Posture-Induced Changes in Intraocular, Orbital, Cranial, Jugular Vein, and Arterial Pressures in a Porcine Model.

Purpose: The purpose of this study was to determine posture-induced changes in arterial blood pressure (ABP), intraocular pressure (IOP), orbital pressure (Porb), intracranial pressure (ICP), and jugular vein pressure (JVP) at various tilt angles in an in vivo pig. Methods: Anesthetized and ventilated pigs (n = 8) were placed prone on a tiltable operating table. ABP, IOP, Porb, ICP, and JVP were monitored while the table was tilted at various angles between 15 degrees head up tilt (HUT) and 25 degrees head down tilt (HDT) either in stepwise changes (5 degrees per step) or continuously. The mean pressure was calculated from digitized pressure waveforms from each compartment. For stepwise changes in tilt angle the pressures were plotted as a function of tilt angle. For continuous tilt changes, the pressures were plotted as a function of time. Results: In the case of stepwise changes, ABP remained relatively stable whilst IOP, Porb, ICP, and JVP demonstrated significant differences between most angles (typically P < 0.0001). The difference was greatest for IOP (P < 0.0001) where the average IOP increased from 13.1 ± 1.23 mm Hg at 15 degrees HUT to 46.3 ± 2.03 mm Hg at 25 degrees HDT. The relationship between pressure and tilt angle was almost linear for ICP and JVP, and sigmoidal for IOP and Porb. Interestingly, the effect of changes in tilt angle occurred very rapidly, within a few seconds. Conclusions: Our results in a pig model demonstrate that changes in posture (tilt angle) induce rapid changes in IOP, Porb, ICP, and JVP, with IOP affected most severely.

Quantitative study of spatial and temporal variation in retinal capillary network perfusion in rat eye by in vivo confocal imaging.

Microvascular dysfunction is the underlying pathological process in many systemic diseases. However, investigation into its pathogenesis is impeded by the accessibility and complexity of the microvasculature within different organs, particularly for the central nervous system. The retina as an extension of the cerebrum provides a glimpse into the brain through which the microvasculature can be observed. Two major questions remain unanswered: How do the microvessels regulate spatial and temporal delivery to satisfy the varying cellular demands, and how can we quantify blood perfusion in the 3D capillary network? Here, quantitative measurements of red blood cell (RBC) speed in each vessel in the field were made in the in vivo rat retinal capillary network using an ultrafast confocal technique with fluorescently labelled RBCs. Retinal RBC speed and number were found to vary remarkably between microvessels ranging from 215 to 6641 microns per second with significant variations spatially and temporally. Overall, the RBC speed was significantly faster in the microvessels in the superficial retina than in the deep retina (estimated marginal means of 2405 ± 238.2 µm/s, 1641 ± 173.0 µm/s respectively). These observations point to a highly dynamic nature of microvasculature that is specific to its immediate cellular environment and is constantly changing.

An assessment of microvascular hemodynamics in human macula.

An adequate blood supply to meet the energy demands is essential for any tissue, particularly for high energy demand tissues such as the retina. A critical question is: How is the dynamic match between neuronal demands and blood supply achieved? We present a quantitative assessment of temporal and spatial variations in perfusion in the macular capillary network in 10 healthy human subjects using a non-invasive and label-free imaging technique. The assessment is based on the calculation of the coefficient of variation (CoV) of the perfusion signal from arterioles, venules and capillaries from a sequence of optical coherence tomography angiography images centred on the fovea. Significant heterogeneity of the spatial and temporal variation was found within arterioles, venules and capillary networks. The CoV values of the capillaries and smallest vessels were significantly higher than that in the larger vessels. Our results demonstrate the presence of significant heterogeneity of spatial and temporal variation within each element of the macular microvasculature, particularly in the capillaries and finer vessels. Our findings suggest that the dynamic match between neuronal demands and blood supply is achieved by frequent alteration of local blood flow evidenced by capillary perfusion variations both spatially and temporally in the macular region.

Optimal Calculation of Mean Pressure From Pulse Pressure.

