Finally, because of multiple organ damage, people with T2DM have poor balance and gait and impaired exercise performance.

Niacin improves small artery vasodilatory function and compliance in statin-treated type 2 diabetic patients

We investigated the effect of niacin (nicotinic acid prolonged release) on forearm vasodilatory function and arterial compliance in statin-treated type 2 diabetic patients with endothelial dysfunction. In a parallel group study, we randomised 15 subjects, with LDL-cholesterol ≤2.5 mmol/L, to niacin (dose titrated to 1500 mg/day over 8 weeks, then maintained for a further 12 weeks) or no additional treatment. Niacin increased maximal post-ischaemic forearm blood flow (mean ± SEM 6.4±2.4 vs. -2.3±1.2 ml/100 ml/min, p = 0.001) and small artery compliance (1.3±0.8 vs. -2.3±1.1 ml/mmHg, p = 0.01) compared with no additional treatment, but did not alter large artery compliance, blood pressure nor heart rate. Niacin decreased serum triglycerides by 47% (p = 0.04), with no change in LDL-cholesterol, HDL-cholesterol, apolipoprotein (Apo) B-100 nor ApoA-I (p > 0.05). Adding niacin to statin therapy improves small artery vasodilatory function and compliance in type 2 diabetes. This may relate to a decrease in serum triglycerides and/or a direct benefit of niacin on vascular biology.

Does compensatory nitric oxide and angiotensin II receptor activity reduce arterial stiffness in early-stage insulin resistance?

Increased arterial stiffness is influenced by both functional and structural properties of the vessel wall, including changes in content of smooth muscle, elastin and collagen, reduced endothelial production of NO and increased release of endothelin-1 or AngII (angiotensin II). The RAS (renin-angiotensin) system is likely to be central to increases in arterial stiffness, since the changes in arterial structure observed with enhanced AngII activity are similar to the same pathophysiological changes that contribute to arterial stiffness. The role of AT(1)R and AT(2)R (AngII type 1 and type 2 receptors respectively) in the development of arterial stiffening, particularly in the early stages of insulin resistance, is however unclear. In this issue of Clinical Science, Brillante and co-workers have observed that in insulin-resistant subjects exhibiting reduced arterial stiffness, wave reflection from small-to-medium-sized, but not large, arteries was increased following separate intravenous infusions of AngII, the selective AT(2)R inhibitor PD123319 and the NO inhibitor L-NMMA (N(G)-monomethyl-L-arginine) in comparison with normal healthy age- and sex-matched controls. These increases probably reflect increased AT(1)R and AT(2)R expression/activity in addition to up-regulation of basal NO release in the small-to-medium-sized arteries. These changes may be compensatory mechanisms related to early vascular damage and may have clinical implications for treatment in hypertensive patients with evidence of the metabolic syndrome.