Nitric oxide in retinal and choroidal blood flow autoregulation in newborn pigs: interactions with prostaglandins

Choroidal measurements in decision making for retinopathy of prematurity: a decision tree analysis

BACKGROUND

To compare the choroidal thickness and vascular profile of premature infants with ROP (retinopathy of prematurity) using a handheld SD-OCT device.

METHODS

We performed horizontal SD-OCT scans through the fovea in 115 eyes of 66 premature infants. Premature infants included 2 groups [infants with ROP requiring treatment (as treatment group) vs. infants without ROP or with ROP not- requiring treatment (as no-treatment group)] Choroidal thicknesses (CT) were measured at 5 points, including the fovea, 250 µm, and 500 µm mm nasal and temporal to the fovea. The choroidal vascularity index (CVI) and choroidal stromal index (CSI) were also calculated. The classification and regression tree (CRT) algorithm was used to predict the need for treatment based on all OCT characteristics.

RESULTS

Mean CT was higher in 500 µm nasal to the fovea compared to temporal CT (275.8 ± 64.8 and 257.1 ± 57.07, P value < 0.03). No statistically significant difference was found regarding CVI, corrected CVI, and temporal and nasal CT in the treatment group versus the no-treatment group. The foveal CT was significantly lower in ROP patients with the plus disease compared to not-plus ROP (P value = 0.03. ANOVA, Bonferroni posthoc test). CT was not significantly different between plus and pre-plus patients (P-value = 0.9, ANOVA, Bonferroni posthoc test). No significant relationship was found between the stage of ROP and choroidal thickness (P value > 0.05, GEE). The decision tree analysis showed that in infants with ROP, the most important predictor for the need for treatment is CSI.

CONCLUSION

This study delineated the possible effectiveness of choroidal measurements as an additive to decision-making for ROP. We also demonstrated that choroidal involution is associated with the presence of plus disease, not with the stage of ROP. We demonstrated that choroidal measurements are very sensitive but not specific tools for assessing the need for treatment in ROP patients.

Retinopathy of prematurity: A review of pathophysiology and signaling pathways

Retinopathy of prematurity (ROP) is a vasoproliferative disorder of the retina and a leading cause of visual impairment and childhood blindness worldwide. The disease is characterized by an early stage of retinal microvascular degeneration, followed by neovascularization that can lead to subsequent retinal detachment and permanent visual loss. Several factors play a key role during the different pathological stages of the disease. Oxidative and nitrosative stress and inflammatory processes are important contributors to the early stage of ROP. Nitric oxide synthase and arginase play important roles in ischemia/reperfusion-induced neurovascular degeneration. Destructive neovascularization is driven by mediators of the hypoxia-inducible factor pathway, such as vascular endothelial growth factor and metabolic factors (succinate). The extracellular matrix is involved in hypoxia-induced retinal neovascularization. Vasorepulsive molecules (semaphorin 3A) intervene preventing the revascularization of the avascular zone. This review focuses on current concepts about signaling pathways and their mediators, involved in the pathogenesis of ROP, highlighting new potentially preventive and therapeutic modalities. A better understanding of the intricate molecular mechanisms underlying the pathogenesis of ROP should allow the development of more effective and targeted therapeutic agents to reduce aberrant vasoproliferation and facilitate physiological retinal vascular development.

Retinal vessel architecture in retinopathy of prematurity and healthy controls using swept-source optical coherence tomography angiography

PURPOSE

To determine microvascular changes in children with a history of retinopathy of prematurity (ROP) and in a control group of full-term children.

METHODS

In a cross-sectional study, 30 eyes of 15 children aged 6-8 years with a history of ROP were evaluated with swept-source optical coherence tomography angiography (SS-OCTA). Twenty-eight eyes of 22 age-matched full-term children served as a healthy control group. The foveal avascular zone (FAZ), vessel density (VD) and choroidal vascular flow area (VFA) were evaluated on OCTA and correlated with central retinal thickness (CRT), visual acuity (VA), birth weight (BW), gestational age (GA) and ROP stages.

RESULTS

Twenty-two eyes of 14 children with a history of ROP (stage 1-3) and 25 eyes of 19 full-term children were available for evaluation. In the ROP group, the gestational age was 27 ± 2 weeks and birth weight was 781 ± 164 g. In the ROP group, CRT was higher in the central ETDRS segment (mean difference [95% CI]: 32.8 µm [18.7; 47.0], p = 0.0002) compared to the controls. Smaller mean FAZ area (-0.12 [-0.19; -0.04], p = 0.004) and perimeter (-662 [-1228; -96], p = 0.03) was found in comparison to the control group. An oval shape of the FAZ was observed among patients with a history of ROP. The mean central VD of the superficial plexus was 28 ± 8/23 ± 8% and of the deep plexus 7 ± 7/3 ± 5% (ROP group/control group; p > 0.05). No statistically significant difference was found regarding the choroidal VFA. Only weak correlation of FAZ and VD with function was observed.

CONCLUSIONS

Swept-source optical coherence tomography angiography imaging revealed significant microvascular anomalies in children with a history of ROP indicating disturbance of early morphological development of the central retina.

Biomarkers for central serous chorioretinopathy

Central serous chorioretinopathy (CSCR) is a common chorioretinal disease characterized by serous retinal detachment that most commonly involves the macular region. Although the natural history of the acute form shows a self-limiting course, a significant number of patients suffer from recurrent episodes leading to chronic disease, often leaving patients with residual visual impairment. Visual morbidity is often worsened by a delay in the diagnosis due to the incorrect understanding of the particular biomarkers of the disease. The aim of this review is to provide clinical understanding of the biomarkers of CSCR with an emphasis on the most recent findings in patient demographics, risk factors, clinical imaging findings, and management options. Patients with these biomarkers, age 30–44 years, male gender, increased stress levels, hypercortisolism (endogenous and exogenous exposures), sleep disturbance, pregnancy, and genetic predisposition have increased susceptibility to CSCR. Also, biomarkers on optical coherence tomography (OCT) such as choroidal thickness (CT) and choroidal vascularity index (CVI) showed good diagnostic and prognostic significance in the management of CSCR. There are nonspecific features of CSCR on OCT and OCT angiography such as choroidal neovascularization, photoreceptor alteration/cone density loss, and flat irregular pigment epithelium detachment. We described rare complications of CSCR such as cystoid macular edema (CME) and cystoid macular degeneration (CMD). Patients with CME recovered some vision when treated with anti-vascular endothelial growth factors (anti-VEGFs). Patients with CMD had irreversible macular damage even after treatment with anti-VEGFs.

