Retinal arterial tone is controlled by a retinal-derived relaxing factor

Neuropeptide Y treatment induces retinal vasoconstriction and causes functional and histological retinal damage in a porcine ischaemia model

PURPOSE

To investigate the effects of intravitreal neuropeptide Y (NPY) treatment following acute retinal ischaemia in an in vivo porcine model. In addition, we evaluated the vasoconstrictive potential of NPY on porcine retinal arteries ex vivo.

METHODS

Twelve pigs underwent induced retinal ischaemia by elevated intraocular pressure clamping the ocular perfusion pressure at 5 mmHg for 2 hr followed by intravitreal injection of NPY or vehicle. After 4 weeks, retinas were evaluated functionally by standard and global-flash multifocal electroretinogram (mfERG) and histologically by thickness of retinal layers and number of ganglion cells. Additionally, the vasoconstrictive effects of NPY and its involved receptors were tested using wire myographs and NPY receptor antagonists on porcine retinal arteries.

RESULTS

Intravitreal injection of NPY after induced ischaemia caused a significant reduction in the mean induced component (IC) amplitude ratio (treated/normal eye) compared to vehicle-treated eyes. This reduction was accompanied by histological damage, where NPY treatment reduced the mean thickness of inner retinal layers and number of ganglion cells. In retinal arteries, NPY-induced vasoconstriction to a plateau of approximately 65% of potassium-induced constriction. This effect appeared to be mediated via Y1 and Y2, but not Y5.

CONCLUSION

In seeming contrast to previous in vitro studies, intravitreal NPY treatment caused functional and histological damage compared to vehicle after a retinal ischaemic insult. Furthermore, we showed for the first time that NPY induces Y1- and Y2- but not Y5-mediated vasoconstriction in retinal arteries. This constriction could explain the worsening in vivo effect induced by NPY treatment following an ischaemic insult and suggests that future studies on exploring the neuroprotective effects of NPY might focus on other receptors than Y1 and Y2.

Characterization of the retina-induced relaxation in mice

PURPOSE

The retinal relaxing factor (RRF) is a continuously released factor from the retina that causes vasorelaxation, the identity and potential role in physiology of which remain largely unknown. Experiments were performed to find out whether the RRF-induced relaxation is influenced by serotonin, glutamate, L-cysteine, the cytochrome P450 pathway, the cyclooxygenase pathway, or oxidative stress. In addition, the sensitivity of retinal and non-retinal arteries towards the RRF was compared.

METHODS

In vitro tension measurements were performed on isolated mouse femoral or bovine retinal arteries to study the vasorelaxing effect of the RRF, induced by mouse or bovine retinas.

RESULTS

The presence of serotonin, glutamate, or L-cysteine did not alter the RRF-induced relaxation. Increasing oxidative stress by hydroquinone and diethyldithiocarbamic acid sodium salt enhanced the RRF response. Inhibition of the cytochrome P450 or the cyclooxygenase pathway did not cause any alteration. Surprisingly, the RRF-induced relaxation was enhanced by the presence of flufenamic acid or carbenoxolone. Furthermore, bringing retinal tissue in close contact with retinal or non-retinal arteries induced comparable relaxations.

CONCLUSIONS

Serotonin, glutamate, L-cysteine, the cytochrome P450, and the cyclooxygenase pathway do not influence the RRF-induced relaxation and the RRF-induced relaxation seems to be resistant to oxidative stress. The mechanism responsible for the enhanced RRF-induced relaxation in the presence of flufenamic acid or carbenoxolone remains elusive and the RRF does not show more effectivity on retinal arteries.

Search for the Source of the Retinal Relaxing Factor

Purpose/Aim of the study: the retinal relaxing factor (RRF) is an unidentified paracrine factor, which is continuously released from retinal tissue and causes smooth muscle cell relaxation. This study tried to identify the cellular source of the RRF. Furthermore, the possible RRF release by voltage-dependent sodium channel activation and the calcium-dependency of the RRF release were investigated.

MATERIALS AND METHODS

mouse femoral arteries were mounted in myograph baths for in vitro isometric tension measurements. The vasorelaxing effect of chicken retinas, which contain no vascular cells, and of solutions incubated with MIO-M1 or primary Müller cell cultures were evaluated. The RRF release of other retinal cells was investigated by using cell type inhibitors. Concentration-response curves of veratridine, a voltage-dependent sodium channel activator, were constructed in the presence or absence of mouse retinal tissue to evaluate the RRF release. The calcium-dependency of the RRF release was investigated by evaluating the vasorelaxing effect of RRF-containing solutions made out of chicken retinas in the absence or presence of calcium.

