Effects of high-dose prednisolone on optic nerve head blood flow in patients with acute optic neuritis.

BACKGROUND: In this study, patients with optic neuritis were treated with high-dose prednisolone. Little information is available about the effects of this treatment on ocular blood flow. We set out to investigate the effects of high-dose prednisolone on optic nerve head (ONH) blood flow in patients with acute optic neuritis. METHODS: Thirteen patients with acute optic neuritis were included in the study. 1000 mg of prednisolone was infused intravenously over 30 minutes on 3 consecutive days. On each study day, ONH blood flow was measured using laser Doppler flowmetry. The ocular hemodynamic measurements were performed on the unaffected eye of the patients with unilateral acute optic neuritis before and immediately after cessation of the infusion. Intraocular pressure (IOP) and systemic blood pressure was measured before and after the infusion on each study day. Data was analyzed using a repeated measures ANOVA model. RESULTS: Prednisolone increased ONH blood flow in the patients under study (p = 0.04), although the effects were in generally small. No significant change in mean arterial pressure (p = 0.70) or IOP (p = 0.20) could be detected in the patients treated with high-dose prednisolone. CONCLUSIONS: A small but significant increase in ONH blood flow resulted from infusion of high-dose prednisolone. Further studies are required to investigate whether this effect contributes to the therapeutic efficacy of cortisone in patients with optic neuritis.

Response of retinal vessel diameters to flicker stimulation in patients with early open angle glaucoma.

INTRODUCTION: Diffuse luminance flicker increases retinal vessel diameters in animals and humans, indicating the ability of the retina to adapt to different metabolic demands. The current study seeks to clarify whether flicker-induced vasodilatation of retinal vessels is diminished in glaucoma patients. METHODS: Thirty-one patients with early stage glaucoma (washout for antiglaucoma medication) and 31 age- and sex- matched healthy volunteers were included in the study. Retinal vessel diameters were measured continuously with a Retinal Vessel Analyzer. During these measurements three episodes of square wave flicker stimulation periods (16, 32, and 64 secs; 8 Hz) were applied through the illumination pathway of the retinal vessel analyser. RESULTS: Flicker-induced vasodilatation in retinal veins was significantly diminished in glaucoma patients as compared with healthy volunteers (ANOVA, P < 0.01). In healthy volunteers, retinal venous vessel diameters increased by 1.1 +/- 1.8% (16 seconds, P < 0.001), 2.0 +/- 2.6 (32 seconds, P < 0.001), and 2.1 +/- 2.1% (64 seconds, P < 0.001) during flicker stimulation. In glaucoma patients, venous vessel diameters increased by 0.2 +/- 1.7% (16 seconds, P < 0.6), 1.1 +/- 2.1% (32 seconds, P < 0.01), and 0.8 +/- 2.5 (64 seconds, P < 0.09). In retinal arteries, no significant difference in flicker response was noticed between the two groups (ANOVA, P < 0.6). In healthy controls, flicker stimulation increased retinal arterial vessel diameters by 1.0 +/- 2.4% (P < 0.03), 1.6 +/-3.2% (P < 0.004) and 2.4 +/- 2.6% (P < 0.001) during 16, 32, and 64 seconds of flicker, respectively. In glaucoma patients, flickering light changed arterial vessel diameters by 0.3 +/-2.6% (16 seconds, P = 0.4), 1.3 +/-3.1% (32 seconds, P = 0.03), and 1.8 +/- 3.8% (64 seconds, P = 0.005). CONCLUSION: Flicker-induced vasodilatation of retinal veins is significantly diminished in patients with glaucoma compared with healthy volunteers. This indicates that regulation of retinal vascular tone is impaired in patients with early glaucoma, independently of antiglaucoma medication.

Reduced response of retinal vessel diameters to flicker stimulation in patients with diabetes.

