ATP-induced relaxation of porcine retinal arterioles depends on the perivascular retinal tissue and acts via an adenosine receptor

The vasoactive effects of bradykinin, vasoactive intestinal peptide, calcitonin gene-related peptide and neuropeptide Y depend on the perivascular tissue in porcine retinal arterioles in vitro

PURPOSE

The retina contains a number of vasoactive neuropeptides and corresponding receptors, but the role of these neuropeptides for tone regulation of retinal arterioles has not been studied in detail.

METHODS

Porcine arterioles with preserved perivascular retinal tissue were mounted in a wire myograph, and the tone was measured after the addition of increasing concentrations of bradykinin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP) and brain natriuretic peptide (BNP). The experiments were performed during inhibition of the synthesis of nitric oxide (NO), prostaglandins and dopamine and were repeated after removal of the perivascular retinal tissue.

RESULTS

Bradykinin, VIP and CGRP induced significant concentration-dependent dilatation and NPY significant concentration-dependent contraction of the arterioles in the presence of perivascular retinal tissue (p < 0.03 for all comparisons) but not on isolated arterioles. BNP and SP had no effect on vascular tone. The NOS inhibitor L-NAME reduced bradykinin- and VIP-induced relaxation (p < 0.001 for both comparisons), whereas none of the other inhibitors influenced the vasoactive effects of the studied neuropeptides.

CONCLUSION

The effects of neuropeptides on the tone of retinal arterioles depend on the perivascular retinal tissue and may involve effects other than those mediated by nitric oxide, prostaglandins and adrenergic compounds. Investigation of the mechanisms underlying the vasoactive effect of neuropeptides may be important for understanding and treating retinal diseases where disturbances in retinal flow regulation are involved in the disease pathogenesis.

Differential effects of purines and prostaglandins on hypoxia induced dilatation of porcine retinal vessels at different branching level ex vivo

The metabolic pathways leading from hypoxia to retinal vasodilatation can involve effects of both purines and prostaglandins, but the effects of these compounds at different vascular branching levels are unknown. The purpose of the present study was to investigate differential effects of purines and prostaglandins in hypoxia-induced dilatation of retinal arterioles, precapillary arterioles and capillaries ex vivo. Porcine hemiretinas were mounted in a tissue chamber while monitoring temperature, pH, and oxygen tension. The effect of hypoxia on the diameter of larger arterioles, precapillary arterioles and capillaries was studied in the presence of the ecto-nucleotidase inhibitor AOPCP, the nonselective P2 purinoreceptor antagonist PPADS, the A2B adenosine receptor antagonist MRS 1754, the A3 adenosine receptor antagonist MRS 1523, the EP1 receptor antagonist SC-19220, the EP2 receptor antagonist PF-04418948, the EP3 receptor antagonist L-798,106, the EP4 receptor antagonist L-161-982, the prostaglandin synthesis inhibitor ibuprofen, and ibuprofen combined with AOPCP or ATP. Hypoxia-induced dilatation in arterioles was reduced by the A2B adenosine receptor antagonist (p < 0.01) and increased by the EP2 and the EP3 receptor antagonists (p < 0.01 for both comparisons). In precapillary arterioles the dilatation was reduced by the EP2 receptor antagonist (p < 0.04) and increased by the EP1 receptor antagonist (p < 0.03), whereas in capillaries the dilatation was increased by both the A3 adenosine receptor antagonist (p < 0.01), by ibuprofen in combination with the unspecific ecto-nucleotidase inhibitor AOPCP (p = 0.04) and by the prostaglandin EP3 receptor antagonist. Hypoxia-induced dilatation of retinal vessels is influenced by adenosine A2B and A3 receptors, and by the prostaglandin EP1, EP2 and EP3 receptors. The effects mediated by these receptors differ at different branching levels of the resistance vessels.

Keep an eye on adenosine: Its role in retinal inflammation

Adenosine is an endogenous purine nucleoside ubiquitously distributed throughout the body that interacts with G protein-coupled receptors, classified in four subtypes: A₁R, A_2AR, A_2BR and A₃R. Among the plethora of functions of adenosine, it has been increasingly recognized as a key mediator of the immune response. Neuroinflammation is a feature of chronic neurodegenerative diseases and contributes to the pathophysiology of several retinal degenerative diseases. Animal models of retinal diseases are helping to elucidate the regulatory roles of adenosine receptors in the development and progression of those diseases. Mounting evidence demonstrates that the adenosinergic system is altered in the retina during pathological conditions, compromising retinal physiology. This review focuses on the roles played by adenosine and the elements of the adenosinergic system (receptors, enzymes, transporters) in the neuroinflammatory processes occurring in the retina. An improved understanding of the molecular and cellular mechanisms of the signalling pathways mediated by adenosine underlying the onset and progression of retinal diseases will pave the way towards the identification of new therapeutic approaches.

