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      Chorioretinal Vascular Oxygen Tension in Spontaneously Breathing Anesthetized Rats

      , , ,

      Ophthalmic Research

      S. Karger AG

      Choroid, Oxygen, Retina

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          Abstract

          Purpose: To establish baseline and variability of oxygen tension (PO<sub>2</sub>) measurements in the choroid, retinal arteries, capillaries, and veins of spontaneously breathing anesthetized rats and determine the effect of a moderate surgical procedure on the chorioretinal PO<sub>2</sub>. Methods: Our previously established optical section phosphorescence imaging technique was utilized to measure PO<sub>2</sub> in the chorioretinal vasculatures. Imaging was performed in 29 spontaneously breathing rats under ketamine/xylazine anesthesia. In 7 rats, blood was drawn using a surgically implanted femoral arterial catheter and analyzed to determine the systemic arterial PO<sub>2</sub>. The PO<sub>2</sub> measurements in 22 rats without surgery (group 1) and 7 surgically instrumented rats (group 2) were statistically compared. The intrasubject variability was calculated by the average standard deviation (SD) of repeated measurements. Results: The average systemic arterial PO<sub>2</sub> was 52 ± 7 mm Hg (mean ± SD) in group 2. In group 1, the average PO<sub>2</sub> measurements in the choroid, retinal arteries, capillaries, and veins were 50 ± 11, 40 ± 5, 39 ± 6, and 30 ± 5 mm Hg, respectively. No statistically significant PO<sub>2</sub> differences in any of the chorioretinal vasculatures were found between the two groups (p > 0.4). The intrasubject variability was 3 mm Hg in the choroid, retinal arteries, capillaries, and veins. Conclusions: Chorioretinal PO<sub>2</sub> measurements in spontaneously breathing anesthetized rats have a relatively low variability, indicating that PO<sub>2</sub> changes due to various physiological alterations can be reliably assessed.

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          Most cited references 21

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          Effects of light and darkness on oxygen distribution and consumption in the cat retina

          These experiments were done to investigate the effects of light and darkness on the oxygenation of the retina in anesthetized cats. Measurements were made with double-barreled oxygen microelectrodes capable of recording both oxygen tension (PO2) and local voltages. Diffuse white illumination presented to a dark-adapted retina led to an increase in PO2 of up to 30 mmHg in the outer half of the retina. Changes were maximal at approximately 75% depth, corresponding to the outer nuclear layer. No change or decrease in PO2 was observed in the inner retina. Light-evoked increases in outer retinal PO2 were graded with the duration and strength of illumination, and were maximal in response to 60 s of illumination at rod saturation. For these stimuli, the increase at the onset of illumination was slower (average half- time, 12.2 s) than the recovery at the end of illumination (average half-time, 5.9 s), but for stimuli above rod saturation, PO2 recovered much more slowly. The profile of PO2 was measured during electrode penetration and withdrawal and during light and dark adaptation. Dark- adapted profiles were characterized by a minimum PO2 of nearly 0 mmHg at depths of 65-85%, and a steep gradient from the minimum to the choroid. During light adaptation at rod saturation, PO2 was elevated in the outer half of the retina and the minimum was eliminated. Fits of the profiles to a one-dimensional model of oxygen diffusion indicated that light reduced the oxygen consumption of the outer retina to approximately 50% of its dark-adapted value.
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            Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia

            Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark- adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark- adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.
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              Intraretinal oxygen distribution in the monkey retina and the response to systemic hyperoxia.

              To measure the intraretinal oxygen distribution and consumption in the fovea, the parafovea, and the inferior retina in the monkey eye and determine the influence of graded systemic hyperoxia. Oxygen sensitive microelectrodes were used to measure the oxygen tension as a function of depth through the retina in anesthetized monkeys (n = 8) under normoxic and hyperoxic conditions. Oxygen consumption rates in the avascular regions of retina were determined by fitting the oxygen profiles to established oxygen consumption models. Under normoxic conditions, in the foveal area, the intraretinal oxygen distribution reflected the absence of retinal capillaries and the predominantly choroidal origin of retinal oxygenation. A similar shape of oxygen distribution was seen in the parafoveal retina with the addition of local perturbations in the inner retina attributed to the presence of retinal capillaries. In the inferior retina the same general shape was found. Oxygen consumption in the outer retina was higher in the parafoveal region, and the minimum oxygen tension was lower. During hyperoxia, choroidal oxygen levels in all areas increased dramatically, but the increase in oxygen tension in the inner retina was much less. The avascular nature of the foveal area allowed oxygen consumption analysis of both the inner and outer retina and showed that inner retinal oxygen consumption increased significantly during hyperoxic ventilation to a level equivalent to that of the outer retina. In the outer retina of the monkey the P(O2) minimum is lower, and the oxygen consumption rate is higher in the parafoveal region. During systemic hyperoxia, outer retinal oxygen consumption is unaffected, but in the foveal area, total oxygen consumption increases. This regulation of oxygen consumption in the monkey retina is comparable to that reported in lower mammals and may represent an important mechanism in retinal homeostasis.
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                Author and article information

                Journal
                ORE
                Ophthalmic Res
                10.1159/issn.0030-3747
                Ophthalmic Research
                S. Karger AG
                0030-3747
                1423-0259
                2007
                March 2007
                02 February 2007
                : 39
                : 2
                : 103-107
                Affiliations
                Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Ill., USA
                Article
                99246 PMC2883832 Ophthalmic Res 2007;39:103–107
                10.1159/000099246
                PMC2883832
                17284937
                © 2007 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 2, Tables: 2, References: 35, Pages: 5
                Categories
                Original Paper

                Vision sciences, Ophthalmology & Optometry, Pathology

                Oxygen, Retina, Choroid

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