Retinopathy of prematurity: inflammation, choroidal degeneration, and novel promising therapeutic strategies

Retinopathy of prematurity (ROP) is an important cause of childhood blindness globally, and the incidence is rising. The disease is characterized by initial arrested retinal vascularization followed by neovascularization and ensuing retinal detachment causing permanent visual loss. Although neovascularization can be effectively treated via retinal laser ablation, it is unknown which children are at risk of entering this vision-threatening phase of the disease. Laser ablation may itself induce visual field deficits, and there is therefore a need to identify targets for novel and less destructive treatments of ROP. Inflammation is considered a key contributor to the pathogenesis of ROP. A large proportion of preterm infants with ROP will have residual visual loss linked to loss of photoreceptor (PR) and the integrity of the retinal pigment epithelium (RPE) in the macular region. Recent studies using animal models of ROP suggest that choroidal degeneration may be associated with a loss of integrity of the outer retina, a phenomenon so far largely undescribed in ROP pathogenesis. In this review, we highlight inflammatory and neuron-derived factors related to ROP progression, as well, potential targets for new treatment strategies. We also introduce choroidal degeneration as a significant cause of residual visual loss following ROP. We propose that ROP should no longer be considered an inner retinal vasculopathy only, but also a disease of choroidal degeneration affecting both retinal pigment epithelium and photoreceptor integrity.

CHOROIDAL THICKNESS IN INFANTS WITH RETINOPATHY OF PREMATURITY

PURPOSE

To evaluate choroidal thickness in premature infants and its relationship with stage of retinopathy of prematurity (ROP) using spectral domain optical coherence tomography (SD-OCT).

METHODS

Spectral domain optical coherence tomography imaging for measuring subfoveal choroidal thickness was performed for 80 premature infants. Subfoveal choroidal thickness was defined as the distance from the hyperreflective line of the outermost retinal pigment epithelium (RPE) to the innermost hyperreflective line of the choroidoscleral junction. Each measurement was performed at the central fovea (CF) and 0.75 mm to 1.5 mm nasal (N1 and N2) and temporal (T1 and T2) to the fovea. Subfoveal choroidal thickness and grading of cystoid macular edema (CME) were analyzed statistically.

RESULTS

Choroidal thickness of CF was found to be significantly greater than nasal (N1 and N2) and temporal (T1 and T2) choroidal thickness (P < 0.05). There was no significant relationship between stage of ROP and nasal (N1 and N2) choroidal thickness (P = 0.057, P = 0.282, respectively). However, CF and temporal (T1 and T2) choroidal thickness was found to be significantly lower at a higher stage of ROP (P = 0.005, P = 0.01 and P = 0.001). No significant relationship was found between subfoveal choroidal thickness and the grades of cystoid macular edema (P > 0.05). The choroidal thickness of CF was found to be correlated with birth weight (r = 0.267, P = 0.017) but not birth week (r = 0.140, P = 0.217). Maximum stage of ROP was found to be negatively correlated with choroidal thickness, at N1, T1, and T2 (r < -0.250, P < 0.02).

CONCLUSION

The subfoveal choroid in premature infants can be effectively evaluated using a portable SD-OCT device. Choroidal thickness gets thinner with the severity of ROP and the decrease is more prominent at the central and temporal location. Cystoid macular edema is not correlated with choroidal thickness in premature infants.

[Analysis of choroidal thickness in AP-ROP, threshold disease and ROP without laser photocoagulation]

BACKGROUND

Enhanced depth imaging (EDI) and spectral domain optical coherence tomography (SD-OCT) provide high-definition cross-sectional images of the choroid. Information on alterations in choroidal thickness (CT) after laser photocoagulation (LC) in aggressive posterior retinopathy of prematurity (APROP) and threshold disease (TD) is rare.

PATIENTS AND METHODS

A total of 75 eyes were retrospectively analyzed in 4 groups. Groups 1 and 2 included patients with APROP and TD, respectively, who underwent LC. Group 3 included ROP children who did not undergo LC and group 4 included full-term children. Infants aged ≥4 < 7, who had examination of subfoveal (SF) CT with SD-EDI-OCT, visual acuity (VA), spherical equivalent (SE), anterior segment and fundus examination, axial lenght (AXL) were included. The results of SFCT, VA and SE at the age of ≥ 4 < 7 years, AXL, gestational age (GA), birth weight (BW) and age at examination were compared between the groups. Potential risk factors (GA, BW, SE, AXL and SFCT) influencing visual acuity were evaluated by using multivariate linear regression analysis.

RESULTS

The results of SFCT and AXL were not significantly different between groups 2 and 3 or between groups 3 and 4. There was a significant difference between the other groups for SFCT and AXL and VA was significantly different between all groups. The SE was not significantly different between groups 3 and 4 but there was a significant difference for SE, BW and GA between the groups. Age at examination was not significantly different between the groups. Multivariate linear regression analysis revealed SFCT for groups 1 and 2, GA for group 3 and GA, SFCT and AXL for group 4 as independent risk factors influencing visual acuity.

CONCLUSION

The regression model used for groups 1-4 explains the variation of the dependent risk factor LogMar VA for groups 1-4 with 31.2 %, 43.5 %, 9.6 % and 69.4 %, respectively. These values expressed in percentage demonstrate that even more predictors may influence the dependent factor LogMar VA than evaluated in the study.

