Effect of NO synthase inhibition on retinal vessel reaction to isometric exercise in healthy humans.

PURPOSE: It has been shown that retinal blood flow is autoregulated, meaning that flow is independent of perfusion pressure within a certain range. We tested the hypothesis that nitric oxide (NO) synthase inhibition alters the response of retinal arterial and venous vessels during isometric exercise. METHODS: In this study, nine healthy subjects were included. Each subject received the NO synthase inhibitor Ng-monomethyl-l-Arginine (l-NMMA, the α-receptor agonist phenylephrine or placebo intravenously on three study days. Retinal vessel diameter was assessed with the retinal vessel analyser (RVA), at baseline and during a squatting period of 6-7 min in absence or presence of l-NMMA, phenylephrine or placebo. RESULTS: Mean arterial pressure (MAP) and pulse rate (PR) increased significantly during all pretreatment squatting periods (p < 0.001) Retinal venous and arterial diameters showed a continuous decrease during squatting (p < 0.001). Phenylephrine increased MAP and PR but did not alter the retinal vessel diameter response to squatting. Administration of l-NMMA lead to a significant decrease in venous diameter before isometric exercise (p = 0.004). In addition, the retinal venous diameter response during administration of the NO synthase inhibitor was less pronounced than during phenylephrine or placebo (p < 0.001). CONCLUSION: Our study confirms that NO plays an important role in the control of retinal vascular tone at rest. In addition, the present data indicate a role of NO in retinal autoregulation, because the response of retinal venous diameters was altered after NO synthase inhibition. The nature of involvement, however, appears to be complex and requires further studies.

Effect of intravitreal bevacizumab (Avastin) in neovascular age-related macular degeneration using a treatment regimen based on optical coherence tomography: 6- and 12-month results.

PURPOSE: To study the effect of intravitreal bevacizumab therapy on visual and anatomical outcomes in patients with neovascular age-related macular degeneration (AMD) within a follow-up period of 6 and 12 months. METHODS: A retrospective analysis of 102 eyes of 102 consecutive patients with neovascular AMD evaluated repeated intravitreal bevacizumab (1 or 2.5 mg) injections. Retreatment was performed following an optical coherence tomography (OCT)-based regimen. Ophthalmic examination included best-corrected visual acuity (BCVA), dilated fundus examination and OCT imaging. Data were analysed at baseline, 6 months (24 weeks) and 12 months (48 weeks) after treatment initiation. RESULTS: BCVA remained stable at 6 months (mean: 0.00+/-0.41 logMAR; p=0.95) and 12 months (mean: +0.02+/-0.43 logMAR; loss of approximately 1 letter; p=0.70) after the first treatment. OCT retinal thickness decreased by a mean of -37.8+/-101.6 microm (p<0.05) compared to baseline at month 6 and -38.6+/-93.3 microm (p<0.05) at month 12. A mean of 2.6+/-1.2 injections were needed to obtain absence of fluid by OCT, and the time to recurrence was 23+/-11 weeks thereafter. There was no difference in BCVA and OCT outcomes between treatment-naive eyes and eyes that had undergone prior treatment. CONCLUSION: The 6- and 12-month follow-up of repeated intravitreal bevacizumab therapy in eyes with neovascular AMD demonstrated stabilization of vision and no safety concerns. An OCT-based retreatment strategy appears appropriate in the management of patients treated with intravitreal bevacizumab.

Time-course and characteristic morphology of retinal changes following combination of verteporfin therapy and intravitreal triamcinolone in neovascular age-related macular degeneration.

