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

Prostaglandin E2 Exposure Disrupts E-Cadherin/Caveolin-1-Mediated Tumor Suppression to Favor Caveolin-1-Enhanced Migration, Invasion, and Metastasis in Melanoma Models

Caveolin-1 (CAV1) is a membrane-bound protein that suppresses tumor development yet also promotes metastasis. E-cadherin is important in CAV1-dependent tumor suppression and prevents CAV1-enhanced lung metastasis. Here, we used murine B16F10 and human A375 melanoma cells with low levels of endogenous CAV1 and E-cadherin to unravel how co-expression of E-cadherin modulates CAV1 function in vitro and in vivo in WT C57BL/6 or Rag-/- immunodeficient mice and how a pro-inflammatory environment generated by treating cells with prostaglandin E2 (PGE2) alters CAV1 function in the presence of E-cadherin. CAV1 expression augmented migration, invasion, and metastasis of melanoma cells, and these effects were abolished via transient co-expression of E-cadherin. Importantly, exposure of cells to PGE2 reverted the effects of E-cadherin expression and increased CAV1 phosphorylation on tyrosine-14 and metastasis. Moreover, PGE2 administration blocked the ability of the CAV1/E-cadherin complex to prevent tumor formation. Therefore, our results support the notion that PGE2 can override the tumor suppressor potential of the E-cadherin/CAV1 complex and that CAV1 released from the complex is phosphorylated on tyrosine-14 and promotes migration/invasion/metastasis. These observations provide direct evidence showing how a pro-inflammatory environment caused here via PGE2 administration can convert a potent tumor suppressor complex into a promoter of malignant cell behavior.

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Neuroaxonal and cellular damage/protection by prostanoid receptor ligands, fatty acid derivatives and associated enzyme inhibitors

Cellular and mitochondrial membrane phospholipids provide the substrate for synthesis and release of prostaglandins in response to certain chemical, mechanical, noxious and other stimuli. Prostaglandin D₂, prostaglandin E₂, prostaglandin F_2α, prostaglandin I₂ and thromboxane-A₂ interact with five major receptors (and their sub-types) to elicit specific downstream cellular and tissue actions. In general, prostaglandins have been associated with pain, inflammation, and edema when they are present at high local concentrations and involved on a chronic basis. However, in acute settings, certain endogenous and exogenous prostaglandins have beneficial effects ranging from mediating muscle contraction/relaxation, providing cellular protection, regulating sleep, and enhancing blood flow, to lowering intraocular pressure to prevent the development of glaucoma, a blinding disease. Several classes of prostaglandins are implicated (or are considered beneficial) in certain central nervous system dysfunctions (e.g., Alzheimer’s, Parkinson’s, and Huntington’s diseases; amyotrophic lateral sclerosis and multiple sclerosis; stroke, traumatic brain injuries and pain) and in ocular disorders (e.g., ocular hypertension and glaucoma; allergy and inflammation; edematous retinal disorders). This review endeavors to address the physiological/pathological roles of prostaglandins in the central nervous system and ocular function in health and disease, and provides insights towards the therapeutic utility of some prostaglandin agonists and antagonists, polyunsaturated fatty acids, and cyclooxygenase inhibitors.

Role of Prostaglandins in Nitric Oxide-Induced Glial Cell-Mediated Vasodilation in Rat Retina

We previously identified that NO derived from neuronal cells acts on glial cells and causes vasodilation in the healthy rat retina via the release of epoxyeicosatrienoic acids (EETs) and prostaglandins (PGs) by activation of the arachidonic acid cascade. However, it is not clear which PG types are involved in these responses. The aim of the present study was to identify prostanoid receptors involved in glial cell-derived vasodilation induced by NO in rat retina. Male Wistar rats were used to examine the effects of intravitreal pretreatment with indomethacin, a cyclooxygenase inhibitor; PF-04418948, a prostanoid EP₂ receptor antagonist; and CAY10441, a prostanoid IP receptor antagonist, on the changes in the retinal arteriolar diameter induced by intravitreal administration of NOR3, an NO donor. Retinal arteriolar diameters were measured using ocular fundus images captured with a high-resolution digital camera in vivo. The increase in the retinal arteriolar diameter induced by intravitreal injection of NOR3 was significantly suppressed by intravitreal pretreatment with indomethacin and PF-04418948, but not by CAY10441. The dose of PF-04418948 and CAY10441 injected intravitreally in the present study significantly reduced the increase in the retinal arteriolar diameter induced by prostaglandin E₂ (PGE₂) and prostaglandin I₂ (PGI₂), respectively. These results suggest that activation of the arachidonic acid cascade and subsequent stimulation of prostanoid EP₂ receptors are involved in rat retinal vasodilatory responses evoked by NO-induced glial cell stimulation. Therefore, glial cell-derived PGE₂, similar to EETs, may play an important role in retinal vasodilatory mechanisms.

Roles of EP Receptors in the Regulation of Fluid Balance and Blood Pressure

Prostaglandin E2 (PGE2) is an important prostanoid expressing throughout the kidney and cardiovascular system. Despite the diverse effects on fluid metabolism and blood pressure, PGE2 is implicated in sustaining volume and hemodynamics homeostasis. PGE2 works through four distinct E-prostanoid (EP) receptors which are G protein-coupled receptors. To date, pharmacological specific antagonists and agonists of all four subtypes of EP receptors and genetic targeting knockout mice for each subtype have helped in uncoupling the diverse functions of PGE2 and discriminating the respective characteristics of each receptor. In this review, we summarized the functions of individual EP receptor subtypes in the renal and blood vessels and the molecular mechanism of PGE2-induced fluid metabolism and blood pressure homeostasis.

