Interaction between leukocytes and erythrocytes in the human retina: effects of pentoxifylline on hyperoxia-induced vasoconstriction during increased neutrophil counts.

PURPOSE: Pentoxifylline, a nonselective phosphodiesterase inhibitor, shows vasodilator effects in certain vascular beds and reduces blood viscosity. We have previously shown that under states of vasoconstriction an interaction between circulating erythrocytes and leukocytes may play a role in the control of blood flow. The reason for this observation is not entirely clear but may be related to a mechanical interaction between red and white blood cells. In the present study we hypothesized that pentoxifylline may alter this interaction during oxygen-induced vasoconstriction. METHODS: 24 healthy male subjects participated in this double masked, randomized, placebo-controlled 2 way cross over trial. In order to increase white blood cell count (WBC) count, 300 µg of G-CSF was administered intravenously. Vasoconstriction of retinal vessels was induced by oxygen inhalation. 400mg of pentoxifylline or placebo was infused at two different study days. White blood cell flux was assessed with the blue-field entoptic technique. Vessel calibers were measured with a dynamic vessel analyzer (DVA) and red blood cell velocity (RBCV) was determined with laser Doppler velocimetry (LDV). Retinal blood flow was calculated based on retinal vessel diameters and RBCV. RESULTS: Administration of G-CSF induced a significant increase in WBC, both in the placebo and the pentoxifylline group (p<0.01 for both groups). Retinal vessel diameter, RBCV, calculated retinal blood flow and white blood cell flow were not altered by administration of pentoxifylline. Hyperoxia induced a pronounced decrease in retinal blood flow parameters. No difference was observed between groups during oxygen breathing in vessel diameters (p=0.54), RBCV (p=0.34), calculated retinal blood flow (p=0.3) and white blood cell flow (p=0.26). CONCLUSION: Our data indicate that short time administration of pentoxifylline does not alter the oxygen-induced effect on ocular blood flow parameters during leukocytosis. Whether long-term treatment could improve retinal blood flow under states of vasoconstriction remains to be investigated.

Role of nitric oxide in optic nerve head blood flow regulation during isometric exercise in healthy humans.

PURPOSE: We determined whether administration of a nitric oxide synthase (NOS) inhibitor alters optic nerve head blood flow (ONHBF) regulation during isometric exercise in healthy subjects. METHODS: Our study was done in a randomized, placebo-controlled, double-masked, three-way crossover design. A total of 18 healthy subjects was randomized to receive either placebo, phenylephrine, or an inhibitor of NOS (L-NMMA) on three different study days. ONHBF was measured with laser Doppler flowmetry while the study participants performed isometric exercise (squatting). This was done before drug administration and during infusion of the study drugs. Mean arterial pressure (MAP) and IOP were measured noninvasively, and ocular perfusion pressure (OPP) was calculated as 2/3 MAP - IOP. RESULTS: The response in ONHBF to isometric exercise was less pronounced than the response in OPP, indicating for some autoregulatory capacity in the ONH. Administration of L-NMMA significantly decreased ONHBF at rest (P < 0.01). In contrast, inhibition of NOS did not alter the pressure-flow relationship in the ONH during an experimental increase in OPP compared to phenylephrine and placebo (P = 0.37 between groups). CONCLUSIONS: The data of our study support previous findings that ONHBF is autoregulated during an experimental increase in OPP. Nitric oxide has an important role in basal ONHBF regulation, but seems not to be involved in the autoregulatory response during an increase in OPP induced by isometric exercise. (ClinicalTrials.gov number, NCT00806741.).

Retinal white blood cell flux and systemic blood pressure in patients with type 1 diabetes.