Role of nitric oxide in optic nerve head blood flow regulation during an experimental increase in intraocular pressure in healthy humans

The present study set out to investigate whether nitric oxide, a potent vasodilator, is involved in the regulatory processes in optic nerve head blood flow during an experimental increase in intraocular pressure (IOP). The study was conducted in a randomized, double-masked, placebo-controlled, three way cross-over design. 12 healthy subjects were scheduled to receive either L-NMMA (an unspecific nitric oxide synthase inhibitor), phenylephrine (an α-adrenoceptor agonist) or placebo on three different study days. Optic nerve head blood flow was measured using laser Doppler flowmetry and IOP was increased stepwise with a suction cup. Mean arterial pressure (MAP) and IOP were measured non-invasively and ocular perfusion pressure (OPP) was calculated as OPP = 2/3 MAP-IOP. Administration of L-NMMA and phenylephrine significantly increased MAP and therefore OPP at rest (p < 0.01). L-NMMA significantly reduced baseline blood flow in the optic nerve head (p < 0.01). Application of the suction cup induced a significant increase in IOP and a decrease in OPP (p < 0.01). During the stepwise increase in IOP, some autoregulatory potential was observed until OPP decreased approximately -30% below baseline. None of the administered substances had an effect on this autoregulatory behavior (p = 0.49). The results of the present study confirm that the human optic nerve head shows some regulatory capacity during a decrease in OPP. Nitric oxide is involved in the regulation of basal vascular tone in the optic nerve head but does not seem to be involved in the regulatory mechanisms during an acute increase in IOP in young healthy subjects.

Assessment of retinal and choroidal blood flow changes using laser Doppler flowmetry in rats

PURPOSE

A new noninvasive laser Doppler flowmetry (LDF) probe (one emitting fiber surrounded by a ring of eight collecting fibers, 1-mm interaxis distance) was tested for its sensitivity to assess the retinal/choroidal blood flow variations in response to hypercapnia, hyperoxia, diverse vasoactive agents and following retinal arteries photocoagulation in the rat.

MATERIALS AND METHODS

After pupil dilation, a LDF probe was placed in contact to the cornea of anesthetized rats in the optic axis. Hypercapnia and hyperoxia were induced by inhalation of CO(2) (8% in medical air) and O(2) (100%) while pharmacological agents were injected intravitreously. The relative contribution of the choroidal circulation to the LDF signal was estimated after retinal artery occlusion by photocoagulation.

RESULTS

Blood flow was significantly increased by hypercapnia (18%), adenosine (14%) and sodium nitroprusside (16%) as compared to baseline values while it was decreased by hyperoxia (-8%) and endothelin-1 (-11%). Photocoagulation of retinal arteries significantly decreased blood flow level (-45%).

CONCLUSIONS

Although choroidal circulation most likely contributes to the LDF signal in this setting, the results demonstrate that LDF represents a suitable in vivo noninvasive technique to monitor online relative reactivity of retinal perfusion to metabolic or pharmacological challenge. This technique could be used for repeatedly assessing blood flow reactivity in rodent models of ocular diseases.

Retinal arteriolar responses to acute severe elevation in systemic blood pressure in cats: role of endothelium-derived factors

The purpose of this study was to investigate the roles of endothelium-derived factors in the retinal arteriolar responses to acute severe elevation in systemic blood pressure (BP) in cats. Acute elevation of mean arterial BP by 60% for 5 min was achieved by inflating a balloon-tipped catheter in the descending aorta. The retinal arteriolar diameter, flow velocity, wall shear rate (WSR) and blood flow (RBF) changes during BP elevation were assessed with laser Doppler velocimetry 2 h after intravitreal injections of nitric oxide (NO) synthase inhibitor l-NAME, cyclooxygenase inhibitor indomethacin, endothelin-1 receptor antagonists (BQ-123 for type A and BQ-788 for type B), or Rho kinase inhibitor fasudil. BP elevation caused a marked increase in retinal arteriolar flow velocity and WSR with slight vasoconstriction, resulting in an increase in RBF. The increases in velocity, WSR and RBF, but not diameter, were correlated with the increase in ocular perfusion pressure. With l-NAME or indomethacin, the increase in RBF upon BP elevation was significantly attenuated due to enhanced retinal arteriolar vasoconstriction. In contrast, BQ-123 and fasudil potentiated the increased RBF. BQ-788 had no effect on arteriolar diameter and hemodynamics. Our data suggest that acute elevation of BP by 60% leads to an increase in RBF due to the release of NO and prostanoids probably through a shear stress-induced vasodilation mechanism. The release of endothelin-1 and Rho kinase activation help to limit RBF augmentation by counteracting the vasodilation. It appears that the retinal endothelium, by releasing vasoactive substances, contributes to RBF regulation during acute severe elevation of systemic blood pressure.

Pharmacological MRI of the choroid and retina: blood flow and BOLD responses during nitroprusside infusion

Nitroprusside, a vasodilatory nitric oxide donor, is clinically used during vascular surgery and to lower blood pressure in acute hypertension. This article reports a novel application of blood flow (BF) and blood oxygenation level dependent (BOLD) MRI on an 11.7T scanner to image the rat chorioretinal BF and BOLD changes associated with graded nitroprusside infusion. At low doses (1 or 2 μg/kg/min), nitroprusside increased BF as expected but decreased BOLD signals, showing an intriguing BF-BOLD uncoupling. At high doses (3-5 μg/kg/min), nitroprusside decreased BF and markedly decreased BOLD signals. To our knowledge, this is the first pharmacological MRI application of the retina. This approach has potential to open up new avenues to study the drug-related hemodynamic functions and to evaluate the effects of novel therapeutic interventions on BOLD and BF in the normal and diseased retinas.

Retinopathy of prematurity: understanding ischemic retinal vasculopathies at an extreme of life

Retinopathy of prematurity (ROP) is a major complication of preterm birth. It encompasses a spectrum of pathologies that affect vision, from mild disease that resolves spontaneously to severe disease that causes retinal detachment and subsequent blindness. The pathologies are characterized by an arrest in normal retinal vascular development associated with microvascular degeneration. The resulting ischemia and retinal hypoxia lead to excessive abnormal compensatory blood vessel growth. However, this neovascularization can lead to fibrous scar formation and culminate in retinal detachment. Present therapeutic modalities to limit the adverse consequences of aberrant neovascularization are invasive and/or tissue-destructive. In this Review, we discuss current concepts on retinal microvascular degeneration, neovascularization, and available treatments, as well as present future perspectives toward more profound elucidation of the pathogenesis of ROP.

