How does the eye breathe? Evidence for neuroglobin-mediated oxygen supply in the mammalian retina

Visual performance of the vertebrate eye requires large amounts of oxygen, and thus the retina is one of the highest oxygen-consuming tissues of the body. Here we show that neuroglobin, a neuron-specific respiratory protein distantly related to hemoglobin and myoglobin, is present at high amounts in the mouse retina (approximately 100 microm). The estimated concentration of neuroglobin in the retina is thus about 100-fold higher than in the brain and is in the same range as that of myoglobin in the muscle. Neuroglobin is expressed in all neurons of the retina but not in the retinal pigment epithelium. Neuroglobin mRNA was detected in the perikarya of the nuclear and ganglion layers of the neuronal retina, whereas the protein was present mainly in the plexiform layers and in the ellipsoid region of photoreceptor inner segment. The distribution of neuroglobin correlates with the subcellular localization of mitochondria and with the relative oxygen demands, as the plexiform layers and the inner segment consume most of the retinal oxygen. These findings suggest that neuroglobin supplies oxygen to the retina, similar to myoglobin in the myocardium and the skeletal muscle.     

Morphological observations on the unique paired capillaries of the opossum retina

Light-microscopic and ultrastructural analysis of the ocular tissues of the North American opossum (Didelphis virginiana) revealed that the arterial and venous segments of retinal vessels, including capillaries of the smallest calibre, occur in pairs. They do not form anastomotic networks, the common pattern in mammals with vascularised retinae, but instead the two segments of the pair join to form hairpin end loops. The pairedd vessels, with the arteriolar limb usually on the vitread aspect, penetrate the retina and branch to form three distinct layers of capillaries. The most superficial lies in the nerve fiber layer, the middle is situated in the inner nuclear layer and the deepest extends to the external limiting membrane, which is considerably deeper than in normal mammalian holangiotic retinae. The paired capillaries display classical morphological features of central nervous system capillaries, i.e., they are lined by continuous endothelial cells united by tight junctions. The lining endothelium is supported by a distinct basal lamina that splits to envelop pericytes. The latter, although abundant, are invariably interposed between the two vessels that form each vascular unit. Phylogenetic and functional aspects of this unique form of retinal vascularisation are discussed.     

A multi-layer model of retinal oxygen supply and consumption helps explain the muted rise in inner retinal PO(2) during systemic hyperoxia

A multi-layer mathematical model of oxygen supply and consumption in the rat retina is described. The model takes advantage of the highly layered structure of the retina and the compartmentalisation of the available oxygen sources. The retina is divided into eight layers, each with a distinct oxygen consumption or supply rate. When applied to the available data from intraretinal oxygen measurements in the rat under normal physiological conditions, a close fit between the model and the data was achieved (r(2)=0.98+0.005, n=6). The model was then used to investigate recent evidence of oxygen regulating mechanisms in the rat retina during systemic hyperoxia. Fitting our model to the experimental data (r(2)=0.988+0.004, n=25) allowed the relative oxygen delivery or consumption of the key retinal layers to be determined. Two factors combine to produce the relative stability of inner retinal oxygen levels in hyperoxia. The retinal layer containing the outer plexiform layer/deep retinal capillaries, switches from a net source to a net consumer of oxygen, and the oxygen consumption of the outer region of the inner plexiform layer increases significantly. The model provides a useful tool for examining oxygen consumption and supply in all retinal layers, including for the first time, those layers within the normally perfused inner retina.     

