Effect of noradrenaline on retinal blood flow in healthy subjects

The Role of Adrenoceptors in the Retina

The retina is a part of the central nervous system, a thin multilayer with neuronal lamination, responsible for detecting, preprocessing, and sending visual information to the brain. Many retinal diseases are characterized by hemodynamic perturbations and neurodegeneration leading to vision loss and reduced quality of life. Since catecholamines and respective bindings sites have been characterized in the retina, we systematically reviewed the literature with regard to retinal expression, distribution and function of alpha₁ (α₁)-, alpha₂ (α₂)-, and beta (β)-adrenoceptors (ARs). Moreover, we discuss the role of the individual adrenoceptors as targets for the treatment of retinal diseases.

Vascular effects of physical activity are not modified by short-term inhaled diesel exhaust: Results of a controlled human exposure study

BACKGROUND

The combined effects of physical activity and air pollution exposure on vascular function are insufficiently understood, particularly after the inhalation of a β₂-agonist, a vasodilating agent.

OBJECTIVE

To assess the micro- and macrovascular response to physical activity after β₂-agonist use while breathing diesel exhaust (DE) in individuals with exercise-induced bronchoconstriction.

METHODS

On four exposure visits, eighteen adults inhaled either 400 μg of the β₂-agonist salbutamol or placebo before resting for 60 min, followed by a 30-min cycling bout. During rest and cycling, participants inhaled filtered air (FA) or DE (300 μg/m³ of PM_2.5). Microvascular (central retinal arteriolar and venular equivalents, CRAE and CRVE, respectively) and macrovascular parameters (blood pressure (BP)) and heart rate (HR)) were assessed at baseline (T₁), 10 min (T₂) and 70 min (T₃) after cycling.

RESULTS

The cycling bout increased CRAE (T₂-T₁ difference (95th % confidence interval): 4.88 μm (4.73, 5.00 μm), p < 0.001; T₃-T₁ difference: 2.10 μm (1.62, 2.58 μm), p = 0.031) and CRVE (T₂-T₁ difference: 3.78 μm (3.63, 3.92 μm), p < 0.001; T₃-T₁ difference: 3.73 μm (3.63, 3.92 μm), p < 0.001). The exposure to DE had no effect on CRAE (FA-DE difference at T₂: 0.46 μm (-0.02, 0.92 μm); p = 0.790; FA-DE difference at T₃: 1.76 μm (1.36, 2.16 μm), p = 0.213) and CRVE (FA-DE difference at T₂: 0.26 μm (-0.35, 0.88 μm), p = 0.906; FA-DE difference at T₃: 0.55 μm (0.05, 1.06 μm), p = 0.750). Compared to T₁, systolic BP was decreased at T₂ by 2.5 mmHg (2.8, 2.3 mmHg, p = 0.047), independent of inhaled exposure. Heart rate at T₂ was significantly increased by 3 bpm (2, 3 bpm, p = 0.025) after the DE-exposure when compared to FA.

DISCUSSION

Acute physical activity induces a vasodilatory response in the micro- and macrovasculature in healthy adults by increasing CRAE and CRVE, and by reducing systolic BP post exercise, despite breathing DE. The DE-associated increase in HR might be indicative of an increased sympathetic response to physical activity while breathing DE.

Effects of acute bouts of endurance exercise on retinal vessel diameters are age and intensity dependent

Alterations of retinal vessel diameters are associated with increased cardiovascular risk. We aimed to investigate changes in retinal vessel diameters in response to acute dynamic exercise of different intensities and whether these changes are age dependent. Seventeen healthy seniors (median (IQR) age 68 (65, 69) years) and 15 healthy young adults (median (IQR) age 26 (25, 28) years) first performed a maximal treadmill test (MTT) followed by a submaximal treadmill test (SMTT) and a resting control condition in randomised order. Central retinal arteriolar (CRAE) and central retinal venular (CRVE) diameter equivalents were measured before as well as 5 (t5) and 40 (t40) minutes after exercise cessation using a static retinal vessel analyser. Both exercise intensities induced a significant dilatation in CRAE and CRVE at t5 compared to the control condition (P < 0.001). At t40, the mean increase in CRAE and CRVE was greater for MTT compared to that for SMTT (CRAE 1.7 μm (95 % confidence interval (CI) -0.1, 3.6; P = 0.061); CRVE 2.2 μm (95 % CI 0.4, 4.1; P = 0.019)). However, the estimated difference at t5 between seniors and young adults in their response to MTT compared to SMTT was 5.3 μm (95 % CI 2.0, 8.5; P = 0.002) for CRAE and 4.1 μm (95 % CI -0.4, 8.6; P = 0.076) for CRVE. Wider arteries and veins after maximal versus submaximal exercise for seniors compared to young adults suggest that myogenic vasoconstriction in response to exhaustive exercise may be reduced in seniors. Age-related loss of vascular reactivity has clinical implications since the arteriolar vasoconstriction protects the retinal capillary bed from intraluminal pressure peaks.