Translational research in retinal vascular disease. An approach

The clinical presentation of the most frequent vision threatening retinal diseases is dominated by lesions indicating that disturbances in retinal blood flow are involved in the pathogenesis of these diseases. The present review describes the experience from a translational strategy pursued to investigate retinal vascular diseases with diabetic retinopathy as the main object. The normal regulation of retinal blood flow is investigated in porcine retinal vessels in vitro and ex vivo. Subsequently, the in vitro findings are translated to clinical studies in normal persons in vivo, and it is investigated whether the mechanisms are disturbed in retinal vascular disease. This is followed by clinical intervention studies on these diseases. The approach has been used to investigate pressure autoregulation, metabolic autoregulation and vasomotion in retinal vessels. The investigations have shown that retinal vascular tone can be regulated by receptor-specific agonists and antagonists to vasoactive compounds such as purines, prostaglandins and nitric oxide synthesis and that the vasoactive effects can be modulated by the concentration and the mode of administration of these compounds. Additionally, it has been shown that retinal precapillary arterioles and capillaries not visible by ophthalmoscopy may play an important role for the pathophysiology of retinal vascular disease and its treatment. Future studies should focus on investigating normal and pathological regulation of retinal blood flow in these smaller vessels.

The Diameter of Retinal Arterioles Is Unaffected by Intravascular Administration of the Adenosine A2A Receptor Agonist Regadenoson in Normal Persons

Background

The neurotransmitter adenosine has been proposed to be involved in the pathogenesis of diabetic retinopathy, which may be due to the vasoactive properties of the compound. Previous studies have shown that adenosine can affect the tone of retinal arterioles in vitro to induce dilatation mediated by A_2A and A_2Breceptors and constriction mediated by A₁ and A₃ receptors.

Purpose

To investigate effects of intravenous administration of the adenosine A_2A receptor agonist regadenoson on the diameter of retinal vessels in vivo.

Method

The diameter responses of larger retinal arterioles and venules were evaluated using the dynamic vessel analyser in 20 normal persons (age 22–31 years) after intravenous administration of the adenosine A_2A receptor agonist regadenoson during exposure to systemic normoxia and hypoxia.

Results

The diameter of retinal arterioles and venules increased significantly during stimulation with flickering light (p < 0.0001). Regadenoson reduced the flicker-induced dilatation of venules during normoxia (p = 0.0006), but otherwise had no effect on vessel diameters (p > 0.08 for all comparisons).

Conclusions

Intravenous administration of the adenosine A_2A receptor agonist regadenoson had no significant effect on the diameter of retinal arterioles. Future studies should investigate differential effects of intra- and extravascular administration of adenosine receptor agonists on retinal vessels.

The vasodilating effect of glucose differs among vessels at different branching level in the porcine retina ex vivo

Diabetic retinopathy is characterized by retinal lesions related to disturbances in retinal blood flow. The metabolic dysregulation in diabetes involves hyperglycemia which in both clinical and experimental studies has been shown to induce dilatation of larger retinal vessels, which has been suggested to be mediated by nitric oxide (NO). However, the effects of glucose on the diameter of smaller retinal vessels that are the site of development of diabetic retinopathy are unknown. Diameter changes in porcine retinal arterioles, pre-capillary arterioles and capillaries were studied ex vivo during acute changes in intraluminal glucose concentrations that mimicked changes in plasma glucose in diabetic patients. The experiments were repeated during blocking of NO-synthesis. Intravascular application of 2 mM glucose dilated arterioles and capillaries significantly, while 20 mM glucose dilated precapillary arterioles significantly. Intravascular application of 20 mM glucose dilated precapillary arterioles previously exposed to 2 mM glucose, while no significant diameter changes were observed after application of 2 mM glucose in vessels previously exposed to 20 mM glucose. No diameter changes were observed after application of 5.5 mM glucose in vessels previously exposed to both 2 mM and 20 mM glucose in either order. There was no significant difference between the diameter responses in the absence and presence of NO-synthesis blocker. Glucose induced dilatation of porcine precapillary arterioles ex vivo differs from the response in larger arterioles and capillaries, and the response is unaffected by the blocking of NO-synthesis. This may have implications for understanding the pathophysiology of diseases in the retinal microcirculation, such as diabetic retinopathy.

