An experimental study of VEGF induced changes in vasoactivity in pig retinal arterioles and the influence of an anti-VEGF agent

BACKGROUND: Vascular endothelial growth factor (VEGF) plays an important role in ocular physiology. Anti-VEGF agents are now used for treatment of common retinal diseases. This study characterises the vasoactive properties of VEGF in isolated perfused pig retinal arterioles under normal tone or endothelin-1 (ET-1) pre-contracted conditions and determines the influence of an anti VEGF agent on VEGF induced vasoactivity. METHODS: An isolated perfused retinal arteriole preparation was used. The outer diameter of retinal vessels was monitored at 2 second intervals in response to VEGF and the anti VEGF agent, bevacizumab. The effect of intraluminal delivery of VEGF was determined over a wide concentration range (10(-16) to 10(-7) M) both with and without pre-contraction with ET-1 (3 x 10(-9) M). Bevacizumab (0.35 mg mL(-1)) was applied extraluminally to determine the influence of bevacizumab on VEGF induced vasoactive changes on ET-1 pre-contracted vessels. RESULTS: In retinal arterioles with normal tone, VEGF induced a concentration dependent contraction at low concentrations, reaching 93.5% at 10(-11) M and then contraction was reduced at higher concentrations, recovering to 98.1% at 10(-7) M. VEGF produced a potent concentration dependent vasodilatation in arterioles pre-contracted with ET-1. VEGF induced vasodilatation in arterioles pre-contracted with ET-1 was significantly inhibited by bevacizumab. CONCLUSIONS: VEGF induced vasoactive changes in pig retinal arterioles are dependent on concentration and vascular tone. Bevacizumab inhibits VEGF-induced vasodilatation in pre-contracted arterioles

Laser-Induced Changes in Intraretinal Oxygen Distribution in Pigmented Rabbits

PURPOSE. To make the first measurements of intraretinal oxygen distribution and consumption after laser photocoagulation of the retina and to compare the efficiency of micropulsed (MP) and continuous wave (CW) laser delivery in achieving an oxygen benefit in the treated area. METHODS. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of retinal depth before and after laser treatment in anesthetized, mechanically ventilated, Dutch Belted rabbits (n ϭ 11). Laser lesions were created by using a range of power levels from an 810-nm diode laser coupled with an operating microscope delivery system. MP duty cycles of 5%, 10%, and 15% were compared with CW delivery in each eye. RESULTS. Sufficient power levels of both the CW and MP laser reduced outer retinal oxygen consumption and increased oxygen level within the retina. At these power levels, which correlated with funduscopically visible lesions, there was histologically visible damage to the RPE and photoreceptors. Retinal damage was energy dependent but short-duty-cycle MP delivery was more selective in terms of retinal cell damage, with a wider safety range in comparison with CW delivery. CONCLUSIONS. The relationship between laser power level and mode of delivery and the resultant changes in oxygen metabolism and oxygen level in the retina was determined. Only partial destruction of RPE and photoreceptors is necessary, to produce a measurable oxygen benefit in the treated area of retina. (Invest Ophthalmol Vis Sci. 2005;46:988 -999

An Explanation of the "Supernormal" B-Wave In Vitro

The isolated arterially perfused eye preparation has proven to be comparable to the in vivo model in many respects. However, the existence of "supernormal" b-wave amplitudes in the perfused eyes has remained an unexplained functional difference between the two preparations. The term "supernormal" reflected the observation that at high perfusate flow rates the amplitude of the b-wave from the perfused eyes was frequently larger than that recorded in vivo under the same stimulus and adaptation conditions. Recent investigations in this laboratory have demonstrated that the position of the scleral electrode on the isolated eye greatly influences the amplitude of the b-wave obtained. The simple comparison of b-wave amplitudes in vivo and in vitro was therefore not appropriate, due to the different electrode locations used in the two situations. In addition, the relationship between perfusate flow rate and b-wave amplitude at a fixed location has been reinvestigated. In our perfusion system the b-wave amplitude has been shown to saturate at moderate flow rates (1.5 ml/min), considerably lower than those required to maximize the b-wave amplitude in earlier studies. This difference is due to the higher oxygen tension of our perfusate at the entry point to the eye. It is concluded that b-wave stability with increasing perfusate flow can be achieved in vitro, and that the apparently supernormal b-wave amplitudes observed under these conditions can be explained in terms of the different electrode environment in the in vivo and in vitro preparations. The implications of these findings with regard to autoregulation of the retinal circulation are discussed. Invest Ophthalmol Vis Sci 29: [1044][1045][1046][1047][1048][1049]1988 Following the first use of the isolated arterially perfused eye technique, 4 - 6 This description resulted from the observation that increasing the perfusate flow rate, or the oxygen con