BACKGROUND/AIM: Stimulation of the retina with flickering light increases retinal arterial and venous diameters in animals and humans, indicating a tight coupling between neural activity and blood flow. The aim of the present study was to investigate whether this response is altered in patients with insulin dependent diabetes mellitus. METHODS: 26 patients with diabetes mellitus with no or mild non-proliferative retinopathy and 26 age and sex matched healthy volunteers were included in the study. Retinal vessel diameters were measured continuously with the Zeiss retinal vessel analyser. During these measurements three episodes of square wave flicker stimulation periods (16, 32, and 64 seconds; 8 Hz) were applied through the illumination pathway of the vessel analyser. RESULTS: In retinal arteries, the response to stimulation with diffuse luminance flicker was significantly diminished in diabetic patients compared to healthy volunteers (ANOVA, p<0.0031). In non-diabetic controls flicker stimulation increased retinal arterial diameters by +1.6% (1.8%) (mean, p<0.001 v baseline), +2.8% (SD 2.2%) (p<0.001) and +2.8% (1.6%) (p<0.001) during 16, 32, and 64 seconds of flicker stimulation, respectively. In diabetic patients flicker had no effect on arterial vessel diameters: +0.1% (3.1%) (16 seconds, p = 0.9), +1.1% (2.7%) (32 seconds, p = 0.07), +1.0% (2.8%) (64 seconds, p = 0.1). In retinal veins, the response to flicker light was not significantly different in both groups. Retinal venous vessel diameters increased by +0.7% (1.6%) (16 seconds, p<0.05), +1.9% (2.3%) (32 seconds, p<0.001) and 1.7% (1.8%) (64 seconds, p<0.001) in controls during flicker stimulation. Again, no increase was observed in the patients group: +0.6% (2.4%), +0.5% (1.5%), and +1.2% (3.1%) (16, 32, and 64 seconds, respectively). CONCLUSION: Flicker responses of retinal arteries and veins are abnormally reduced in patients with IDDM with no or mild non-proliferative retinopathy. Whether this diminished response can be attributed to altered retinal vascular reactivity or to decreased neural activity has yet to be clarified.

Diffuse luminance flicker increases blood flow in major retinal arteries and veins.

It has been shown that diffuse luminance flicker increases optic nerve head blood flow. The current study has been performed to quantify changes in retinal blood flow during flicker stimulation. In a group of 11 healthy volunteers, red blood cell velocity and retinal vessel diameters were assessed with bi-directional laser Doppler velocimetry and the Zeiss retinal vessel analyzer before, during and after stimulation with diffuse luminance flicker. Retinal blood flow was calculated for each condition. Flicker stimulation increased retinal blood flow by +59 +/- 20% (p<0.01) in arteries and by +53 +/- 25% (p<0.01) in retinal veins. These results demonstrate that diffuse luminance flicker increases retinal blood flow in the human retina.

Choroidal blood flow and arterial blood pressure.

PURPOSE: Untreated hypertension is associated with ocular complications and is a risk factor for the development and progression of vascular ocular pathologies. We set out to investigate the association between systemic blood pressure and choroidal blood flow. METHODS: All subjects were male non-smokers, who did not receive any medication and had normal or slightly elevated blood pressure (systolic blood pressure < or = 160 mmHg; diastolic blood pressure < or = 100 mmHg). The association between systemic blood pressure and fundus pulsation amplitude, a measure of pulsatile choroidal blood flow, was investigated in 318 volunteers. In addition, the association between systemic blood pressure and blood flow velocities in the posterior ciliary arteries supplying the choroid was investigated in these subjects. RESULTS: Ocular fundus pulsation amplitude (r = 0.252; P < 0.001) and mean flow velocity in the posterior ciliary arteries (r = 0.346, P < 0.001) were significantly associated with mean arterial pressure. The correlation of ocular haemodynamic variables with systolic and diastolic blood pressure was in the same range. CONCLUSIONS: Our data indicate a small, but significant increase in choroidal blood flow with increasing blood pressure.

Effect of inhalation of different mixtures of O(2) and CO(2) on retinal blood flow.