Choroidal blood flow compensation in rats for arterial blood pressure decreases is neuronal nitric oxide-dependent but compensation for arterial blood pressure increases is not

Choroidal blood flow (ChBF) compensates for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a +/-40 mmHg range of basal ABP in rabbits, humans and pigeons. In the present study, we investigated if ChBF can compensate for increases and decreases in ABP in rats. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized rats, and ABP measured via the femoral artery. At multiple intervals over a 2-4 h period during which ABP varied freely, ChBF and ABP were sampled and the results compiled across rats. We found that ChBF remained near baseline over an ABP range from 40 mmHg above basal ABP (90-100 mmHg) to 40 mmHg below basal ABP, but largely followed ABP linearly below 60 mmHg. Choroidal vascular resistance increased linearly as BP increased above 100 mmHg, and decreased linearly as BP declined from basal to 60 mmHg, but resistance declined no further below 60 mmHg. Inhibition of nitric oxide (NO) formation by either a selective inhibitor of neuronal nitric oxide synthase (NOS) (N(omega)-propyl-L-arginine) or a nonselective inhibitor of both neuronal NOS and endothelial NOS (N(omega)-nitro-l-arginine methyl ester) did not affect compensation above 100 mmHg ABP, but did cause ChBF to linearly follow declines in BP below 90 mmHg. In NOS-inhibited rats, vascular resistance increased linearly with BP above 100 mmHg, but remained at baseline below 90 mmHg. These findings reveal that ChBF in rats, as in rabbits, humans and pigeons, compensates for rises and/or declines in arterial blood pressure so as to remain relatively stable within a physiological range of ABPs. The ChBF compensation for low ABP in rats is dependent on choroidal vasodilation caused by neuronal NO formation but not the compensation for elevated BP, implicating parasympathetic nervous system vasodilation in the ChBF compensation to low ABP.

Intravenously administered vasodilatory prostaglandins increase retinal and choroidal blood flow in rats

We established an experimental system for measuring blood flow in the rat fundus and examined whether intravenously administered vasodilatory prostaglandins (PGE(1), PGE(2), and PGI(2)), 8-(4-chlorophenylthio)-cAMP (a cAMP analogue), and nicardipine (a Ca(2+)-channel blocker) increase fundus blood flow (FBF). Under artificial ventilation, rats were injected with tetrodotoxin (50 microg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye. After tetrodotoxin, the rats were infused with norepinephrine (0.3 - 0.5 microg . kg(-1) . min(-1)) and epinephrine (2.7 - 4.5 microg . kg(-1) . min(-1)) simultaneously to maintain adequate systemic circulation. We found that intravenous infusion of PGE(1) (2 - 10 microg . kg(-1) . min(-1)), PGE(2) (3 - 30 microg . kg(-1) . min(-1)), and PGI(2) (1 - 10 microg . kg(-1) . min(-1)) increased the FBF in a dose-dependent manner. The vasodilatory PGs decreased arterial pressure, whereas they did not affect heart rate. Like vasodilatory PGs, 8-(4-chlorophenylthio)-cAMP (30 micromol/kg, i.v.) increased FBF and decreased arterial pressure. While infusion of nicardipine (0.3 - 3 microg . kg(-1) . min(-1)) produced comparable depressor responses with those to vasodilatory PGs and the cAMP analogue, it did not increase FBF. These results suggest that vasodilatory PGs and cAMP act more selectively than Ca(2+)-channel blockers on retinal/choroidal blood vessels. Therefore, the vasodilatory PGs might be considered to be possible candidates for the therapeutics to treat disorders of retinal/choroidal circulation.

New insights into the retinal circulation: inflammatory lipid mediators in ischemic retinopathy

Ischemic proliferative retinopathy develops in various retinal disorders, including retinal vein occlusion, diabetic retinopathy and retinopathy of prematurity. Ischemic retinopathy remains a common cause of visual impairment and blindness in the industrialized world due to relatively ineffective treatment. Oxygen-induced retinopathy (OIR) is an established model of retinopathy of prematurity associated with vascular cell injury culminating in microvascular degeneration, which precedes an abnormal neovascularization. The retina is a tissue particularly rich in polyunsaturated fatty acids and the ischemic retina becomes highly sensitive to lipid peroxidation initiated by oxygenated free radicals. Consequently, the retina constitutes an excellent model for testing the functional consequences of membrane lipid peroxidation. Retinal tissue responds to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. Activation of phospholipase A(2) is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and a membrane-derived phospholipid mediator platelet-activating factor (PAF). These lipid mediators accumulate in the retina in response to injury and a physiologic role of these metabolites in retinal vasculature remains for the most part to be determined; albeit proposed roles have been suggested for some. The eicosanoids, in particular the prostanoids, thromboxane (TXA2) and PAF are abundantly generated following an oxidant stress and contribute to neurovascular injury. TXA2 and PAF play an important role in the retinal microvacular degeneration of OIR by directly inducing endothelial cell death and potentially could contribute to the pathogenesis of ischemic retinopathies. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. This review focuses on mechanisms that precede the development of neovascularization, most notably regarding the role of lipid mediators that partake in microvascular degeneration.

An association between central serous chorioretinopathy and gastroesophageal reflux disease

PURPOSE

To explore a clinically observed association between central serous chorioretinopathy (CSC) and gastroesophageal reflux disease (GERD) DESIGN: A retrospective case-control study.

PARTICIPANTS AND CONTROLS

Sixty-nine consecutive patients diagnosed with CSC were compared with a control group of 55 non-CSC patients.

METHODS

The records of 69 patients with CSC were retrospectively reviewed and compared with the records of 55 controls. All patients and controls were examined in a referral setting.

RESULTS

Patients with CSC were significantly more likely to have GERD compared with controls (odds ratio 6.05; 95% confidence interval 2.14-17.11; P =.0003). Central serous chorioretinopathy patients were also more likely than controls to have used oral corticosteroid medications (odds ratio 16.30; 95% confidence interval 2.09-127.33; P =.0006) and antacid/antireflux medications (odds ratio 15.00; 95% confidence interval 1.91-117.58; P =.001).