PURPOSE: To identify characteristic morphological changes of the retina over time and the association with visual function after combined photodynamic therapy (PDT) and intravitreal triamcinolone (IVTA). METHODS: In this retrospective study, 40 patients (40 eyes) were treated with PDT and same-day IVTA. Optical coherence tomography (OCT), fluorescein angiography (FA) and evaluation of distance visual acuity (VA) were performed. The anatomical changes within intra- and subretinal compartments and their detailed analysis and grading were the main outcome measures. RESULTS: Intraretinal fluid (IRF) and subretinal fluid (SRF) by OCT decreased until 3 months (p < 0.01). At month 3, intraretinal cystoid spaces (ICS) had resolved or decreased in 84% of eyes, SRF in 58% and pigment epithelial detachment (PED) in 50%. Mean best-corrected VA (BCVA) improved significantly at month 1 (p < 0.01). Mean central retinal thickness (CRT) increased from 334 microm at baseline to 439 microm at day 1 (p = 0.03) before decreasing to 286 microm at day 7 (p = 0.06), 233 microm at month 1 (p = 0.001) and 255 microm at month 3 (p = 0.001). CONCLUSION: Combined verteporfin/IVTA therapy induces distinct time-related effects on the retina within the different intra- and subretinal compartments.

Neovascular age-related macular degeneration: intraocular cytokines and growth factors and the influence of therapy with ranibizumab.

PURPOSE: To investigate concentrations of growth factors and inflammatory cytokines in eyes with neovascular age-related macular degeneration (AMD) before and during therapy with intravitreal ranibizumab and to identify associations with disease activity. DESIGN: Prospective clinical trial. PARTICIPANTS AND CONTROLS: Twenty-eight eyes of patients with neovascular AMD were compared with 28 eyes of age-matched patients with cataract as control. METHODS: Ranibizumab was administered intravitreously once at baseline, and retreatments were given at monthly visits if optical coherence tomography (OCT) revealed macular edema or vision loss had occurred. Aqueous humor samples were taken each time intravitreal interventions were performed. Follow-up was 12 months. Luminex (Luminex Inc., Austin, TX) multiplex assays were used for measurement of 29 different growth factors and cytokines, including vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF). MAIN OUTCOME MEASURES: Differences in the concentrations of growth factors and inflammatory cytokines in eyes with neovascular AMD compared with control eyes and the influence of therapy with intravitreal ranibizumab. RESULTS: A significantly increased expression of VEGF (P = 0.033) and a significantly decreased expression of PDGF (P = 0.038) were measured in the aqueous humor of eyes with neovascular AMD. Furthermore, a significant decrease of VEGF (P<0.001) was observed after intravitreal injection of ranibizumab along with significant changes in visual acuity and central retinal thickness (P = 0.039 and P<0.001). During follow-up with a flexible regimen, a correlation was identified between increased VEGF levels and persistent or recurrent macular edema. Changes in PDGF levels were strongly associated with alterations in VEGF concentration. CONCLUSIONS: Vascular endothelial growth factor and PDGF-AA seemed to be associated with disease activity of neovascular AMD. Intravitreal anti-angiogenic treatment with ranibizumab resulted in significantly decreased intraocular VEGF expression below physiologic levels compared with controls. This effect was measurable as long as 4 weeks after each injection and was prolonged by consecutive retreatment. With recurrence after discontinuation of treatment, VEGF levels increased again.

Twelve-hour reproducibility of retinal and optic nerve blood flow parameters in healthy individuals.

PURPOSE: The aim of the present study was to investigate the reproducibility and potential diurnal variation of optic nerve head and retinal blood flow parameters in healthy individuals over a period of 12 hr. METHODS: We measured optic nerve head and retinal blood flow parameters in 16 healthy male non-smoking individuals at five time-points during the day (08:00, 11:00, 14:00, 17:00 and 20:00 hr). Outcome parameters were perimacular white blood cell flux (as assessed with the blue field entoptic technique), blood velocities in retinal veins (as assessed with bi-directional laser Doppler velocimetry), retinal arterial and venous diameters (as assessed with the retinal vessel analyser), optic nerve head blood flow, volume and velocity (as assessed with single point and scanning laser Doppler flowmetry) and blood velocities in the central retinal artery (as assessed with colour Doppler imaging). The coefficient of variation and the maximum change from baseline in an individual were calculated for each outcome parameter. RESULTS: No diurnal variation in optic nerve head or retinal blood flow was observed with any of the techniques employed. Coefficients of variation were between 1.6% and 18.5% for all outcome parameters. The maximum change from baseline in an individual was much higher, ranging from 3.7% to 78.2%. CONCLUSION: Our data indicate that in healthy individuals the selected techniques provide adequate reproducibility to be used in clinical studies. However, in patients with eye diseases and reduced vision the reproducibility may be considerably worse.