Impairment of endothelium-dependent vasodilator function of retinal blood vessels in adult rats with a history of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a proliferative retinal vascular disease, initiated by delayed retinal vascular growth after premature birth. In the majority of cases, ROP resolves spontaneously; however, a history of ROP may increase the risk of long-term visual problems. In this study, we evaluated the endothelial function of retinal blood vessels in adult rats with a history of ROP. ROP was induced in rats by subcutaneous injection of a vascular endothelial growth factor receptor tyrosine kinase inhibitor (KRN633) on postnatal day (P) 7 and P8. On P56, vasodilator responses to acetylcholine, GSK1016790A (an activator of transient receptor potential vanilloid 4 channels), NOR3 (a nitric oxide [NO] donor), and salbutamol (a β2-adrenoceptor agonist) were assessed. Compared to age-matched controls, retinal vasodilator responses to acetylcholine and GSK1016790A were attenuated in P56 rats with a history of ROP. No attenuation of acetylcholine-induced retinal vasodilator response was observed under inhibition of NO synthase. Retinal vasodilator responses to NOR3 and salbutamol were unaffected. These results suggest that the production of and/or release of NO is impaired in retinal blood vessels in adult rats with a history of ROP. A history of ROP might increase the risk of impaired retinal circulation in adulthood.

Erlanger Glaucoma Registry: Effect of a Long-Term Therapy with Statins and Acetyl Salicylic Acid on Glaucoma Conversion and Progression

Purpose: Drugs with cardiovascular protective properties (statins, acetylsalicylic acid (ASS)) were assumed to have positive effects on patients suffering from glaucoma disease. The present retrospective study aimed to investigate the influence of statins, ASS or a combination of both on the glaucoma conversion and progression rate in glaucoma suspects and glaucoma patients with a 20-year follow-up period. Methods: A retrospective analysis of 199 eyes of 120 patients (63 male, 57 female) of the Erlanger Glaucoma Registry (EGR; ClinicalTrials.gov Identifier: NCT00494923; ISSN 2191-5008, CS-2011) was performed considering systemic therapy with statins, ASS or a combination of both: 107 eyes with ocular hypertension (OHT) and 92 eyes with pre-perimetric primary open-angle glaucoma (pre-POAG). All patients received an ophthalmological examination including morphometric and functional glaucoma diagnostics. Glaucoma conversion was defined as the conversion of OHT to pre-POAG. Glaucoma progression was defined as confirmed visual field loss. Data were shown as percentages. Statistical analysis was performed by Chi-Quadrat tests. Results: 1. Glaucoma conversion/progression was observed in 46.7% of the subjects, additionally in combination with hypercholesterinemia in 76.8%. 2. Statins: 27.3% of eyes under systemic statin therapy showed a conversion/progression. Patients taking statins ≥ 10 years yielded a reduced conversion/progression rate (p = 0.028, non-significant after Bonferroni-Holm). 3. ASS: 34.7% of eyes under systemic ASS therapy showed a conversion/progression. A significantly lower conversion/progression rate was observed after ASS therapy ≥ 12 years (p = 0.017, significant after Bonferroni-Holm). 4. ASS and statins: 25.0% of eyes under combined therapy showed a conversion/progression. A significantly reduced conversion/progression rate was reached after 8 years of combined therapy (p = 0.049, non-significant after Bonferroni-Holm). Conclusions: Patients with ocular hypertension and early glaucoma seem to benefit from adjuvant cardiovascular protective therapy. However, the benefits and disadvantages of treatment with statins and/or ASS should be kept in mind. Thus, a thorough risk-benefit evaluation has to be performed for each patient individually to avoid unwanted side effects.

L-Citrulline ameliorates the attenuation of acetylcholine-induced vasodilation of retinal arterioles in diabetic rats

In our previous study, we found that the vasodilation of retinal arterioles induced by acetylcholine and BMS-191011, a large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel opener, were diminished in diabetic rats. Currently, few agents ameliorate the impaired vasodilator responses of retinal blood vessels. Our recent finding that the intravenous infusion of L-citrulline dilated retinal arterioles, suggests that L-citrulline could be a potential therapeutic agent for circulatory disorders of the retina. In this study, we determined the effect of an oral L-citrulline treatment on impaired acetylcholine- and BMS-191011-induced vasodilation in the retinal arterioles of diabetic rats. To induce diabetes, rats were administered an intravenous dose of streptozotocin (65 mg/kg) and a 5% D-glucose solution as drinking water. The L-citrulline (2 g/kg/day) and L-arginine (2 g/kg/day) treatments commenced either 15 days before or just after the streptozotocin injection and continued throughout the experimental period. A 29-day treatment with L-citrulline, but not L-arginine, significantly ameliorated the impaired acetylcholine- and BMS-191011-induced retinal vasodilation in diabetic rats without affecting their plasma glucose levels. The 2-week L-citrulline treatment tended to ameliorate the dysfunction of the acetylcholine-induced retinal vasodilation in diabetic rats. In conclusion, these results showed that the retinal blood vessel dysfunction induced by diabetes mellitus could be prevented by the long-term administration of L-citrulline and suggest that the latter could play a potentially prophylactic role in diabetic retinopathy.