Activation of Veratridine Sensitive Sodium Channels, But not Electrical Field Stimulation, Dilates Porcine Retinal Arterioles with Preserved Perivascular Tissue

PURPOSE

Disturbances in retinal blood flow are a prominent feature of vision threatening retinal diseases. The regulation of tone in retinal resistance vessels involves the perivascular retinal tissue, but it is unknown to what extent neurons or glial cells contribute to the effect. Therefore, the purpose of the present study was to study the contribution of neurons in the perivascular retina to vascular tone during activation of voltage-gated sodium channels with veratridine and electrical field stimulation (EFS).

METHODS

Porcine retinal arterioles with and without perivascular tissue were mounted in an isometric myograph system for studying the effects of the voltage-gated sodium channel opener veratridine and EFS on retinal vascular tone.

RESULTS

Veratridine induced concentration-dependent relaxation of retinal arterioles which was more pronounced in arterioles with preserved perivascular retinal tissue than in isolated vessels. In the presence of this tissue, veratridine-induced relaxation was inhibited by the voltage-gated sodium channel blocker tetrodotoxin and the nitric oxide synthase inhibitor, N_ω-Nitro-L-arginine methyl ester (L-NAME), but was unaffected by the inhibition of the cyclo-oxygenase inbitior ibuprofen and by blocking of adenosine receptors with 8-(p-Sulfophenyl)theophylline hydrate (8-PSPT). Electrical field stimulation induced no changes in retinal vascular tone.

CONCLUSIONS

Sodium channels of neuronal origin are likely to be involved in the regulation of retinal vascular tone. The lack of effect of EFS on retinal vascular tone may be due to the lack of autonomic nerves in the retina.

Differential effects of nitric oxide and cyclo-oxygenase inhibition on the diameter of porcine retinal vessels with different caliber during hypoxia ex vivo

BACKGROUND

Hypoxia induced relaxation of larger retinal arterioles has been shown to be mediated by nitric oxide (NO) and cyclo-oxygenase (COX) products both in vivo and in vitro. However, the involvement of smaller retinal vessels in the response is unknown. Therefore, the purpose of the present study was to investigate the effect of blocking the synthesis of NO and COX on hypoxia induced changes in the diameter of smaller porcine retinal vessels at different branching level.

METHODS

Porcine hemiretinas were mounted in a tissue chamber and were constricted with the prostaglandin agonist U46619. Changes in the diameter of arterioles, pre-capillary arterioles and capillaries were studied during hypoxia, in the presence of the COX inhibitor ibuprofen and the NO synthase inhibitor L-NAME.

RESULTS

In the presence of L-NAME hypoxia induced dilatation was significantly smaller in arterioles and capillaries than in precapillary arterioles (p < 0.04), whereas in the presence of ibuprofen the dilatation was significantly smaller in capillaries and pre-capillary arterioles than in arterioles (p < 0.04).

CONCLUSIONS

The mechanisms underlying hypoxia induced dilatation differ among smaller porcine retinal vessels with different caliber ex vivo. This may reflect differences in the responses of retinal vessels to changes in metabolism, and may point to possible targets for pharmacological intervention on the diameter of retinal vessels with different caliber in vivo.

The vasodilating effects of insulin and lactate are increased in precapillary arterioles in the porcine retina ex vivo

PURPOSE

Disturbances in the diameter regulation of the larger retinal vessels are involved in the pathophysiology of a number of vision threatening retinal diseases, but the role of the smaller vessels has received less attention. Therefore, an experimental model was developed for studying diameter changes in retinal vessels at all branching levels secondary to pharmacological interventions ex vivo.

METHODS

Porcine hemiretinas were mounted in a tissue chamber that allowed the control of temperature, pH and oxygen saturation. The chamber was positioned in a fluorescence microscope, and changes in the diameter of larger arterioles, precapillary arterioles and capillaries were studied after intravascular and extravascular application of the thromboxane analogue U46619, lactate, the glutamate agonist N-Methyl-D-aspartic acid (NMDA) and insulin.