Intravitreal bevacizumab therapy for neovascular age-related macular degeneration with large submacular hemorrhage.

PURPOSE: To evaluate functional and anatomic effects of intravitreal bevacizumab (Avastin; Roche Pharma, Vienna, Austria) in patients with neovascular age-related macular degeneration (AMD) with large submacular hemorrhages. DESIGN: Retrospective, clinical study. METHODS: Twenty-one eyes of 19 AMD patients with choroidal neovascularization and large submacular hemorrhage involving the fovea comprising more than 50% of the total lesion area were evaluated. All patients completed at least four months of follow-up; 12 patients fulfilled 12 months or more of follow-up. Patients were treated with up to six intravitreal bevacizumab injections (1 mg/0.04 ml) at a minimum of four-week intervals. Changes from baseline visual acuity (VA) scores, retinal measurements by optical coherence tomography (OCT), angiographic lesion characteristics, and hemorrhage size were analyzed. A safety assessment was performed at all visits. RESULTS: Intravitreal bevacizumab injections were well tolerated in all patients. At month 4, VA was stable or improved (visual loss of 3 acuity lines or fewer) in 100% and improved by at least 3 lines in 9.5%. Comparable results were found at month 12. On average, the central foveal thickness decreased significantly by 55 microm four weeks after the first injection (P < .001) and by 52 microm at month 4 (P = .002). A significant anatomic improvement also was found for maximum retinal thickness, minimum retinal thickness, and foveal volume (P < .05) and was maintained during four months of follow-up. Mean size of hemorrhage was significantly reduced from 19.7 mm(2) at baseline to 2.5 mm(2) at the four-month follow-up (P < .001). CONCLUSIONS: Intravitreal bevacizumab seems to be a promising therapeutic option in eyes with neovascular AMD and large submacular hemorrhages, with a stabilization in VA and anatomic improvement.

Altered nitric oxide system in patients with open-angle glaucoma.

OBJECTIVE: To investigate the ocular blood flow response to systemic nitric oxide synthase inhibition in patients with primary open-angle glaucoma. METHODS: In 12 patients with glaucoma and 12 age-matched control subjects, subfoveal choroidal blood flow, optic nerve head blood flow, ocular fundus pulsation amplitude, intraocular pressure, and systemic hemodynamic parameters were measured at baseline and after inhibition of nitric oxide synthase by intravenous administration of NG-monomethyl-L-arginine. RESULTS: The increase in blood pressure in response to NG-monomethyl-L-arginine was comparable between the 2 study cohorts. In patients with glaucoma, the decrease of optic nerve head blood flow (P = .03) and fundus pulsation amplitude (P<.001) during nitric oxide synthase inhibition was significantly less pronounced than in healthy control subjects. A tendency toward a reduced response in choroidal blood flow was seen (P = .051 between groups) in patients with glaucoma. CONCLUSIONS: This is the first in vivo study providing evidence for an altered ocular L-arginine/nitric oxide system in patients with glaucoma. Normalization of the ocular nitric oxide production may be beneficial in terms of normalization of ocular blood flow and neuroprotection of retinal ganglion cells.

Influence of Ginkgo biloba on ocular blood flow.