Role of Epoxyeicosatrienoic Acids in Acetylcholine-Induced Dilation of Rat Retinal Arterioles in Vivo

CYP epoxygenase-derived epoxyeicosatrienoic acids (EETs) contribute to endothelium-dependent hyperpolarization (EDH)-related dilation in multiple vascular beds. The present study aimed to determine the role of EETs in the acetylcholine (ACh)-induced dilation of retinal arterioles in rats in vivo. The vasodilator responses were assessed by determining the change in diameter of the retinal arterioles on images of the ocular fundus. The intravitreal injection of 17-octadecynoic acid (1.4 nmol/eye), an inhibitor of CYP epoxygenase, and 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EE-5(Z)-E; 2 nmol/eye), an antagonist of EETs, reduced the ACh (0.3-10 µg/kg/min)-induced dilation of the retinal arterioles. The EET antagonist attenuated the vasodilator response to ACh under blockade of nitric oxide (NO) synthases and cyclooxygenases with N^G-nitro-L-arginine methyl ester (30 mg/kg) plus indomethacin (5 mg/kg). Intravitreal injection of 14,15-EET (0.5 nmol/eye) dilated retinal arterioles and the response was prevented by iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels (20 pmol/eye). These results suggest that ACh stimulates the production of EETs, thereby dilating the retinal arterioles via activation of BK_Ca channels. CYP epoxygenase-derived EETs may be involved in the EDH-related component of the ACh-induced dilation of the retinal arterioles.

Evaluation of Patterns and Correlations of the Degree of Conjunctival Hyperemia Induced by Omidenepag Isopropyl 0.002% and Ripasudil 0.4

PURPOSE

To evaluate the pattern of conjunctival hyperemia induced by omidenepag isopropyl 0.002% and ripasudil 0.4%, and its correlation with the degree of hyperemia.

SUBJECTS AND METHODS

We previously reported the time course of conjunctival hyperemia induced by administering one drop of omidenepag isopropyl to one eye and one drop of ripasudil to the other eye in 34 healthy subjects (mean age: 29.7 years; 22 females, 12 males). We assessed the degree of hyperemia by slit-lamp photography of the frontal and temporal conjunctiva 0, 15, 30, 60, 120, 180, and 360 min after the administration of one drop of omidenepag isopropyl and ripasudil. The data were used to compare the frontal photographs before and at the peak of hyperemia according to the clinical hyperemia score (0-3) and classify the pattern of developing hyperemia due to both drugs. We also examined the correlation between the degree of hyperemia by comparing the images captured at the peak of hyperemia in both groups, using clinical hyperemia score and "percent coverage" of conjunctival hyperemia by using an automated hyperemia analysis software program; this program provides the pixel coverage of the conjunctival vessels in the region of interest.  Results: There were significant differences in the developmental pattern of hyperemia between omidenepag isopropyl-administered and ripasudil-administered eyes (P<0.001, χ2 test), with dilation of large blood vessels only (N=2 vs. 1, respectively), small blood vessels only (N=17 vs. 5), both large and small blood vessels (N=8 vs. 27), and no change (N=6 vs. 0). The degree of hyperemia between the two groups was positively correlated with the hyperemia score (rs=0.344, P=0.055) in the frontal conjunctival photographs and the percent coverage of conjunctival blood vessels (r=0.510, P=0.003) in the temporal conjunctival photographs.

CONCLUSIONS

The pattern of conjunctival hyperemia induced by omidenepag isopropyl predominantly involved small blood vessels, whereas that of ripasudil involved both large and small blood vessels. The eyes that were hyperemic with omidenepag isopropyl also tended to be hyperemic with ripasudil.

Time course of conjunctival hyperemia induced by omidenepag isopropyl ophthalmic solution 0.002%: a pilot, comparative study versus ripasudil 0.4

Objective

We investigated the detailed time course of conjunctival hyperemia induced by omidenepag isopropyl ophthalmic solution 0.002% (omidenepag), a selective prostaglandin E2 receptor 2 agonist.

Methods and analysis

We recruited 34 healthy subjects and administered omidenepag in the right eye and ripasudil 0.4% in the left eye. We evaluated conjunctival hyperemia using slit-lamp photography at baseline and after 15, 30, 60, 120, 180 and 360 min. The conjunctival hyperemia score was graded by three independent observers using a scale from 0 (none) to 3 (severe). We also evaluated conjunctival hyperemia by the pixel coverage of conjunctival blood vessels (per cent coverage) determined using a conjunctival hyperemia-analysing software.

Results

In omidenepag, the conjunctival hyperemia score and per cent coverage peaked at both 30 min (mean score±SD: 1.57±0.67 and 11.90%±3.66%, respectively) and then gradually decreased at 60 min (10.79%±3.32%) and 120 min (1.10±0.52) when they reached a level that was not significantly different from the baseline values. For ripasudil 0.4%, the peak time of the conjunctival hyperemia score and per cent coverage were both at 15 min (score: 2.42±0.54 and 15.26%±3.38%). The degree of conjunctival hyperemia was significantly higher for ripasudil 0.4% than that for omidenepag from 15 to 30 min in both the conjunctival hyperemia score and per cent coverage (p<0.007 by Bonferroni correction).

Conclusion

Conjunctival hyperemia induced by omidenepag gradually peaks to moderate severity, though weaker compared with that induced by ripasudil 0.4%, and subsides relatively quickly.