RESULTS

Chicken retinas induced vasorelaxation, whereas solutions incubated with Müller cell cultures did not. Moreover, the gliotoxin DL-α-aminoadipic acid, the microglia inhibitor minocycline, and the tetrodotoxin-resistant voltage-dependent sodium channel 1.8 inhibitor A-803467 could not reduce the RRF-induced relaxation. Concentration-response curves of veratridine were not enlarged in the presence of retinal tissue, and RRF-containing solutions made in the absence of calcium induced a substantial, but reduced vasorelaxation.

CONCLUSIONS

the RRF is not released from vascular cells and probably neither from glial cells. The retinal cell type that does release the RRF remains unclear. Veratridine does not stimulate the RRF release in mice, and the RRF release in chickens is calcium-dependent as well as calcium-independent.

Constriction of porcine retinal arterioles induced by endothelin-1 and the thromboxane analogue U46619 in vitro decreases with increasing vascular branching level

PURPOSE

The retinal blood flow depends on the diameter of retinal arterioles, but diameter changes in these vessels have hitherto only been assessed in vessels larger than approximately 100 μm. Therefore, a new method was developed for studying diameter changes along the vascular tree of arterioles in whole perfused segments of porcine retinas, and the effect of known vasoconstrictors on the diameter of retinal arterioles at different branching levels were studied.

METHODS

Thirty-four whole-mounted porcine retinas were placed in a specially designed tissue chamber. On the basis of video recordings through an inverted microscope, the diameter of retinal arterioles was measured at five different branching levels before and after addition of a high potassium concentration, or increasing concentrations of endothelin-1, the prostaglandin analogue U46619, noradrenaline or none (time controls).

RESULTS

The baseline diameter ranged from 136 μm (95% CI 132-140 μm) for 1st order arterioles to 33 μm (95% CI 21-44 μm) for 5th order arterioles. In 1st order arterioles, endothelin produced 56.6% (95% CI 47.6-64.0) and U46619 14.6% (95% CI 5.7-22.6) relative constriction compared with baseline, which for both compounds decreased significantly with increasing branching level (p<0.0001 and p<0.0001, respectively). The change in diameter during addition of noradrenaline did not differ significantly from the time controls (p=0.07).

CONCLUSIONS

The effect of retinal vasoconstrictors differs among larger and smaller arterioles. The study highlights the need for investigating diameter regulation in smaller retinal arterioles as a basis for understanding normal and pathological changes in retinal blood flow.

[Elucidation of dysfunctional mechanisms of retinal circulation in the rat models of glaucoma and exploration of novel therapeutic drugs]

In recent times, glaucoma has become the leading cause of acquired blindness among the Japanese. As visual disorders markedly decrease the quality of life (QOL), it is important to develop new strategies for preventing the onset of and delaying the progression of glaucoma. Glaucoma has long since been recognized as a serious disease caused by increased intraocular pressure and subsequent injury and death of the neuronal retinal cells. Therefore, numerous studies have focused on the mechanisms that damage neuronal cells and on the drugs that possess protective effects in reversing this damage. However, injury to the retinal vasculature has been recently shown in animal models of glaucoma. Hence, thus far, only few papers have been published on retinal circulation in glaucoma. These study results have indicated that retinal circulation is altered in glaucoma and that this vascular abnormality may be the cause of and/or may accelerate retinal degeneration. In this report, we have attempted to elucidate the mechanisms of retinal circulation and explore novel drugs for the treatment of retinal circulation disorders. We have also introduced here our previous research results on retinal circulation. We reported that the drugs that improved retinal circulation, by intravitreal injection, in the rat model of glaucoma also inhibited retinal nerve injury, thereby representing possibilities that they might be novel candidate drugs for glaucoma prevention and treatment.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.

Theoretical analysis of vascular regulatory mechanisms contributing to retinal blood flow autoregulation

PURPOSE

To study whether impaired retinal autoregulation is a risk factor for glaucoma, the relationship between vascular regulatory mechanisms and glaucoma progression needs to be investigated. In this study, a vascular wall mechanics model is used to predict the relative importance of regulatory mechanisms in achieving retinal autoregulation.

METHODS

Resistance vessels are assumed to respond to changes in pressure, shear stress, carbon dioxide (CO2), and the downstream metabolic state communicated via conducted responses. Model parameters governing wall tension are fit to pressure and diameter data from porcine retinal arterioles. The autoregulation pressure range for control and elevated levels of IOP is predicted.

RESULTS

The factor by which flow changes as the blood pressure exiting the central retinal artery is varied between 28 and 40 mm Hg is used to indicate the degree of autoregulation (1 indicates perfect autoregulation). In the presence of only the myogenic response mechanism, the factor is 2.06. In the presence of the myogenic and CO2 responses, the factor is 1.22. The combination of myogenic, shear, CO2, and metabolic responses yields the best autoregulation (factor of 1.10).