AIMS: There is evidence that altered retinal blood flow and altered retinal blood flow regulation play a role in the development and progression of diabetic retinopathy. We compared the association between systemic blood pressure and retinal white blood cell flux in patients with type 1 diabetes and healthy control subjects. METHODS: The study was performed in 100 patients with type 1 diabetes with no or minimal diabetic retinopathy and a group of 313 age-matched healthy controls. Inclusion criteria were systolic blood pressure ≤ 160 mmHg and diastolic blood pressure ≤ 95 mmHg. None of the subjects took vasoactive medication except insulin. The blue field entoptic technique was used to assess retinal white blood cell flux, velocity and density in the perimacular region. Pressure-flow relationships were calculated for both groups to assess differences in blood flow regulation. RESULTS: Retinal white blood cell flux was comparable between the two study groups. Both type 1 diabetic patients and healthy subjects showed a significant positive correlation between retinal white blood cell flux and mean arterial pressure (diabetic patients: r=0.48; p<0.05, healthy subjects r=0.28). The correlation coefficients between mean arterial pressure and white blood cell flux were significantly higher in patients with diabetes than in the healthy control group (p=0.0459). CONCLUSION: Retinal white blood cell flux, as assessed with the blue-field entoptic technique, is not significantly different between type 1 diabetic patients with no or minimal retinopathy and healthy control subjects. Type 1 diabetic subjects do, however, show an abnormal association between systemic blood pressure and retinal white blood cell flux. This indicates altered autoregulation in early diabetic retinopathy.

Effects of increased white blood cell count on endothelin-induced vasoconstriction in healthy subjects.

It is known that administration of granulocyte-colony stimulating factor is followed by an increase of white blood cell count. There is evidence from other vascular beds that an increase in white blood cell count impairs blood flow regulation especially in the microcirculation. Whether this also holds true for the ocular circulation is unknown. In the following study we investigated whether an increase in white blood cell count alters the endothelin-1 induced vasoconstriction in humans. Neither granulocyte-colony stimulating factor nor endothelin-1 had any consistent effect on blood pressure, pulse rate or intraocular pressure. Administration of granulocyte-colony stimulating factor induced a pronounced increase in retinal white blood cell density (p < 0.01). Administration of endothelin-1 decreased choroidal (p < 0.01) and retinal blood flow (p < 0.01). The change in choroidal blood flow in response to endothelin-1 was not altered by pre-treatment with granulocyte-colony stimulating factor. By contrast, the decrease in retinal blood flow was more pronounced during an increase in white blood cell count (p = 0.02) when compared to placebo. Our data indicates that during pronounced vasoconstriction, as induced by administration of endothelin-1, vascular regulation can be altered by the number of circulating white blood cells. Whether this effect is caused by an interaction of red and white blood cells in the microcirculation or a yet unknown mechanism needs further investigation.

Effect of nifedipine on choroidal blood flow regulation during isometric exercise.

PURPOSE: To determine whether nifedipine, an L-type calcium channel blocker, alters choroidal blood flow (ChBF) regulation during isometric exercise in healthy subjects. METHODS: The study was carried out in a randomized, placebo-controlled, double-masked, two-way crossover design. Fifteen healthy male subjects were randomly assigned to receive either placebo or nifedipine on two different study days. Subfoveal ChBF was measured with laser Doppler flowmetry while the study participants performed isometric exercise (squatting). This was performed before drug administration and during infusion of nifedipine and placebo, respectively. Mean arterial pressure (MAP) and intraocular pressure (IOP) were measured noninvasively, and ocular perfusion pressure (OPP) was calculated as ⅔ MAP-IOP. RESULTS: MAP and OPP increased significantly during all squatting periods (P < 0.01). The increase in ChBF was less pronounced than the increase in OPP during isometric exercise. Nifedipine did not alter the OPP increase in response to isometric exercise, but it significantly augmented the exercise-induced increase in ChBF (P < 0.001 vs. placebo). Although ChBF increased by a maximum of 14.2% ± 9.2% during the squatting period when placebo was administered, the maximum increase during administration of nifedipine was 23.2% ± 7.2%. CONCLUSIONS: In conclusion, the data of the present study suggest that nifedipine augments the ChBF response to an experimental increase in OPP. In addition, it confirms that the choroidal vasculature has a significant regulatory capacity over wide ranges of OPPs during isometric exercise. (ClinicalTrials.gov number, NCT00280462.).

Effects of increased white blood cell count on retinal perfusion during hyperoxia-induced vasoconstriction.