AIM: To determine the effects of various mixtures of O(2) and CO(2) on retinal blood flow in healthy subjects. METHODS: A randomised, double masked, four way crossover trial was carried out in 12 healthy male non-smoking subjects. Gas mixtures (100% O(2), 97.5% O(2) + 2.5% CO(2), 95% O(2) + 5% CO(2), and 92% O(2) + 8% CO(2)) were administered for 10 minutes each. Two non-invasive methods were used: laser Doppler velocimetry (LDV) for measurement of retinal blood velocity and fundus imaging with the Zeiss retinal vessel analyser (RVA) for the assessment of retinal vessel diameters. Arterial pH, pCO(2), and pO(2) were determined with an automatic blood gas analysis system. Retinal blood flow through a major temporal vein was calculated. RESULTS: Retinal blood velocity, retinal vessel diameter, and retinal blood flow decreased during all breathing periods (p <0.001 each). Administration of 92% O(2) + 8% CO(2) significantly increased SBP, MAP, and PR (p <0.001 each, versus baseline), whereas the other gas mixtures had little effect on systemic haemodynamics. Addition of 2.5%, 5%, and 8% CO(2) to oxygen caused a marked decrease in pH and an increase in pCO(2) (p <0.001 versus pure oxygen). CONCLUSIONS: Breathing of pure oxygen and oxygen in combination with carbon dioxide significantly decreases retinal blood flow. Based on these data the authors speculate that hyperoxia induced vasoconstriction is not due to changes in intravascular pH and cannot be counteracted by an intravascular increase in pCO(2).

Influence of diffuse luminance flicker on choroidal and optic nerve head blood flow.

PURPOSE: In the retina there is general agreement that blood flow adapts in response to different conditions of light and darkness including diffuse luminance flicker. By contrast, regulation of choroidal blood flow in response to different light conditions is still a matter of controversy. Thus, we investigated the effect of diffuse luminance flicker on choroidal and optic nerve head blood flow. METHODS: In a group of 14 healthy volunteers, choroidal blood flow and ocular fundus pulsation amplitude were assessed with laser Doppler flowmetry and laser interferometry, respectively. Measurements were done before, during and after stimulation with diffuse luminance flicker. Furthermore, the response of optic nerve head blood flow (ONHBF) to flicker stimulation was measured. Flicker stimuli were generated by a Grass PS2 photostimulator, stimulating at a frequency of 8 Hz. Flicker light consisted of light flashes at a wavelength below 550 nm and produced a retinal irradiance of 140 microW/cm( 2). Blood pressure and pulse rate were measured non-invasively. Paired t-test was used for statistical analysis. RESULTS: ONHBF increased immediately after onset of flicker stimulation. The maximum increase in ONHBF was 30% +/- 10% (mean +/- SEM, p < 0.008). Both choroidal perfusion parameters were only slightly increased during flicker stimulation, by 2 +/- 2% (laser Doppler flowmetry, p < 0.5) and by 4 +/- 1% (laser interferometry, p < 0.12). After the end of stimulation all values returned to baseline levels. CONCLUSION: Our study clearly demonstrates that diffuse luminance flicker increases optic nerve head blood flow. In contrast, increased neural activity in the retina has no effect on choroidal blood flow. Thus, choroidal blood flow appears to be largely independent of alterations in retinal metabolism.

Response of retinal blood flow to CO2-breathing in humans.

PURPOSE: To investigate the effect of systemic hypercapnia on retinal hemodynamics in humans. METHODS: We studied the effect of breathing a mixture of normal air with 5% CO2 for 13 minutes in ten healthy young male volunteers, using the Zeiss retinal vessel analyzer for continuous measurement of retinal vessel diameter and the blue-field entoptic technique to quantify retinal white blood cell flux. In eight other subjects the effect of hypercapnia was measured with the Zeiss retinal vessel analyzer and by laser Doppler velocimetry to establish retinal blood flow velocity. RESULTS: Retinal arterial and venous vessel diameters increased by a maximum of 4.2% and 3.2%, respectively. Peak effect was observed after 3 minutes of breathing the mixture of normal air with 5% CO2. During hypercapnia red blood cell velocity increased 11.7% and, accordingly, retinal blood flow increased 19.1%. White blood cell density and velocity rose significantly during hypercapnia, resulting in an increase in white blood cell flux (19.2%). CONCLUSIONS: Our data indicate that CO2 induces vasodilation in retinal arteries and retinal veins. Retinal blood flow and perimacular white blood cell flux increased to the same extent in subjects breathing a mixture of normal air with 5% CO2.