4-Aminopyridine, a Voltage-Gated K+ Channel Inhibitor, Attenuates Nitric Oxide-Mediated Vasodilation of Retinal Arterioles in Rats

Nitric oxide (NO) is an important regulator of the retinal blood flow. The present study aimed to determine the role of voltage-gated K⁺ (K_V) channels and ATP-sensitive K⁺ (K_ATP) channels in NO-mediated vasodilation of retinal arterioles in rats. In vivo, the retinal vasodilator responses were assessed by measuring changes in the diameter of retinal arterioles from ocular fundus images. Intravitreal injection of 4-aminopyridine (a K_V channel inhibitor), but not glibenclamide (a K_ATP channel blocker), significantly attenuated the retinal vasodilator response to the NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Intravitreal injection of indomethacin (a non-selective cyclooxygenase inhibitor) also reduced the NOR3-induced retinal vasodilator response. The combination of 4-aminopyridine and indomethacin produced a greater reduction in the NOR3-induced response than either agent alone. 4-Aminopyridine had no significant effect on pinacidil (a K_ATP channel opener)-induced response. These results suggest that the vasodilatory effects of NO are mediated, at least in part, through the activation of 4-aminopyridine-sensitive K_V channels in the retinal arterioles of rats. NO exerts its dilatory effect on the retinal vasculature of rats through at least two mechanisms, activation of the K_V channels and enhancement of prostaglandin production.

Activation of transient receptor potential vanilloid 4 channels dilates rat retinal arterioles through nitric oxide- and BKCa channel-dependent mechanisms in vivo

Transient receptor potential vanilloid 4 (TRPV4) channel, a cation channel expressed in nearly all cell types, plays an important role in the regulation of vascular tone. In the present study, we examined the effect of GSK1016790A, an activator of TRPV4 channels, on the diameter of retinal blood vessels in rats and the underlying mechanisms. Ocular fundus images were captured with an original high-resolution digital fundus camera in vivo and diameters of retinal blood vessels were measured. Intravenous infusion of GSK1016790A (0.2-2 μg kg^-1 min^-1) increased retinal arteriolar diameter in a dose-dependent manner. The higher dose of GSK1016790A (2 μg kg^-1 min^-1) slightly decreased blood pressure. These responses to GSK1016790A were significantly attenuated by intravenous injection of GSK2193874 (0.3 mg/kg), an antagonist of TRPV4 channels. Intravitreal injection of N^ω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide (NO) synthase or iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel, significantly attenuated the GSK1016790A-induced increases in retinal arteriolar diameter. These results suggest that activation of TRPV4 channels dilates rat retinal arterioles through NO- and BK_Ca channel-dependent mechanisms in vivo.

l-Citrulline ameliorates cerebral blood flow during cortical spreading depression in rats: Involvement of nitric oxide- and prostanoids-mediated pathway

l-Citrulline is a potent precursor of l-arginine, and exerts beneficial effect on cardiovascular system via nitric oxide (NO) production. Migraine is one of the most popular neurovascular disorder, and imbalance of cerebral blood flow (CBF) observed in cortical spreading depression (CSD) contributes to the mechanism of migraine aura. Here, we investigated the effect of l-citrulline on cardiovascular changes to KCl-induced CSD. in rats. Intravenous injection of l-citrulline prevented the decrease in CBF, monitored by laser Doppler flowmetry, without affecting mean arterial pressure and heart rate during CSD. Moreover, l-citrulline attenuated propagation velocity of CSD induced by KCl. The effect of l-citrulline on CBF change was prevented by l-NAME, an inhibitor of NO synthase, but not by indomethacin, an inhibitor of cyclooxygenase. On the other hand, attenuation effect of l-citrulline on CSD propagation velocity was prevented not only by l-NAME but also by indomethacin. In addition, propagation velocity of CSD was attenuated by intravenous injection of NOR3, a NO donor, which was diminished by ODQ, an inhibitor of soluble guanylyl cyclase. These results suggest that NO/cyclic GMP- and prostanoids-mediated pathway differently contribute to the effect of l-citrulline on the maintenance of CBF.

Anti-diabetic drug metformin dilates retinal blood vessels through activation of AMP-activated protein kinase in rats

The aim of this study was to examine whether metformin, a biguanide anti-hyperglycemic drug, dilates retinal blood vessels in rats. Ocular fundus images were captured with an original high-resolution digital fundus camera in vivo and diameters of retinal blood vessels were measured. Both systemic blood pressure and heart rate were continuously recorded. Metformin (0.01-0.3mg/kg/min) increased diameters of retinal blood vessels in a dose-dependent manner. This retinal vasodilator effect of metformin was abolished by compound C, an inhibitor of AMP-activated protein kinase (AMPK), and N^G-nitro-L-arginine methyl ester, an inhibitor of nitric oxide (NO) synthase. Similar results were obtained with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleoside (AICAR, 0.01-1mg/kg/min). Neither metformin nor AICAR exerted significant effect on mean blood pressure and heart rate. However, a significant pressor response to AICAR was observed upon inhibition of NO synthase. These results suggest that metformin dilates retinal blood vessels through activation of AMPK, and NO plays an important role in the retinal vasodilator response following AMPK activation.

Vasodilator Effects of Elcatonin, a Synthetic Eel Calcitonin, on Retinal Blood Vessels in Rats

The aim of this study was to examine the effects of elcatonin, a synthetic derivative of eel calcitonin, on rat retinal blood vessels, and to determine how diabetes affects the retinal vascular responses. Ocular fundus images were captured with an original high-resolution digital fundus camera in vivo. The retinal vascular responses were evaluated by measuring the diameter of retinal blood vessels contained in the digital images. Both systemic blood pressure and heart rate were continuously recorded. Elcatonin increased the diameter of retinal blood vessels but decreased mean blood pressure in a dose-dependent manner, whereas it had no significant effect on heart rate. A diminished retinal vasodilator response and significant pressor response to elcatonin were observed in rats injected intravenously with N(G)-nitro-L-arginine methyl ester, a nitric oxide (NO) synthase inhibitor. Intravitreal injection of indomethacin, a non-selective cyclooxygenase (COX) inhibitor, and SQ22536, an adenylyl cyclase inhibitor, markedly attenuated the vasodilator effects of elcatonin on retinal blood vessels. The retinal vasodilator responses to elcatonin were unaffected 2 weeks after the induction of diabetes by a combination of streptozotocin treatment and D-glucose feeding. These results suggest that elcatonin dilates rat retinal blood vessels via NO- and COX-dependent mechanisms and that the adenylyl cyclase-adenosine 3',5'-cyclic monophosphate system plays a major role in the vasodilator mechanisms. The retinal vasodilatory effects of elcatonin seem to be preserved at early stages of diabetes.