PURPOSE: Several studies have shown that administration of granulocyte-colony stimulating factor (G-CSF) is followed by an increase of white blood cell (WBC) count. There is evidence from other vascular beds that an increase in WBC count impairs blood flow regulation especially in the microcirculation. Whether this also holds true for the ocular circulation is yet unknown. In the following trial we investigated whether an increase in WBC count alters the oxygen induced vasoconstriction of retinal vessels. METHODS: The study design was randomized, double-masked, placebo-controlled with two parallel groups. 24 healthy, male subjects were included. Measurements of retinal white blood cell flux with the blue-field entoptic technique, red blood cell velocity using the laser Doppler velocimeter and retinal vessel diameter using a Retinal Vessel Analyzer, were assessed at baseline and after breathing of 100% oxygen over 20 min. Thereafter 300 µg of G-CSF or placebo was administered. Measurements were repeated after another inhalation of 100% oxygen 8h later. RESULTS: G-CSF did not show any influence on systemic hemodynamics. WBC count increased significantly from 5.7 ± 1.6 × 10(9)/L at baseline up to 19.5 ± 4.8 × 10(9)/L 8h after G-CSF administration. As expected, oxygen breathing induced a pronounced vasoconstriction and a decrease red and white cell flux in both, the placebo and the G-CSF group (p<0.01 for both groups). Administration of G-CSF increased WBC flux, but did not affect red blood cell flux. The response of red blood cell flux and retinal vessel diameters to hyperoxia was not altered by G-CSF administration. However, leukocytosis leads to a more pronounced oxygen induced reduction in red blood cell velocity compared to the placebo group (p=0.024). CONCLUSION: Our data indicate that increased WBC count as induced with G-CSF, leads to a more pronounced reduction in retinal red blood cell flux during states of vasoconstriction. This indicates that, as in other vascular beds, an increase in WBC leads to an altered blood flow regulation.

Correlation of optic disc morphology and ocular perfusion parameters in patients with primary open angle glaucoma.

PURPOSE: Little information is available about the relationship between glaucomatous visual field defects, morphological changes of the optic disc and ocular blood flow. In this study, ocular blood flow parameters were correlated with parameters of optic nerve head (ONH) morphology and visual field performance in a cross-sectional study. METHODS: A total of 103 patients with primary open angle glaucoma were included. Choroidal and ONH blood flow was assessed using laser Doppler flowmetry. Retinal blood velocities and retinal vessel diameters were measured with laser Doppler velocimetry and a Retinal Vessel Analyzer, respectively. To evaluate the ONH morphology, fundus photographs were taken and confocal laser scanning tomography was performed. RESULTS: Among all measured ocular hemodynamic parameters, the ONH blood flow was most strongly correlated to structural parameters of ONH damage and visual field loss. Reduced retinal vessel diameters were only slightly correlated with the degree of glaucomatous damage. CONCLUSION: Reduced blood flow in the ONH was associated with increasing amount of visual field defect and morphological changes of the ONH. Retinal vessel diameters were only marginally associated with glaucomatous optic nerve damage. Based on retinal vessel diameter determination alone, it is not possible to assess whether reduced retinal blood flow is causative or secondary in glaucoma.

Effect of latanoprost on choroidal blood flow regulation in healthy subjects.

PURPOSE: The present study tested the hypothesis that human choroidal blood flow (ChBF) regulation in the face of changes in ocular perfusion pressure (OPP) may be modified by a drug-induced decrease in intraocular pressure (IOP). METHODS: This hypothesis was tested in a double-masked, randomized, placebo-controlled, parallel-group trial in 24 healthy volunteers. OPP was manipulated by 6 minutes of squatting and a subsequent period of artificial increase in IOP induced with a suction cup. These interventions were repeated after 14 days of treatment with either latanoprost or placebo. ChBF was measured continuously with a portable laser Doppler flowmeter. RESULTS: As expected, latanoprost significantly reduced IOP compared with placebo (P = 0.008). The relative increases in OPP during squatting (P = 0.97) and an artificial IOP increase (P = 0.75), however, were comparable after placebo and latanoprost. The response of ChBF was, in contrast, different between the two treatment groups. During the squatting-induced elevation of OPP, ChBF increased less after latanoprost than after placebo treatment (P = 0.049). During the suction cup-induced increase in IOP, the decrease in ChBF was less pronounced after latanoprost than after placebo (P = 0.026). Latanoprost, however, did not modify baseline ChBF at rest (P = 0.30). CONCLUSIONS: The data indicate that latanoprost improves ChBF regulation during both an increase and a decrease in OPP. Since latanoprost did not affect baseline ChBF, the authors assume that this effect is related to the decrease in IOP. This finding has important implications for understanding the relation between IOP and vascular factors in glaucoma, because it indicates that a reduction in IOP itself improves ChBF regulation.