Outer retinal anoxia during dark adaptation is not a general property of mammalian retinas

The purpose of this study was to measure the intraretinal oxygen distribution across the retina under conditions, which maximise outer retinal oxygen consumption. In particular, we looked for evidence of increased oxygen delivery from the choroid and the deep retinal capillary layer, and whether or not this was sufficient to avoid the development of intraretinal anoxia. Under dark-adapted conditions the photoreceptors need additional energy, at least part of which is derived from increased oxidative metabolism. In earlier studies in the cat retina it was revealed that dark adaptation could render some regions of the outer retina anoxic. The present study of the in vivo oxygen distribution across the rat retina in light and dark found no evidence of outer retinal anoxia in the dark. This was despite a mean increase of 52.6+/-11.4% (n=7) in outer retinal oxygen consumption in the dark. The mean value for the minimum outer retinal PO(2) in the dark was 5.2+1.2 mmHg. Oxygen delivery from both the choroid and the deep retinal capillary layer increased in the dark (P<0.01, and P<0.001, respectively). It is argued that the ability of the deep capillary layer to compensate for changes in oxygen demand in the outer retina is an important element in the maintenance of homeostasis in the retina. This is in addition to the role of the deep capillary layer in supplying oxygen to the highly consuming plexiform layers within the inner retina. These findings in the rat retina also demonstrate that intraretinal anoxia in the dark, is not, as implied by earlier work in the cat, a general feature of mammalian retinas.     

Distribution of mitochondria within Muller cells – I. Correlation with retinal vascularization in different mammalian species

The distribution of mitochondria within retinal glial (Muller) cells and neurons was studied by electron microscopy, by confocal microscopy of a mitochondrial dye and by immunocytochemical demonstration of the mitochondrial enzyme GABA transaminase (GABA-T). We studied sections and enzymatically dissociated cells from adult vascularized (human, pig and rat) and avascular or pseudangiotic (guinea-pig and rabbit) mammalian retinae. The following main observations were made. (1) Muller cells in adult euangiotic (totally vascularized) retinae contain mitochondria throughout their length. (2) Muller cells from the periphery of avascular retinae display mitochondria only within the sclerad-most end of Muller cell processes. (3) Muller cells from the vascularized retinal rim around the optic nerve head in guinea-pigs contain mitochondria throughout their length. (4) Muller cells from the peripapillar myelinated region (‘medullary rays’) of the pseudangiotic rabbit retina contain mitochondria up to their soma. In living dissociated Muller cells from guinea-pig retina, there was no indication of low intracellular pH where the mitochondria were clustered. These data support the hypothesis that Muller cells display mitochondria only at locations of their cytoplasm where the local O2 pressure (pO2) exceeds a certain threshold. In contrast, retinal ganglion cells of guinea-pig and rabbit retinae display many mitochondria although the local pO2 in the inner (vitread) retinal layers has been reported to be extremely low. It is probable that the alignment of mitochondria and the expression of mitochondrial enzymes are regulated by different mechanisms in various types of retinal neurons and glial cells.     

Low oxygen consumption in the inner retina of the visual streak of the rabbit

The oxygen requirements of different retinal layers are of interest in understanding the vulnerability of the retina to hypoxic damage in retinal diseases with an ischemic component. Here, we report the first measurements of retinal oxygen consumption in the visual streak of the rabbit retina, the region with the highest density of retinal neurons, and compare it with that in the less-specialized region of the retina underlying the vascularized portion of the rabbit retina. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of retinal depth in anesthetized animals. Measurements were performed in the region of the retina containing overlying retinal vessels and in the center of the visual streak. Established mathematical analyses of the intraretinal oxygen distribution were used to quantify the rate of oxygen consumption in the inner and outer retina and the relative oxygen contributions from the choroidal and vitreal sides. Outer retinal oxygen consumption was higher in the visual streak than in the vascularized area (means +/- SE, 284 +/- 20 vs. 210 +/- 23 nl O2.min(-1) x cm(-2), P = 0.026, n = 10). However, inner retinal oxygen consumption in the visual streak was significantly lower than in the vascular area (57 +/- 4.3 vs. 146 +/- 12 nl O2 x min(-1) x cm(-2), P < 0.001). We conclude that despite the higher processing requirements of the inner retina in the visual streak, it has a significantly lower oxygen consumption rate than the inner retina underlying the retinal vasculature. This suggests that the oxygen uptake of the inner retina is regulated to a large degree by the available oxygen supply rather than the processing requirements of the inner retina alone.     