BACKGROUND: There are six different formulae for estimating mean arterial pressure (MAP) from systolic and diastolic pressure readings. This study is to determine the optimum formula for calculating MAP when compared to the gold standard approach, which is the area under the curve of an invasively measured pulse waveform divided by the cardiac cycle duration. METHODS: Eight live pigs were used as the experimental model for the invasive measurement of femoral artery pressure (AP) by a fluid filled catheter connected with a pressure transducer. In addition, intraocular pressure (IOP) and jugular vein pressure (JVP) were also recorded. The mean pressure (MP) was calculated from digital waveforms sampled at 1,000 points per second with the six formulae and area method for AP, IOP and JVP. RESULTS: The absolute mean difference between the area MAP and each formula's MAP ranged from 0.98 to 3.23 mm Hg. Our study also found that even under physiological conditions, area MAP can vary between successive pulses by up to 5 mm Hg. For mean IOP and JVP, the mean difference between a formula's MP and the area method's was less than 1 mm Hg for most formulae. With the pooled data, there was excellent agreement amongst all formulae for MAP with the intra-class correlation coefficient (ICC) ranging from 0.97 to 0.99, while the ICC of most formulae for IOP and JVP was 1.0. CONCLUSIONS: Our study suggests that all current formulae are adequate for estimating MAP, though some formulae are not suitable for mean IOP and JVP.

Endothelial contraction of retinal veins.

We have previously reported that porcine retinal veins can be contracted by vasoactive factors such as endothelin-1, but it is still unknown which cells play the major role in such contraction responses. This study seeks to confirm whether retinal vein endothelial cells play a significant role in the endothelin-1 induced contraction of porcine retinal veins. This is a novel study which provides confirmation of the endothelial cells' ability to contract retinal veins using a live vessel preparation. Retinal veins were isolated from porcine retina and cannulated for perfusion. The vessels were exposed to extraluminal delivery of endothelin-1 (10-8 M) and change in vessel diameter recorded automatically every 2 s. A phase contrast objective lens was also used to capture images of the endothelial cell morphometries. The length, width, area, and perimeter were assessed. In addition, vein histology and immuno-labeling for contractile proteins was performed. With 10-8 M endothelin-1 contractions to 63.6% of baseline were seen. The polygonal shape of the endothelial cells under normal tone became spindle-like after contraction. The area, width, perimeter and length were significantly reduced by 54.8%, 48.1%, 28.5% and 10.5% respectively. Three contractile proteins, myosin, calponin and alpha-SMA were found in retinal vein endothelial cells. Retinal vein endothelial cells contain contractile proteins and can be contracted by endothelin-1 administration. Such contractile capability may be important in regulating retinal perfusion but could also be a factor in the pathogenesis of retinal vascular diseases such as retinal vein occlusion. As far as we are aware, this is the first study on living isolated veins to confirm that endothelial cells contribute to the endothelin-1 induced contraction.

Topographic distribution and phenotypic heterogeneity of Schlemm's canal endothelium in human donor eyes.

Endothelium phenotype is known to be closely associated with flow shear stress. This study is to determine the topographic distribution of endothelial cells and the phenotype of different quadrants and regions of Schlemm's canal using human donor eyes. This study infers differences in flow dynamics based on cell shape and intracellular structure. The Schlemm's canal from 15 human donor eyes were either perfusion labelled using silver stain or dissected for float labeling with Phalloidin to enable visualization of endothelial cell border and intracellular structure. Data were acquired for endothelial cells from the outer and inner wall of Schlemm's canal and grouped according to quadrant of origin. Measurements included endothelial cell length, width, area, and aspect ratio and compared between quadrants. Endothelial cells are mostly spindle-shape and the cell size on the outer wall are larger and longer than those from the inner wall. Significant differences in endothelial cell size and shape were seen in different quadrants. The endothelial cells have varied shapes and orientations close to large ostia in the outer wall and remarkably long endothelial cells were found in the walls of collector channels. F-actin aggregation was found at all endothelial cell borders, and inside some of the endothelial cytoplasm. The presence of various spindle shapes, significant phenotype heterogeneity and F-actin aggregation of endothelial cells indicates aqueous humor flow likely creates variations in shear stress within Schlemm's canal. Further investigation of the relationship between the phenotype heterogeneity and hydrodynamics of aqueous flow may help us understand the mechanisms of outflow resistance changes in glaucoma.

Topographic Distribution of Contractile Protein in the Human Macular Microvasculature.