RESULTS

U46619 induced significant contraction of all vessel calibres after extravascular application, but had no significant effect on vessel diameters after intravascular application. In contracted vessels, lactate induced significant dilatation in all vessel calibres, and the effect was more pronounced after intravascular than after extravascular application where only precapillary arterioles dilated significantly. N-Methyl-D-aspartic acid (NMDA) induced similar dilatation in the three vessel types after intra- and extravascular application, whereas the dilating effect of insulin was significantly more pronounced in the precapillary arterioles than in the other vessel types.

CONCLUSION

The diameter regulation in precapillary retinal arterioles differs from that of other retinal vascular branching levels and may play a special role in the pathophysiology of retinal vascular disease.

Purinergic mechanisms and prostaglandin E receptors involved in ATP-induced relaxation of porcine retinal arterioles in vitro

PURPOSE

Adenosine triphosphate (ATP) is involved in the tone regulation of retinal arterioles, and the effect may be direct, through ATP degradation or mediated by cyclo-oxygenase products. However, the relative contribution of these mechanisms and the extent to which the mechanisms are active in the retinal vascular wall or depend on the perivascular retinal tissue are unknown.

METHODS

Porcine retinal arterioles with perivascular retinal tissue were mounted in a wire myograph for isometric tone recordings. The relaxing effects of ATP and the non-degradable analogue ATP-x03B3;S were studied in the presence of antagonists to ATP, adenosine and prostaglandin E (EP) receptors. The experiments were repeated after removal of the perivascular retinal tissue.

RESULTS

ATP induced a significant concentration-dependent relaxation of retinal arterioles (p < 0.05) which was reduced after removal of perivascular retinal tissue. The effect was due to non-degraded ATP and a degradation product of ATP acting via adenosine receptors. Relaxation was reduced by ibuprofen and blocking of EP1 receptors.

CONCLUSION

ATP-induced relaxation of retinal arterioles is mediated by ATP, ATP degradation products and by stimulation of EP1 receptors, involving both the perivascular retina and the vascular wall. The findings emphasize the complexity of purinergic effects in the regulation of retinal vascular tone.

Regional morphology and pathophysiology of retinal vascular disease

Disturbances in the retinal vascular supply are involved in the pathophysiology of the most frequent diseases causing visual impairment and blindness in the Western World. These diseases are diagnosed by noting how morphological lesions in the retina vary in shape, size, location and dynamics, and subsequently concluding the presence of a specific disease entity. This diagnostic approach can be used to identify the site of a retinal vascular occlusion, to assess whether retinal diseases are primarily due to changes in the larger retinal vessels or the microcirculation, and to differentiate the relative involvement of the choroidal and the retinal vascular systems. However, a number of morphological manifestations of retinal vascular disease cannot presently be related to the underlying pathophysiology. The review concludes that there is a need for developing new methods for assessing vascular structure and function in the ciliary vascular system supplying the choroid and the optic nerve head. Presently, the study of these structures relies on imaging techniques with limited penetration and resolution into the tissue. Secondly, there is a need for studying oscillations in retinal vascular function occurring within days to weeks, and for studying regional manifestations of retinal vascular disease. This may constitute the basis for future research in retinal vascular pathophysiology and for the development of new treatment modalities to reduce blindness secondary to retinal vascular disease.

Relaxation of porcine retinal arterioles exposed to hypercholesterolemia in vivo is modified by hepatic LDL-receptor deficiency and diabetes mellitus

Metabolic disturbances in diabetes mellitus include changes in the type and concentration of lipids in the blood plasma which may contribute to the development of diabetic retinopathy. This disease is characterized by changes in retinal blood flow secondary to changes in the tone of retinal arterioles which is regulated by compounds such as adenosine, adenosine triphosphate (ATP), the glutamate agonist N-methyl-d-aspartate (NMDA) and prostaglandin E2 (PGE2). However, the relation between increased plasma low density lipoprotein (LDL) and tone regulation in retinal resistance vessels has not been studied in detail. Twelve male and nine female Yucatan minipigs overexpressing a gain-of-function mutant (D374Y) of the human gene PCSK9 that blocks LDL transport into the liver and twelve wild-type males were studied. The animals were fed a cholesterol rich diet from the age of 60 days, followed by induction of diabetes mellitus in twelve of the transgenic animals. The animals were sacrificed at a mean age of 51 weeks (range 26-60 weeks), followed by inspection and histological examination of retinal vessels, and examination of the changes in vascular tone induced by adenosine, ATP, NMDA and PGE2. In the transgenic pigs without diabetes mellitus ATP-induced relaxation was reduced in isolated arterioles, and a whitish infiltration in an arteriole was observed in 4/8 (50%) of the animals, whereas these changes were not found in the other groups. Histological examination of one of the infiltrations showed staining with Oil Red O representing foamy cells sub-endothelially in the vascular wall indicating atheromatosis. Adenosine, ATP and PGE2 induced a significant concentration-dependent relaxation of retinal arterioles in all groups. The presence of perivascular retinal tissue had no effect on the relaxing effect of adenosine, but increased the relaxing effect of ATP and PGE2 in the two transgenic animal groups, whereas NMDA had no significant effect on vascular tone in any of the groups. Relaxation of porcine retinal arterioles exposed to hypercholesterolemia in vivo is modified by hepatic LDL-receptor deficiency and diabetes mellitus. This suggests that transgenic animal models are suitable for studying the influence of systemic diseases on retinal vascular function.