Angiopoietin 2 expression in the retina: upregulation during physiologic and pathologic neovascularization

Vascular development in the embryo requires coordinated signaling through several endothelial cell-specific receptors; however, it is not known whether this is also required later during retinal vascular development or as part of retinal neovascularization in adults. The Tie2 receptor has been implicated in stabilization and maturation of vessels through action of an agonist ligand, angiopoietin 1 (Ang1) and an antagonistic ligand, Ang2. In this study, we have demonstrated that ang2 mRNA levels are increased in the retina during development of the deep retinal capillaries by angiogenesis and during pathologic angiogenesis in a model of ischemic retinopathy. Mice with hemizygous disruption of the ang2 gene by insertion of a promoterless beta-galactosidase (beta gal) gene behind the ang2 promoter, show constitutive beta gal staining primarily in cells along the outer border of the inner nuclear layer identified as horizontal cells by colocalization of calbindin. During development of the deep capillary bed or retinal neovascularization, other cells in the inner nuclear layer and ganglion cell layer, in regions of neovascularization, stain for beta gal. Thus, there is temporal and spatial correlation of Ang2 expression with developmental and pathologic angiogenesis in the retina, suggesting that it may play a role.     

Zebrafish reveals different and conserved features of vertebrate neuroglobin gene structure, expression pattern, and ligand binding

Neuroglobin has been identified as a respiratory protein that is primarily expressed in the mammalian nervous system. Here we present the first detailed analysis of neuroglobin from a non-mammalian vertebrate, the zebrafish Danio rerio. The zebrafish neuroglobin gene reveals a mammalian-type exon-intron pattern in the coding region (B12.2, E11.0, and G7.0), plus an additional 5'-non-coding exon. Similar to the mammalian neuroglobin, the zebrafish protein displays a hexacoordinate deoxy-binding scheme. Flash photolysis kinetics show the competitive binding on the millisecond timescale of external ligands and the distal histidine, resulting in an oxygen affinity of 1 torr. Western blotting, immune staining, and mRNA in situ hybridization demonstrate neuroglobin expression in the fish central nervous system and the retina but also in the gills. Neurons containing neuroglobin have a widespread distribution in the brain but are also present in the olfactory system. In the fish retina, neuroglobin is mainly present in the inner segments of the photoreceptor cells. In the gills, the chloride cells were identified to express neuroglobin. Neuroglobin appears to be associated with mitochondria-rich cell types and thus oxygen consumption rates, suggesting a myoglobin-like function of this protein in facilitated oxygen diffusion.     

DNA inhibition and development of the pigment epithelium and receptors in rat retina]

In the retina of the guinea pig the outer segments of the receptor cells are damaged by cyclophosphamid, but the inner segments and the centriole are not altered. The disturbed membranes of the outer segments are incorporated by pigment epithelium. The progress of degeneration of outer segments corresponds with the increase of the number of Lysosomes in the pigmemt epithelium. The structure of mitochondria is disturbed by blocking the DNA. The unaltered mitochondria show a normal reaction of succinode hydrogenase.     

Oxygen supply from the bird's eye perspective: globin E is a respiratory protein in the chicken retina

The visual process in the vertebrate eye requires high amounts of metabolic energy and thus oxygen. Oxygen supply of the avian retina is a challenging task because birds have large eyes, thick retinae, and high metabolic rates but neither deep retinal nor superficial capillaries. Respiratory proteins such as myoglobin may enhance oxygen supply to certain tissues, and thus the mammalian retina harbors high amounts of neuroglobin. Globin E (GbE) was recently identified as an eye-specific globin of chicken (Gallus gallus). Orthologous GbE genes were found in zebra finch and turkey genomes but appear to be absent in non-avian vertebrate classes. Analyses of globin phylogeny and gene synteny showed an ancient origin of GbE but did not help to assign it to any specific globin type. We show that the photoreceptor cells of the chicken retina have a high level of GbE protein, which accumulates to ～10 μM in the total eye. Quantitative real-time RT-PCR revealed an ～50,000-fold higher level of GbE mRNA in the eye than in the brain. Spectroscopic analysis and ligand binding kinetics of recombinant chicken GbE reveal a penta-coordinated globin with an oxygen affinity of P(50) = 5.8 torrs at 25 °C and 15 torrs at 41 °C. Together these data suggest that GbE helps to sustain oxygen supply to the avian retina.     