Purpose: We studied the topographic distribution of contractile protein in different orders of the human macular microvasculature to further understanding of the sites for capillary blood flow regulation. Methods: Nine donor eyes from eight donors were cannulated at the central retinal artery and perfusion labeled for alpha smooth muscle actin (αSMA) and filamentous actin (F-actin). Confocal images were collected from the macula region, viewed, projected, and converted to a 255 grayscale for measurements. The mean intensity was measured for macular arterioles, venules, and capillary segments. The diameter of each vessel segment measured was recorded. The normalized mean intensity values from all images were ranked according to vessel types and size with a total of nine categories. Results: F-actin was present throughout the macular microvasculature whereas αSMA labeling showed variations. Overall, αSMA has a more prominent presence in the macular arterioles than in the macular capillaries and venules, and αSMA strongly labeled the smaller macular arterioles. Some capillaries also labeled positive for αSMA, including some of the capillaries in the innermost capillary ring surrounding the foveola. It was weakly present in the capillaries on the venous side and larger venules. In the larger macular arterioles closer to 100 µm in diameter, αSMA labeling was weakly present and not as ubiquitous as in the smaller arterioles. Conclusions: Nonuniform distribution of contractile proteins in the different types, orders, and sizes of macular microvasculature indicates that these vessels may have different contractile capability and roles in macular flow regulation.

Retinal capillary perfusion: Spatial and temporal heterogeneity.

The central role of the cardiovascular system is to maintain adequate capillary perfusion. The spatially and temporally heterogeneous nature of capillary perfusion has been reported in some organs. However, such heterogeneous perfusion properties have not been sufficiently explored in the retina. Arguably, spatial and temporal heterogeneity of capillary perfusion could be more predominant in the retina than that in other organs. This is because the retina is one of the highest metabolic demand neural tissues yet it has a limited blood supply due to optical requirements. In addition, the unique heterogeneous distribution of retinal neural cells within different layers and regions, and the significant heterogeneity of intraretinal oxygen distribution and consumption add to the complexity. Retinal blood flow distribution must match consumption of nutrients such as oxygen and glucose within the retina at the cellular level in order to effectively maintain cell survival and function. Sophisticated local blood flow control in the microcirculation is likely required to control the retinal capillary perfusion to supply local retinal tissue and accommodate temporal and spatial variations in metabolic supply and demand. The authors would like to update the knowledge of the retinal microvessel and capillary network and retinal oxidative metabolism from their own studies and the work of others. The coupling between blood supply and energy demands in the retina is particularly interesting. We will mostly describe information regarding the retinal microvessel network and retinal oxidative metabolism relevant to the spatial and temporal heterogeneity of capillary perfusion. We believe that there is significant and necessary spatial and temporal heterogeneity and active regulation of retinal blood flow in the retina, particularly in the macular region. Recently, retinal optical coherence tomography angiography (OCTA) has been widely used in ophthalmology, both experimentally and clinically. OCTA could be a valuable tool for examining retinal microvessel and capillary network structurally and has potential for determining retinal capillary perfusion and its control. We have demonstrated spatial and temporal heterogeneity of capillary perfusion in the retina both experimentally and clinically. We have also found close relationships between the smallest arterioles and capillaries within paired arterioles and venules and determined the distribution of smooth muscle cell contraction proteins in these vessels. Spatial and temporal heterogeneity of retinal capillary perfusion could be a useful parameter to determine retinal microvessel regulatory capability as an early assay for retinal vascular diseases. This topic will be of great interest, not only for the eye but also other organs. The retina could be the best model for such investigations. Unlike cerebral vessels, retinal vessels can be seen even at the capillary level. The purpose of this manuscript is to share our current understanding with the readers and encourage more researchers and clinicians to investigate this field. We begin by reviewing the general principles of microcirculation properties and the spatial and temporal heterogeneity of the capillary perfusion in other organs, before considering the special requirements of the retina. The local heterogeneity of oxygen supply and demand in the retina and the need to have a limited and well-regulated retinal circulation to preserve the transparency of the retina is discussed. We then consider how such a delicate balance of metabolic supply and consumption is achieved. Finally we discuss how new imaging methodologies such as optical coherence tomography angiography may be able to detect the presence of spatial and temporal heterogeneity of capillary perfusion in a clinical setting. We also provide some new information of the control role of very small arterioles in the modulation of retinal capillary perfusion which could be an interesting topic for further investigation.

Regulation of Oxygen Tension in the Mammalian Retina During Systemic Hyperoxia Is Species Dependent.