The Gatekeepers in the Mouse Ophthalmic Artery: Endothelium-Dependent Mechanisms of Cholinergic Vasodilation

Cholinergic regulation of arterial luminal diameter involves intricate network of intercellular communication between the endothelial and smooth muscle cells that is highly dependent on the molecular mediators released by the endothelium. Albeit the well-recognized contribution of nitric oxide (NO) towards vasodilation, the identity of compensatory mechanisms that maintain vasomotor tone when NO synthesis is deranged remain largely unknown in the ophthalmic artery. This is the first study to identify the vasodilatory signalling mechanisms of the ophthalmic artery employing wild type mice. Acetylcholine (ACh)-induced vasodilation was only partially attenuated when NO synthesis was inhibited. Intriguingly, the combined blocking of cytochrome P₄₅₀ oxygenase (CYP450) and lipoxygenase (LOX), as well as CYP450 and gap junctions, abolished vasodilation; demonstrating that the key compensatory mechanisms comprise arachidonic acid metabolites which, work in concert with gap junctions for downstream signal transmission. Furthermore, the voltage-gated potassium ion channel, K_v1.6, was functionally relevant in mediating vasodilation. Its localization was found exclusively in the smooth muscle. In conclusion, ACh-induced vasodilation of mouse ophthalmic artery is mediated in part by NO and predominantly via arachidonic acid metabolites, with active involvement of gap junctions. Particularly, the K_v1.6 channel represents an attractive therapeutic target in ophthalmopathologies when NO synthesis is compromised.

4-Hydroxy-2-nonenal attenuates β2-adrenoceptor-mediated vasodilation of rat retinal arterioles

4-Hydroxy-2-nonenal (4-HNE) is a major reactive aldehyde formed by lipid peroxidation, and it plays an important role in the pathogenesis of several vascular diseases, including diabetes mellitus. In this study, we examined the effects of 4-HNE on the vasodilatory mechanisms of rat retinal arterioles. The retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles in the fundus images. Intravitreal injection of 4-HNE significantly prevented the vasodilation of retinal arterioles induced by the β2-adrenoceptor agonist salbutamol but not the nitric oxide donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Iberiotoxin, an inhibitor of large-conductance KCa (BKCa) channels, significantly reduced the salbutamol-induced vasodilation of retinal arterioles. The vasodilator effect of BMS-191011, a BKCa channel opener, on retinal arterioles was significantly attenuated by 4-HNE. These results suggest that 4-HNE attenuates retinal vasodilator responses to β2-adrenoceptor agonists through the impairment of the BKCa pathway. The direct effect of 4-HNE on retinal blood vessels may, therefore, contribute to the retinal vascular dysfunction observed in patients with diabetes mellitus.

Activation of the TRPV1 channel attenuates N-methyl-D-aspartic acid-induced neuronal injury in the rat retina

Capsaicin, a transient receptor potential vanilloid type1 (TRPV1) agonist, has been reported to protect against ischemia-reperfusion injury in various organs, including the brain, heart, and kidney, whereas activation of TRPV1 was also reported to contribute to neurodegeneration, including pressure-induced retinal ganglion cell death in vitro. We histologically investigated the effects of capsaicin and SA13353, TRPV1 agonists, on retinal injury induced by intravitreal N-methyl-d-aspartic acid (NMDA; 200 nmol/eye) in rats in vivo. Under ketamine/xylazine anesthesia, male Sprague-Dawley rats were subjected to intravitreal NMDA injection. Capsaicin (5.0 nmol/eye) was intravitreally admianeously with NMDA injection. SA13353 (10mg/kg) was intraperitoneally administered 15 min before NMDA injection. Morphometric evaluation at 7 days after NMDA injection showed that intravitreal NMDA injection resulted in ganglion cell loss. Capsaicin and SA13353 almost completely prevented this damage. Treatment with capsazepine (TRPV1 antagonist, 0.5 nmol/eye), CGRP (8-37) (calcitonin gene-related peptide (CGRP) receptor antagonist, 0.5 pmol/eye), or RP67580 (tachykinin NK1 receptor antagonist, 0.5 nmol/eye) almost completely negated the protective effect of capsaicin in the NMDA-injected rats. Seven days after intravitreal NMDA injection, the cell number of retinal ganglion cell was significantly smaller than in the eye that had received capsaicin in B6.Cg-TgN(Thy1-CFP)23Jrs/J transgenic mice that express the enhanced cyan fluorescent protein in retinal ganglion cells in the retina. These results suggested that activation of TRPV1 protects retinal neurons from the injury induced by intravitreal NMDA in rats in vivo. Activation of CGRP and tachykinin NK1 receptors is possibly involved in underlying protective mechanisms.