Retinal blood flow and systemic blood pressure in healthy young subjects.

BACKGROUND: The aim of the present study was to investigate the association between systemic blood pressure and retinal blood flow in healthy young subjects. METHODS: Three independent study cohorts were included. A cross-sectional study was performed in 420 young male subjects with systolic blood pressure < 160 mmHg and diastolic blood pressure <100 mmHg. Retinal white blood cell flux (n=210) and blood velocity in the central retinal artery (n=210) were measured. In addition, a longitudinal study was performed in 40 young male subjects in whom retinal and systemic haemodynamic parameters were measured thrice within 6 weeks. Retinal white blood cell flux was measured with the blue-field entoptic technique. Blood flow velocity in the central retinal artery was measured by means of colour Doppler imaging. RESULTS: Retinal white blood cell flux (r=0.262; P<0.001) and mean flow velocity in the central retinal artery (r=0.174, P=0.010) were significantly associated with mean arterial pressure in the cross-sectional study. In the longitudinal study retinal white blood cell flux and mean flow velocity in the central retinal artery were also correlated with systemic blood pressure. CONCLUSIONS: Our data indicate a slight but significant increase in retinal blood flow with blood pressure. Whether this is of clinical relevance in eye diseases with altered retinal perfusion, such as diabetic retinopathy, remains to be established.

Perinatal 'programming' of insulin resistance in childhood: critical impact of neonatal insulin and low birth weight in a risk population.

AIM: Low birth weight may predispose to later insulin resistance and hyperinsulinaemia, but the pathophysiological mechanisms are unclear. The perinatal endocrine situation may play an important role, but has been little studied. Children of mothers with diabetes during pregnancy are an important risk population for later insulin resistance and hyperinsulinaemia. We therefore examined relationships between birth weight, insulin and insulin resistance at birth, and insulin secretion and insulin resistance in infancy in these children. METHODS: We studied 104 infants of mothers with Type 1 diabetes mellitus during pregnancy. Oral glucose tolerance tests (area under the curve of glucose, AUCG) with determination of insulin (area under the curve of insulin, AUCI) were performed at 1-5 years of age. Using correlation and regression analysis, birth data were related to insulin secretion (AUCI) and stimulated insulin/glucose ratio (AUCI/AUCG) in childhood. RESULTS: Children with an AUCI in the highest tertile of distribution had the lowest birth weights. Birth weight was negatively correlated to AUCI in childhood (P = 0.03). Insulin/glucose ratio at birth was raised in infants with an AUCI in the upper tertile, accompanied by a positive correlation between insulin/glucose ratio at birth and AUCI (P = 0.02). Insulin and insulin/glucose ratio at birth were both positively correlated to AUCI/AUCG (P = 0.04 and P = 0.02 respectively), while the correlation between birth weight and AUCI/AUCG was not significant (P = 0.12). In a stepwise regression analysis, insulin/glucose ratio contributed as much as birth weight to AUCI in childhood. Birth weight, however, was significantly negatively related to AUCI/AUCG only when the insulin/glucose ratio at birth was included in the regression model. CONCLUSIONS: Insulin and insulin resistance at birth show a positive relation to insulin secretion and insulin resistance in later life, in addition to the influence of a low birth weight, but independent of it. Perinatal and neonatal insulin, known to be of critical importance for long-term outcome, should be evaluated in future studies on the 'small baby syndrome'.

Effects of atropine and propranolol on retinal vessel diameters during isometric exercise.