Retrobulbar blood flow velocities in open angle glaucoma and their association with mean arterial blood pressure.

PURPOSE: A number of previous studies have shown that blood velocities in retrobulbar arteries, as assessed with color Doppler imaging (CDI), are reduced in primary open angle glaucoma (POAG) patients, indicative of reduced blood flow to the eye. In the present study, the authors hypothesized that patients with POAG show an abnormal association between retrobulbar blood flow velocities as assessed with CDI and blood pressure, indicative of vascular dysregulation. METHODS: A total of 252 POAG patients and 198 healthy age-matched control subjects were included. Mean flow velocity (MFV) in the ophthalmic artery (OA), posterior ciliary artery (PCA), and central retinal artery (CRA) were measured with CDI. Mean arterial blood pressure was measured noninvasively using automated oscillometry, and intraocular pressure was measured using Goldmann applanation tonometry. RESULTS: Intraocular pressure was increased in POAG patients compared with healthy controls (P < 0.01). Mean arterial blood pressure was not different between groups. All blood flow velocities were significantly reduced in POAG patients compared with healthy control subjects (P < 0.01 each). The correlation between MFV and mean arterial blood pressure in the CRA was stronger in POAG subjects than in healthy control subjects. CONCLUSIONS: These data indicate that blood flow velocities in retrobulbar vessels are reduced in POAG patients compared with healthy control subjects. In addition, an abnormal correlation between blood velocities and mean arterial blood pressure was found in POAG. This suggests vascular dysregulation and supports the concept that reduced ocular blood flow in glaucoma is not solely a consequence of the disease.

Effect of dorzolamide and timolol on ocular pressure: blood flow relationship in patients with primary open-angle glaucoma and ocular hypertension.

PURPOSE: The authors have reported previously that a study population, consisting of patients with glaucoma and ocular hypertension, is characterized by an impaired association between ocular blood flow parameters and systemic blood pressure, indicative of abnormal autoregulation. Here they report on the effects of dorzolamide and timolol on ocular pressure/flow relationships to test the hypothesis that these drugs improve autoregulation. METHODS: One hundred forty patients with primary open-angle glaucoma or ocular hypertension were included in a clinical trial in a controlled, randomized double-masked study in two parallel groups. Seventy patients were randomly assigned to receive timolol, and 70 patients were randomly assigned to receive dorzolamide for a 6-month period. Scanning laser Doppler flowmetry was used to measure blood flow in the temporal neuroretinal rim and the cup of the optic nerve head. Pulsatile choroidal blood flow was assessed using laser interferometric measurement of fundus pulsation amplitude. The association between blood flow parameters and systemic blood pressure was compared before and after the 6-month treatment period. RESULTS: Before treatment a significant association was observed between ocular blood flow parameters and systemic blood pressure in both parallel groups (r = 0.23-0.42). All regression lines between ocular hemodynamic parameters and systemic blood pressure were less steep after treatment with either dorzolamide or timolol (r = 0.03-0.24). CONCLUSIONS: The present study indicates that intraocular pressure reduction with timolol or dorzolamide is associated with normalization of the ocular pressure/flow relationship. Whether this is related to the beneficial effects of IOP-lowering therapy in glaucoma remains to be established. (ClinicalTrials.gov number, NCT00991822.).

Role of NO in the control of choroidal blood flow during a decrease in ocular perfusion pressure.

