The ocular hemodynamic response to nitric oxide synthase inhibition is unaltered in patients with early type I diabetes

Microvascular function and oxidative stress in adult individuals with early onset of cardiovascular disease

The current study aims to investigate retinal vascular function and its relationship with systemic anti-oxidative defence capacity in normal individuals versus those with early hypertensive changes according to the current ESC/ESH guidelines. Retinal microvascular function was assessed in 201 participants by means of dynamic retinal vessel analysis. Blood pressure, lipid panel, oxidized (GSH) & reduced glutathione (GSSG) were also evaluated for each participant. Individuals classed as grade 1 hypertension demonstrated higher retinal arterial baseline diameter fluctuation (p = 0.0012), maximum dilation percentage (p = 0.0007), time to maximum constriction (p = 0.0003) and lower arterial constriction slope (p = 0.0131). Individuals classed as high normal and grade 1 hypertension also demonstrated higher time to maximum dilation than individuals classed as optimal or normal. GSH levels correlated negatively with SBP, DBP and MBP values in all participants (p = 0.0010; p = 0.0350 and p = 0.0050) as well as with MBP values in high normal and grade 1 hypertension (p = 0.0290). The levels of GSSG correlated positively with SBP, DBP and MBP values in all participants (p = 0.0410; p = 0.0330 and, p = 0.0220). Our results point to the fact that microvascular alterations can be identifiable at BP values still considered within normal values and go in parallel with the changes observed in the level of oxidative stress.

The diameter response of retinal arterioles in diabetic maculopathy is reduced during hypoxia and is unaffected by the inhibition of cyclo-oxygenase and nitric oxide synthesis

PURPOSE

Diabetic retinopathy is accompanied with changes in the diameter regulation and oxygenation of retinal vessels. Previous studies have shown that in normal persons and in diabetic patients without retinopathy hypoxia-induced vasodilatation is mediated by cyclo-oxygenase (COX) products and nitric oxide (NO). The purpose of the present study was to study whether these effects can be reproduced in patients with diabetic maculopathy.

METHODS

Eighteen patients with diabetic maculopathy aged 29-57 years were examined using the Dynamic Vessel Analyzer. The resting diameter and the diameter changes of retinal arterioles during isometric exercise and flicker stimulation were studied before and during breathing a hypoxic gas mixture. The examinations were also performed before and during intravenous infusion of the NOS inhibitor L-NMMA, and were repeated on a second day after topical administration of the COX-inhibitor diclofenac.

RESULTS

The diameter of retinal arterioles showed no significant change during hypoxia or L-NMMA infusion, or after topical application of diclofenac (p > 0.25 for all comparisons). The resting diameter of the venules was significantly increased during hypoxia (p = 0.003) and decreased during L-NMMA infusion (p < 0.0001). The diameter of retinal venules during isometric exercise increased significantly during hypoxia (p = 0.01). Flicker stimulation induced significant dilatation of the venules, which was significantly reduced during hypoxia and increased during L-NMMA infusion (p < 0.0001 for all comparisons).

CONCLUSION

Hypoxia-induced dilatation of retinal arterioles is severely reduced in patients with diabetic maculopathy. Future intervention studies aimed at normalizing the diameter regulation of retinal arterioles in diabetic patients should preferentially be conducted in the early stages of the disease where the potential for changing the vessel diameter is preserved. ClinicalTrials.gov identifier: NCT01689090.

Insulin is a key determinant of elevated retinal arteriolar flicker response in insulin-resistant individuals

AIMS/HYPOTHESIS

Insulin may link metabolic disorders to retinal microvascular pathology. The aim of the present study was to investigate the impact of early insulin resistance on retinal microcirculation.

METHODS

Retinal diameter responses to flicker-light stimulation were investigated in 81 clinically healthy participants (32 ± 6 years [mean ± SD], 59% women) who were recruited according to their BMI. All participants underwent an OGTT and euglycaemic-hyperinsulinaemic clamp (40 mU/m(2) · min(-1) insulin dose). After stratification by low and high insulin sensitivity based on a clamp-derived glucose disposal rate of ≤ or >4.9 mg/kg body mass, respectively, baseline retinal diameters and their relative changes to flicker stimulation were compared while controlling for mean arterial pressure, BMI and sex.

RESULTS

The arterial vasodilator response at the end of flicker stimulation (p = 0.044) and the area under the arterial reaction curve during flicker stimulation (p = 0.015) were significantly higher in individuals with low vs high insulin sensitivity. Vasodilatory responses of retinal veins to flicker stimulation and baseline retinal diameters did not differ between insulin-sensitive and insulin-resistant participants (p > 0.05). In a stepwise linear regression analysis, fasting insulin remained the only predictor of the arterial vasodilator response to flicker-light (p < 0.01). Waist circumference also contributed, although to a lesser extent, to the arterial vasodilator response (p = 0.023).

CONCLUSIONS/INTERPRETATION

Insulin sensitivity is an important determinant of retinal microvascular function. We propose that the elevated arterial flicker response in insulin-resistant states is a result of higher circulating insulin levels.