PURPOSE: There is controversy regarding the nervous control of retinal blood vessels in humans. Most in vitro studies indicate that the intraocular part of the central retinal artery lacks autonomic innervation. We investigated the response of retinal vessels to isometric exercise during blockade of beta-receptors (propranolol) or muscarinic receptors (atropine). METHODS: Twelve healthy subjects performed squatting for 6 min during infusion of either propranolol atropine or placebo. Blood pressure and pulse rate were measured non-invasively. Retinal vessel diameters were measured continuously using the Zeiss Retinal Vessel Analyser. RESULTS: Squatting induced a significant increase in blood pressure and pulse rate, which was paralleled by a decrease in retinal vein and artery diameters. Atropine did not change the retinal vessel response to isometric exercise. Propranolol significantly blunted the exercise-induced vasoconstriction in retinal arteries. CONCLUSION: This result likely indicates propranolol-evoked vasoconstriction in the extraocular parts of the central retinal artery during isometric exercise.

Similar effects of atorvastatin, simvastatin and pravastatin on thrombogenic and inflammatory parameters in patients with hypercholesterolemia.

BACKGROUND: Previous studies have suggested that statins exert beneficial effects beyond their favorable lipid lowering effect. Particularly, the modification of thrombus formation and degradation, alteration in inflammatory response, plaque stabilization and improved endothelial function are thought to be responsible for additional reduction of morbidity and mortality due to cardiovascular events. To date, however, it is still unclear whether these effects are elicited by all statins. METHODS AND RESULTS: We set out to compare in a controlled, randomized, double-blind study design the effects of almost equieffective cholesterol lowering doses of three chemically and pharmacokinetically different statins (atorvastatin, simvastatin, pravastatin) on hemostatic and inflammatory markers in 99 hypercholesterolemic patients. At entry and 3 months after onset of statin therapy plasma cholesterol and von Willebrand factor antigen (vWf-Ag), fibrinogen, d-dimer, prothrombin fragment 1+2 (F1.2) and C-reactive protein (CRP) were measured. The effect on plasma values of F1.2, vWf-Ag, d-dimer and CRP was not significantly different between the three treatment groups. The effect of simvastatin on fibrinogen (p = 0.005) was more pronounced than the effects of atorvastatin (p = 0.48 n.s.) and pravastatin (p = 0.15 n.s.). Plasma levels of F1.2 and vWf-Ag (when data of all statins were pooled) were significantly reduced by 7% and 10% versus baseline, respectively. No significant reduction was observed for d-dimer (p = 0.26) and CRP (p = 0.5). Total plasma cholesterol levels decreased significantly (p < 0.0001 in all groups) between 22% and 29% compared to baseline. CONCLUSION: The present study shows similar short-term (3 months) effects of atorvastatin, simvastatin and pravastatin on selected hemostatic and inflammatory parameters in plasma in patients with hypercholesterolemia. Thus, chemical and pharmacological differences between statins appear to exert no major influence on these parameters.

Female infertility--effect of perinatal xenoestrogen exposure on reproductive functions in animals and humans.

Environmental chemicals with inherent estrogenic activity are supposed to be responsible for the decrease of quantity and quality of human sperms during the past 40 years. The current hypothesis is that estrogenic agents acting during fetal life may lead to impaired development of the testes and of the male reproductive tract in the human as well as in several wildlife species. However, from clinical and experimental data it is known, that estrogens may also lead to impairment of female reproductive functions. Perinatal application of DDT, octylphenol and benzylbutylphthalate resulted in persistent estrus in rats, in- and subfertility, respectively, and impaired sexual behaviour. Epidemiological and experimental data speak in favour of environmental estrogens to be one cause for the development of polycystic ovaries, being the most important reason for female infertility in the human.

Genetic and epigenetic effects on sexual brain organization mediated by sex hormones.