PURPOSE: To investigate the effect of Ginkgo biloba extract (EGb761) on ocular blood flow. METHODS: This randomized, double-masked, placebo-controlled, two-way crossover study included 15 healthy male volunteers. Measurements were taken with laser Doppler flowmetry, laser Doppler velocimetry, a retinal vessel analyser, laser interferometry and applanation tonometry, before and up to 3 hours after oral intake of 240 mg EGb761. RESULTS: At baseline, no significant differences in ocular and systemic haemodynamic parameters were observed between the two study days. Ginkgo biloba significantly decreased retinal venous diameters (p < 0.05 versus baseline), but there was no significant difference between the two groups. Blood pressure, retinal arterial and venous diameters, choroidal blood flow, fundus pulsation amplitude, intraocular pressure and retinal blood flow remained unchanged in both groups and did not differ between groups. Optic nerve head blood flow significantly increased in response to Ginkgo biloba (p < 0.002 versus baseline), but this effect was not significant compared with that of placebo. CONCLUSIONS: The results of this study indicate that a single administration of Ginkgo biloba does not influence ocular blood flow to a relevant degree. Whether the drug may influence ocular blood flow in patients with ocular vascular disease after longterm treatment remains to be investigated in a randomized, placebo-controlled clinical trial.

ETa-receptor blockade, but not ACE inhibition, blunts retinal vessel response during isometric exercise.

Angiotensin II and endothelin-1 are potent vasoconstrictors that appear to play a role in retinal blood flow regulation. In the present study, we investigated the possible role of the angiotensin and the endothelin system in the regulation of retinal vessel diameters during isometric exercise in healthy humans. The study design was randomized, double-masked, placebo-controlled, and three-way cross over. Twelve healthy subjects performed squatting exercises for 6 min during infusion of either an angiotensin-converting enzyme inhibitor (enalapril), an ET(A)-receptor antagonist (BQ-123), or placebo. Retinal vessel diameters were measured continuously with the Zeiss retinal vessel analyzer. Systemic hemodynamics were assessed noninvasively, and intraocular pressure was measured with applanation tonometry. Squatting induced a significant increase in blood pressure and pulse rate, which was paralleled by a vasoconstriction in retinal arteries and veins. Intraocular pressure was only slightly increased during the squatting periods. BQ-123 significantly blunted the exercise-induced decrease in venous (P < 0.01) and arterial (P < 0.02, ANOVA) vessel diameters but had no effect on basal retinal diameters. By contrast, enalapril did neither influence vessel diameter at baseline conditions nor in response to isometric exercise. The data of the present study indicate that retinal vasoconstriction during isometric exercise is modified by ET(A)-receptor blockade, whereas it is not altered by angiotensin-converting enzyme inhibition. Hence, the present data indicate that endothelin-1, but not angiotensin II, is involved in retinal blood flow regulation during isometric exercise.

Response of retinal blood flow to systemic hyperoxia in smokers and nonsmokers.

BACKGROUND: We investigated the influence of chronic smoking on ocular vascular reactivity during breathing of 100% oxygen. METHODS: Retinal vascular reactivity was tested during inhalation of 100% oxygen over 10 min. The observer-masked two-cohort study was performed in 24 healthy male volunteers (12 smokers and 12 nonsmokers) using the Zeiss Retinal Vessel Analyzer and laser Doppler velocimetry. From these parameters retinal blood flow was calculated. RESULTS: Hyperoxia significantly decreased arterial (smokers: p<0.001 vs baseline; nonsmokers: p=0.003 vs baseline) and venous (smokers: p<0.001 vs baseline; nonsmokers: p<0.001 vs baseline) diameters. This decrease was significantly more pronounced in smokers (arterial diameter: p<0.001, venous diameter: p=0.003). Hyperoxia decreased venous blood flow velocity (smokers: p=0.02 vs baseline; nonsmokers: p<0.001 vs baseline) to a comparable degree (p=0.51). The two groups showed a comparable decrease in retinal blood flow during hyperoxia (smokers: p<0.001 vs baseline; nonsmokers: p<0.001 vs baseline; p=0.76 between groups). The decrease of PCO(2) during inhalation of 100% oxygen was significantly more pronounced in smokers than in nonsmokers (p=0.038). CONCLUSION: The present study indicates an abnormal retinal vascular response to hyperoxia in smokers. Further studies are needed to identify possible neural or humoral factors involved in this shifted vasoconstrictory status in smokers.

