Intrinsic vasomotricity and adrenergic effects in a model of isolated rabbit eye

Ex vivo ocular perfusion model to study vascular physiology in the mouse eye

Several hypotheses have been tested to understand whole organ regulation in other organs such as the brain and kidney, but no such hypothesis has yet been proposed for ocular circulations. To some extent resolve this deficit our ex vivo mouse eye perfusion model takes the first step in elucidating the mechanisms controlling the individual components of the ocular circulation. Various isolated ocular vascular preparations have been utilized in studies of ocular vascular biology, physiology, and pharmacology, including studies on both normal and pathological conditions. However, there is still significant potential for further studies to improve our understanding of ocular circulation and its regulation. The choroid specifically is inaccessible to direct visualization due to the retina's high metabolic requirement with a transparency that cannot be compromised by an overly rich vascular network on the inner retinal side hindering the visualization of the choroid. In this technical paper, we provide a detailed description of all the steps to be followed from the enucleation of mouse eyes to cannulation of the ophthalmic artery and perfusion and ex vivo confocal microscopy imaging of the dynamic nature of the choroid circulation.

The influence of hemorrhagic shock on ocular microcirculation by obtained by laser speckle flowgraphy in a white rabbit model

PURPOSE

To clarify the continuous changes in the retinal vessels' and choroid's microcirculation during hemorrhagic shock and resuscitation in a rabbit model.

METHODS

Hemorrhagic shock by the removal of blood (30 mL) and resuscitation by a blood-return technique was induced in anesthetized male New Zealand White rabbits (n = 10). We evaluated the retinal vessel blood flow (relative flow volume: RFV) and choroidal blood flow (mean blur rate in the choroid area: MBR-CH) by laser speckle flowgraphy (LSFG), with simultaneous measurements of systemic hemodynamics and laboratory parameters.

RESULTS

RFV and MBR-CH showed significant decreases immediately after the initiation of blood removal and recovered by blood return. The lactate concentration tended to increase from baseline by the blood-removal operation, and it was significantly higher at the end of observation period. The %RFV and %MBR-CH each showed a significant positive correlation with mean arterial blood pressure, cardiac output, carotid blood flow, and central venous pressure. %RFV showed a significant positive correlation with %central venous oxygen saturation and negatively correlated with %lactate. The %hemoglobin did not show a significant correlation with %RFV or %MBR-CH.

CONCLUSION

This rabbit hemorrhagic shock model confirmed that ocular microcirculation measurements by LSFG feasibly reflect variations in systemic hemodynamics during hemorrhagic shock and recovery.

Retinal Organ Cultures as Alternative Research Models

Ex vivo organ cultures represent unique research models, as they combine the advantages of cell cultures with those of animal models. Being able to mimic in vivo situations through the use of organ cultures provides an excellent opportunity to investigate cellular processes, molecular pathways and cell–cell interactions, as well as structural and synaptic organisation. Human and animal organ cultures are now well established and comprise sensitive, easy-to-manipulate experimental systems that raise minimal ethical concerns. The eye, in particular, is a very complex organ that is not easy to reproduce in vitro. However, a lot of research has been dedicated to the development of suitable ocular organ cultures. This review covers the various ex vivo retinal organ culture systems available for use in ophthalmology research and compares them with commonly used animal models. In particular, bovine and porcine retinal organ culture systems are described, because the size, anatomy, physiology and vessel morphology of bovine and porcine eyes are similar to the human eye in an undisputed way, thus making them good models. In addition, these animals are widely used by the food industry and the eyes are considered surplus material. A short overview of murine, rat, rabbit, cat, canine and simian retinal organ cultures is also provided.

Hypoxia challenge test and retinal circulation changes - a study using ocular coherence tomography angiography

PURPOSE

Previous studies report that the response of retinal vessels to a decrease in oxygen (hypoxia) is vasodilation, thus increasing blood flow. We aimed to characterize the changes in retinal microvasculature induced by a mild hypoxia stress test in a healthy population, using ocular coherence tomography angiography (OCT-A) technology.