Alterations of sex hormone levels during pre- or perinatal sexual brain organization - responsible for long-term changes of gonadotropin secretion, sexual orientation, and gender role behavior - can be caused by: 1. Genetic effects, i.e. mutations or polymorphisms of a) 21-hydroxylase genes on chromosome 6, b) 3beta-hydroxysteroid dehydrogenase genes in chromosome 1 or c) X-chromosomal genes, and 2. Epigenetic effects, such as a) stressful situations - especially in combination with mutations - and b) endocrine disrupters, e.g. the pesticide DDT and its metabolites, which display estrogenic, antiandrogenic, and inhibitory effects on the enzyme 3beta-hydroxysteroid dehydrogenase leading to increased levels of dehydroepiandrosterone and its sulfate as precursors of endogenous androgens and estrogens. In connection with the introduction and extensive use of the pesticide DDT, the following findings were obtained in subjects born before as compared to those born during this period: 1. The prevalence of patients with polycystic ovaries (PCO), idiopatic oligospermia (IO), and transsexualism (TS) increased significantly (about 3-4 fold). 2. Partial 21-hydroxylase deficiencies were observed in most patients with PCO and TS and some patients with IO born before this period. 3. In contrast, most patients with PCO and TS and several patients with IO born during the period of massive use of DDT displayed clearly increased plasma levels of dehydroepiandrosterone sulfate (DHEA-S) and DHEA-S/cortisol ratios suggesting partial 3beta-hydroxsteroid dehydrogenase (3beta-HSD) deficiencies. Interestingly enough, geneticists could not find any mutations of 3beta-HSD genes in such subjects. However, o,p'-DDT and/or its metabolite o,p'-DDD are strong inhibitors of 3beta-HSD, indicating their possible co-responsibility for such life-long ontogenetic alterations. Finally, some data suggest that endocrine disrupters may also be able to affect the development of sexual orientation.

Assessment of optic disk blood flow in patients with open-angle glaucoma.

PURPOSE: To characterize optic disk blood flow in patients with open-angle glaucoma compared with age-matched healthy control subjects. METHODS: In this prospective cross-sectional study, 90 eyes of 90 patients with open-angle glaucoma and 61 eyes of 61 age-matched healthy control subjects were evaluated. Flow in the optic disk cup and the neuroretinal rim were assessed with scanning laser Doppler flowmetry. Fundus pulsation amplitude in the cup and the macula were assessed with laser interferometry. Visual field mean deviation was measured with the Humphrey 30 to 2 program. RESULTS: Flow in the neuroretinal rim (-18%, P =.002), and in the cup (-46%, P <.001) and fundus pulsation amplitude in the cup (-33%, P <.001) and in the macula (-24%, P <.001) were significantly lower in patients with open-angle glaucoma compared with healthy control subjects. A significant association between blood flow measurements in the cup and fundus pulsation amplitudes in the cup was observed in both study cohorts. A significant association was also observed between the mean defect from visual field testing and ocular hemodynamic parameters. CONCLUSIONS: Reduced optic disk perfusion in patients with open-angle glaucoma is evidenced from two independent methods in the present study. Moreover, our data indicate that reduced ocular blood flow in these patients is linked to visual field changes. It remains to be established whether compromised optic disk and choroidal blood flow contributes to optic disk damage in glaucomatous eyes or is a secondary functional phenomenon.

Hypothalamic nuclei are malformed in weanling offspring of low protein malnourished rat dams.