PURPOSE: The study was conducted to investigate whether the L-arginine/nitric oxide system plays a role in choroidal blood flow (ChBF) regulation during a decrease in ocular perfusion pressure (OPP). METHODS: Experiments were performed on 3 days in a randomized double-masked, placebo-controlled, three-way crossover design. On different study days, subjects received intravenous infusions of N(G)-monomethyl-L-arginine (L-NMMA), phenylephrine, or placebo. Intraocular pressure was raised in stepwise increments using the suction cup METHOD: Choroidal blood flow (ChBF, laser Doppler flowmetry), mean arterial blood pressure (MAP), and IOP were assessed. Ocular perfusion pressure was calculated as OPP = 23(MAP - IOP). For correlation analysis all OPP/ChBF data pairs from all subjects were pooled independent of time point of measurement. Then, the pooled data were sorted according to OPP, and correlation analyses were performed. RESULTS: L-NMMA and phenylephrine increased resting OPP by +17% +/- 18% and +14% +/- 21%, respectively (P < 0.05). L-NMMA reduced resting ChBF by -21% +/- 17% (P < 0.05). The relative decrease in OPP during suction cup application was comparable with all drugs administered. The decrease in OPP was paralleled by a significant decrease in ChBF (maximum between -39% and -47%), which was less pronounced, however, than the decrease in OPP (maximum between -69% and -74%). Neither placebo nor L-NMMA, nor phenylephrine, influenced the OPP/ChBF relationship. CONCLUSIONS: The data confirm previously published observations that the choroid shows some regulatory capacity during reduced OPP. The L-arginine/nitric oxide-system plays a role in the maintenance of basal vascular tone but seems not to be involved in the choroidal vasodilator response when IOP is increased.

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.

Endothelial dysfunction in glaucoma.

Glaucoma is a group of ocular diseases characterized by optic neuropathy associated with loss of the retinal nerve fibre layer and re-modelling of the optic nerve head, and a subsequent particular pattern of visual field loss. Increased intraocular pressure is the most important risk factor for the disease, but the pathogenesis of glaucoma is not monofactorial. Among other factors, ischaemia and vascular dysregulation have been implicated in the mechanisms underlying glaucoma. The vascular endothelium plays an important role in the regulation of ocular blood flow and pathological alterations of vascular endothelial cells may induce ischaemia and dysregulation. The present review summarizes our current evidence of endothelial dysfunction in glaucoma. This is of interest because endothelial dysfunction is a good prognostic factor for progression in several diseases. Although such data are lacking for glaucoma, endothelial dysfunction may provide an attractive target for therapeutic intervention in open-angle glaucoma and other vascular disorders of the eye.

Reactivity of retinal blood flow to 100% oxygen breathing after lipopolysaccharide administration in healthy subjects.

Administration of low doses of Escherichia coli endotoxin (LPS) to humans enables the study of inflammatory mechanisms. The purpose of the present study was to investigate the retinal vascular reactivity after LPS infusion. In a randomized placebo-controlled cross-over study, 18 healthy male volunteers received 20 IU/kg LPS or placebo as an intravenous bolus infusion. Outcome parameters were measured at baseline and 4h after LPS/placebo administration. At baseline and at 4h after administration a short period of 100% oxygen inhalation was used to assess retinal vasoreactivity to this stimulus. Perimacular white blood cell velocity, density and flux were assessed with the blue-field entoptic technique, retinal branch arterial and venous diameters were measured with a retinal vessel analyzer and red blood cell velocity in retinal branch veins was measured with laser Doppler velocimetry. LPS is associated with peripheral blood leukocytosis and increased white blood cell density in ocular microvessels (p<0.001). In addition, retinal arterial (p=0.02) and venous (p<0.01) diameters were increased. All retinal hemodynamic parameters showed a decrease during 100% oxygen breathing. This decrease was significantly blunted by LPS for all retinal outcome parameters except venous diameter (p=0.04 for white blood cell velocity, p=0.0002 for white blood cell density, p<0.0001 for white blood cell flux, p=0.01 for arterial diameter, p=0.02 for red blood cell velocity and p=0.006 for red blood cell flux). These data indicate that LPS-induced inflammation induces vascular dysregulation in the retina. This may provide a link between inflammation and vascular dysregulation. Further studies are warranted to investigate whether this model may be suitable to study inflammation induced vascular dysregulation in the eye.

Estimation of ocular rigidity based on measurement of pulse amplitude using pneumotonometry and fundus pulse using laser interferometry in glaucoma.

