[The myogenic response of retinal arterioles in diabetic retinopathy]

Loss of TRPV2-mediated blood flow autoregulation recapitulates diabetic retinopathy in rats

Loss of retinal blood flow autoregulation is an early feature of diabetes that precedes the development of clinically recognizable diabetic retinopathy (DR). Retinal blood flow autoregulation is mediated by the myogenic response of the retinal arterial vessels, a process that is initiated by the stretch‑dependent activation of TRPV2 channels on the retinal vascular smooth muscle cells (VSMCs). Here, we show that the impaired myogenic reaction of retinal arterioles from diabetic animals is associated with a complete loss of stretch‑dependent TRPV2 current activity on the retinal VSMCs. This effect could be attributed, in part, to TRPV2 channel downregulation, a phenomenon that was also evident in human retinal VSMCs from diabetic donors. We also demonstrate that TRPV2 heterozygous rats, a nondiabetic model of impaired myogenic reactivity and blood flow autoregulation in the retina, develop a range of microvascular, glial, and neuronal lesions resembling those observed in DR, including neovascular complexes. No overt kidney pathology was observed in these animals. Our data suggest that TRPV2 dysfunction underlies the loss of retinal blood flow autoregulation in diabetes and provide strong support for the hypothesis that autoregulatory deficits are involved in the pathogenesis of DR.

Reduced baseline diameter and contraction of peripheral retinal arterioles immediately after remote ischemia in diabetic patients

PURPOSE

Remote ischemic conditioning (RIC) implies that transient ischemia in one organ can affect blood flow and protect from ischemia in another remote organ such as the retina. The purpose of the present study was to investigate the effect of RIC on the diameter of retinal arterioles in patients with diabetic retinopathy and whether this effect differs among peripheral and macular vessels.

METHODS

In twenty type 1 diabetes patients aged 20-31 years, the Dynamic Vessel Analyzer (DVA) was used to measure diameters of peripheral and macular arterioles during rest, isometric exercise, and flicker stimulation. Measurements were obtained before, immediately after, and 1 h after RIC, and were compared to responses obtained from normal persons.

RESULTS

The reduced baseline diameter (p < 0.009) and contraction of peripheral retinal arterioles during isometric exercise (p = 0.025) observed immediately after RIC in normal persons were absent in the studied diabetic patients.

CONCLUSIONS

RIC affects the diameter of peripheral but not macular arterioles in normal persons, but the response is abolished in diabetic patients.

TRIAL REGISTRATION

NCT03906383.

Retinal arterial hypertrophy: the new LVH?

Prevention of target organ damage represents the El Dorado for clinicians who treat hypertension. Although many of the cardiovascular sequelae of chronic hypertension are due to large artery atherosclerosis, an equal number are due to small artery dysfunction. These microvascular complications include eye disease (retinopathy), kidney failure, diastolic dysfunction of the heart and small vessel brain disease leading to stroke syndromes, dementia and even depression. Examination of the retinal vasculature represents the only way to reliably derive information regarding small arteries responsible for these diverse pathologies. This review aims to summarise the rapidly accruing evidence indicating that easily observable abnormalities of retinal arteries reflect target organ damage elsewhere in the body of hypertensive patients. In tandem, we also present putative mechanisms by which hypertension and diabetes fundamentally change small artery structure and function and how these processes may lead to target organ damage.

Long-term results of the effect of intravitreal ranibizumab on the retinal arteriolar diameter in patients with neovascular age-related macular degeneration

PURPOSE

To study the effect of intravitreal (IVT) ranibizumab on the retinal arteriolar diameter in patients with neovascular age-related macular degeneration (AMD).

METHODS

Ten eyes of 10 patients with previously untreated neovascular AMD were included. All eyes had three monthly IVT injections of ranibizumab and then were retreated as needed, based on visual acuity and optical coherence tomography (OCT) criteria. The diameter of the retinal arterioles was measured in vivo with a retinal vessel analyser (RVA) before the first IVT injection, 7 and 30 days after the first, the second and the third injection, and at month 12 of follow-up.