CONCLUSIONS

This study identifies an association between CSC and GERD. There are biochemical factors common to both diseases that support this association, giving further insight into the pathogenesis of CSC.

Hyperoxia impairs airway relaxation in immature rats via a cAMP-mediated mechanism

Hyperoxic exposure enhances airway reactivity in newborn animals, possibly due to altered relaxation. We sought to define the role of prostaglandinand nitric oxide-mediated mechanisms in impaired airway relaxation induced by hyperoxic stress. We exposed 7-day-old rat pups to either room air or hyperoxia (>95% O2) for 7 days to assess airway relaxation and cAMP and cGMP production after electrical field stimulation (EFS). EFS-induced relaxation of preconstricted trachea was diminished in hyperoxic vs. normoxic animals (P < 0.05). Indomethacin (a cyclooxygenase inhibitor) reduced EFS-induced airway relaxation in tracheae from normoxic (P < 0.05), but not hyperoxic, rat pups; however, in the presence of NG-nitro-L-arginine methyl ester (a nitric oxide synthase inhibitor) EFS-induced airway relaxation was similarly decreased in tracheae from both normoxic and hyperoxic animals. After EFS, the increase from baseline in the production of cAMP was significantly higher in tracheae from normoxic than hyperoxic rat pups, and this was accompanied by greater prostaglandin E2 release only in the normoxic group. cGMP production after EFS stimulation did not differ between normoxic and hyperoxic groups. We conclude that hyperoxia impairs airway relaxation in immature animals via a mechanism primarily involving the prostaglandin-cAMP signaling pathway with an impairment of prostaglandin E2 release and cAMP accumulation.

Choroidal blood flow in pigeons compensates for decreases in arterial blood pressure

While it had once been thought that choroidal blood flow (ChBF) does not compensate for changes in perfusion pressure, recent studies have shown that ChBF in rabbits and humans does compensate for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a physiological range of ABPs. In this study, we sought to determine if ChBF in birds can compensate for decreases in ABP, either spontaneously occuring or caused by blood withdrawal. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized pigeons, and at the same time ABP was measured via the brachial artery. In studies of spontaneous fluctuation in ABP, ChBF and ABP were analyzed at regular intervals over a 2-3 hr period, while for blood withdrawal studies, blood was transiently withdrawn via the brachial artery. In both paradigms, ChBF remained near baseline over an ABP range from basal (about 90 mmHg) to about 55 mmHg, followed ABP nearly linearly below 50 mmHg, and showed no compensation below 40 mmHg. The blood withdrawal studies further showed that the compensation was more rapid with small acute declines in ABP than with larger declines. These findings reveal that ChBF in pigeons, as in rabbits and humans, compensates for declines in ABP so as to remain relatively stable within a physiological range of ABPs. Given the phylogenetic distance between humans and rabbits on one hand and birds on the other, these results suggest that choroidal compensation for ABP declines may be a common ocular mechanism among warm-blooded vertebrates.

Central serous chorioretinopathy and glucocorticoids

Central serous chorioretinopathy is a relatively common retinal disease characterized by the accumulation of subretinal fluid at the posterior pole of the fundus, creating a circumscribed area of serous retinal detachment. It typically affects young and middle-aged men with no previous medical and family history, and no systemic symptoms or signs. However, it has been noted that central serous chorioretinopathy is associated with different conditions, characterized by exposure to increased levels of endogenous or exogenous glucocorticoids. In fact, central serous chorioretinopathy has been described in patients with endogenous Cushing's syndrome. It is also prevalent in patients with type-A behavior, and following stressful events, and pregnancy probably represents a risk factor for central serous chorioretinopathy; these conditions are characterized by endogenous hypercortisolism. In addition, many cases of central serous chorioretinopathy have been described during or following treatment with glucocorticoids, administrated by any route, for various systemic or ocular conditions. Central serous chorioretinopathy, when related to the exposure to exogenous glucocorticoids, has a less prominent male predilection, presents more often with a chronic or atypical form, and is frequently bilateral. Furthermore, treatment of central serous chorioretinopathy with glucocorticoids was found to exacerbate the clinical picture. Based on these observations it could be suggested that glucocorticoids may be involved in the development of central serous chorioretinopathy, even though the exact pathogenic mechanism remains unclear. Glucocorticoids should not be used in the treatment of central serous chorioretinopathy and central serous chorioretinopathy should be added to the list of ocular complications of glucocorticoids.

Major role for neuronal NO synthase in curtailing choroidal blood flow autoregulation in newborn pig

We examined whether nitric oxide (NO) generated from neuronal NO synthase (nNOS) contributes to the reduced ability of the newborn to autoregulate retinal blood flow (RBF) and choroidal blood flow (ChBF) during acute rises in perfusion pressure. In newborn pigs (1-2 days old), RBF (measured by microsphere) is autoregulated over a narrow range of perfusion pressure, whereas ChBF is not autoregulated. N(G)-nitro-L-arginine methyl ester (L-NAME) or specific nNOS inhibitors 7-nitroindazole, 3-bromo-7-nitroindazole, and 1-(2-trifluoromethyl-phenyl)imidazole as well as ganglionic blocker hexamethonium, unveiled a ChBF autoregulation as observed in juvenile (4- to 6-wk old) animals, whereas autoregulation of RBF in the newborn was only enhanced by L-NAME. All NOS inhibitors and hexamethonium prevented the hypertension-induced increase in NO mediator cGMP in the choroid. nNOS mRNA expression and activity were three- to fourfold higher in the choroid of newborn pigs than in tissues of juvenile pigs. It is concluded that increased production of NO from nNOS curtails ChBF autoregulation in the newborn and suggests a role for the autonomic nervous system in this important hemodynamic function, whereas, for RBF autoregulation, endothelial NOS seems to exert a more important contribution in limiting autoregulation.