PURPOSE: There is evidence from theoretical models and animal studies that the biomechanical properties of the optic nerve head and the sclera play a role in the pathophysiology of glaucoma. There are, however, only a few data available that demonstrate such biomechanical alterations in vivo. In this study, the hypothesis was that patients with primary open-angle glaucoma (POAG) have an abnormal ocular structural stiffness based on measurements of intraocular pressure amplitude and ocular fundus pulsation amplitude (FPA). METHODS: Seventy patients with POAG and 70 healthy control subjects matched for age, sex, intraocular pressure and systemic blood pressure were included. The ocular PA and pulsatile ocular blood flow were assessed with pneumotonometry. The FPA was measured by using laser interferometry. Based on the Friedenwald equation, a coefficient of ocular rigidity (E1) was calculated relating PA to FPA. RESULTS: There was no difference in systemic blood pressure, intraocular pressure, and ocular perfusion pressure (OPP) between the patients with glaucoma and the healthy control subjects. Both, FPA and PA were lower in the patients with glaucoma than in the control subjects. The calculated factor E1 was significantly higher in the patients with POAG (0.0454 +/- 0.0085 AU) than in the control subjects (0.0427 +/- 0.0058 AU, P = 0.03). Multiple regression analysis revealed that E1 was independent of age and sex, and correlated only slightly with OPP. CONCLUSIONS: The present study indicates increased ocular rigidity in patients with POAG. This is compatible with a number of previous animal experiments and supports the concepts that the biomechanical properties of ocular tissues play a role in the diseases process.

Is neurovascular coupling of relevance in glaucoma?

Rapid development of functional neuroimaging techniques have brought about a growing interest for neurovascular coupling in neuroresearch, which, in turn, has prompted similar research in ophthalmology. There are now hints that neurovascular coupling is disturbed in glaucoma. The contact of the nerve terminals to the cortical blood vessels is mostly realized through astrocytes. A major defining property of glaucoma, cupping of the optic disk, implies tissue remodeling of the optic nerve head and involves an astrocytic responses. A malfunction of the astrocytes in glaucoma may lead not only to the hallmark of glaucoma-cupping and death of retinal ganglion cells-but also to an accompanying or even preceding disturbance in ocular neurovascular coupling. This article is a short overview of research published in this field so far.

Role of nitric oxide in choroidal blood flow regulation during light/dark transitions.

PURPOSE: Several studies have recently shown that a transition from light to dark is associated with a reduction in choroidal blood flow. The mechanism underlying this effect is unclear but may be related to changes in neural input. In the present study, the authors hypothesized that either the alpha-receptor agonist phenylephrine or the nitric oxide synthase (NOS) inhibitor L-NMMA may alter the choroidal blood flow response during a transition from light to dark. METHODS: In 15 healthy male nonsmoking subjects, the response of choroidal perfusion was studied in a randomized placebo-controlled three-way crossover study. Phenylephrine, L-NMMA or placebo was administered on different study days, and the effect of a light/dark transition on choroidal perfusion parameters was studied. Subfoveal choroidal blood flow and fundus pulsation amplitude were assessed with laser Doppler flowmetry and laser interferometry, respectively. RESULTS: Before drug administration, a transition from light to dark reduced both choroidal hemodynamic parameters by 11% to 20%. Neither phenylephrine nor placebo altered basal choroidal blood flow or choroidal blood flow responses to the light/dark transitions. By contrast, the NOS inhibitor L-NMMA significantly reduced basal choroidal blood flow by 20.5% +/- 5.9% (P < 0.001) and basal fundus pulsation amplitude by 21.5% +/- 4.8% (P < 0.001). In addition, the response of subfoveal choroidal blood flow (-6.2% +/- 3.2%; P = 0.008) and fundus pulsation amplitude (-4.2% +/- 2.4%; P < 0.001) to the light/dark transition was significantly diminished. CONCLUSIONS: The present study indicates that NO plays a role in the choroidal blood flow decrease during a transition from light to dark. Given that L-NMMA is a nonspecific inhibitor of NOS, the present study does not clarify whether this NO is from endothelial or neural sources.

Effects of topical clonidine versus brimonidine on choroidal blood flow and intraocular pressure during squatting.