RESULTS

  A significant vasoconstriction of the retinal arterioles was observed following each one of the first three IVT injections of ranibizumab. Thirty days following the first, second and third injection, there was a mean decrease of 8.4 ± 3.2%, 11.9 ± 4.5% and 18.5 ± 7.2%, respectively, of the retinal arteriolar diameter compared with baseline (p < 0.01). At month 12, the vasoconstriction was still present with a mean decrease of 19.1 ± 8.3% of the retinal arteriolar diameter compared with baseline (p < 0.01). Median number of ranibizumab injections was 4 (range 3-10). There was no correlation between the number of injections and percentage diameter decrease at month 12 (r = -0.54, p > 0.1). There was no significant change in mean arterial pressure (MAP) during the period of follow-up (p > 0.05).

CONCLUSIONS

  These results suggest that IVT ranibizumab induces sustained retinal arteriolar vasoconstriction in eyes with neovascular AMD.

Differential diameter responses in macular and peripheral retinal arterioles may contribute to the regional distribution of diabetic retinopathy lesions

BACKGROUND

Diabetic retinopathy is assumed to be due to impaired retinal autoregulation, involving both pressure autoregulation and metabolic autoregulation. The disease displays regional differences, with signs of hyperperfusion in the macular area and capillary occlusion with retinal ischemia in the peripheral retinal areas. It can be hypothesized that these regional differences in the occurrence of retinopathy lesions may reflect differences in the capacity of retinal arterioles to autoregulate the diameter of retinal arterioles.

METHODS

Seventeen normal persons and two matched groups of patients with respectively diabetic maculopathy and proliferative diabetic retinopathy were examined. The diameter change of a macular and a peripheral retinal arteriole during an increase in the arterial blood pressure induced by isometric exercise, during an increase in retinal metabolism induced by flicker stimulation, and during both stimulus paradigms simultaneously were studied using the dynamic vessel analyzer (DVA).

RESULTS

During isometric exercise, the diameter response was reduced in both macular and peripheral retinal arterioles in the two groups of patients with diabetes mellitus. During flicker stimulation, the diameter response was significantly reduced in patients with proliferative diabetic retinopathy, but there was no significant difference between the responses of macular and peripheral arterioles. During simultaneous isometric exercise and flicker stimulation, there was no difference between the diameter response of macular arterioles in the three groups, whereas the diameter response of macular arterioles was significantly lower in normal persons and significantly higher in persons with proliferative diabetic retinopathy as compared to peripheral arterioles.

CONCLUSIONS

Regional differences in the disturbances of the diameter response to increased blood pressure may contribute to the regional differences in the distribution of diabetic retinopathy lesions. In the central retinal areas, the diameter response to increased blood pressure and retinal metabolism interacted in a way that may potentially protect this area from ischaemia, whereas this protective mechanism was absent in the peripheral retinal arterioles. An elucidation of the mechanisms underlying diameter regulation to increased blood pressure and retinal metabolism, and the interaction between these two mechanisms, may help in understanding the pathophysiology of diabetic retinopathy.

Lack of correlation between short-term dynamics of diabetic retinopathy lesions and the arterial blood pressure

INTRODUCTION

Diabetic retinopathy is characterized by morphological lesions in the ocular fundus related to impairment of the retinal blood flow. The morphological lesions display a dynamic pattern of new formation and resorption of lesions, with a turn-over within days to weeks that may reflect basic pathophysiological mechanisms underlying the disease. However, a possible relationship between short-term fluctuations in diabetic retinopathy and risk factors for the long-term prognosis of diabetic retinopathy, such as the arterial blood pressure, has not been investigated.

METHODS

Five patients with moderate non-proliferative diabetic retinopathy were examined by fundus photography and measurement of the blood pressure at 1- to 2-week intervals for up to 1 year. The numbers of microaneurysms/haemorrhages and hard exudates in the macular area were counted, and changes in these counts were related to changes in the arterial blood pressure.