Vascular tone pathway polymorphisms in relation to primary open-angle glaucoma

AIMS

Vascular perfusion may be impaired in primary open-angle glaucoma (POAG); thus, we evaluated a panel of markers in vascular tone-regulating genes in relation to POAG.

METHODS

We used Illumina 660W-Quad array genotype data and pooled P-values from 3108 POAG cases and 3430 controls from the combined National Eye Institute Glaucoma Human Genetics Collaboration consortium and Glaucoma Genes and Environment studies. Using information from previous literature and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, we compiled single-nucleotide polymorphisms (SNPs) in 186 vascular tone-regulating genes. We used the 'Pathway Analysis by Randomization Incorporating Structure' analysis software, which performed 1000 permutations to compare the overall pathway and selected genes with comparable randomly generated pathways and genes in their association with POAG.

RESULTS

The vascular tone pathway was not associated with POAG overall or POAG subtypes, defined by the type of visual field loss (early paracentral loss (n=224 cases) or only peripheral loss (n=993 cases)) (permuted P≥0.20). In gene-based analyses, eight were associated with POAG overall at permuted P<0.001: PRKAA1, CAV1, ITPR3, EDNRB, GNB2, DNM2, HFE, and MYL9. Notably, six of these eight (the first six listed) code for factors involved in the endothelial nitric oxide synthase activity, and three of these six (CAV1, ITPR3, and EDNRB) were also associated with early paracentral loss at P<0.001, whereas none of the six genes reached P<0.001 for peripheral loss only.

DISCUSSION

Although the assembled vascular tone SNP set was not associated with POAG, genes that code for local factors involved in setting vascular tone were associated with POAG.

Constriction of porcine retinal arterioles induced by endothelin-1 and the thromboxane analogue U46619 in vitro decreases with increasing vascular branching level

PURPOSE

The retinal blood flow depends on the diameter of retinal arterioles, but diameter changes in these vessels have hitherto only been assessed in vessels larger than approximately 100 μm. Therefore, a new method was developed for studying diameter changes along the vascular tree of arterioles in whole perfused segments of porcine retinas, and the effect of known vasoconstrictors on the diameter of retinal arterioles at different branching levels were studied.

METHODS

Thirty-four whole-mounted porcine retinas were placed in a specially designed tissue chamber. On the basis of video recordings through an inverted microscope, the diameter of retinal arterioles was measured at five different branching levels before and after addition of a high potassium concentration, or increasing concentrations of endothelin-1, the prostaglandin analogue U46619, noradrenaline or none (time controls).

RESULTS

The baseline diameter ranged from 136 μm (95% CI 132-140 μm) for 1st order arterioles to 33 μm (95% CI 21-44 μm) for 5th order arterioles. In 1st order arterioles, endothelin produced 56.6% (95% CI 47.6-64.0) and U46619 14.6% (95% CI 5.7-22.6) relative constriction compared with baseline, which for both compounds decreased significantly with increasing branching level (p<0.0001 and p<0.0001, respectively). The change in diameter during addition of noradrenaline did not differ significantly from the time controls (p=0.07).

CONCLUSIONS

The effect of retinal vasoconstrictors differs among larger and smaller arterioles. The study highlights the need for investigating diameter regulation in smaller retinal arterioles as a basis for understanding normal and pathological changes in retinal blood flow.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.

Innervation pattern of the preocular human central retinal artery

The central retinal artery (CRA) is the main vessel for inner retinal oxygen and nutrition supply. While the intraocular branches lack autonomic innervation, the innervation pattern of the extra-ocular part of this vessel along its course within the optic nerve is poorly investigated. This part however is essential for maintenance of retinal blood supply, in physiological and pathological conditions. Therefore, the aim of this study was the characterization of the autonomic innervation of the preocular CRA in humans with morphological methods. Meeting the Declaration of Helsinki, eyes of body or cornea donors were processed for single or double immunohistochemistry against tyrosine hydroxilase (TH), dopamine-β-hydroxylase (DBH), choline acetyl-transferase (ChAT), vesicular acetylcholine transporter (VAChT), neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), and cytochemistry for NADPH-diaphorase (NADPH-d). For documentation, light-, fluorescence-, and confocal laser-scanning microscopy were used. TH and DBH immunoreactive nerve fibres were detected in the CRA vessel wall, although a distinct perivascular plexus was missing. Further, nerve fibres immunoreactive for ChAT and VAChT were found, while CGRP, SP, and VIP were not detected. NADPH-d staining revealed scattered nerve fibres in the adventitia of the CRA and in close vicinity; however, nNOS-immunostaining could not confirm this finding. The CRA receives adrenergic and cholinergic innervations, indicating sympathetic and parasympathetic components, respectively. Remarkably, a peptidergic primary afferent innervation was missing. Since clinical results suggest an autoregulation of intraretinal vessels, further studies are needed to clarify the impact of CRA innervation for retinal perfusion.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