Dominant role of an endothelium-derived hyperpolarizing factor (EDHF)-like vasodilator in the ciliary vascular bed of the bovine isolated perfused eye

1. The roles of the endothelium-derived nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor (EDHF) in mediating vasodilator responses to acetylcholine and bradykinin were assessed in the ciliary vascular bed of the bovine isolated perfused eye preparation. 2. Vasodilatation to acetylcholine or bradykinin was unaffected by the nitric oxide synthase inhibitor, L-NAME (100 microM), or the cyclo-oxygenase inhibitor, flurbiprofen (30 microM), but was virtually abolished following treatment with a high concentration of KCl (30 mM), or by damaging the endothelium with the detergent, CHAPS (0.3%, 2 min). 3. Acetylcholine-induced vasodilatation was unaffected by glibenclamide (10 microM), an inhibitor of ATP-sensitive K(+) channels (K(+)(ATP)), but was significantly attenuated by TEA (10 mM), a non-selective inhibitor of K(+) channels. 4. The small conductance calcium-sensitive K(+) channel (SK(+)(Ca)) inhibitor, apamin (100 nM), and the large conductance calcium-sensitive K(+) channel (BK(+)(Ca)) inhibitor, iberiotoxin (50 nM), had no significant effect on acetylcholine-induced vasodilatation. In contrast, the intermediate (IK(+)(Ca))/large conductance calcium-sensitive K(+) channel inhibitor, charybdotoxin (50 nM), powerfully blocked these vasodilator responses, and uncovered a vasoconstrictor response. 5. The combination of apamin (100 nM) with a sub-threshold concentration of charybdotoxin (10 nM) significantly attenuated acetylcholine-induced vasodilatation, but the combination of apamin (100 nM) with iberiotoxin (50 nM) had no effect. 6. In conclusion, blockade by a high concentration of KCl, by charybdotoxin, or by the combination of apamin with a sub-threshold concentration of charybdotoxin, strongly suggests that vasodilatation in the bovine isolated perfused eye is mediated by an EDHF.

Regulation of endothelial nitric oxide synthase by PGD(2) in the developing choroid

We investigated if prostaglandins might regulate the increased choroidal endothelial (e) nitric oxide synthase (NOS) expression in the perinate. Prostaglandins, eNOS mRNA, immunoreactive protein and activity, and nitrite [stable metabolite of nitric oxide (NO)] production were markedly higher in newborn (1 day old) than juvenile (6-8 wk old) pig choroid. Treatment of isolated newborn choroids with the prostaglandin synthase inhibitor ibuprofen for 24 h reduced eNOS mRNA and nitrite production to values in juveniles. This effect was equally observed with the PGD(2) receptor (DP) blocker BW A868C and was prevented by cotreatment with PGD(2) but not other prostaglandins; similar observations were made on NOS activity in vivo. PGD(2) also increased eNOS expression on choroids of juveniles, and this effect was blocked by BW A868C. The manifestation of this upregulation of eNOS by PGD(2) on the control of choroidal vasomotor response was tested by using NO-dependent vasorelaxants, ACh, bradykinin (Bk), and substance P (SP). ACh-, Bk-, and SP-elicited choroidal vasorelaxation was greater in saline-treated newborn than juvenile pigs. Ibuprofen (24 h) decreased ACh-, Bk-, and SP-evoked vasorelaxation in newborns, whereas PGD(2) increased that in juveniles and prevented the ibuprofen-induced attenuated relaxation in newborns; infusion of N(omega)-monomethyl-L-arginine in choroids of those animals treated with PGD(2) reversed the augmented vasorelaxation to ACh, Bk, and SP. Finally, PGD(2)-induced upregulation of NOS in the perinate was also reflected by curtailed choroidal blood flow autoregulatory response to increased perfusion pressure. In conclusion, PGD(2) exhibits a major role in upregulating eNOS expression and activity in the choroid, which in turn results in greater NO-mediated vasorelaxation; a new mechanism for eNOS regulation via DP is hereby disclosed. The relationship between PGD(2) and eNOS in the developing subject provides an explanation for the interactive role of these two factors in the absent choroidal blood flow autoregulation in the perinate.

Developmentally increased cerebrovascular NO in newborn pigs curtails cerebral blood flow autoregulation

We tested the hypothesis that a reduced ability of the newborn (1-2 d old) to autoregulate cerebral blood flow (CBF) during acute hypertension is contributed by an increased synthesis of nitric oxide (NO) from endothelial (e) and neuronal NO synthase (nNOS). As previously reported, CBF (measured by radiolabeled microsphere technique) in newborn pigs remained constant only between 50 and 90 mm Hg of mean arterial blood pressure. Treatment of newborn pigs with Nomega-monomethyl-L-arginine or specific nNOS inhibitors 7-nitroindazole monosodium, 3-bromo-7-nitroindazole, and 1-(2-trifluoromethylphenyl) imidazole extended the upper limit of CBF autoregulation as seen in saline-treated (control) juvenile (4-6-wk-old) animals. Cerebrovascular production of nitrite (stable NO oxidation product) in vivo was markedly increased during hypertension (mean arterial blood pressure > 90 mm Hg) in newborn but not in the juvenile pigs. Inhibition of NOS with Nomega-monomethyl-L-arginine, 7-nitroindazole monosodium, 3-bromo-7-nitroindazole, or 1-(2-trifluoromethylphenyl) imidazole prevented the hypertension-induced increase in nitrite levels. In addition, eNOS and nNOS protein expression and activity were 2- to 3-fold higher (p < 0.05) in the cerebral microvasculature of newborn than in the tissues of juvenile pigs. It is concluded that during acute hypertension, excess production of NO associated with increased activity of NOS curtails the upper limit of CBF autoregulation in the newborn subject; in addition, nNOS seems to serve a significant role in this important physiologic function.

Systemic findings associated with central serous chorioretinopathy

PURPOSE

To determine systemic factors associated with central serous chorioretinopathy.

METHODS

In a retrospective study, 230 consecutive patients with central serous chorioretinopathy examined in a referral setting were compared with a historical gender-matched and age-matched control group of 230 patients with ocular findings who were examined in the same referral setting.

RESULTS

The median age of the patients was 49.8 years, and of the control subjects, 50.0 years. The male-female ratio for both groups was 2.7:1. Patients with central serous chorioretinopathy were more likely to use psychopharmacologic medications (odds ratio = 2.6; 95% confidence interval = 1.30 to 5.19; P = .0049) and corticosteroids (odds ratio = 3.17; 95% confidence interval = 1.30 to 7.70; P = .0067) and were more likely to have hypertension (odds ratio = 2.25; 95% confidence interval = 1.39 to 3.63; P = .0008) than were the control subjects.