METHODS

Interventional repeated-measures study. The standardized hypoxia challenge test (HCT) was performed to all volunteers, according to the British Thoracic Society protocol. Ocular coherence tomography angiography (OCT-A) was performed at three time-points (baseline, during HCT and 30' posthypoxia). Macular and peripapillary vessel densities were assessed using the built-in software. To minimize bias, analysis was performed separately in right (OD) and left (OS) eyes. Repeated-measures anova and mean comparison analysis were used as statistical tests (stata v13).

RESULTS

Studied population included 30 healthy subjects (14 women), with a mean age of 28.8 ± 4.2 [range 22-37] years. Baseline vessel density increased in hypoxic conditions and subsequently decreased to near-baseline values in posthypoxia conditions. This pattern was observed for both eyes in both parafovea (OD: 55.3 ± 2.3 to 56.7 ± 1.9 to 55.8 ± 1.9, p < 0.05; OS: 56.9 ± 2.1 to 57.9 ± 1.9 to 57.3 ± 1.7, p < 0.05) and peripapillary (OD: 60.5 ± 0.5 to 62.6 ± 0.5 to 60.1 ± 0.4, p < 0.05; OS: 60.4 ± 0.4 to 62.3 ± 0.5 to 60.7 ± 0.4, p < 0.05) areas.

CONCLUSION

To our knowledge, there are no published data specifically addressing mild hypoxia conditions and retinal microvasculature changes, using OCT-A. This pilot study may pave way to better understand vascular responses in disease setting.

Evaluation of choroidal thickness in patients with scleroderma

PURPOSE

To investigate the choroidal thickness in patients with scleroderma and to compare them with healthy control subjects.

METHODS

Forty-six patients with scleroderma (3 male and 43 female) and 31 healthy controls (6 male and 25 female) were included in the study. Twenty-five patients had limited-type and 21 patients had diffuse-type scleroderma. Only left eyes of the patients and control subjects were used in the analysis. Demographic features of all the patients and control subjects were recorded. Each subject underwent ophthalmological examinations including refraction, visual acuity, intraocular pressure, axial length (AXL) measurement, slit-lamp biomicroscopy, and fundus examination. Body mass index (BMI) was estimated for all participants.

RESULTS

There were no significant differences between the patients with scleroderma and the control subjects in terms of age, gender, BMI, mean AXL, and mean spherical equivalent refractive error (SE) (P=0.1, P=0.086, P=0.37, P=0.55, and P=0.072 respectively). The patients with scleroderma had significantly thinner nasal, temporal, and subfoveal choroid than the healthy control subjects (P1=0.012, P2=0.046, and P3<0.001, respectively). There were no significant differences between the patients with limited-type and diffuse-type scleroderma in terms of age, gender, BMI, mean AXL, mean SE, nasal, temporal, and subfoveal choroidal thicknesses (all P>0.05).

CONCLUSIONS

Choroidal thickness in patients with scleroderma was significantly less than healthy control subjects. Vasculopathy in scleroderma is characterized by obliteration of arterioles and reduced capillary density may cause atrophy of choroid in patients with scleroderma.

Choroidal thickness measurement in myopic eyes by enhanced depth optical coherence tomography

PURPOSE

To measure choroidal thickness (CT) in myopic eyes using enhanced depth imaging (EDI).

DESIGN

A cross-sectional study.

PARTICIPANTS

Fifty-six consecutive patients with spherical equivalent refractive error of at least 6 diopters (D) were evaluated.

METHODS

Enhanced depth imaging optical coherence tomography (OCT) images were obtained by positioning the spectral-domain OCT device close enough to the eye to acquire an enhanced signal of the choroidal layer. Choroidal depth was measured as the distance between the outer reflective retinal pigment epithelium (RPE) layer and the inner sclera border. Measurements were made in a horizontal fashion across the fovea at 500-μm intervals of the sections. The CT was measured at the subfoveal region in a horizontal fashion, 3 mm temporal to fovea and 3 mm nasal to fovea.