Functional in vivo assessment of retinal artery microirregularities in glaucoma

PURPOSE

We investigated whether retinal branch arteries in healthy subjects, and non-treated and treated primary open-angle glaucoma (POAG) patients show irregular local patterns during dynamic reaction to acute increases of different magnitudes in intraocular pressure (IOP).

METHODS

Nine POAG patients and nine age-matched normal volunteers were examined with the retinal vessel analyser (RVA) using a suprasystolic IOP increase (Study 1). Fourteen POAG patients and 13 age-matched controls were examined using a moderate IOP increase for 100 seconds (Study 2). Longitudinal arterial profiles were obtained for the chosen time intervals. The high-frequency waviness (HFW) of these profiles was analysed quantitatively.

RESULTS

No significant changes in HFW were found in controls in different phases of the arterial reaction. Significant increases in HFW from baseline to dilation (Study 1, P < 0.03) and from dilation to constriction (Study 2, P < 0.05) were found in POAG patients. High-frequency waviness was higher in POAG patients than in controls during dilation (P < 0.05) in both studies.

CONCLUSIONS

Our results indicate a local vessel wall difference in glaucoma patients compared with age-matched controls. Increasing HFW might worsen hydraulic resistance of the vessel segment to blood flow. Significant increase of arterial microirregularities in the POAG retina during vascular dilation might be an indication for vascular endothelial alterations in glaucoma, leading to impaired perfusion in response to IOP increase.

Effects of dopamine on retinal and choroidal blood flow parameters in humans

AIM

To investigate the effect of dopamine on retinal and choroidal blood flow in humans.

METHODS

We investigated the effect of two doses of intravenous dopamine (5 and 10 microg/kg/min) via a randomised double-masked crossover study in 12 healthy subjects chosen from a total of 16. Blood flow parameters in retina, optic nerve head and choroid were assessed with bi-directional laser Doppler velocimetry, laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude, respectively.

RESULTS

Intravenous dopamine dose-dependently increased retinal blood cell velocity and fundus pulsation amplitude (p<0.001). At the highest administered dose red blood cell velocity in retinal vessels increased by 37% and fundus pulsation amplitude by 24%. By contrast, optic nerve head blood flow did not change with dopamine administration.

CONCLUSIONS

Our data indicate that dopamine has a pronounced enhancing effect on the retinal perfusion in humans. Further studies are required to establish the exact role of dopamine in the regulation of choroidal and optic nerve head blood flow.

Response of retinal arteriole diameter to increased blood pressure during acute hyperglycaemia

PURPOSE

To study retinal response in terms of arteriole diameter and retinal thickness secondary to an increase in arterial blood pressure during acute hyperglycaemia.

METHODS

In a randomized, double-blinded, cross-over study, nine healthy persons were subjected to clamping of blood glucose to either 5 mmol/l or 15 mmol/l using somatostatin to control endogenous insulin secretion. The response of retinal arterioles in terms of diameter as determined with the retinal vessel analyser (RVA) and retinal thickness as assessed by optical coherence tomography (OCT) were measured after an increase in arterial blood pressure induced by isometric exercise. Arterial feeding pressure in the eye was assessed from the ophthalmic artery pressure and pulse amplitude measured by ophthalmodynamometry.

RESULTS

Isometric exercise induced a significant increase in mean arterial blood pressure and a significant contraction of the retinal arterioles. An acute increase in blood glucose from 5 mmol/l to 15 mmol/l did not affect either the diameter of retinal vessels or retinal thickness.

CONCLUSIONS

Acute hyperglycaemia per se does not change isometric exercise-induced retinal arteriolar contraction. Metabolic factors other than blood glucose are suspected to be involved in the impairment of retinal autoregulation as seen in hyperglycaemia induced by oral glucose intake.