CONCLUSIONS

This study identified psychopharmacologic medication use, corticosteroid use, and hypertension as factors associated with central serous chorioretinopathy. These findings reinforce the concept that stress and adaptations to stress play a role in this disorder. The findings of possible associations between central serous chorioretinopathy and both hypertension and corticosteroid usage suggest that these modifiable factors may influence morbidity of central serous chorioretinopathy.

Functional role of nitric oxide in regulation of ocular blood flow

Nitric oxide generated by three distinct enzyme systems appears to play a critical role in many diverse physiological processes. Using both conventional and immunohistochemical techniques, nitric oxide synthases have been identified throughout the body, including all regions of the eye. A large number of in vitro and in vivo preparations have been utilized showing nitric oxide to have an important role in regulation of regional ocular blood flow. Nitric oxide-mediated control of basal ocular blood flow is demonstrated by vasoconstriction seen in experiments where vascular endothelial cells are removed, or when nitric oxide synthase is inhibited. The endogenous source of nitric oxide in the eye appears to be both endothelial and neural. In addition, administration of drugs that can 'donate' nitric oxide produces vasodilation of the eye vasculature. Local vasodilation in response to illumination of the retina is controlled by generation and release of nitric oxide, whereas most other physiological adjustments of ocular blood flow (i.e., autoregulation and responses to altered blood gas levels) seem to be relatively independent of nitric oxide mechanisms. Nitric oxide is implicated in a variety of ocular pathophysiological states including uveitis, retinal ischemic disease, diabetes and glaucoma.

Physiology of perfusion as it relates to the optic nerve head

Blood flow in the optic nerve has been demonstrated to be autoregulated, and, thus, within certain limits, to be independent of the local perfusion pressure. As in the brain, a close coupling of neuronal activity and optic nerve head blood flow has been demonstrated. A number of regulatory systems and factors participate in the regulation of vascular tone in various organs, including the optic nerve. Metabolic and myogenic mechanisms keep local perfusion constant or adapted to the local metabolic needs. Such mechanisms seem to be involved in the regulation of optic nerve blood flow as well. In contrast, neuronal blood flow regulation is of minor importance in the optic nerve. Many of the regulatory modalities induce a response of vascular smooth muscle cells through stimulation of factors produced by the endothelial cell layer. Indeed, endothelial factors are of utmost importance in the regulation of optic nerve blood flow. The facts that there is a basal formation of nitric oxide, which leads to an active dilation of the ocular vasculature, and that endothelin-1 decreases blood flow to the anterior optic nerve in a dose-dependent manner suggest that alterations in these regulatory mechanisms might be relevant for optic nerve blood flow alterations as they relate to glaucomatous optic neuropathy. It is hoped that a detailed knowledge of blood flow regulation in the optic nerve might initiate new treatment modalities in optic neuropathies that are hemodynamic and vascular in nature.

Mechanism for substance P-induced relaxation of precontracted airway smooth muscle during development

Release of substance P (SP) from sensory nerve endings of the tracheobronchial system modulates airway smooth muscle contraction and may cause relaxation of precontracted airways. We sought to elucidate the effect of postnatal maturation on SP-induced relaxation of precontracted airways and determine the roles of endogenously generated nitric oxide (NO) and prostaglandins (PGs). Cylindrical airway segments were isolated from the midtrachea of rats at four different ages, 1, 2, and 4 wk and 3 mo, and contracted to 50-75% of the maximum response induced by bethanechol. SP was then administered in the absence and presence of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), the PG inhibitor indomethacin, or both. Relaxation of airways with SP decreased significantly with advancing postnatal age. SP-induced tracheal relaxation was consistently attenuated by pretreatment with L-NAME, indomethacin, or both. In a different group of animals, L-NAME significantly attenuated the relaxant response of airways to PGE2 exposure, but indomethacin had no significant effect on the relaxant response to exogenous NO. We conclude that SP induces a relaxant effect on precontracted airway smooth muscle, which decreases with advancing age and is mediated via SP-induced release of NO and/or PG.

PGE2, via EP3 receptors, regulates brain nitric oxide synthase in the perinatal period

We tested the hypothesis that high prostaglandin levels during the perinatal period might regulate brain nitric oxide synthase (nNOS) expression. nNOS and cyclooxygenase (COX)-2 mRNAs were higher in brain cortex and the periventricular area of newborn rats and pigs compared with adult brain. Nitric oxide synthase activity was also 2. 5- to 4-fold higher in newborn than in adult brain. Administration of nonselective COX inhibitor ibuprofen or COX-2 inhibitor nimesulide every 8 h for 24 h to newborn rats and pigs reduced prostaglandin levels and caused comparable reductions in nNOS mRNA, protein, and activity to levels of adults; COX inhibitor-induced changes were prevented by cotreatment with PGE2 analog, 16, 16-dimethyl-PGE2, and agonist for the EP3 receptor of PGE2, sulprostone, but not by PGI2 analog carbaprostacyclin, PGD2, EP1 receptor agonist 17-phenyl trinor-PGE2, and EP2 agonist butaprost. Concordant observations were made in vitro and revealed that nNOS expression (detected by NADPH diaphorase reactivity) mostly present in neurons of the deeper cortical layers was reduced by COX inhibitor, and this effect was prevented by EP3 agonist. In conclusion, high levels of PGE2 in neonatal brain contribute to the increased expression of nNOS by acting on EP3 receptors; this positive interaction between PGE2 and nNOS might be required physiologically for normal brain development.