MAIN OUTCOME MEASURES

Correlations among CT with age, refractive error in diopters, and visual acuity in logarithm of the minimum angle of resolution (logMAR) were analyzed with linear mixed models.

RESULTS

The mean age of the 56 patients was 50.4 years (± 2.03 years standard deviation; interquartile range [IQR], 42-62 years), and the mean refractive error was -8.7 D (IQR, -6.1 to -11 D). The mean subfoveal CT was 118 μm (± 68 μm) and correlated negatively with age (P = 0.032) and refractive error (P = 0.011). Regression analysis suggested that subfoveal CT decreased by 11.9 μm for each decade of life and by 6.205 μm for each diopter of myopia. The subfoveal CT was inversely correlated with the logMAR visual acuity (P = 0.008), and visual acuity improved by 0.02 (logMAR) in a 10-μm increase in CT.

CONCLUSIONS

Choroidal thickness decreases with age and severity of myopia. Visual acuity decreases in line with decreasing subfoveal CT. A reduction in CT is related to aging and the severity of myopia, whereas visual acuity depends on subfoveal CT. Our study supports the theory that choroidal abnormality may play a key role in the pathogenesis of myopic degeneration.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Modulation of vasomotive activity in rabbit external ophthalmic artery by neuropeptides

Purpose. To investigate the vasomotive activity upon the external ophthalmic artery of vasointestinal peptide (VIP) and neuropeptide Y (NPY) using a previously developed model. Methods. Isolated rabbit eyes (n = 12) were perfused in situ with tyrode through the external ophthalmic artery. Effects of intra-arterial injections of NPY 200 μg/ml (Group A; n = 6) and VIP 200 μg/ml (Group B; n = 6) on the recorded pressure were obtained. For statistical analysis, Student's paired t-test and Fast Fourier Transform were used. Results. Spontaneous oscillations were observed before any drug administration in the 12 rabbit models. NPY produced an increase in total vascular resistance and a higher frequency and amplitude of oscillations, while VIP evoked the opposite effects. Conclusions. This study provides evidence of vasomotion in basal conditions in rabbit external ophthalmic artery. Concerning drug effects, NPY increased arterial resistance and enhanced vasomotion while VIP produced opposite effects which demonstrates their profound influence in arterial vasomotion.

Endothelin-1 effects on spontaneous oscillations in choroidal arterioles

PURPOSE

This study characterizes the effects of endothelin-1 (ET-1) on the perfusion pressure of the choroidal vasculature using in situ perfused isolated rabbit eyes.

METHODS

Rabbit external ophthalmic arteries (n = 12) in a head-mounted preparation were cannulated and perfused with warmed tyrode. The three-way polypropylene catheter was further connected to a pressure transducer and the effect of intraluminal pressure as a measure of total vascular resistance was assessed. Response curves to intra-arterial injections of ET-1 (group A; n = 6) and to an intravitreal injection followed by an intra-arterial injection of ET-1 (group B; n = 6) were obtained. Data were studied using paired t-test and fast Fourier transform.

RESULTS

Before any drugs were administered, spontaneous oscillations were observed in the 12 rabbit models. In group A, ET-1 induced a short and weak vasodilating effect followed by a strong and long-lasting vasoconstrictor tone. Vasomotion became more evident, showing a higher frequency and shorter amplitude of oscillations. In group, B the intravitreal injection produced no significant changes in registered pressure or vasomotion characteristics. The intra-arterial injection produced effects similar to those observed in group A.

CONCLUSION

Our study has three main findings: (i) the choroidal vasculature demonstrated spontaneous oscillations in perfusion pressure in basal conditions in all rabbit eye models; (ii) ET-1 applied intra-arterially induced a short drop in perfusion pressure followed by a long withstanding contraction; and (iii) intra-arterial ET-1 modulated the frequency and amplitude of the spontaneous oscillations, causing a faster rate of pulsatility.