Experimental Retinal Ablation Using a Fourth-Harmonic 266 nm Laser Coupled with an Optical Fiber Probe

PURPOSE. To explore the ablation potential of 266 nm laser pulses, with an intact porcine retina preparation. METHODS. Segments of porcine eyes were used in an in vitro preparation in which localized areas of intact retina and choroid could be exposed to 266 nm laser irradiation. The segments of ocular tissue were bathed in fluid, to mimic the intraocular environment. Contact between the probe and the retinal surface was established before the first laser pulse. Single or multiple pulses (5-7 ns duration) at fluence levels of 0.4 to 1.2 J/cm 2 were delivered via a tapered fiber optic probe with a tip size of approximately 110 m. The retinal tissue was then fixed and sectioned for histologic examination. The ablation depth and extent of damage were measured and related to fluence level and the number of pulses applied. RESULTS. Ablation of the inner retina was achieved by single pulses at fluence levels of 0.6 J/cm 2 and higher. The depth of retinal ablation was highly dependent on fluence for lesions generated with a single pulse but less so for multiple pulses (3-10), particularly at lower fluence levels. Higher numbers of pulses (50 -100) did not increase ablation depth in a predictable manner. CONCLUSIONS. Pulsed laser (266 nm) irradiation at low pulse counts and high fluence levels is a possible alternative for localized retinal ablation with minimal collateral damage in a fluid environment. (Invest Ophthalmol Vis Sci. 2006;47: 1587-159

Intraretinal oxygen levels before and after photoreceptor loss

PURPOSE. To measure the intraretinal oxygen environment at different stages in the Royal College of Surgeons (RCS) rat model of retinal degeneration to determine whether changes in oxygen level are an important aspect of the disease. METHODS. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of depth through the retina of anesthetized, mechanically ventilated RCS rats at ages ranging from postnatal day (P)20 to P104. The oxygen profiles were correlated with histologic observations of the cellular changes within the dystrophic retinas and compared with those in RCS-rdy ϩ control animals and published values in normal mature rats. RESULTS. Although the youngest rats studied exhibited some differences in intraretinal oxygen distribution compared with mature animals, the distribution in dystrophic RCS rats at P20 was not significantly different from that in age-matched control subjects. However, the intraretinal oxygen distribution in dystrophic RCS rats was clearly affected after approximately P30, reflecting a loss of photoreceptor oxygen consumption consistent with histologic observations. In contrast, oxygen uptake by the inner retina was still evident long after the loss of photoreceptors was essentially complete. CONCLUSIONS. There was no significant tissue hypoxia during photoreceptor degeneration in the dystrophic RCS rat. The changes in intraretinal oxygen distribution are consistent with the loss of outer retinal oxygen uptake but the preservation of inner retinal oxygen metabolism. (Invest Ophthalmol Vis Sci. 2000;41:3999 -4006) R etinitis pigmentosa (RP) refers to a heterogeneous group of inherited human diseases that cause primary degeneration of rod and cone photoreceptors. 1 The Royal College of Surgeons (RCS) rat is commonly used as an animal model of photoreceptor degeneration of relevance to RP. 5 A similar explanation was proposed for the sparing of photoreceptors around the edges of photocoagulation lesions in dystrophic RCS rats. 6 Manipulation of environmental oxygen conditions at key stages in the dystrophic RCS rat has reportedly been able to modulate the rate of photoreceptor cell death. Systemic hyperoxia has been shown to slow down and systemic hypoxia to accelerate the rate of cell death in the dystrophic RCS rat model. 5,7 Despite these compelling arguments for a possible role for oxygen in the photoreceptor degeneration in the dystrophic RCS rat, there has, to date, been no direct measurement of retinal oxygen levels at any stage of the disease. The present study therefore set out to characterize for the first time the intraretinal oxygen distribution at different stages in the RCS rat model of photoreceptor degeneration

An assessment of microvascular hemodynamics in human macula

Abstract 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

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