A major role for prostacyclin in nitric oxide-induced ocular vasorelaxation in the piglet

We studied the mechanisms of retinal and choroidal vasorelaxation elicited by nitric oxide (NO) using piglet eyes. The NO donors sodium nitroprusside (SNP) and diethylamine-NONOate caused comparable concentration-dependent relaxation that was partially (approximately 40%) attenuated by the guanylate cyclase inhibitors methylene blue and LY83583 and reduced to a lesser extent (approximately 25%) by the inhibitor of cGMP-dependent kinase, KT 5823. In contrast, NO-induced dilatation (by NO donors and endogenous NO after stimulation with bradykinin) was substantially (approximately 70%) diminished by the KCa channel blockers tetraethylammonium (TEA), charybdotoxin, and iberiotoxin; by the cyclooxygenase inhibitors indomethacin and ibuprofen; by the prostaglandin I (PGI2) synthase inhibitor trans-2-phenyl cyclopropylamine (TPC); and by the removal of endothelium; whereas relaxation of endothelium-denuded vasculature to SNP was unaltered by indomethacin, TPC, and charybdotoxin but was nearly nullified by methylene blue and the Kv channel blocker 4-aminopyridine. NO donors significantly increased PGI2 synthesis and the putative PGI2 receptor-coupled second messenger cAMP, from ocular vasculature (retinal microvessels and choroidal perfusate), and this increase in PGI2 formation was markedly reduced by TPC, tetraethylammonium, charybdotoxin, and/or the removal of endothelium, but it was only slightly reduced by methylene blue and LY83583. Also, SNP and KCa channel openers NS1619 and NS004 caused an increase in PGI2 synthesis in cultured endothelial cells, which was virtually abolished by KCa blockers. Finally, vasorelaxation to a cGMP analogue, 8-bromo cGMP, and protein kinase G stimulant beta-phenyl-1,N2-etheno-8-bromoguanosine 3':5'-cyclic monophosphate was mostly Kv dependent and, in contrast to NO, largely unrelated to PGI2 formation. In conclusion, data indicate that NO-induced ocular vasorelaxation is partly mediated by cGMP through its action on smooth muscle, and more importantly, by stimulating PGI2 formation of endothelial origin via a mechanism mostly independent of guanylate cyclase, which involves the opening of a KCa channel.

Key role for cyclooxygenase-2 in PGE2 and PGF2alpha receptor regulation and cerebral blood flow of the newborn

Ibuprofen, a cyclooxygenase (COX) inhibitor nonselective for either COX-1 or COX-2 isoform, upregulates cerebrovascular prostaglandin E2 (PGE2) and PGF2alpha receptors in newborn pigs. COX-2 was shown to be the predominant form of COX and the main catalyst of prostaglandin synthesis in the newborn brain. We proceeded to establish direct evidence that COX-2-generated prostaglandins govern PGE2 and PGF2alpha receptor density and function in the cerebral vasculature of the newborn. Hence, we determined PGE2 and PGF2alpha receptor density and functions in brain vasculature by using newborn pigs treated with saline, ibuprofen, COX-1 inhibitor (valerylsalicylate), or COX-2 inhibitors (DUP-697 and NS-398). Newborn brain PGE2 and PGF2alpha concentrations were significantly reduced by ibuprofen, DUP-697, and NS-398 but not by valerylsalicylate. In newborn pigs treated with DUP-697, NS-398, and ibuprofen, PGE2 and PGF2alpha receptor densities in brain microvessels were increased to adult levels; there was also a significant increase in inositol 1,4,5-trisphosphate (IP3) production and cerebral vasoconstrictor effects of 17-phenyl trinor PGE2 (EP1 receptor agonist), M&B-28767 (EP3 receptor agonist), PGF2alpha, and fenprostalene (PGF2alpha analog). Treatment with ibuprofen or DUP-697 also increased the upper blood pressure limit of cerebral cortex and periventricular blood flow autoregulation from 85 to > or = 125 mmHg (uppermost blood pressure studied). However, valerylsalicylate treatment did not affect cerebrovascular PGE2 and PGF2alpha receptors, IP3 production, or vasoconstrictor effects in newborn animals. These in vivo and in vitro observations indicate that COX-2 is mainly responsible for the regulation of PGE2 and PGF2alpha receptors and their functions in the newborn cerebral vasculature.

Control of cerebral and ocular blood flow autoregulation in neonates

Several factors have been implicated in the regulation of cerebral and ocular vasomotor tone in the newborn: the interrelationship between prostanoids, NO, and other vasoactive mediators remains a subject of interest and active investigation. Pharmacologic modulation may provide new treatment modalities for diseases of the newborn that are mostly hemodynamic and vascular in nature.

Increased nitric oxide synthesis and action preclude choroidal vasoconstriction to hyperoxia in newborn pigs

We tested the hypothesis that hyperoxia does not cause adequate constriction of choroidal vessels of the newborn (1 to 5 days old) pig, resulting in increased O2 delivery to the retina, possibly due to excess production and/or effects of vasodilators such as nitric oxide (NO). Hyperoxia (100% O2, 45 minutes) led to a decrease in retinal blood flow (RBF) of both newborn and juvenile (5 to 6 weeks old) pigs and also reduced choroidal blood flow (ChBF) in juvenile but not in newborn pigs; the absence of hyperoxia-induced ChBF response in the newborn was associated with a rise in choroidal O2 delivery. Ibuprofen (prostaglandin G/H synthase inhibitor) and 1,3-dimethyl-2-thiourea (a free radical scavenger) did not modify the choroidal hemodynamic responses to hyperoxia in newborn pigs. However, in newborn animals treated with the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME), hyperoxia caused a decrease in blood flow and O2 delivery to the choroid. Consistent with these effects of L-NAME, hyperoxia induced an increase in choroidal cGMP in newborn pigs ventilated with 100% O2 and stimulated nitrite production in isolated choroids exposed to hyperoxia from newborn but not juvenile pigs; these effects were inhibited by NOS blockers. Also, both constitutive and inducible NOS activities were higher in choroidal tissues from newborn than from juvenile animals. In addition, the vasorelaxant effect of the NO donor sodium nitroprusside in vitro was also greater on choroids from newborn than from juvenile pigs. Finally, L-NAME prevented the hyperoxia-induced increase in peroxidation products in the choroid of newborns. It is concluded that hyperoxia does not lead to a decrease in blood flow and O2 delivery to the choroid of the newborn because of increased NO synthesis and effects; since the choroid is the main source of O2 supply to the retina, the present data contribute in providing an explanation for the increased susceptibility of the immature neonate to hyperoxia-induced retinopathy.