The oxygen supply to the retina in man and most mammals is derived from both the retinal and choroidal circulations. However, some mammals have only a partially vascularized retina, and some have a completely avascular retina. Here we contrast the retinal oxygen levels during systemic hyperoxia in a fully vascularized retina (rat), a partially vascularized retina (rabbit), and an avascular retina (guinea pig). Oxygen sensitive microelectrodes were used to measure the intraretinal oxygen distribution in anaesthetized rats, rabbits and guinea pigs during air breathing and 100% oxygen ventilation. In the vascularized rat retina the increase in oxygen tension in the choroid, reflected the increase in systemic oxygen levels during hyperoxic ventilation. However, the rise in oxygen levels in the inner retina was muted. In the avascular region of the partially vascularized rabbit retina, the increase in choroidal oxygen tension resulted in a large increase in oxygen tension across the full thickness of the retina. In the avascular retina of the guinea pig, very little change in choroidal or retinal oxygen tension was seen during systemic hyperoxia. Remarkably different responses to systemic hyperoxia are evident in the rat, rabbit, and guinea pig, three conventional laboratory animals that are commonly used in ophthalmic research. Neither the regulatory mechanisms responsible for the increase in oxygen consumption in the rat retina, or the stability of the choroidal oxygen tension in the guinea pig during systemic hyperoxia are currently understood. A better understanding of oxygen regulation in the mammalian retina could open up new avenues for improving the oxygen environment in the human retina in a range of ischaemic retinal diseases that account for the majority of blindness in the developed world.

Regional differences in endothelial cell cytoskeleton, junctional proteins and phosphorylated tyrosine labeling in the porcine vortex vein system.

We previously demonstrated endothelial phenotype heterogeneity in the vortex vein system. This study is to further determine whether regional differences are present in the cytoskeleton, junctional proteins and phosphorylated tyrosine labeling within the system. The vortex vein system of twenty porcine eyes was perfused with labels for f-actin, claudin-5, VE-Cadherin, phosphorylated tyrosine and nucleic acid. The endothelial cells of eight different regions (choroidal veins, pre-ampulla, anterior ampulla, mid-ampulla, posterior ampulla, post-ampulla, intra-scleral canal and the extra-ocular vortex vein) were studied using confocal microscopy. There were regional differences in the endothelial cell structures. Cytoskeleton labeling was relatively even in intensity throughout Regions 1 to 6. Overall VE-Cadherin had a non-uniform distribution and thicker width endothelial cell border staining than claudin-5. Progressing downstream there was an increased variation in thickness of VE-cadherin labeling. There was an overlap in phosphorylated tyrosine and VE-Cadherin labeling in the post-ampulla, intra-scleral canal and extra-ocular vortex vein. Intramural cells were observed that were immune-positive for VE-Cadherin and phosphorylated tyrosine. There were significant differences in the number of intramural cells in different regions. Significant regional differences with endothelial cell labeling of cytoskeleton, junction proteins, and phosphorylated tyrosine were found within the vortex vein system. These findings support existing data on endothelial cell phenotype heterogeneity, and may aid in the knowledge of venous pathologies by understanding regions of vulnerability to endothelial damage within the vortex vein system. It could be valuable to further investigate and characterize the VE-cadherin and phosphotyrosine immune-positive intramural cells.

Microvascular Network and Its Endothelial Cells in the Human Iris.

PURPOSE: The iris allows effective delivery of nutrients into the aqueous humor supplying the surrounded avascular tissues. However, possible underlying mechanisms of the iris vasculature have not been well established. This study aims to quantitatively assess the human iris vascular network, endothelial cell morphometries, and characterize endothelial junctions to better understand the properties of the iris vasculature. MATERIALS AND METHODS: The irises from human donor eyes were dissected and short fixed before float staining for VE-cadherin and claudin-5, f-actin and nuclei and flat-mounted for confocal imaging. The iris microvasculature was studied for its distribution and branch orders. The endothelial and nuclear morphometrics were measured for each vessel order. Characteristics of cellular junction staining and intracellular cytoskeleton were investigated. RESULTS: The human iris vasculature was found to comprise of six orders of arteries, three orders of veins, and capillaries. The endothelial cell shape was long and narrow in all arteries, suggesting a high hemodynamic shear stress. Relatively large vessels ran radially in the superficial two-thirds of the iris, while smaller and denser vessels ran in the deepest third. Significant heterogeneity in vascular diameter, shape of the endothelia and nuclei, and the nuclear position was evident between artery, capillary and vein. Staining of junction proteins VE-cadherin and claudin-5 appeared non-uniform at the cell borders, especially in large veins. CONCLUSIONS: High rates of blood flow and special barrier properties are indicated by the morphological properties of the human iris vasculature. Detailed information of the iris vasculature combined with the inter- and intra-endothelial structure may help us further understand the physiological and pathogenic roles of the iris.