PURPOSE: Clonidine and brimonidine, two alpha-2 agonists, have been shown to reduce intraocular pressure (IOP) in patients with glaucoma. Little is known, however, about the exact role of alpha receptors in the control of ocular blood flow in the posterior pole of the eye. Hence, the study was conducted to investigate the effects of topical clonidine versus topical brimonidine on choroidal blood flow and intraocular pressure during squatting. METHODS: This was a randomized, double-masked, controlled, two-way crossover study. Twelve healthy male nonsmoking volunteers, aged between 19 and 35 years were included in the study. Two drops of clonidine or brimonidine were administered in the subjects' study eyes. Continuous measurement using the compact laser Doppler flowmeter was performed during a 6-minute squatting period, to assess choroidal blood flow regulation during an increase in ocular perfusion pressure. RESULTS: Both substances induced a pronounced but comparable (P = 0.8) decrease in IOP. Squatting increased mean arterial pressure (MAP) and ocular perfusion pressure (P < 0.01). This increase was comparable between the clonidine and the brimonidine study day (P = 0.88). Squatting induced an increase in choroidal blood flow that was less pronounced than the increase in ocular perfusion pressure. Compared with baseline the alpha-2 agonists decreased choroidal blood flow during squatting (P = 0.0026) to a comparable degree (P = 0.86). Vascular resistance increased at baseline and during squatting after administration of the alpha-2 agonists (P < 0.01) in both groups to a comparable degree (P = 0.56). CONCLUSIONS: Topical alpha-2 agonists may induce changes in choroidal blood flow, even after a single administration. Long-term studies are needed to study potential effects of brimonidine and clonidine in the clinical setting.

Intravenous administration of clonidine reduces intraocular pressure and alters ocular blood flow.

AIM: To evaluate the effect of intravenously administered clonidine on ocular blood flow in healthy volunteers. METHODS: A randomised, double-masked, placebo-controlled, two-way crossover study was performed in 12 healthy young volunteers. Clonidine (0.2 microg/kg/min) or placebo was administered intravenously over 10 minutes. The effects of clonidine were studied at baseline and up to 150 minutes after infusion. Ocular haemodynamics were measured using laser Doppler flowmetry, laser Doppler velocimetry and a retinal vessel analyser. RESULTS: Clonidine significantly decreased mean arterial pressure (MAP) and intraocular pressure (IOP). Calculated ocular perfusion pressure decreased significantly by -8.7+/-8.7% after infusion of clonidine (p<0.01 vs placebo). Retinal arterial diameters increased by +4.4+/-2.7% (p = 0.012 vs placebo), whereas no significant change was observed in retinal veins. Red blood cell velocity decreased by -16+/-14% (p<0.01 vs placebo) after infusion of clonidine. Hence, calculated retinal blood flow decreased by -14+/-12% (p = 0.033 vs placebo). Choroidal blood flow increased by +18+/-19% (p<0.01 vs placebo) and optic nerve head blood flow increased by +16+/-23% (p = 0.046 vs placebo) 30 minutes after administration of clonidine but both returned to baseline thereafter. CONCLUSION: The short-time increase in choroidal and optic nerve head blood flow indicates a transient vasodilatory effect of clonidine due to an unknown mechanism. The decrease in retinal blood flow indicates clonidine-induced vasoconstriction in the retinal microvasculature.

Polymorphism of the beta-2 adrenoceptor and IOP lowering potency of topical timolol in healthy subjects.

PURPOSE: Beta-2 adrenoceptor antagonists such as timolol have been used in the treatment of glaucoma for more than 30 years. Several functionally important polymorphisms for the beta-2 receptor have been described. In the present study we hypothesized that a relation between the intraocular pressure (IOP) lowering effect of timolol and beta-2 adrenoceptor polymorphisms may exist. METHODS: A total of 270 healthy nonsmoking subjects were screened and individuals homozygous for the wild beta-2 Adrenoceptor (Arg16/Gln27) and two polymorphisms (Gly16/Gln27 or Gly16/Glu27) were included. In these subjects the IOP lowering effect of timolol was compared. RESULTS: Twenty-four subjects were included in the group Arg16/Gln27, 18 subjects in the group Gly16/Gln27, and 47 subjects in the group Gly16/Glu27. The ocular hypotensive effect of timolol was between 40 and 45% in all groups, but not significantly different between the three study groups (p=0.979). CONCLUSIONS: The present study indicates that the beta-2 adrenoceptor polymorphism does not influence the ocular hypotensive effects of topical beta adrenoceptor antagonists. Accordingly, other factors appear to be responsible for the intersubject variability seen with timolol in glaucoma subjects.