Oxygen Distribution and Consumption within the Retina in Vascularised and Avascular Retinas and in Animal Models of Retinal Disease

Abstract

Maintenance of an adequate oxygen supply to the retina is critical for retinal function. In species with vascularised retinas, such as man, oxygen is delivered to the retina via a combination of the choroidal vascular bed, which lies immediately behind the retina, and the retinal vasculature, which lies within the inner retina. The high-oxygen demands of the retina, and the relatively sparse nature of the retinal vasculature, are thought to contribute to the particular vulnerability of the retina to vascular disease. A large proportion of retinal blindness is associated with diseases having a vascular component, and disrupted oxygen supply to the retina is likely to be a critical factor. Much attention has therefore been directed at determining the intraretinal oxygen environment in healthy and diseased eyes. Measurements of oxygen levels within the retina have largely been restricted to animal studies in which oxygen sensitive microelectrodes can be used to obtain high-resolution measurements of oxygen tension as a function of retinal depth. Such measurements can immediately identify which retinal layers are supplied with oxygen from the different vascular elements. Additionally, in the outer retinal layers, which do not have any intrinsic oxygen sources, the oxygen distribution can be analysed mathematically to quantify the oxygen consumption rate of specific retinal layers. This has revealed a remarkable heterogeneity of oxygen requirements of different components of the outer retina, with the inner segments of the photoreceptors being the dominant oxygen consumers. Since the presence of the retinal vasculature precludes such a simple quantitative analysis of local oxygen consumption within the inner retina, our understanding of the oxygen needs of the inner retinal components is much less complete. Although several lines of evidence suggest that in the more commonly studied species such as cat, pig, and rat, the oxygen demands of the inner retina as a whole is broadly comparable to that of the outer retina, exactly which cell layers within the inner retina have the most stringent oxygen demands is not known. This may be a critical issue if the cell types most at risk from disrupted oxygen supply are to be identified. This paper reviews our current understanding of the oxygen requirements of the inner and outer retina and presents new data and mathematical models which identify three dominant oxygen-consuming layers in the rat retina. These are the inner segments of the photoreceptors, the outer plexiform layer, and the deeper region of the inner plexiform layer.

We also address the intriguing question of how the oxygen requirements of the inner retina are met in those species which naturally have a poorly vascularised, or even totally avascular retina. We present measurements of the intraretinal oxygen distribution in two species of laboratory animal possessing such retinas, the rabbit and the guinea pig. The rabbit has a predominantly avascular retina, with only a narrow band of retinal vasculature, and the guinea pig retina is completely avascular. Both these animals demonstrate species adaptations in which the oxygen requirement of their inner retinas are extremely low when compared to that of their outer retinas. This finding both uncovers a remarkable ability of the inner retina in avascular species to function in a low-oxygen environment, and also highlights the dangers of extrapolating findings from avascular retinas to infer metabolic requirements of vascularised retinas.

Different species also demonstrate a marked diversity in the manner in which intraretinal oxygen distribution is influenced by increases in systemic oxygen level. In the vascularised rat retina, the inner retinal oxygen increase is muted by a combination of increased oxygen consumption and a reduction of net oxygen delivery from the retinal circulation. The avascular retina of the guinea pig demonstrated a novel and powerful regulatory mechanism that prevents any dramatic rise in choroidal oxygen levels and keeps retinal oxygen levels within the normal physiological range. In contrast, in the avascular regions of the rabbit retina the choroidal oxygen level passively follows the increase in systemic oxygenation, and there is a dramatic rise in oxygen level in all retinal layers. The presence or absence of oxygen-regulating mechanisms may well reflect important survival strategies for the retina which are not yet understood.

Intraretinal oxygen measurements in rat models of retinal disease are also presented. We describe how oxygen distribution across the rat retina is influenced by manipulation of systemic blood pressure. We examine the effect of acute and chronic occlusion of the retinal vasculature, and explore the feasibility of meeting the oxygen needs of the ischemic retina from the choroid. We also describe how oxygen metabolism in the rat retina is affected in two different models of outer retinal degeneration. A urethane model of outer retinal degeneration, in which the oxygen uptake of the remaining inner retina is compromised, and the RCS model of outer retinal degeneration, in which the oxygen uptake of the inner retina is largely maintained. These contrasting findings of the oxygen metabolism of the inner retina in two models of outer retinal degeneration, clearly point to important mechanistic differences in the retinal pathology in each case.

The presented studies of intraretinal oxygen distribution have identified a marked heterogeneity of oxygen uptake in different regions of the vascularised rat retina. In an avascular retina, or in an avascular region of a partially vascularised retina, the inner retina has a dramatically lower oxygen demand. Mechanisms capable of regulating intraretinal oxygen tension are identified and they are based on both the regulation of oxygen supply and the local regulation of oxygen consumption. The diversity of intraretinal oxygen changes in different models of retinal disease further highlights the fact that there is clearly still a great deal to be learnt about the role of retinal oxygen supply and consumption in retinal disease.

Introduction

Oxygen is the only molecule serving as the primary biological oxidant (Vanderkooi et al., 1991). In man, lack of oxygen is known to be the primary cause of visual loss following total ischemia of the intraocular vasculature. This is readily demonstrated by the increased time to visual blackout when the retina is loaded with additional oxygen prior to the ischemic insult (Anderson and Saltzman, 1964). Under conditions of systemic hypoxia, a non-ischemic form of oxygen deprivation, retinal function is also impaired (McFarland and Forbes, 1940). We can thus conclude that oxygen is a key supply limited metabolite essential for normal retinal function in man. Since the retinal oxygen requirements must be derived from the local blood supply, it is not surprising that tissue hypoxia is thought to be an important factor in retinal diseases with a vascular component. Such diseases account for the majority of retinal blindness in our community (Cooper, 1990). The energy demands of the visual process are high, and much of the energy is derived from oxidative metabolism coupled to ATP synthesis. The oxygen consumption of the retina on a per gram basis has been described as higher than that of the brain (Anderson and Saltzman, 1964; Ames, 1992). Given that the brain consumes a highly disproportionate share of the total body oxygen uptake (Coyle and Puttfarcken, 1993), this places the retina as one of the highest oxygen consuming tissues in the body (Anderson, 1968). Since oxygen cannot be “stored” in tissue, a constant and adequate supply must be guaranteed in order to preserve function (Vanderkooi et al., 1991). Oxygen supply to the retina is arguably more vulnerable to vascular deficiencies than any other organ. Whilst in all mammals, there is a very rich vascular bed lying immediately behind the retina, the extent of vascularisation within the retina itself varies considerably between species (Chase, 1982), and sometimes in different regions of the same retina. The requirement for a relatively unobstructed light path to the photoreceptors presumably places a constraint on the degree to which the retina can be vascularised. This results in a very delicate balance between the available oxygen supply and the consumption of oxygen within the retina.

In contrast to the brain, the retina has a highly layered structure in which the different cell types and the supporting vascular components are spatially separated. The likelihood of differing oxygen requirements of different cell types and their relationship to the nearest oxygen source makes it difficult to predict the oxygen environment in any particular retinal layer. Fortunately, the retina in vivo is reasonably amenable to direct measurement of the intraretinal oxygen distribution. This review outlines a series of studies in which the intraretinal oxygen environment has been explored in a range of species with differing levels of retinal vascularisation, and under both normal and pathological conditions. The findings illustrate that oxygen metabolism of the retina is more complex than may have been anticipated, and the presence of powerful oxygen regulating mechanisms clearly indicates that an appropriate oxygen environment is critical to the function of some classes of retinal cells. The implications of these findings are discussed and future avenues for research identified.