Maternal low protein malnutrition during gestation and lactation (LP) is an animal model frequently used for the investigation of long-term deleterious consequences of perinatal growth retardation. Both perinatal malnutrition and growth retardation at birth are risk factors for diabetic and cardiovascular disturbances in later life. The pathophysiologic mechanisms responsible are unknown. Hypothalamic nuclei are decisively involved in the central nervous regulation of food intake, body weight and metabolism. We investigated effects of a low protein diet (8% protein; control diet, 17% protein) during gestation and lactation in rat dams on the organization of hypothalamic regulators of body weight and metabolism in the offspring at weaning (d 20 of life). LP offspring had significantly lower body weight than control offspring (CO; P: < 0.001), associated with hypoglycemia and hypoinsulinemia (P: < 0. 005) on d 20 of life. This was accompanied by a greater relative volume of the ventromedial hypothalamic nucleus (P: < 0.01) and a greater numerical density of Nissl-stained neurons in this nucleus (P: < 0.01) as well as in the paraventricular hypothalamic nucleus (PVN; P: < 0.001). In contrast, no significant differences in neuronal densities were observed generally in the lateral hypothalamic area, arcuate hypothalamic nucleus (ARC), and dorsomedial hypothalamic nucleus between LP offspring and CO offspring. On the other hand, LP offspring displayed fewer neurons immunopositive for neuropeptide Y in the ARC (P: < 0.05), whereas in the PVN, lower neuronal densities of neurons immunopositive for galanin were found in LP offspring compared with CO offspring (P: < 0.001). On the contrary, in the PVN, no significant group difference in the numerical density of cholecystokinin-8S-positive neurons was present. A long-term effect of these specific hypothalamic alterations on body weight and metabolism in LP offspring during later life is suggested.

Evaluation of the Zeiss retinal vessel analyser.

AIM: To investigate the reproducibility and sensitivity of the Zeiss retinal vessel analyser, a new method for the online determination of retinal vessel diameters in healthy subjects. METHODS: Two model drugs were administered, a peripheral vasoconstrictor (the alpha receptor agonist phenylephrine) and a peripheral vasodilator (the nitric oxide donor sodium nitroprusside) in stepwise increasing doses. Nine healthy young subjects were studied in a placebo controlled double masked three way crossover design. Subjects received intravenous infusions of either placebo or stepwise increasing doses of phenylephrine (0.5, 1, or 2 microg/kg/min) or sodium nitroprusside (0.5, 1, or 2 microg/kg/min). Retinal vessel diameters were measured with the new Zeiss retinal vessel analyser. Retinal leucocyte velocity, flow, and density were measured with the blue field entoptic technique. The reproducibility of measurements was assessed with coefficients of variation and intraclass correlation coefficients. RESULTS: Placebo and phenylephrine did not influence retinal haemodynamics, although the alpha receptor antagonist significantly increased blood pressure. Sodium nitroprusside induced a significant increase in retinal venous and arterial diameters (p<0.001 each), leucocyte density (p=0.001), and leucocyte flow (p=0.024) despite lowering blood pressure to a significant degree. For venous and arterial vessel size measurements short term coefficients of variation were 1.3% and 2.6% and intraclass correlation coefficients were 0.98 and 0.96, respectively. The sensitivity was between 3% and 5% for retinal veins and 5% and 7% for retinal arteries. CONCLUSIONS: These data indicate that the Zeiss retinal vessel analyser is an accurate system for the assessment of retinal diameters in healthy subjects. In addition, nitric oxide appears to have a strong influence on retinal vascular tone.

Effects of systemic NO synthase inhibition on choroidal and optic nerve head blood flow in healthy subjects.

PURPOSE: There is evidence from animal studies that nitric oxide (NO) is a major determinant of ocular blood flow. In humans NO synthase inhibition reduces pulsatile choroidal blood flow, but no data on optic nerve head (ONH) vasculature are available yet. The goal of this study was to investigate the effects of NO synthase inhibition on human choroidal and ONH blood flow using laser Doppler flowmetry. METHODS: The study design was a randomized, placebo-controlled, double-masked, balanced three-way crossover. On separate study days 12 healthy male subjects received infusions of N:(G)-nitro-L-arginine (L-NMMA; either 3 mg/kg over 5 minutes followed by 30 microg/kg per minute over 55 minutes or 6 mg/kg over 5 minutes followed by 60 microg/kg per minute over 55 minutes) or placebo. The effects of L-NMMA or placebo on choroidal and ONH blood flow were measured with laser Doppler flowmetry. In addition, laser interferometric measurement of fundus pulsation was performed in the macula to assess pulsatile choroidal blood flow. RESULTS: L-NMMA reduced all outcome parameters in the choroid and the ONH. The higher dose of L-NMMA caused a significant decrease in blood flow in the choroid (-26% +/- 9%; P: < 0.001) and the ONH (-20% +/- 16%; P: < 0.001) as evidenced from laser Doppler flowmetry and a significant decrease in fundus pulsation amplitude (-26% +/- 5%; P: < 0.001). CONCLUSIONS: These results indicate that NO is continuously released in human choroidal and ONH vessels.