CONCLUSIONS

Model results are compared with flow and pressure data from multiple patient studies, and the combined effects of the metabolic and CO2 responses are predicted to be critical for achieving retinal autoregulation. When IOP is elevated, the model predicts a decrease in the autoregulation range toward low perfusion pressure, which is consistent with observations that glaucoma is associated with decreased perfusion pressure.

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.

An experimental study of VEGF induced changes in vasoactivity in pig retinal arterioles and the influence of an anti-VEGF agent

BACKGROUND

Vascular endothelial growth factor (VEGF) plays an important role in ocular physiology. Anti-VEGF agents are now used for treatment of common retinal diseases. This study characterises the vasoactive properties of VEGF in isolated perfused pig retinal arterioles under normal tone or endothelin-1 (ET-1) pre-contracted conditions and determines the influence of an anti VEGF agent on VEGF induced vasoactivity.

METHODS

An isolated perfused retinal arteriole preparation was used. The outer diameter of retinal vessels was monitored at 2 second intervals in response to VEGF and the anti VEGF agent, bevacizumab. The effect of intraluminal delivery of VEGF was determined over a wide concentration range (10(-16) to 10(-7) M) both with and without pre-contraction with ET-1 (3 x 10(-9) M). Bevacizumab (0.35 mg mL(-1)) was applied extraluminally to determine the influence of bevacizumab on VEGF induced vasoactive changes on ET-1 pre-contracted vessels.

RESULTS

In retinal arterioles with normal tone, VEGF induced a concentration dependent contraction at low concentrations, reaching 93.5% at 10(-11) M and then contraction was reduced at higher concentrations, recovering to 98.1% at 10-7 M. VEGF produced a potent concentration dependent vasodilatation in arterioles pre-contracted with ET-1. VEGF induced vasodilatation in arterioles pre-contracted with ET-1 was significantly inhibited by bevacizumab.

CONCLUSIONS

VEGF induced vasoactive changes in pig retinal arterioles are dependent on concentration and vascular tone. Bevacizumab inhibits VEGF-induced vasodilatation in pre-contracted arterioles.

Noradrenaline contracts rat retinal arterioles via stimulation of α(1A)- and α(1D)-adrenoceptors

The aim of this study was to characterize the α₁-adrenoceptor subtype(s) involved in the noradrenaline-induced contraction of retinal arterioles in rats. In vivo ocular fundus images were captured with a digital camera equipped with a special objective lens. By measuring changes in diameter of retinal arterioles in the fundus images, retinal vascular response was assessed. The systemic blood pressure and heart rate in the animals were also continuously recorded. Following blockade of β₁/β₂-adrenoceptors with propranolol, noradrenaline (0.03-3 μg/kg/min, i.v.) decreased the diameter of retinal arterioles and increased the mean blood pressure in a dose-dependent manner. The highest dose (3 μg/kg/min, i.v.) of noradrenaline caused a small increase in heart rate. The α(1A)-adrenoceptor antagonist RS100329 (0.1 mg/kg, i.v.) and the α(1D)-adrenoceptor antagonist BMY 7378 (1 mg/kg, i.v.) significantly prevented noradrenaline-induced contraction of retinal arterioles and pressor responses whereas the α(1B)-adrenoceptor antagonist L-765314 (1 mg/kg, i.v.) did not. The α(1A)-adrenoceptor agonist, A 61603 (0.03-0.3 μg/kg/min, i.v.), also caused contractile responses of retinal arterioles and pressor responses. These responses were almost completely prevented by RS100329 (0.1 mg/kg, i.v.), but not by BMY 7378 (1 mg/kg, i.v.). These results suggest that the contractile effects of noradrenaline on retinal arterioles and peripheral resistance vessels are, at least in part, mediated by stimulation of α(1A)- and α(1D)-adrenoceptors. Furthermore, it is likely that the α₁-adrenoceptor subtype(s) involved in rat vascular responses are similar in both retinal and peripheral circulation.

Role of β3-adrenoceptors in regulation of retinal vascular tone in rats

The aim of this study was to determine the role of β(3)-adrenoceptors in the action of endogenous catecholamines (adrenaline and noradrenaline) on rat retinal arterioles in vivo. Using an original high-resolution digital fundus camera, the rat ocular fundus images were captured. The diameter of retinal arterioles contained in the images was measured. Both systemic blood pressure and heart rate were recorded continuously. Adrenaline (0.3-5.0 μg/kg/min, i.v.) increased the diameter of retinal arterioles, mean blood pressure and heart rate in a dose-dependent manner. Under blockade of β(1)/β(2)-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 μg/kg/min infusion), adrenaline decreased the diameter of retinal arterioles. Similar observation was made under treatment with the β(3)-adrenoceptor antagonist L-748337 (50 μg/kg, i.v.). The pressor response to adrenaline was enhanced by propranolol, but not by L-748337. The positive chronotropic action of adrenaline was markedly prevented by propranolol, whereas it was unaffected by L-748337. Noradrenaline (0.03-1.0 μg/kg/min, i.v.) decreased the diameter of retinal arterioles but increased the mean blood pressure and heart rate. The effects of noradrenaline on retinal arteriolar diameter and blood pressure were unaffected by propranolol or L-748337. The positive chronotropic action of noradrenaline was almost completely abolished by propranolol. These results suggest that β(3)-adrenoceptors play crucial roles in vasodilator responses to adrenaline of retinal arterioles but have minor or no effect on noradrenaline-induced responses. The results also indicate that the functional role of β(3)-adrenoceptors may be more important than that in peripheral resistance vessels.