RESULTS

The five patients discontinued the study after 1-12 months, which resulted in between 5 and 39 examinations per patient, and amounted to a total of 101 examinations. There was a significant covariation between microaneurysms/haemorrhages and hard exudates, whereas there was no significant covariation between the number of either type of lesion and the blood pressure.

CONCLUSIONS

The cause of dynamic changes in diabetic retinopathy lesions occurring over days to weeks may involve other factors than changes in the arterial blood pressure.

Intravitreal ranibizumab may induce retinal arteriolar vasoconstriction in patients with neovascular age-related macular degeneration

OBJECTIVE

To study the effect of intravitreal (IVT) ranibizumab (Lucentis; Genentech, Inc, San Francisco, CA) on the retinal arteriolar diameter in patients with neovascular age-related macular degeneration (AMD).

DESIGN

Prospective consecutive interventional case series.

PARTICIPANTS

Eleven eyes of eleven patients with previously untreated neovascular AMD.

METHODS

All eyes had 3 monthly IVT injections of ranibizumab. The diameter of the retinal arterioles was measured in vivo with a retinal vessel analyzer (RVA) before the first IVT injection and then 7 and 30 days after the first, second, and third injections.

MAIN OUTCOME MEASURES

Primary end points were changes in retinal arteriolar diameter and mean arterial pressure (MAP) after IVT ranibizumab. Secondary end points were changes in best-corrected visual acuity (BCVA), central retinal thickness, and intraocular pressure after IVT ranibizumab, and appearance of adverse events during the follow-up period.

RESULTS

A significant decrease of the retinal arteriolar diameter was observed after each IVT injection of ranibizumab. Thirty days after the first, second, and third injections, there was a mean decrease of 8.1+/-3.2%, 11.5+/-4.4%, and 17.6+/-7.4%, respectively, of the retinal arteriolar diameter compared with baseline values (P<0.01). There was no significant change in MAP during the period of follow-up (P>0.05). Thirty days after the third IVT injection of ranibizumab, mean BCVA improved by 6.5+/-4.9 Early Treatment Diabetic Retinopathy Study (ETDRS) letters, and central retinal thickness decreased by 91+/-122 microm (P = 0.03).

CONCLUSIONS

These results suggest that IVT ranibizumab may induce retinal arteriolar vasoconstriction in patients with neovascular AMD after IVT ranibizumab. Further studies evaluating larger sample sizes are needed to confirm these results and potential adverse effects on the retinal circulation in patients with AMD and retinal vascular diseases.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Myeloid-related protein 8/14 complex describes microcirculatory alterations in patients with type 2 diabetes and nephropathy

Background

Inflammation contributes to cardiovascular complications in type 2 diabetes, which are often characterized by microvascular alterations. We investigated whether myeloid-related protein 8/14 complex (MRP8/14) secreted by transmigrating monocytes and granulocytes may represent a biomarker for microvascular alterations in patients with type 2 diabetes and nephropathy.

Methods

MRP8/14 was measured in 43 patients with type 2 diabetes and nephropathy. Additionally, the inflammatory markers Interleukin-6 (IL-6), Tumor necrosis factor-α (TNF-α) and C-reactive protein (CRP) were quantified. To detect microvascular alterations proteinuria and retinal vessel caliber were used as classical and novel marker, respectively. Proteinuria was quantified by protein-creatinine ratio (PCR); retinal vessel caliber was quantified after retina photography on digitalized retina pictures.

Results

MRP8/14 was positively associated with inflammation (r = 0.57), proteinuria (r = 0.40) and retinal arterial caliber (r = 0.48). Type 2 diabetic patients with MRP8/14 values above the median of 5.8 μg/ml demonstrated higher proteinuria and larger retinal artery caliber than patients with MRP8/14 values below the median (logPCR: -0.51 ± 0.52 versus -0.96 ± 0.46, P < 0.01; retinal artery lumen (μm): 178.3 ± 14.1 versus 162.7 ± 14.9 P < 0.01). Both groups did not differ with regard to metabolic factors and blood pressure. MRP8/14 was an independent predictor of retinal artery caliber in multivariate stepwise regression analysis (β = 0.607) and was positively associated with IL-6 (r = 0.57, P < 0.001) and TNF-α (r = 0.36, P < 0.05).