High blood pressure and visual sensitivity

The study had two main purposes: (1) to determine whether the foveal visual sensitivities of people treated for high blood pressure (vascular hypertension) differ from the sensitivities of people who have not been diagnosed with high blood pressure and (2) to understand how visual adaptation is related to standard measures of systemic cardiovascular function. Two groups of middle-aged subjects--hypertensive and normotensive--were examined with a series of test/background stimulus combinations. All subjects met rigorous inclusion criteria for excellent ocular health. Although the visual sensitivities of the two subject groups overlapped extensively, the age-related rate of sensitivity loss was, for some measures, greater for the hypertensive subjects, possibly because of adaptation differences between the two groups. Overall, the degree of steady-state sensitivity loss resulting from an increase of background illuminance (for 580-nm backgrounds) was slightly less for the hypertensive subjects. Among normotensive subjects, the ability of a bright (3.8-log-td), long-wavelength (640-nm) adapting background to selectively suppress the flicker response of long-wavelength-sensitive (LWS) cones was related inversely to the ratio of mean arterial blood pressure to heart rate. The degree of selective suppression was also related to heart rate alone, and there was evidence that short-term changes of cardiovascular response were important. The results suggest that (1) vascular hypertension, or possibly its treatment, subtly affects visual function even in the absence of eye disease and (2) changes in blood flow affect retinal light-adaptation processes involved in the selective suppression of the flicker response from LWS cones caused by bright, long-wavelength backgrounds.

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.

Effects of dopamine on human retinal vessel diameter and its modulation during flicker stimulation

We performed a randomized, subject-blinded, placebo and time-controlled, two-way crossover study in 12 healthy male subjects. Placebo or dopamine was administered on two separate study days. After saline infusion, dopamine hydrochloride was infused in three consecutive doses (5, 10, and 15 microg x kg(-1) x min(-1)). Plasma levels of dopamine were determined at each perfusion step. Arterial and venous retinal vessel diameters were measured with the use of a Zeiss retinal vessel analyzer. Diffuse luminance flicker stimuli of 8 Hz were applied for 60 s. Blood pressure and pulse rate were monitored continuously. Flicker stimulation (8 Hz) increased retinal vessel diameters under basal conditions. The response to 8-Hz flicker light was significantly reduced by dopamine administration. In addition, dopamine slightly but significantly increased retinal vessel diameters. Dopamine hydrochloride significantly increased systolic but not diastolic or mean arterial pressure. The present study indicates that dopamine has a distinct effect on retinal vessel diameters also attenuating the flicker-induced response reactivity of retinal vessels. This implies a role of dopamine in retinal blood flow hemodynamics.

Does glaucoma medication influence the diameter of the retinal arteriole in the human eye? (A pilot study using the retinal vessel analyser)

PURPOSE

To investigate the potential in vivo influence of different topical glaucoma medications on the diameter of the retinal arterioles of healthy volunteers and glaucoma patients.

METHODS

The diameter of one pre-selected retinal arteriole per eye was measured using the Retinal Vessel Analyser (RVA), an instrument developed for non-invasive clinical measurement of the diameter of the main retinal vessels. The instrument contains a video system, and the integrated software recognises the boundaries of the retinal vessels by detecting their light-transmission profile. The vessel diameter (in arbitrary units) is plotted against time (seconds) on a separate display screen. In Study I the vessel diameter was measured in 12 eyes of six healthy volunteers (age 21-26 years, mean age 24.0 years) on six occasions each separated by 14 days. In a double-masked fashion, each subject's right eye was treated with one of 5 glaucoma medications (brinzolamide 1%, timolol 0.5%, betaxolol 0.5%, brimonidine 0.2% or latanoprost 0.005%) and the left eye always received balanced salt solution. In Study II, one randomly selected eye of 16 patients (age 50-79 years, mean age 65.2 years) suffering from primary open-angle glaucoma controlled with topical monotherapy was investigated, in an unmasked fashion. Four patients were on betaxolol 0.5% treatment, six subjects were receiving non-selective topical beta receptor blockers and six subjects were being treated with once daily latanoprost 0.005%.

RESULTS

The coefficient of variation for the arteriole diameter in the healthy volunteers was less than 12% in each case. No significant post-treatment change of the diameter of the pre-selected arteriole was found for any topical medication investigated, either in the healthy volunteers (Study I) or in the patients suffering from glaucoma (Study II) (p>0.05, paired t-test). In addition, in Study I no difference was observed in the alteration of the arteriole diameter between the baseline and the hour 2 measurements when the values from the drug-treated and placebo treated eyes were compared (p>0.05, two-way ANOVA).

CONCLUSION

In the present investigations it was not possible to detect any statistically meaningful change of the arteriole diameter at two hours after the instillation of any of several topical antiglaucoma drugs widely used in clinical practice. Further investigations are necessary to clarify whether the lack of observed change is due to the lack of retinal vascular effects of the drugs investigated, or is due to an inability of the RVA instrument in practice to detect alterations between time-points separated by several hours.