Retina evokes biphasic relaxations in retinal artery unrelated to endothelium, K(V), K(ATP), K(Ca) channels and methyl palmitate

Retinal relaxing factor (RRF) is suggested to be released from the retina and to contribute in the maintenance of retinal arterial tone. Herein, we aimed to clarify the effects of retinal tissue in isolated bovine retinal arteries in comparison with choroidal tissue and to evaluate the possible role of endothelium and potassium channels. In parallel, the effects of palmitic acid methyl ester (PAME), a putative vasodilator proposed to be released from the retina, was also examined. A piece of bovine retinal or choroidal tissue was placed within a close proximity on top of retinal arteries mounted in a wire myograph and precontracted with noradrenaline, prostaglandin F(2α), endothelin-1, thromboxane A(2) mimetic, U46619 or potassium (K(+)). To elucidate possible mechanisms in the effects of retinal tissue, retinal arteries were either deendothelized or incubated with inhibitors of endothelial vasodilators, i.e. nitric oxide (NO) and prostaglandins, or K(+) channels. Unlike the choroid, retinal tissue produced rapid, biphasic and complete relaxations in isolated bovine retinal arteries precontracted with various spasmogens acting on distinct receptors. Endothelium removal or preincubation of retinal arteries with inhibitors of NO synthase; L-NOARG (10(-4)M), guanylate cyclase; ODQ (10(-5)M) and cyclooxygenase; indomethacin (10(-5)M), did not cause a significant difference in the relaxation profile. Additionally, retinal relaxations remained unchanged in the presence of respective inhibitors of ATP-sensitive (K(ATP)) (glibenclamide, 10(-5)M), voltage-dependent (K(V)) (4-aminopyridine, 2×10(-3)M), and calcium-activated (K(Ca)) (tetraethylammonium 10mM; charybdotoxin, 10(-7)M; and apamin, 5×10(-7)M) K(+) channels. Thus, our results provide novel evidence regarding the biphasic relaxing profile of RRF in the retinal artery which was unrelated to endothelium and K(+) channels (K(ATP), K(V) and K(Ca)). Interestingly, PAME (10(-14)-10(-5)M) did not provoke a relaxation in bovine retinal artery suggesting no association with RRF.

Pharmacological evidence for the presence of functional beta(3)-adrenoceptors in rat retinal blood vessels

The aim of this study was to examine whether stimulation of beta(3)-adrenoceptors dilates rat retinal blood vessels and how diabetes affects the vasodilator responses. Images of ocular fundus were captured with an original high-resolution digital fundus camera in vivo. The retinal vascular responses were evaluated by measuring diameter of retinal blood vessels contained in the digital images. Both systemic blood pressure and heart rate (HR) were continuously recorded. The beta(3)-adrenoceptor agonist CL316243 (0.3-10 microg/kg/min, i.v.) increased diameter of retinal arterioles (at 10 microg/kg/min, a 31% increase) and decreased mean blood pressure (at 10 microg/kg/min, a 21% decrease) in a dose-dependent manner. CL316243 produced a small but significant increase in HR (at 10 microg/kg/min, a 9% increase). Both SR59230A (1 mg/kg, i.v.) and L-748337 (50 microg/kg, i.v.), beta(3)-adrenoceptor antagonists, significantly prevented CL316243-induced retinal vasodilator responses. Similar observations were made with another beta(3)-adrenoceptor agonist, BRL37344. The beta(2)-adrenoceptor agonist salbutamol also increased diameter of retinal arterioles (at 10 microg/kg/min, a 43% increase), whereas the drug produced greater decrease in blood pressure (at 10 microg/kg/min, a 46% decrease) and increase in HR (at 10 microg/kg/min, a 16% increase), compared with beta(3)-adrenoceptor agonists. The retinal vasodilator responses to CL316243 and BRL37344 observed under blockade of beta(1)/beta(2)-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 microg/kg/min infusion) were unaffected 2 weeks after induction of diabetes by the combination of streptozotocin treatment and D: -glucose feeding. On the other hand, the vasodilator responses to salbutamol of retinal arterioles were significantly reduced in diabetic rats. These results suggest that stimulation of beta(3)-adrenoceptors causes the vasodilation of retinal arterioles in vivo and the vasodilator responses are unaffected at the early stage of diabetes.