Specific endothelin ET(A) receptor antagonism does not modulate insulin-induced hemodynamic effects in the human kidney, eye, or forearm.

There is evidence that hyperinsulinemia may stimulate endothelin-1 (ET-1) generation or release, which may affect diabetic vascular complications. BQ-123, a specific ET(A) receptor antagonist, was used to investigate if insulin-induced vascular effects are influenced by an acute ET-1 release. Two randomized, placebo-controlled, double-blind, cross-over studies were performed. In protocol 1, 12 healthy subjects received, on separate study days, infusions of BQ-123 (60 microg/min for 30 min) during placebo clamp conditions, BQ-123 during euglycemic hyperinsulinemia (3 mU/kg/min for 390 min), or placebo during euglycemic hyperinsulinemia. Fundus pulsation amplitude (FPA) was measured to assess pulsatile choroidal blood flow, and mean flow velocity (MFV) of the ophtalmic artery was measured by color Doppler imaging. In protocol 2, eight healthy subjects received, on separate study days, intra-arterial infusions of BQ-123 (32 microg/min for 120 min) during placebo or insulin clamp. Forearm blood flow was measured with bilateral plethysmography, expressing the ratio of responses in the intervention arm and in the control arm. Insulin alone increased FPA (+10%, p < 0.001) and forearm blood flow (+19%). BQ-123 increased FPA, MFV, and forearm blood flow ratio in the absence and presence of exogenous insulin, but this effect was not different between normo- and hyperinsulinemic conditions. ET-1 plasma concentrations were not affected by insulin. In conclusion, these data do not support the concept that hyperinsulinemia increases ET-1 generation in healthy subjects. Our results, however, cannot necessarily be extrapolated to diabetic and obese subjects.

Influence of operator experience on the performance of ultrasound biometry compared to optical biometry before cataract surgery.

PURPOSE: To compare measurements performed with the IOLMaster (Carl Zeiss, Meditec AG) with those obtained by applanation ultrasound (US) and manual keratometry and to evaluate the effect of operator experience on US biometry. SETTING: Department of Ophthalmology, University of Vienna, Vienna, Austria. METHODS: The axial length (696 eyes) and anterior chamber depth (ACD) (462 eyes) were measured in 377 patients with cataract using the IOLMaster and applanation US. To assess the effect of operator experience on the biometric results, the operators were divided into 2 groups: experienced and less experienced in performing US biometry. The difference in measurements between the methods and the variability of the difference were compared between the 2 groups. RESULTS: Applanation US measured axial length and ACD shorter than the IOLMaster; the mean numerical difference was 0.13 mm and 0.19 mm, respectively (P<.01). For axial length, the absolute difference was smaller with experienced operators than with less experienced operators (0.15 mm versus 0.22 mm) (P<.01). For ACD, experienced operators obtained a smaller difference between measurement techniques (0.21 mm versus 0.29 mm; P<.05). CONCLUSIONS: Experienced US operators had less difference and lower variability in the difference between applanation US and IOLMaster readings for axial length and ACD measurements. The noncontact optical method, which is essentially operator independent, gave significantly more reliable biometry before cataract surgery, especially in the case of less experienced operators.

Effects of isometric exercise on subfoveal choroidal blood flow in smokers and nonsmokers.