Prostanoid receptor EP2 as a therapeutic target

Cycoloxygenase-2 (COX-2) induction is prevalent in a variety of (brain and peripheral) injury models where COX-2 levels correlate with disease progression. Thus, COX-2 has been widely explored for anti-inflammatory therapy with COX-2 inhibitors, which proved to be effective in reducing the pain and inflammation in patients with arthritis and menstrual cramps, but they have not provided any benefit to patients with chronic inflammatory neurodegenerative disease. Recently, two COX-2 drugs, rofecoxib and valdecoxib, were withdrawn from the United States market due to cardiovascular side effects. Thus, future anti-inflammatory therapy could be targeted through a specific prostanoid receptor downstream of COX-2. The PGE2 receptor EP2 is emerging as a pro-inflammatory target in a variety of CNS and peripheral diseases. Here we highlight the latest developments on the role of EP2 in diseases, mechanism of activation, and small molecule discovery targeted either to enhance or to block the function of this receptor.

[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.

Dual effect of prostaglandins on isolated intraocular porcine ciliary arteries

PURPOSE

Prostaglandin analogues are used to reduce the intraocular pressure (IOP) in glaucoma, but also affect the tone of the arteries supplying the ciliary body. Previously, the effect of prostaglandins has been studied on the extraocular ciliary arteries, whereas the effect on intraocular ciliary arteries has not been studied in detail.

METHODS

Intraocular long posterior porcine ciliary arteries were isolated and mounted in a myograph system for isometric tension recording, and the effects of PGF2α , the prostaglandin analogue latanoprost, PGD2 , PGE2 , PGI2 and the thromboxane analogue U46619 were studied in the presence and absence of selective receptor antagonists.

RESULTS

The prostaglandins PGD2 and PGE2 induced relaxation at low concentrations (10(-9) - 3 × 10(-7) m), which could be inhibited by blocking either the DP or the EP4 receptor, whereas PGD2 , PGE2 , PGF2α , latanoprost and U46619 induced contraction at high concentrations (10(-6) - 10(-5) m), which could be inhibited by blocking the TP receptor. Additionally, blocking of the FP receptor induced a right-shift of latanoprost-induced contraction.

CONCLUSIONS

Prostaglandins with affinity to DP1 and EP4 receptors induce relaxation at low concentrations, and prostaglandins with affinity to TP and FP receptors induce contraction at high concentrations of intraocular porcine ciliary vessels in vitro. The findings may contribute to understanding the regulation of blood flow to the ciliary body.

Retinal toxicity associated with high dose of meclofenamic acid

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used medications because they provide effective relief of chronic pain and inflammation through inhibition of cyclooxygenase (COX). However, visual side effects also have been reported, such as temporary blindness, visual field defect, blurred vision, scotomata, and color vision changes subsequent to short- or long-term use. Our aim was to investigate the effect of a high dose of meclofenamic acid (MFA) on the retina. In our study, we applied electroretinography (ERG) and histologic examination to study functional and morphological damage of the retina in rabbits after MFA treatment. We reveal that MFA markedly decreased the amplitudes of b-wave of Rod-response and a- and b-wave of the scotopic standard combined ERG, respectively, and induced morphological destruction of the retina, especially photoreceptor cells. We conclude that a high dose of MFA causes retinal toxicity and impairs visual transduction. These findings may explain, at least partially, the vision problems of certain clinically used NSAIDs.

Dietary fish oil reduces glomerular injury and elevated renal hydroxyeicosatetraenoic acid levels in the JCR:LA-cp rat, a model of the metabolic syndrome

We have previously shown nutritional intervention with fish oil (n-3 PUFA) to reduce numerous complications associated with the metabolic syndrome (MetS) in the JCR:LA-corpulent (cp) rat. In the present study, we sought to explore the potential role of fish oil to prevent glomerulosclerosis in JCR:LA-cp rats via renal eicosanoid metabolism and lipidomic analysis. Male lean and MetS JCR:LA-cp rats were fed a lipid-balanced diet supplemented with fish oil (5 or 10 % of total fat). After 16 weeks of feeding, albuminuria was significantly reduced in MetS rats supplemented with 5 or 10 % fish oil ( − 53 and − 70 %, respectively, compared with the untreated MetS rats). The 5 % fish oil diet resulted in markedly lower glomerulosclerosis ( − 43 %) in MetS rats and to a lesser extent in those supplemented with 10 % fish oil. Interestingly, untreated MetS rats had higher levels of 11- and 12-hydroxyeicosatetraenoic acids (HETE) v. lean rats. Dietary fish oil reduced these levels, as well as other (5-, 9- and 15-) HETE. Whilst genotype did not alter prostanoid levels, fish oil reduced endogenous renal levels of 6-keto PGF1α (PGI2 metabolite), thromboxane B2 (TxB2), PGF2α and PGD2 by approximately 60 % in rats fed 10 % fish oil, and TxB2 ( − 50 %) and PGF2α ( − 41 %) in rats fed 5 % fish oil. In conclusion, dietary fish oil prevented glomerular damage in MetS rats and mitigated the elevation in renal HETE levels. These results suggest a potential role for dietary fish oil to improve dysfunctional renal eicosanoid metabolism associated with kidney damage during conditions of the MetS.

Roles of prostaglandin E2 in cardiovascular diseases

Prostaglandin E2 (PGE(2)) is produced in inflammatory responses and regulates a variety of immunological reactions through 4 different receptor subtypes; EP1, 2, 3 and 4. However, the precise role of each receptor in cardiovascular disease has not yet been elucidated. Enhanced expression of some EPs has been observed in clinical and experimental cardiovascular diseases. EP agonists have been developed to clarify the role of each receptor. Recently, we developed a novel selective agonist to examine the effects of EP4 on cardiac transplantation, myocardial ischemia, and myocarditis. Of note, a selective EP4 agonist attenuated inflammatory cytokines and chemokines via attenuation of macrophage activation in inflammatory heart diseases. In this review article, we discuss the effects of PGE(2) receptor agonists on the development of cardiovascular diseases.