Clozapine Directly Relaxes Bovine Retinal Arteries

PURPOSE

It was suggested that clozapine might be helpful in the development of new antiglaucoma agents, as it combines lowering the intraocular pressure after topical instillation with vasodilation. This study aimed to evaluate and characterize the vasodilatory effect of clozapine in isolated bovine retinal arteries (BRAs).

METHODS

Retinal arteries were isolated from bovine eyes and mounted in the organ bath of a small vessel myograph.

RESULTS

Cumulative addition of clozapine (1 nM to 10 microM) caused a concentration-dependent relaxation of the BRAs. Removal of the endothelium, inhibition of nitric oxide synthase and of soluble guanylyl cyclase reduced the clozapine response, whereas cyclooxygenase inhibition had no influence. A Ca2+ channel activator, a 5-hydroxytryptamine receptor antagonist, and an adenosine receptor antagonist failed in affecting the clozapine-induced relaxations.

CONCLUSIONS

Clozapine relaxes bovine retinal arteries. Endothelium-derived NO seems to be involved, whereas prostanoids, calcium entry blockade, 5-HT7 receptor stimulation, and adenosine receptor stimulation do not.

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.

Beta-adrenoceptor-mediated vasodilation of retinal blood vessels is reduced in streptozotocin-induced diabetic rats

We investigated the effects of epinephrine and dopamine on retinal blood vessels in streptozotocin (STZ, 80 mg/kg, i.p.)-treated rats and age-matched control rats to determine whether diabetes mellitus alters the retinal vascular responses to circulating catecholamines. Experiments were performed 6-8 weeks after treatment with STZ or the vehicle. The fundus images were captured with the digital fundus camera system for small animals we developed and diameters of retinal blood vessels contained in the digital images were measured. Epinephrine increased the diameters of retinal blood vessels, but the vasodilator responses were reduced in diabetic rats. Dopamine produced a biphasic retinal vascular response with an initial vasoconstriction followed by a vasodilation. The vasoconstrictor effects of dopamine on retinal arterioles were enhanced in diabetic rats, whereas the difference between the two groups was abolished by treatment with propranolol. The vasodilator effect of isoproterenol, but not of the activator of adenylyl cyclase colforsin, on retinal blood vessels was reduced in diabetic rats. No difference in vasoconstriction of retinal blood vessels to phenylephrine between non-diabetic and diabetic rats was observed. The vasodilator responses of retinal blood vessels to 1,1-dimethyl-4-phenylpiperazinium, a ganglionic nicotinic receptor agonist, were also attenuated in diabetic rats. These results suggest that diabetes mellitus alters the retinal vascular responses to circulating catecholamines and the impairment of vasodilator responses mediated by beta-adrenoceptors contributes to the alteration.

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.

The relaxing effect of perivascular tissue on porcine retinal arterioles in vitro is mimicked by N-methyl-D-aspartate and is blocked by prostaglandin synthesis inhibition

PURPOSE

Retinal hyperperfusion resulting from disturbances in the regulation of arteriolar tone is involved in the pathophysiology of a variety of retinal diseases. The mechanisms underlying this regulation of tone involve cellular components in both the vascular wall and the perivascular tissue. However, previous in vitro studies of the influence of perivascular retinal tissue on retinal tone regulation have been hampered by the release of an endogenous relaxing factor that renders the arteriole insensitive to vasoconstrictors. The purpose of the present study was to test whether N-methyl-D-aspartate (NMDA) and gamma-amino butyric acid (GABA) receptors, and a cyclooxygenase (COX) product influence this effect of perivascular retinal tissue in vitro.

METHODS

Porcine retinal arterioles were mounted in a wire myograph for isometric force measurements. The contractile effect of the prostaglandin analogue U46619 was studied on vessels with preserved perivascular retinal tissue and after this tissue had been removed. The influence of the perivascular tissue was studied after addition of NMDA (a specific agonist for a subtype of the glutamate receptor), DL-amino-5-phosphonovaleric acid (DL-APV, an antagonist at the same receptor), the natural inhibitory transmitter GABA, and picrotoxin (an antagonist at ionotropic GABA receptors). These experiments were made in the absence and presence of the COX inhibitor, ibuprofen.

RESULTS

U46619 caused a concentration-dependent contraction of isolated retinal arterioles. This vasoconstriction was significantly smaller in the presence of perivascular tissue. The NMDA-receptor antagonist, DL-APV, reduced this attenuating influence of the perivascular tissue on the response to U46619, and the response could be modified by NMDA and GABA, but not by picrotoxin. However, ibuprofen totally blocked the attenuating influence of the perivascular tissue on the response to U46619.