PURPOSE: Little is known about potential effects of smoking on ocular blood flow regulation. In the present study, the hypothesis was that choroidal blood flow (CBF) changes during an increase in ocular perfusion pressure induced by isometric exercise are altered in chronic smokers. METHODS: The study was performed in 24 (12 smokers and 12 nonsmokers) healthy male volunteers in an observer-masked, two-cohort study design. The difference in CBF regulation between smokers and nonsmokers was tested during isometric exercise over a period of 6 minutes. CBF was assessed with laser Doppler flowery (LDF), and ocular perfusion pressure (OPP) was calculated from mean arterial pressure (MAP) and intraocular pressure (IOP). RESULTS: Six minutes of isometric exercise induced a significant increase in MAP, pulse rate (PR), OPP, and CBF in smokers and nonsmokers (each P<0.001). The increase in CBF was significantly higher in the smoking group (P<0.001) than in the healthy control group, whereas a comparable increase in MAP (P=0.18), PR (P=0.18), and OPP (P=0.43) occurred in smokers and nonsmokers. IOP remained unchanged during isometric exercise in both groups. Moreover, in smokers, CBF started to increase at OPPs more than 49% above baseline, whereas CBF in nonsmokers remained stable until an increase in OPP of 74% over baseline. This difference between the two groups was significant (P<0.001). CONCLUSIONS: These data indicate abnormal CBF regulation in chronic smokers compared with age-matched nonsmoking subjects during isometric exercise. The pathways responsible for this abnormal blood flow response remain to be elucidated.

Effects of sildenafil on retinal blood flow and flicker-induced retinal vasodilatation in healthy subjects.

PURPOSE: Sildenafil is a specific inhibitor of phosphodiesterase V, which is widely used for the treatment of erectile dysfunction. Sildenafil has been shown to induce vasodilation in several vascular beds by inhibiting the cGMP breakdown. The present study was conducted to investigate whether sildenafil increases blood flow in the human retina. METHODS: In a randomized, double-masked, placebo-controlled, two-way crossover study in 12 healthy male volunteers the effects of a single dose of 100 mg sildenafil were studied. Subjects received sildenafil or placebo on two different study days. After administration, retinal hemodynamic parameters were measured every 20 minutes. Retinal vessel diameters and retinal blood velocity were assessed with the retinal vessel analyzer and bidirectional laser Doppler flowmetry, respectively. In addition, the response of retinal vessel diameters to stimulation with diffuse flicker light was studied. Blood pressure and intraocular pressure were measured with noninvasive techniques. RESULTS: Sildenafil had no effect on mean arterial pressure, pulse rate, intraocular pressure, retinal blood velocity, or retinal arterial diameter. However, a significant increase in retinal venous diameters (4.7% +/- 3.2%; P=0.0028 versus placebo) and retinal blood flow 15.7% +/- 18.0%; P=0.029 versus placebo) was observed. Sildenafil had no effect on flicker-induced vasodilation in retinal arteries or veins. CONCLUSIONS: The data indicate that sildenafil increases retinal venous diameters and retinal blood flow in healthy subjects. By contrast, it does not affect intraocular pressure and flicker-induced retinal vasodilation. Further studies are needed to elucidate whether this drug may be therapeutically used in retinal ischemic disease.

Increased plasma levels of plasminogen activator inhibitor-1 and soluble vascular cell adhesion molecule after triacylglycerol infusion in man.

Increased plasma plasminogen activator inhibitor-1 (PAI-1) has been implicated in the development of vascular disease. In type 2 diabetes mellitus high PAI-1 levels are associated with increased plasma concentrations of free fatty acids (FFA) and triacylglycerol indicating an association or a causal relationship. To answer that question, the effect of FFA/triacylglycerol on plasma PAI-1 was examined. Ten healthy male volunteers were studied for 6 h during infusion of triacylglycerol [1.5 ml/min]/heparin [0.2 IU/(kg.min)] (LIP; n=10), saline only (SAL; n=10), and saline/heparin (HEP; n=5). Plasma insulin concentrations were kept constant at approximately 35 pmol/l by intravenous somatostatin-insulin infusions and there was no significant change in plasma glucose levels during any of the study protocols. LIP increased plasma triacylglycerol and FFA approximately 3- (p < 0.001) and approximately 8- (p < 0.000001) fold, respectively, within 90 min. Baseline plasma PAI-1 measured by a bio-immunoassay was similar in HEP (11.4 +/- 2.8 ng/ml), SAL (16.6 +/- 3.6 ng/ml), and LIP studies (15.2 +/- 3.4 ng/ml). Since studies were initiated in the morning, PAI-1 decreased (p < 0.025) over time following its normal diurnal variation to 6.4 +/- 2.0 ng/ml and 4.0 +/- 2.4 ng/ml at 360 min in SAL and HEP, respectively. During LIP, however, PAI-1 increased to approximately 2.6 fold higher levels than during SAL at 360 min (16.4 +/- 4.0 ng/ml, p < 0.01). While tissue plasminogen activator (tPA) and adipsin, an adipocyte derived protease, were unaffected by LIP, changes in soluble vascular cell adhesion molecule-1 (sVCAM-1) were significantly correlated (p = 0.02) with those seen for PAI-1. This suggests that hyperlipidemia independent of insulin and plasma glucose levels stimulates vascular tissue and in turn might induce an increase in plasma PAI-1. PAI-1 then could contribute to the development of atherothrombotic vascular disease.