Inter-Relationship of Arterial Supply to Human Retina, Choroid, and Optic Nerve Head Using Micro Perfusion and Labeling.

Purpose: The prevailing view is that the human retina is supplied by the central retinal artery (CRA), the short posterior ciliary arteries (SPCAs) support the choroid, and both the CRA and the SPCAs are so-called "end artery" systems. In this study, we investigate whether vascular connections among the retina, choroid, and the optic nerve head (ONH) exist, using selective cannulation and microperfusion-labeling techniques. Methods: The CRA and/or one or more of the SPCAs were selected for cannulation in 18 human donor eyes. Fluorescent probes with different excitation wavelengths were perfused through different arteries on the same eye to distinguish the supply sources of different vascular beds. After labeling and fixation, the ONH region was dissected either longitudinally or transversely as thick sections for confocal microscopy. Retina, choroid, and ONH were imaged from whole-mount specimens. Results: Probes perfused through the CRA or the SPCA alone labeled the microvessels in the retina, choroid, and ONH regions, as well as the optic nerve trunk. The vessels of the lamina cribrosa and the optic nerve trunk were labeled when probes were perfused through the SPCA. Perfusion through both the CRA and SPCA produced double labeling of vessels in the retina, the choroid, and the ONH. Conclusions: The results indicate an inter-relationship of arterial supply to the retina, choroid, and ONH in the human eye. This has important implications in understanding clinical observations and disease mechanisms such as that of glaucoma and ischemic optic nerve disease.

Primary angle closure glaucoma: What we know and what we don't know.

Primary angle-closure glaucoma (PACG) is a common cause of blindness. Angle closure is a fundamental pathologic process in PAGC. With the development of imaging devices for the anterior segment of the eye, a better understanding of the pathogenesis of angle closure has been reached. Aside from pupillary block and plateau iris, multiple-mechanisms are more common contributors for closure of the angle such as choroidal thickness and uveal expansion, which may be responsible for the presenting features of PACG. Recent Genome Wide Association Studies identified several new PACG loci and genes, which may shed light on the molecular mechanisms of PACG. The current classification systems of PACG remain controversial. Focusing the anterior chamber angle is a principal management strategy for PACG. Treatments to open the angle or halt the angle closure process such as laser peripheral iridotomy and/or iridoplasty, as well as cataract extraction, are proving their effectiveness. PACG may be preventable in the early stages if future research can identify which kind of angles and/or persons are more likely to benefit from prophylactic treatment. New treatment strategies like adjusting the psychological status and balancing the sympathetic-parasympathetic nerve activity, and innovative medicines are needed to improve the prognosis of PACG. In this review, we intend to describe current understanding and unknown aspects of PACG, and to share the clinical experience and viewpoints of the authors.

Local Modulation of Retinal Vein Tone.

PURPOSE: To determine whether vascular tone of isolated porcine retinal veins can be modulated by tissue-generated vasoactive factors such as endothelin-1 and adenosine. Such information may be useful in understanding the role of the retinal veins in regulating blood flow, and also provide a model for investigating new hypotheses suggesting a role for vasoactive factors in retinal vascular diseases such as retinal vein occlusion. METHODS: An isolated perfused retinal vein preparation was used for this study. Segments of porcine retinal veins were dissected, cannulated, and perfused, and their diameter was monitored during vasoactive agent application of increasing doses of endothelin-1 (10(-12)-10(-8) M) or adenosine (10(-10)-10(-4) M). Adenosine (10(-6) M) was also applied on veins during preconstriction with endothelin-1 (10(-8) M). The significance of any induced change in vessel diameter was assessed in relation to the baseline vessel diameter prior to any drug delivery. RESULTS: Dose-dependent vasocontractile responses were induced by endothelin-1 administration. Endothelin-1 produced a significant contraction at doses of 10-11 M and above. At 10(-8) M the maximal endothelin-1-induced contractions were to 70.2 ± 2.1% of baseline. Adenosine produced a dose-dependent dilation reaching 113.0 ± 2.4% at 10(-4) M. Adenosine (10(-6) M) induced a significant dilation in endothelin-1 (10(-8) M)-contracted vessels. CONCLUSIONS: Porcine retinal veins can be modulated by both vasocontraction and vasodilation agents, suggesting that the retinal veins may play a regulatory role in the retinal circulation, particularly in regard to the capillary pressure upstream from the draining retinal veins. To our knowledge, this is the first study of vasoactivity in isolated perfused retinal veins, providing an opportunity to study the direct vasoactive effects of specific vasoactive agents.