CONCLUSIONS

The inhibition of vascular contractility induced by perivascular retinal tissue in vitro involves NMDA-receptors and an effect of GABA-mimetic substance on retinal tissue. The generation of these effects involves a COX product.

Stimulation of prostanoid IP and EP2 receptors dilates retinal arterioles and increases retinal and choroidal blood flow in rats

We examined the effects of vasodilatory prostaglandins (prostacyclin and prostaglandin E(2)) and selective agonists for prostanoid EP(2) and EP(4) receptor on the diameters of retinal blood vessels and fundus (retinal/choroidal) blood flow in rats. Male Wistar rats (8- to 10-week-old) were treated with tetrodotoxin (50 microg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye and infused with a mixture solution of norepinephrine and epinephrine (1:9) to maintain adequate systemic circulation under artificial ventilation. Fundus images were captured with a digital camera that was equipped with the special objective lens for small animals, and the diameters of retinal arterioles and venules were measured on a personal computer. Fundus blood flow was estimated using a laser Doppler flowmetry. Intravenous infusions of prostacyclin and prostaglandin E(2) dilated retinal blood vessels, increased fundus blood flow and decreased systemic blood pressure in a dose-dependent manner. The effects of vasodilatory prostaglandins on retinal arterioles were greater than those on retinal venules. Similarly, a prostanoid EP(2) receptor agonist (ONO-AE1-259-01) dilated retinal blood vessels, and increased fundus blood flow and decreased systemic blood pressure. However, a prostanoid EP(4) receptor agonist (ONO-AE1-329) failed to increase fundus blood flow, despite its comparable depressor response with those to vasodilatory prostaglandins and the prostanoid EP(2) receptor agonist. The responses to forskolin, an activator of adenylyl cyclase, were very similar to those to prostacyclin and the prostanoid EP(2) receptor agonist. These results suggest that prostacyclin and prostaglandin E(2) act as vasodilators in retinal and choroidal circulation, and prostanoid IP and EP(2) receptors play an important role in the regulation of ocular hemodynamics in rats.

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

PURPOSE

Purinergic compounds and cyclooxygenase inhibitors are involved in the tone regulation of isolated retinal arterioles in vitro, but it is unknown whether the perivascular retinal tissue influences these effects.

METHODS

Adenosine-and ATP-induced vasodilation of porcine retinal arterioles was studied in a wire myograph before and after removal of the perivascular tissue.

RESULTS

Both adenosine and ATP caused relaxation of the studied arterioles. This effect depended on the perivascular tissue and could be blocked by antagonists but was unaffected by ibuprofen.

CONCLUSIONS

The relaxation of porcine retinal arterioles induced by purinergic compounds is modulated by the perivascular retinal tissue.

Vasodilation of Retinal Arteriole Mediated by Corticotropin-Releasing Factor Receptor is Impaired in Streptozotocin-Induced Diabetic Rats

We investigated the vasodilator responses of retinal arterioles induced by stimulating corticotropin-releasing factor receptors in non-diabetic and diabetic rats. Male Wistar rats were treated with streptozotocin (65 mg/kg, i.v.) and experiments were performed 6-8 weeks later. Rats were treated with tetrodotoxin (50 mug/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye and infused with a mixture of norepinephrine and epinephrine to maintain adequate systemic circulation under artificial ventilation. Fundus images were captured with an original high-resolution digital fundus camera system. The vasodilator responses of retinal arterioles were assessed by measuring changes in diameters of retinal arterioles in response to urocortin and urocortin 2. Both urocortin (0.03-1.0 micromol/kg, i.v.) and urocortin 2 (0.1-3.0 micromol/kg, i.v.) increased diameters of retinal arterioles and decreased systemic blood pressure in a dose-dependent manner. The responses to urocortins were reduced in diabetic rats. These results suggest that urocortin and urocortin 2 play as vasodilators in retinal and peripheral resistance arterioles. The impairment of vasodilation mediated by the corticotropin-releasing factor receptors may contribute to the alteration of retinal and systemic circulation in the diabetic state.

Retinopetal axons in mammals: emphasis on histamine and serotonin

Since 1892, anatomical studies have demonstrated that the retinas of mammals, including humans, receive input from the brain via axons emerging from the optic nerve. There are only a small number of these retinopetal axons, but their branches in the inner retina are very extensive. More recently, the neurons in the brain stem that give rise to these axons have been localized, and their neurotransmitters have been identified. One set of retinopetal axons arises from perikarya in the posterior hypothalamus and uses histamine, and the other arises from perikarya in the dorsal raphe and uses serotonin. These serotonergic and histaminergic neurons are not specialized to supply the retina; rather, they are a subset of the neurons that project via collaterals to many other targets in the central nervous system, as well. They are components of the ascending arousal system, firing most rapidly when the animal is awake and active. The contributions of these retinopetal axons to vision may be predicted from the known effects of serotonin and histamine on retinal neurons. There is also evidence suggesting that retinopetal axons play a role in the etiology of retinal diseases.