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.

Vasodilation of retinal arterioles induced by activation of BKCa channels is attenuated in diabetic rats

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the retinal vascular tone, but question of whether the impairment of the channel function contributes to abnormalities of retinal circulation has not yet been completely elucidated. The purpose of this study was to examine effects of diabetes on the vasodilation induced by activation of BK(Ca) channels. Male Wistar rats were treated with streptozotocin and experiments were performed 2 weeks later. The streptozotocin-treated animals were given drinking water containing 5% d-glucose to shorten the term in the development of retinal vascular dysfunction. The retinal vascular responses were assessed by measuring diameter of retinal arterioles in the fundus images that were captured with an original fundus camera system. In non-diabetic rats, vasodilator effects of acetylcholine on retinal arterioles were significantly reduced by iberiotoxin, an inhibitor of BK(Ca) channels. However, the inhibitory effect of iberiotoxin was not observed in diabetic rats, and the responses to the BK(Ca) channel opener BMS-191011 were almost completely abolished. The retinal vasodilator response to acetylcholine, possibly an endothelium-derived hyperpolarizing factor-mediated response, observed after treatment with N(G)-nitro-l-arginine methyl ester and indomethacin was markedly reduced in diabetic rats. The responses to pinacidil, an opener of ATP-sensitive K(+) channels, were unchanged. These results suggest that the retinal vasodilator response mediated through mechanisms involving activation of BK(Ca) channels is diminished at the early stage of diabetes in rats. The impairment of BK(Ca) channel function may contribute to abnormal retinal hemodynamics in diabetes and consequently play an important role in the pathogenesis of diabetic retinopathy.

Prostaglandin E2 modulation of blood pressure homeostasis: studies in rodent models

Hypertension is a well established risk factor for cardiovascular diseases such as stroke and is the leading cause of chronic kidney failure. Although a number of pharmacologic agents are available for the treatment of hypertension including agents that affect the renin-angiotensin-aldosterone system (RAAS), unmet needs in the treatment of hypertension suggest that identification of novel pharmacological targets would be an important healthcare goal. One potential target is prostaglandin E(2) (PGE(2)), a potent lipid mediator with a diverse and sometimes opposing range of biological effects. PGE(2) signals through four subtypes of G-protein coupled receptors designated EP1 through EP4. PGE(2) functions primarily as a vasodepressor; under certain conditions PGE(2) administration mediates vasopressor activity. This review focuses on the current understanding of the roles of PGE(2) receptors in vascular reactivity, hypertension and end-organ damage.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

BMS-191011, an opener of large-conductance Ca2+-activated potassium channels, dilates rat retinal arterioles in vivo

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate vascular smooth muscle tone but the role of BK(Ca) channels in regulation of retinal circulation remains unclear. In the present study, we examined the effects of BMS-191011 and NS 1619, openers of BK(Ca) channels, on rat retinal blood vessels in vivo. Male Wistar rats (8- to 10-week-old) were anesthetized with pentobarbital sodium (50 mg/kg, intraperitoneally (i.p.)) and treated with tetrodotoxin (50 µg/kg, intravenously (i.v.)) to eliminate any nerve activity and prevent movement of the eye under artificial ventilation. A mixture solution of adrenaline and noradrenaline (9:1) was infused to maintain adequate systemic circulation. BMS-191011 (10-100 µg/kg, i.v.) and NS 1619 (0.1-1.0 µg/kg, i.v.) increased the diameter of retinal arterioles without altering systemic blood pressure and heart rate significantly. The vasodilator responses to BMS-191011, but not to NS 1619, were significantly diminished by intravitreal injection of iberiotoxin (an inhibitor of BK(Ca) channels, 20 pmol/eye). These results suggest that BMS-191011 dilates rat retinal arterioles through activation of iberiotoxin-sensitive BK(Ca) channels in vivo. The BK(Ca) channel opener could be considered as a candidate for improving retinal circulation without severe cardiovascular side-effects.

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.

Role of calcium-activated potassium channels in acetylcholine-induced vasodilation of rat retinal arterioles in vivo

The vascular endothelium plays an important role in regulating retinal blood flow via actions of several vasodilators, including nitric oxide (NO), prostaglandin I₂, and an endothelium-derived hyperpolarizing factor (EDHF). Our previous in vivo studies demonstrated that acetylcholine (ACh) dilates the rat retinal arteriole partly through NO- and prostaglandin-independent pathway, possibly the EDHF-mediated pathway, but the underlying mechanism(s) remains to be elucidated. It has been suggested that activation of Ca²+-activated K+ (K(Ca)) channels contributes to the EDHF-mediated responses; therefore, the roles of K(Ca) channels in ACh-induced vasodilation of retinal arterioles were examined in rats. The retinal vascular responses were assessed by determining changes in diameters of retinal arterioles in ocular fundus images that were captured with an original fundus camera system. Intravitreal injection of charybdotoxin, an inhibitor of intermediate- and large-conductance K(Ca) (I/BK(Ca)) channels, or iberiotoxin, an inhibitor of large-conductance K(Ca) (BK(Ca)) channels, significantly reduced the ACh-induced vasodilation of retinal arterioles, whereas neither apamin, an inhibitor of small-conductance K(Ca) (SK(Ca)) channels, nor TRAM-34, an inhibitor of intermediate-conductance K(Ca) (IK(Ca)) channels, altered the response. The vasodilator response to ACh observed under the combined blockade of NO synthase and cyclooxygenase with N(G)-nitro-L-arginine methyl ester plus indomethacin was also diminished by iberiotoxin. Iberiotoxin did not affect the NO donor NOR3-induced vasodilation of retinal arterioles, whereas it significantly reduced the BK(Ca) channel opener BMS-191011-induced responses. These results suggest that activation of BK(Ca) channels is involved in the EDHF-mediated component of the vasodilator response to ACh in the rat retinal arterioles in vivo.