Endothelin-1 contributes to hyperoxia-induced vasoconstriction in the human retina.

PURPOSE: There is evidence that ocular blood flow strongly depends on arterial oxygen tension. Results from recent animal studies indicate that the vasoconstrictor response to hyperoxia may be mediated in part by an increased production of endothelin (ET)-1. In an effort to answer the question whether the retinal vasoconstrictive response to hyperoxia in humans is mediated through ET-1, changes in ocular hemodynamics induced by 100% O2 breathing were studied in the absence and presence of an ET(A) receptor antagonist (BQ-123). METHODS: The study was a randomized, placebo-controlled, double-masked, balanced, three-way crossover design. On separate study days 15 healthy male subjects received infusions of BQ-123 (either 60 microg/min or 120 microg/min) or placebo. The effects of BQ-123 or placebo on hyperoxia-induced (100% O2 breathing) changes in retinal and pulsatile choroidal blood flow were assessed with the blue-field entoptic technique and with laser interferometric measurement of fundus pulsation, respectively. RESULTS: During baseline conditions, hyperoxia caused a decrease in retinal blood flow between -29% and -34% (P<0.001) and a decrease in fundus pulsation amplitude between -7% and -8% (P<0.001). BQ-123 dose dependently blunted the response to hyperoxia in the retina (60 microg/min: -25%, 120 microg/min: -20%; P = 0.003), but not in the choroid. CONCLUSIONS: These results indicate that ET-1 contributes to hyperoxia-induced retinal vasoconstriction in the human retina.

Response of choroidal blood flow in the foveal region to hyperoxia and hyperoxia-hypercapnia.

PURPOSE: Arterial carbon dioxide tension and arterial oxygen tension are important determinants of retinal and cerebral blood flow. In the present study the hypothesis that changes in arterial blood gases also influence choroidal blood flow was tested. METHODS: The effect of breathing different mixtures of oxygen (O(2)) and carbon dioxide (CO(2)) on choroidal blood flow in the foveal region was investigated in healthy subjects. The study was performed in a randomized, double-masked four way cross-over design in 16 subjects. Using a compact laser Doppler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of various mixtures of O(2)and CO(2) (hyperoxia-hypercapnia): 100% O(2), 97%O(2)+3%CO(2), 95%O(2)+5%CO(2) (carbogen) and 92%O(2)+8%CO( 2). Arterial oxygen tension (pO(2)) and carbon dioxide tension (pCO(2)) were measured from arterialized blood samples from the earlobe. RESULTS: Breathing 100% O(2) had no significant effect on ChBVel (-3.7%), ChBVol (+1.7%) and ChBF (-4.3%). Addition of 3% CO(2) to O(2) also produced no significant change on these blood flow parameters. In contrast, carbogen significantly increased ChBVel (10.0 +/- 4.4%, 95% CI, p < 0.001) and ChBF (12.5 +/- 11.7%, p = 0.002). The effect of 92% O(2) + 8% CO(2) was more pronounced since it significantly increased ChBVel and ChBF by 15.5 +/- 7.5% (p < 0.001) and 16.2 +/- 11.0% (p < 0.001), respectively. None of the gas mixtures induced a significant change in ChBVol. The increase in ChBF was approximately 1.5% per 1 mmHg increase in pCO(2). CONCLUSIONS: This study demonstrates that, in healthy subjects, pCO(2) is an important determinant of foveal choroidal blood flow, whereas pO(2) has little impact on it.