Conclusion

MRP8/14 – a marker for transendothelial migration – describes not only the state of inflammation in diabetic nephropathy, but additionally the degree of microvascular alterations in the glomerular and retinal bed. Therefore, MRP8/14 may be a potentially selective novel biomarker for microcirculatory defects in diabetic nephropathy.

Vascular remodeling: implications for small artery function and target organ damage

At the level of the small artery, essential hypertension is associated with eutrophic inward remodeling. This involves reduction in lumen diameter by an increase in wall thickness. Previously thought to involve either hypertrophy or hyperplasia of the vascular smooth muscle cells in the media, it is now felt to be mediated by a functional property of the wall: myogenic tone. This is the ability of an artery to contract in response to an increase in intraluminal pressure. This autoregulatory function is also vital to ensure stabilisation of distal capillary pressures and so prevent, or limit, organ damage. Indeed in any animal model studied, when myogenic autoregulation is affected, target organ damage ensues. We have also observed, in two studies, that when myogenic autoregulation is damaged in the context of hypertension, eutrophic remodeling is replaced by an outward growth of the arterial wall with preservation of lumen diameter. This is called hypertrophic remodeling and, independently, has been observed by a number of groups in small arteries from patients with type 2 diabetes. We believe that this is a key reason for the unique propensity to hypertensive injury seen in patients with diabetes. We also discuss the significance of integrins, transmembrane proteins with wide ranging functions; from initiation of cell migration to intracellular signalling. Two particular integrins, alpha5beta1 and alphanubeta3, have been found to be necessary for both normal myogenic autoregulation and eutrophic remodeling and the possibility that damage to these may occur in diabetes is examined.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Interaction between flicker-induced vasodilatation and pressure autoregulation in early retinopathy of type 2 diabetes

BACKGROUND

Diabetic retinopathy is accompanied with changes in the autoregulation of retinal blood flow secondary to changes in the systemic blood pressure and the retinal metabolism. In the present study we tested the working hypothesis that there is an interaction between these mechanisms that might be relevant for understanding and treating flow disturbances in diabetic retinopathy.

METHODS

Fifty-four persons divided into three age and sex matched groups were studied: Group 1: twenty normal persons. Group 2: fourteen patients with type 2 diabetes mellitus and no diabetic retinopathy. Group 3: twenty type 2 diabetic patients with minimal diabetic retinopathy and a diabetes duration similar to that of the patients in group 2. Using the Retinal Vessel Analyzer (RVA) the diameter response of retinal arterioles was studied in all groups after an increase in the blood pressure by isometric exercise, during exposition to 8 Hz flickering light, and during simultaneous exposition to both stimulus conditions.

RESULTS

The increased blood pressure induced by isometric exercise induced a non-significant vasoconstriction in the normal persons and in the diabetic patients without retinopathy (p=0.10 and p=0.84 respectively), and a non-significant vasodilatation in the diabetic patients with mild retinopathy (p=0.10). The flicker stimulus elicited a significant vasodilatation of retinal arterioles that decreased significantly from the normal persons to the diabetic patients without and with retinopathy (linear regression, p<0.01). The flicker-induced vasodilatation was not significantly affected by a simultaneous increase in the arterial blood pressure in normal persons (p=0.85). Conversely, in the diabetic patients the reduced diameter response during flicker was counteracted by a simultaneous increase in the blood pressure, to a level not differing significantly from the response of normal persons (p=0.75).

CONCLUSIONS

Intervention studies aimed at modifying perfusion in retinal disease should consider the interaction between different mechanisms for autoregulating retinal blood flow. New treatment modalities for retinal vascular disease might need to target several mechanisms of tone control in retinal arterioles simultaneously.