Nitric oxide regulates retinal vascular tone in humans.

The purpose of the present study was to investigate the contribution of basal nitric oxide (NO) on retinal vascular tone in humans. In addition, we set out to elucidate the role of NO in flicker-induced retinal vasodilation in humans. Twelve healthy young subjects were studied in a three-way crossover design. Subjects received an intravenous infusion of either placebo or NG-monomethyl-L-arginine (L-NMMA; 3 or 6 mg/kg over 5 min), an inhibitor of NO synthase. Thereafter, diffuse luminance flicker was consecutively performed for 16, 32, and 64 s at a frequency of 8 Hz. The effect of L-NMMA on retinal arterial and venous diameter was assessed under resting conditions and during the hyperemic flicker response. Retinal vessel diameter was measured with a Zeiss retinal vessel analyzer. L-NMMA significantly reduced arterial diameter (3 mg/kg: -2%; 6 mg/kg: -4%, P < 0.001) and venous diameter (3 mg/kg: -5%; 6 mg/kg: -8%, P < 0.001). After placebo infusion, flicker induced a significant increase in retinal vessel diameter (P < 0.001). At a flicker duration of 64 s, arterial diameter increased by 4% and venous diameter increased by 3%. L-NMMA did not abolish these hyperemic responses but blunted venous vasodilation (P = 0.017) and arterial vasodilation (P = 0.02) in response to flicker stimulation. Our data indicate that NO contributes to basal retinal vascular tone in humans. In addition, NO appears to play a role in flicker-induced vasodilation of the human retinal vasculature.

Regulation of human retinal blood flow by endothelin-1.

There is evidence from in vitro and animal studies that endothelin is a major regulator of retinal blood flow. We set out to characterize the role of the endothelin-system in the blood flow control of the human retina. Two studies in healthy subjects were performed. The study design was randomized, placebo-controlled, double-masked, balanced, two-way crossover in protocol A and three way-way crossover in protocol B. In protocol A 18 healthy male subjects received intravenous endothelin-1 (ET-1) in a dose of 2.5 ng kg (-1)min(-1) for 30 min or placebo on two different study days and retinal vessel diameters were measured. In protocol B 12 healthy male subjects received ET-1 in stepwise increasing doses of 0, 1.25, 2.5 and 5 ng kg (-1)min(-1) (each infusion step over 20 min) in co-infusion with the specific ET(A)-receptor antagonist BQ123 (60 microg min (-1)) or placebo or BQ123 alone investigating retinal vessel diameters, retinal blood velocity and retinal blood flow. Measurements of retinal vessel size were done with the Zeiss retinal vessel analyzer. Measurements of blood velocities were done with bi-directional laser Doppler velocimetry. From these measurements retinal blood flow was calculated. In protocol A exogenous ET-1 tended to decrease retinal arterial diameter, but this effect was not significant versus placebo. No effect on retinal venous diameter was seen. In protocol B retinal venous blood velocity and retinal blood flow was significantly reduced after administration of exogenous ET-1. These effects were significantly blunted when BQ-123 was co-administered. By contrast, BQ-123 alone had no effect on retinal hemodynamic parameters. Concluding, BQ123 antagonizes the effects of exogenously administered ET-1 on retinal blood flow in healthy subjects. In addition, the results of the present study are compatible with the hypothesis that ET-1 exerts its vasoconstrictor effects in the retina mainly on the microvessels.