Attenuation of nitric oxide- and prostaglandin-independent vasodilation of retinal arterioles induced by acetylcholine in streptozotocin-treated rats

Diabetes alters retinal hemodynamics, but little is known about the impact of diabetes on the role of endothelium-derived hyperpolarizing factor (EDHF) in the regulation of retinal circulation. Therefore, we examined how diabetes affects the nitric oxide- and prostaglandin-independent vasodilation of retinal arterioles induced by acetylcholine. Male Wistar rats were treated with streptozotocin (80 mg/kg, i.p.) and experiments were performed 6-8 weeks later. Under artificial ventilation, rats were treated with tetrodotoxin (100 microg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye. Methoxamine was used to maintain adequate systemic circulation. Fundus images were captured by a digital camera that was equipped with a special objective lens. The vasodilator responses of retinal arterioles were assessed by measuring changes in diameters of the vessels. In streptozotocin-induced diabetic rats and the age-matched controls, acetylcholine increased diameters of retinal arterioles in a dose-dependent manner. The vasodilator responses to acetylcholine in diabetic rats were smaller than those in control rats. The nitric oxide- and prostaglandin-independent vasodilation of retinal arterioles observed under treatment with combination of N(G)-nitro-l-arginine methyl ester (30 mg/kg, i.v.) and indomethacin (5 mg/kg, i.v.) were also attenuated by diabetes. Diabetes did not alter the dilator responses of retinal arterioles to sodium nitroprusside and forskolin. These results suggest that diabetes impairs EDHF-mediated vasodilation of retinal arterioles induced by acetylcholine. The impaired EDHF-mediated vasodilation may contribute to alteration of retinal hemodynamics in diabetes.

A method for chorioretinal oxygen tension measurement

PURPOSE

To report an optical imaging system that was developed to measure oxygen tension (pO2) in the chorioretinal vasculatures. The feasibility of the system for the measurement of changes in pO2 separately in the retinal and choroidal vasculatures was established in rat eyes by varying the fraction of inspired oxygen and inhibiting nitric oxide activity.

METHODS

Our optical section phosphorescence imaging system was modified to provide quantitative measurements of pO2 separately in the retinal and choroidal vasculatures. A narrow laser line was projected at an angle on the retina after intravenous injection of an oxygen-sensitive probe (Pd-porphyrin), and phosphorescence emission was imaged. A frequency-domain approach allowed measurements of the phosphorescence lifetime by varying the phase relationship between the modulated excitation laser light and sensitivity of the imaging camera. Chorioretinal pO2 was measured while varying the fraction of inspired oxygen and during intravenous infusion of Nomega-nitro-L-arginine (Nomega-NLA), a nonselective nitric oxide synthase inhibitor.

RESULTS

The systemic arterial pO2 varied according to the fraction of inspired oxygen. The pO2 in the retinal and choroidal vasculatures increased as the fraction of inspired oxygen was increased. Compared with baseline, choroidal pO2 decreased during infusion of Nomega-NLA, whereas the pO2 in the retinal vasculatures remained relatively unchanged. The choroidal pO2 decreased markedly with each incremental increase in Nomega-NLA infusion rate, in the range 1-6 mg/min, and there was no additional change in the choroidal pO2 at Nomega-NLA infusion rates above 6 mg/min.

CONCLUSIONS

An optical method combining pO2 phosphorescence imaging with chorioretinal optical sectioning was established that can potentially be applied for better understanding of retinal and choroidal oxygen dynamics in physiologic and pathologic states.

Effect of acidosis on isolated porcine retinal vessels

PURPOSE

To study the effect of normocapnic (NA) and hypercapnic acidosis (HA) on the tone, the intracellular calcium level ([Ca(2 +)](i)), and the membrane potential of smooth muscle cells in porcine retinal arterioles.

METHODS

Twenty-four porcine retinal arterioles were mounted in a wire myograph for isometric recording of the wall tension. The vessels were precontracted with 0.3 microM U46619 and were exposed to NA (pH = 7.0) and HA (pH = 7.0). Intracellular calcium was measured using the fluorophore Fura-2AM (n = 12). In six vessels, 0.1 mM NG-nitroarginine methyl ester (L-NAME) was added to block NO synthesis. The membrane potential of smooth muscles cells was measured in situ with sharp glass electrodes (n = 12).