Endothelin-induced changes in blood flow in STZ-diabetic and non-diabetic rats: relation to nitric oxide synthase and cyclooxygenase inhibition

In this study, using the microsphere method, the hemodynamic response to endothelin-1 (ET-1) in healthy and streptozotocin (STZ)-diabetic rats was evaluated as well as the influences of inhibition of nitric oxide (NO)-synthase using L-NAME (Nω-nitro-L: -arginine methyl ester) and the cyclooxygenase inhibitor indomethacin. Blood flow (Q) was measured in tissues of interest for vascular complications in diabetes such as kidney, eye, brain, heart and skeletal muscle with the main focus on ophthalmic circulation. Under resting conditions, evidence for renal vasoconstriction was found in diabetic animals. In both groups, administration of L-NAME reduced Q in all investigated tissues indicating a basal NO influence. In the normal rats, ET-1 induced a significant increase in blood pressure and intense vasoconstriction in all tissues except in the choroid of the eye and in the brain, where it induced an increased Q. In the STZ-diabetic rats, effects of ET-1 were less pronounced. Pretreatment with L-NAME, but not the cyclooxygenase inhibitor, abolished the ET-1-induced vasodilation in the choroid of both groups. Administration of ET A receptor antagonist BQ-123 reduced the ET-1-induced vasodilation in the choroid only in diabetic animals. In conclusion, evidence for altered vascular endothelial response to ET-1 in STZ-diabetic animals was found particularly in the ophthalmic circulation. The findings suggest differential involvement of receptors in the response to ET-1 in normal and STZ-diabetic animals.

[Diabetic maculopathy and retinopathy. Functional and sociomedical significance]

OBJECTIVE

The incidence of diabetic microvascular complications is expected to increase by 20-50% in the coming years. Diabetic macular edema (DME) is already a leading cause of blindness in the working-age population in developed countries, and its impact is expected to increase dramatically.

METHODS

Recent literature on the epidemiology and impact of diabetic microangiopathy (maculopathy) on visual function was reviewed to provide a comprehensive overview of the functional and socioeconomic consequences of diabetic retinal microangiopathy and new therapeutic strategies.

RESULTS

The first changes indicating diabetic microangiopathy are detectable shortly after the development of hyperglycemia, and in the long term they induce severe organ damage. More resources are used for this condition's treatment than for the treatment of hyperglycemia, corresponding to an enormous sociomedical burden of disease. Early detection of increased retinal vascular permeability may help control treatment effects. The control of recognized risk factors for the development and progression of DME, namely hyperglycemia and hyperlipidemia, as well as of hypertension has remained the cornerstone of therapy and serves as the basis for preserving visual function.

CONCLUSIONS

Modern treatment options, begun early, may result in a remarkably delayed occurrence of irreversible diabetic microvascular pathologies, particularly diabetic retinopathy and maculopathy. Ophthalmological screening nowadays aims at earlier recognition of at-risk individuals to optimize the therapeutic strategy--that is, before visual impairment is imminent. Close interdisciplinary medical cooperation and implementation of new therapeutic options may provide the foundation for success in terms of maintaining visual function.

Endothelial dysfunction in glaucoma

Glaucoma is a group of ocular diseases characterized by optic neuropathy associated with loss of the retinal nerve fibre layer and re-modelling of the optic nerve head, and a subsequent particular pattern of visual field loss. Increased intraocular pressure is the most important risk factor for the disease, but the pathogenesis of glaucoma is not monofactorial. Among other factors, ischaemia and vascular dysregulation have been implicated in the mechanisms underlying glaucoma. The vascular endothelium plays an important role in the regulation of ocular blood flow and pathological alterations of vascular endothelial cells may induce ischaemia and dysregulation. The present review summarizes our current evidence of endothelial dysfunction in glaucoma. This is of interest because endothelial dysfunction is a good prognostic factor for progression in several diseases. Although such data are lacking for glaucoma, endothelial dysfunction may provide an attractive target for therapeutic intervention in open-angle glaucoma and other vascular disorders of the eye.

C-peptide does not affect ocular blood flow in patients with type 1 diabetes

OBJECTIVE

The aim of the present study was to investigate the effect of intravenous C-peptide infusion on ocular blood flow in patients with type 1 diabetes under euglycemic conditions.

RESEARCH DESIGN AND METHODS

The study was performed in a randomized, placebo-controlled, double-masked, two-way, crossover design in 10 type 1 diabetic patients. C-peptide was intravenously administered at two different dosages (dosage 1: 25 pmol . kg(-1) . min(-1) bolus followed by 5 pmol . kg(-1) . min(-1) continuous infusion; dosage 2: six times higher than dosage 1), each for 60 min. Physiologic saline solution was used as a control for C-peptide on a different study day. On both study days, euglycemic clamps were performed. To assess retinal blood flow, laser Doppler velocimetry (blood flow velocities) and retinal vessel analyzer (vessels diameters) measurements were performed. Laser interferometric measurements of fundus pulsation were used to assess pulsatile choroidal blood flow. Blood velocities in the ophthalmic artery were measured using color Doppler imaging.

RESULTS

Eight patients (two female and six male) completed the study according to the protocol and without adverse events. One patient developed an anaphylactic reaction to C-peptide, which resolved without sequelae. The following results originate from the remaining eight subjects. Systemic hemodynamic parameters remained stable during both study days. Infusion of C-peptide did not affect any ocular hemodynamic parameter.

CONCLUSIONS

The data of the present study indicate that exogenous C-peptide exerts no effect on ocular hemodynamic parameters in type 1 diabetic patients under euglycemic conditions. The maximum detectable change in these parameters was <25%.