Influence of flicker frequency on flicker-induced changes of retinal vessel diameter.

PURPOSE: To investigate the effect of diffuse luminance flicker of different frequencies on retinal vessel diameter in the human eye. METHODS: Nine healthy subjects participated in the study. A retinal vessel analyzer (RVA; Zeiss, Jena, Germany) was modified to allow for continuous measurement of vessel diameter during flicker stimulation. With this technique, the light used for measurement of vessel diameter consists of wavelengths between 567 and 587 nm, whereas the spectrum of the stimulation is low-pass, with a cutoff wavelength at 550 nm. Flicker frequencies ranged from 2 to 64 Hz. RESULTS: In retinal arteries an increase was observed at all flicker frequencies, with a less-pronounced effect at 64 Hz. In retinal veins, all flicker frequencies except 2 and 64 Hz induced vasodilation. Generally, the flicker-induced diameter response was less pronounced in retinal veins than in arteries. CONCLUSIONS: The flicker-induced diameter response of the larger retinal arteries and veins can be continuously measured with a modified RVA system that spectrally separates the flicker stimulation from the fundus illumination light. Vasodilation of retinal arteries was observed in response to short wavelength flicker with frequencies between 2 and 64 Hz, indicating that the parvo- and magnocellular neural pathways are activated with this stimulation.

Retinal blood flow during hyperoxia in humans revisited: concerted results using different measurement techniques.

Retinal vasculature shows pronounced vasoconstriction in response to hyperoxia, which appears to be related to the constant oxygen demand of the retina. However, the exact amount of blood flow reduction and the exact time course of this phenomenon are still a matter of debate. We set out to investigate the retinal response to hyperoxia using innovative techniques for the assessment of retinal hemodynamics. In a total of 48 healthy volunteers we studied the effect of 100% O(2) breathing on retinal blood flow using two methods. Red blood cell movement in larger retinal veins was quantified with combined laser Doppler velocimetry and retinal vessel size measurement. Retinal white blood cell movement was quantified with the blue field entoptic technique. The time course of retinal vasoconstriction in response to hyperoxia was assessed by continuous vessel size determination using the Zeiss retinal vessel analyzer. The response to hyperoxia as measured with combined laser Doppler velocimetry and vessel size measurement was almost twice as high as that observed with the blue field technique. Vasoconstriction in response to 100% O(2) breathing occurred within the first 5 min and no counterregulatory or adaptive mechanisms were observed. Based on these results we hypothesize that hyperoxia-induced vasoconstriction differentially affects red and white blood cell movement in the human retina. This hypothesis is based on the complex interactions between red and white blood cells in microcirculation, which have been described in detail for other vascular beds.

Effect of noradrenaline on retinal blood flow in healthy subjects.

PURPOSE: To gain insight into the role of circulating catecholamines on retinal blood flow in vivo. DESIGN: Nonrandomized, open, crossover design. PARTICIPANTS: In 10 healthy male subjects, tyramine and noradrenaline were administered in stepwise increasing doses. These doses were selected to induce comparable changes in systemic blood pressure. METHODS: During each infusion step, retinal vessel diameter and retinal venous blood speed were measured with the Zeiss retinal vessel analyzer (Zeiss, Jena, Germany) and laser Doppler velocimetry, respectively. MAIN OUTCOME MEASURES: Retinal blood flow through a major temporal vein was calculated. RESULTS: As expected, tyramine and noradrenaline induced a systemic hypertensive response. Tyramine caused a moderate increase in noradrenaline plasma levels, whereas exogenous noradrenaline increased noradrenaline plasma levels more than 10-fold. Nevertheless, neither tyramine nor noradrenaline induced any effect on retinal hemodynamic parameters. CONCLUSIONS: These data indicate that even high levels of circulating noradrenaline have little impact on retinal vascular tone and retinal blood flow. Hence, the adrenergic system appears not to play a major role in retinal blood flow regulation.