Structural characteristics of the optic nerve head influencing human retinal venous pulsations.

The relationship between structural characteristics of the optic nerve head and venous pulsations in the human eye remain unknown. Using photoplethysmographic techniques we investigated whether properties of the human retinal veins and their surrounding structures influence venous pulsation. 448 locations of venous pulsation were analysed from 26 normal human eyes. Green channel densitometry derived from video recordings of venous pulsations were used to generate a map of venous pulsation amplitudes along retinal veins. Optical coherence tomography was used to perform quantitative measurements of tissue characteristics at sites of high and low amplitude points as well as in a second analysis, at maximal amplitude pulsation sites from superior and inferior halves of the eyes. Structural characteristics measured included venous diameter, distance from pulsation point to cup margin, vessel length from pulsation point to vein exit, tissue thickness overlying vein, optic disc diameter and presence of a proximal arteriovenous crossing. Increasing venous pulsation amplitudes were associated with larger applied ophthalmodynamometry force, increasing venous diameter, and decreasing absolute cup margin distance (all p < 0.001). Increasing distance of maximal amplitude pulsation point to cup margin was associated with the presence of a proximal arteriovenous crossing, increasing venous diameter, and decreasing tissue depth (all p ≤ 0.001). Venous diameter and tissue depth alter venous compliance, which is likely to be a major factor determining sites of venous pulsation.

Intracellular cytoskeleton and junction proteins of endothelial cells in the porcine iris microvasculature.

Recently we reported studies of the iris microvasculature and its endothelial cells using intra-luminal micro-perfusion, fixation, and silver staining, suggesting that the iris vascular endothelium may be crucial for maintaining homeostasis in the ocular anterior segment. Here we present information regarding the intracellular structure and cell junctions of the iris endothelium. Thirty-seven porcine eyes were used for this study. The temporal long posterior ciliary artery was cannulated to assess the iris microvascular network and its endothelium using intra-luminal micro-perfusion, fixation, and staining with phalloidin for intracellular cytoskeleton f-actin, and with antibodies against claudin-5 and VE-cadherin for junction proteins. Nuclei were counterstained with Hoechst. The iris was flat-mounted for confocal imaging. The iris microvasculature was studied for its distribution, branch orders and endothelial morphometrics with endothelial cell length measured for each vessel order. Our results showed that morphometrics of the iris microvasculature was comparable with our previous silver staining. Abundant stress fibres and peripheral border staining were seen within the endothelial cells in larger arteries. An obvious decrease in cytoplasmic stress fibres was evident further downstream in the smaller arterioles, and they tended to be absent from capillaries and veins. Endothelial intercellular junctions throughout the iris vasculature were VE-cadherin and claudin-5 immuno-positive, indicating the presence of both adherent junctions and tight junctions between vascular endothelial cells throughout the iris microvasculature. Unevenness of claudin-5 staining was noted along the endothelial cell borders in almost every order of vessels, especially in veins and small arterioles. Our results suggest that significant heterogeneity of intracellular structure and junction proteins is present in different orders of the iris vasculature in addition to vascular diameter and shape of the endothelia. Detailed information of the topography and intracellular structure and junction proteins of the endothelium of the iris microvasculature combined with unique structural features of the iris may help us to further understand the physiological and pathogenic roles of the iris vasculature in relevant ocular diseases.

Role of Endothelium in Abnormal Cannabidiol-Induced Vasoactivity in Retinal Arterioles.