RESULTS

NA and HA induced both a decrease in wall tension from 1.04 +/- 0.06 N/m to 0.65 +/- 0.1 N/m (p < 0.01) (NA) and 0.56 +/- 0.1 N/m (p < 0.01) (HA) and a decrease in [Ca(2 +)](i) as evidenced from the change in the Fura-2 fluorescence emission ratio from 0.66 +/- 0.03 to 0.57 +/- 0.05 (p = 0.005) (NA) and 0.56 +/- 0.05 (p = 0.002) (HA). These results were unaffected by inhibition of NO-synthesis. NA and HA also both induced hyperpolarization of the smooth muscle membrane from -18 +/- 0.7 mV during precontraction to -26 +/- 1.9 mV (p = 0.002) (NA) and -24 +/- 2.6 mV (p = 0.02) (HA).

CONCLUSIONS

Acidosis-induced relaxation of the tone in preconstricted isolated porcine retinal arterioles is associated with a decrease in intracellular calcium and a hyperpolarization of the smooth muscle cells. The acidosis-induced relaxation is independent of CO(2) and is not mediated through NO.

Serotonergic retinopetal axons in the monkey retina

PURPOSE

To describe serotonergic retinopetal axons in monkeys.

METHODS

Whole macaque and baboon retinas, fixed in 4% paraformaldehyde, were labeled with antisera raised against serotonin (5-HT).

RESULTS

Several large-diameter 5-HT-immunoreactive (IR) axons emerged from the optic disk. Most axons ran to the peripheral retina, where they branched extensively. Most terminated in the ganglion cell layer, but a few 5-HT-IR axons terminated in distal inner plexiform or within inner nuclear layer. Some axons branched extensively near the fovea, and a dense plexus of 5-HT-IR axons was also found around the optic disk. Varicose 5-HT-IR axons were also associated with blood vessels, especially in the central retina.

CONCLUSIONS

Immunoreactive serotonin is present in a distinct population of retinopetal axons in the monkey retina. Receptors for serotonin are present in the primate retinas, and based on physiological studies in other mammals, these retinopetal axons are expected to modulate neuronal activity and regulate blood flow.

Effects of melatonin on the nitric oxide treated retina

AIMS

Nitric oxide (NO) is a free radical which reportedly causes damage to living cells. This study evaluated the damaging effect of NO and the protection of melatonin on the retina in vivo.

METHODS

Female Wistar rats (230-250 g) received two intraperitoneal injections of either melatonin (5 mg/kg) or vehicle alone. After general anaesthesia, the animals received 1 microl intravitreal injections of 0.9% saline and 1 mM sodium nitroprusside (SNP) into the right eye and the left eye, respectively. The animals were divided into two groups and then sacrificed after 24 hours (day 1) and 96 hours (day 4). The mean inner retinal layer thickness (mIRLT), the number of retinas expressing hyperchromatic (HC) nuclei in the inner nuclear layer (INL) and the apoptotic ganglion cell detection were compared.

RESULTS

After 1 day, SNP significantly increased the mIRLT by 45% (p = 0.004), initiated more INL nuclear HC expression (p = 0.01) and apoptotic nuclei (p<0.05) compared with the control eyes. Injection of melatonin ameliorated these changes. On day 4, SNP demonstrated similar effects in all parameters on the retina. After the injection of melatonin, both INL HC expression and apoptotic ganglion nuclei in the SNP treated eyes were similar to the controls but the mIRLT was significantly greater than in controls (p = 0.006).

CONCLUSION

Uncontrolled NO elevation caused morphological and nuclear changes in the retina. Melatonin significantly suppressed the NO induced increase in mIRLT, INL HC expression, and apoptotic ganglion cells on day 1, but not after day 4. Melatonin may have a protective role in the NO elevated retina.

Characterization of vasomotion in porcine retinal arterioles

PURPOSE

To characterize vasomotion in porcine retinal arterioles in vitro using isobaric (pressure myograph) and isometric (wire myograph) methods.

METHODS

Pressure myograph: 208 small porcine retinal arterioles (outer diameter 68 +/- 4 microm) were studied under isobaric conditions in a double-barrelled pipette system. Diameter changes of the arterioles were registered by video recordings. Wire myograph: 60 large porcine retinal arterioles (inner diameter 147 +/- 1.6 microm) were studied under isometric conditions in a small vessel myograph for force measurements.

RESULTS

The rates of success in initiating vasomotion were 7.2% using the pressure myograph and 43% using the wire myograph (p < 0.001). The small vessels studied under isobaric conditions oscillated with a frequency of 0.014 Hz and the episodes lasted 6.0 +/- 1.0 min, whereas the large vessels under isometric conditions oscillated with a significantly faster frequency of 0.043 Hz and lasted 32.1 +/- 4.9 min (p = 0.026).

CONCLUSION

Retinal vasomotion can be studied in vitro using both pressure myograph and wire myograph techniques. The wire myograph is superior to the pressure myograph in initiating and maintaining vasomotion in vitro.