Microangiopathy and visual deficits characterize the retinopathy of a spontaneously hypertensive rat model with type 2 diabetes and metabolic syndrome

Retinopathy has been increasing in prevalence as a consequence of type 2 diabetes and a cluster of coexisting risk factors characterized as the metabolic syndrome. However, the combined effects of these conditions on the retina are poorly understood. Therefore, we focused on the spontaneously hypertensive corpulent rat (SHR/N-cp), a model with type 2 diabetes, obesity and features of the metabolic syndrome to characterize retinal changes at a structural and functional level. SHR/N-cp males at 4 and 8 months of age were used in this study. Metabolic parameters and blood pressure were measured by standard methods. Morphology was investigated by histological techniques supplemented by nicotinamide adenine dinucleotide phosphate-diaphorase staining of whole mounts and fluorescein angiography to analyze the retinal vasculature. The in vivo function of the retina was examined by electroretinography (ERG). Obese SHR/N-cp rats were hypertensive and showed significant increases in body weight, serum levels of glucose, triglycerides, total cholesterol and urinary glucose excretion compared with lean controls (P < 0.01 for each). Histology indicated an overall intact integrity of the retina and aspects of microangiopathy in obese SHR/N-cp rats. ERG revealed intact processing of light signals but significantly decreased amplitudes of b-waves for all (P < 0.01) and of a-waves for some examined light intensities (P < 0.05). Oscillatory potentials were significantly protracted (P < 0.01), whereas amplitudes were not reduced. Microangiopathy and electroretinographic deficits combine to produce an early non-proliferative retinopathy phenotype in the obese SHR/N-cp rats. Thus, this model represents a valuable experimental tool to obtain further insights into the mechanisms of retinopathy in the context of obesity, type 2 diabetes and metabolic syndrome.

Nitric oxide dilates rat retinal blood vessels by cyclooxygenase-dependent mechanisms

It has been suggested that nitric oxide (NO) stimulates the cyclooxygenase (COX)-dependent mechanisms in the ocular vasculature; however, the importance of the pathway in regulating retinal circulation in vivo remains to be elucidated. Therefore, we investigated the role of COX-dependent mechanisms in NO-induced vasodilation of retinal blood vessels in thiobutabarbital-anesthetized rats with and without neuronal blockade (tetrodotoxin treatment). Fundus images were captured with a digital camera that was equipped with a special objective lens. The retinal vascular response was assessed by measuring changes in diameter of the retinal blood vessel. The localization of COX and soluble guanylyl cyclase in rat retina was examined using immunohistochemistry. The NO donors (sodium nitroprusside and NOR3) increased the diameter of the retinal blood vessels but decreased systemic blood pressure in a dose-dependent manner. Treatment of rats with indomethacin, a nonselective COX inhibitor, or SC-560, a selective COX-1 inhibitor, markedly attenuated the vasodilation of retinal arterioles, but not the depressor response, to the NO donors. However, both the vascular responses to NO donors were unaffected by the selective COX-2 inhibitors NS-398 and nimesulide. Indomethacin did not change the retinal vascular and depressor responses to hydralazine, 8-(4-chlorophenylthio)-guanosine-3′, 5′-cyclic monophosphate (a membrane-permeable cGMP analog) and 8-(4-chlorophenylthio)-adenosine-3′, 5′-cyclic monophosphate (a membrane-permeable cAMP analog). Treatment with SQ 22536, an adenylyl cyclase inhibitor, but not ODQ, a soluble guanylyl cyclase inhibitor, significantly attenuated the NOR3-induced vasodilation of retinal arterioles. The COX-1 immunoreactivity was found in retinal blood vessels. The retinal blood vessel was faintly stained for soluble guanylyl cyclase, although the apparent immunoreactivities on mesenteric and choroidal blood vessels were observed. These results suggest that NO exerts a substantial part of its dilatory effect via a mechanism that involves COX-1-dependent pathway in rat retinal vasculature.

Loss of retinal capillary vasoconstrictor response to Endothelin-1 following pressure increments in living isolated rat retinas

Increased intraocular pressure (IOP) is a major risk factor for glaucoma, and its contribution to neuronal damage appears multi-factorial. An open issue is whether pressure effects on blood vessels contribute to neuronal damage. In particular, little is known about pressure effects on capillaries, which are the site of most metabolic exchange in the retina, but cannot be easily visualized in vivo. To address this issue, here we have imaged retinal capillaries in acutely isolated living rat retinas, and measured alterations in capillary viability, caliber and response to vasoactive stimuli after controlled pressure stimuli. We found that capillary viability, diameter and response to vasodilator stimulation are not affected after pressure increments; yet, a prolonged lack of capillary response to the vasoconstrictor Endothelin-1 (Et-1) is observed. Considering that Et-1 is a major component of the endogenous control of retinal blood flow the present data lead to the hypothesis that prolonged or repeated IOP elevation could induce capillary disregulation contributing to neuronal damage over time.