PURPOSE: Cannabinoids have been reported to mediate changes in vascular resistance through endothelial receptor targets. We examined involvement of the endothelium in cannabinoid-mediated vasoactive responses in resistance arterioles of the retina. METHODS: Vascular responses to both intraluminal (IL) and extraluminal (EL) administration of the atypical cannabinoid, abnormal cannabidiol (abn-CBD), a prototypical agonist at the non-CB1/CB2 endothelial cannabinoid receptor (CBeR), were studied in endothelial intact and endothelial denuded, isolated perfused porcine retinal arterioles with and without endothelin-1 (ET-1) precontraction. The effects of AM251, a CB1 receptor antagonist, and O-1918, an analog of CBD reported to antagonize CBeR, were also studied. RESULTS: Dose-dependent vasocontractile responses were induced by both IL and EL administration of abn-CBD in the absence of precontraction. Significantly greater vasoconstriction was induced by IL administration of abn-CBD than with EL administration. In contrast, only vasodilation to abn-CBD was observed in ET-1 precontracted retinal arterioles. Endothelium removal significantly reduced abn-CBD-induced vasoactivity when abn-CBD was used IL but not when applied EL. IL abn-CBD-induced vasoactivity was antagonized by O-1918 and AM251. CONCLUSIONS: Cannabinoids show complex vasoactive actions in isolated perfused retinal arterioles. The fact that abn-CBD-mediated vasorelaxation was seen only in precontracted retinal vessels indicates that the abn-CBD-induced vasoactive response is highly dependent on vascular tone. Furthermore, IL and EL administration produced differential responses, and removal of endothelium blunted abn-CBD vasoactivity, highlighting the critical role of endothelium in abn-CBD vasoactivity. AM251 and O-1918 inhibition of abn-CBD-induced vasoactivity suggests the possibility of modulating abn-CBD-induced vasoactivity.

Quantitative study of the microvasculature and its endothelial cells in the porcine iris.

The roles of the iris microvasculature have been increasingly recognised in the pathogenesis of glaucoma and cataract; however limited information exists regarding the iris microvasculature and its endothelium. This study quantitatively assessed the iris microvascular network and its endothelium using intra-luminal micro-perfusion, fixation, and staining of the porcine iris. The temporal long posterior ciliary artery of 11 isolated porcine eyes was cannulated, perfusion-fixed and labelled using silver nitrate. The iris microvasculature was studied for its distribution, orders and endothelial morphometrics. The density of three layers of microvasculature was measured. Endothelial cell length and width were measured for each vessel order. The iris has an unusual vascular distribution which consisted of abundant large vessels in the middle of the iris stroma, branching over a relatively short distance to the microvasculature located in the superficial and deep stroma as well as the pupil edge. The average vascular density of the middle, superficial, and deep layers were 38.9 ± 1.93%, 10.9 ± 1.61% and 8.0 ± 0.79% respectively. Multiple orders of iris vessels (capillary, 6 orders of arteries, and 4 orders of veins) with relatively large capillary and input arteries (319.5 ± 25.6 µm) were found. Significant heterogeneity of vascular diameter and shape of the endothelia was revealed in different orders of the iris vasculature. Detailed information of topography and endothelium of the iris microvasculature combined with unique structural features of the iris may help us to further understand the physiological and pathogenic roles of the iris in relevant ocular diseases.

Correlation between the radial peripapillary capillaries and the retinal nerve fibre layer in the normal human retina.

This study aims to provide evidence of the importance of radial peripapillary capillaries (RPCs) by quantitative study of the relationship between the RPCs and retinal nerve fibre layer (RNFL) in normal human donor eyes. The retinal microvasculature in eleven normal human donor eyes was perfused, fixed and labelled after cannulation of the central retinal artery. The retinas were dissected and whole-mounted for confocal microscopy. Six study regions were taken radially from the edge of the optic disc. RPCs from the optic disc edge to a radial distance up to 2.5 mm were imaged and their diameters, inter-capillary distance and volume occupation measured. These were correlated with the study region as well as thickness of the RNFL. It was found that the pooled average diameter of the RPCs in the first 2.5 mm from the optic disk was 8.9 µm. Significant differences in capillary diameter were present in the six regions, with larger diameter RPCs in the superior, inferior and nasal regions, and significantly smaller diameter in the temporal region. RPCs in the arcuate fibre regions extend the furthest from the optic disc, maintained a close inter-capillary distance for a longer distance than other regions, and have the highest RPCs volume occupancy. The RPCs volume was generally correlated with RNFL thickness. In conclusion, a close correlation between RNFL and RPCs presence has been demonstrated which is supportive of their functional reliance/co-dependence. The significantly smaller temporal RPCs may be a result of the greater presence of RPCs in the two bordering arcuate fibre regions and therefore a richer availability of nutrients diffusing from these two regions.