Relaxation of porcine retinal arterioles during acute hypoxia in vitro depends on prostaglandin and NO synthesis in the perivascular retina

PURPOSE

Disturbances in retinal oxygenation influence retinal function, but are also accompanied by changes in the tone of retinal arterioles. However, the mechanisms underlying these tone changes have not been studied in detail.

MATERIALS AND METHODS

Porcine retinal arterioles were mounted in a wire myograph, and the vasoactive effects of hypoxia and hyperoxia were studied before and after removal of the perivascular retinal tissue. Subsequently, the experiments were repeated in the presence of antagonists to prostaglandins, nitric oxide (NO), adenosine and glutamate.

RESULTS

Hypoxia induced a significant concentration-dependent relaxation of U46619-contracted retinal arterioles which depended on the presence of the perivascular retinal tissue. The relaxation was significantly reduced by inhibiting the synthesis of prostaglandins and NO simultaneously. The recovery of vascular tone after hypoxia was incomplete, but increased to a normal level during the inhibition of prostaglandin synthesis. Hyperoxia induced a slight concentration-dependent contraction of retinal arterioles that was not affected by any of the antagonists used.

CONCLUSIONS

Hypoxia-induced relaxation of porcine retinal arterioles in vitro depends on prostaglandins and NO and the presence of perivascular retinal tissue, whereas recovery of tone after hypoxia depends on the action of prostaglandins. Clinical intervention studies of these effects may help treating retinal diseases where disturbances in tissue oxygenation are involved in the disease pathogenesis.

Adenosine enhances the relaxing influence of retinal tissue

Retinal tissue from different species continuously releases an as yet unidentified retinal relaxing factor (RRF) lowering tone of isolated arteries. The potential influence of adenosine on this relaxing influence was investigated using isometric tension recording of different isolated arteries. The presence of bovine retinal tissue or rat retinal tissue enhanced the vasorelaxing effect of adenosine on isolated bovine retinal artery. In isolated rat carotid artery adenosine elicited no relaxation. However, a small relaxation is observed in the presence of rat retinal tissue, but not in the presence of porcine retina. The fact that adenosine potentiates the effect of rat retinal tissue but not that of a similar piece of porcine retinal tissue indicates species differences. Neither a NO-synthase inhibitor (nitro-L-arginine, 0.1mM), a cyclooxygenase inhibitor (indomethacin, 10 microM) or an epoxygenase inhibitor (miconazole, 10 microM) influenced the enhanced vasodilating effect of adenosine on bovine retinal arteries in the presence of bovine retinal tissue. On the other hand, when the retinal arteries were contracted with 120 mM K(+), adenosine no longer induced relaxation of the preparation with bovine retinal tissue. This is in line with the concept that adenosine enhances the influence of RRF. Also, the fact that rat carotid artery is less sensitive to RRF than bovine retinal artery - corresponding with a less enhanced adenosine response in rat carotid artery - is in line with the potential involvement of the RRF in the enhanced adenosine response. However, experiments using a bioassay setup for RRF gave no evidence for an increased RRF-release from the retina, nor for an increased RRF-sensitivity of the retinal artery in the presence of adenosine. In conclusion, our findings indicate that adenosine potentiates the relaxing influence of bovine and rat retinal tissue. This effect is species dependent as it is not seen with porcine retinal tissue. Neither NO, cyclooxygenase metabolites or epoxyeicosatrienoic acids seem to be involved in this enhanced vasorelaxing response. The involvement of the RRF cannot be excluded.

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.