Amlodipine effects on vasomotion in rabbit external ophthalmic artery

BACKGROUND

In order to understand the role of ocular blood flow in normal and pathological conditions, knowledge of the pharmacological control mechanisms involved in the ocular vascular bed is essential. The present study was designed to investigate the reactivity of the rabbit external ophthalmic artery and its collaterals to amlodipine, in order to answer two questions: (1) What are amlodipine effects upon perfusion pressure and spontaneous oscillations in the in situ perfused rabbit eyes? (2) Can intraarterial amlodipine counteract ET-1 induced vasoconstriction?

METHODS

Rabbit external ophthalmic arteries (n = 12) in a head-mounted preparation were cannulated and perfused with warmed tyrode. Vasomotor response curves to intraarterial injections of amlodipine 3 mg/ml followed by phenylephrine 250 microg (group A, n = 6) and to amlodipine 3 mg/ml after an intraarterial injection of endothelin-1 (ET-1) 27 microg/ml (group B, n = 6) were obtained. For statistical analysis, the paired t-test and Fourier analysis of frequency spectrums of spontaneous oscillations were used.

RESULTS

Before any drug administration, spontaneous oscillations were observed in the 12 rabbit models. In group A, amlodipine elicited vasodilation and a decrease in frequency and amplitude of the oscillations. In group B, ET-1 induced an increase in vasoconstrictor tone and vasomotion became more evident. With amlodipine after ET-1, we obtained vasodilation and abolition of the vasospasm.

CONCLUSIONS

Our study has two main conclusions: (1) amlodipine, an L-type calcium channel blocker, caused intense vasodilation and decreased both frequency and amplitude of the spontaneous oscillations observed in the rabbit external ophthalmic artery and its collaterals, and (2) when we applied amlodipine in arteries previously contracted by the administration of ET-1, vascular resistance greatly decreased and spontaneous oscillations were abolished. Since ET-1 levels are increased in several ischemic ocular diseases, amlodipine might be beneficial in these patients, allowing a protective action against vasospasm.

A pilot study of enhanced depth imaging optical coherence tomography of the choroid in normal eyes

PURPOSE

To measure macular choroidal thickness in normal eyes at different points using enhanced depth imaging (EDI) optical coherence tomography (OCT) and to evaluate the association of choroidal thickness and age.

DESIGN

Retrospective, observational case series.

METHODS

EDI OCT images were obtained in patients without significant retinal or choroidal pathologic features. The images were obtained by positioning a spectral-domain OCT device close enough to the eye to acquire an inverted image. Seven sections were obtained within a 5 x 30-degree area centered at the fovea, with 100 scans averaged for each section. The choroid was measured from the outer border of the retinal pigment epithelium to the inner scleral border at 500-microm intervals of a horizontal section from 3 mm temporal to the fovea to 3 mm nasal to the fovea. Statistical analysis was performed to evaluate variations of choroidal thickness at each location and to correlate choroidal thickness and patient age.

RESULTS

The mean age of the 30 patients (54 eyes) was 50.4 years (range, 19 to 85 years), and 14 patients (46.7%) were female. The choroid was thickest underneath the fovea (mean, 287 microm; standard deviation, +/- 76 microm). Choroidal thickness decreased rapidly in the nasal direction and averaged 145 microm (+/- 57 microm) at 3 mm nasal to the fovea. Increasing age was correlated significantly with decreasing choroidal thickness at all points measured. Regression analysis suggested that the subfoveal choroidal thickness decreased by 15.6 microm for each decade of life.

CONCLUSIONS

Choroidal thickness seems to vary topographically within the posterior pole. The thickness of the choroid showed a negative correlation with age. The decrease in the thickness of the choroid may play a role in the pathophysiologic features of various age-related ocular conditions.