Expression pattern of glycoconjugates in rat retina as analysed by lectin histochemistry

The present study sought to characterize the expression and distribution of complex glycoconjugates in the rat retina by lectin histochemistry, using a panel of 21 different lectins with different carbohydrate specificities. Paraffin sections of Carnoy-fixed Sprague-Dawley rat eyes were stained with various biotinylated lectins, followed by the streptavidin-peroxidase and glucose oxidase-diaminobenzidine-nickel staining procedures. The results showed that the retinal pigment epithelium was stained intensely with LCA, Jacalin, WFA, S-WGA, PWA, DSA, UEA-I, LTA and PHA-E, suggesting that this epithelium contained glycoconjugates with alpha-Man, alpha-Glc, alpha-Gal/GalNAc, beta-GalNAc, alpha-Fuc, NeuAc and other oligosaccharide residues. The outer and inner segments of the photoreceptor layer showed different lectin binding affinities. The outer segments reacted with S-WGA and GS-II, whereas the inner segments reacted with UEA-II, UEA-I, LTA and MAA, suggesting that the inner segments contained glycoconjugates rich in alpha-Fuc and NeuAc(alpha2,3)Gal residues. PNA labelled specifically the cones and could be used as a specific marker for these photoreceptors. RCA-I, WFA, S-WGA, DSA, MAA and PHA-E reacted with both the outer and inner plexiform layers. On the other hand, UEA-I and LTA specifically labelled the outer plexiform layer, while PNA labelled the inner plexiform layer. The retinal microglial cells were labelled specifically by GS-I-B4 and SNA. Interestingly, we also observed that WFA bound specifically to Müller cells and could be used as a novel marker for this retinal glial cell. The capillaries and larger vessels in the retina and choriocapillaris reacted intensely with GS-I-B4, RCA-I, S-WGA, PWA, DSA and PHA-E. No significant differences in lectin binding were observed in the microvessels at these two sites. In summary, the present study demonstrated the expression patterns of glycoconjugates in the rat retina and that certain lectins could be used as histochemical markers for specific structural and cellular components of the rat retina.     

Expression pattern of Kv11 (Ether à-go-go-related gene; erg) K+ channels in the mouse retina

In response to light, most retinal neurons exhibit gradual changes in membrane potential. Therefore K+ channels that mediate threshold currents are well-suited for the fine-tuning of signal transduction. In the present study we demonstrate the expression of the different Kv11 (ether-à-go-go related gene; erg) channel subunits in the human and mouse retina by RT PCR and quantitative PCR, respectively. Immunofluorescence analysis with cryosections of mouse retinae revealed the following local distribution of the three Kv11 subunits: Kv11.1 (m-erg1) displayed the most abundant expression with the strongest immunoreactivity in rod bipolar cells. In addition, immunoreactivity was found in the inner part of the outer plexiform layer (OPL), in the inner plexiform layer (IPL) and in the inner segments of photoreceptors. Immunoreactivity for Kv11.2 (m-erg2) was observed in the outer part of the OPL and throughout the IPL. Double-labeling for vGluT1 or synaptophysin indicated a mainly presynaptic localization of Kv11.2. While no significant staining for Kv11.3 (m-erg3) was detected in the neuronal retina, strong Kv11.3 immunoreactivity was present in the apical membrane of the retinal pigment epithelium. The different expression levels were confirmed by real-time PCR showing almost equal levels of Kv11.1 and Kv11.2, while Kv11.3 mRNA expression was significantly lower. The two main splice variants of Kv11.1, isoforms a and b were detected in comparable levels suggesting a possible formation of cGMP/cGK-sensitive Kv11.1 channels in photoreceptors and rod bipolar cells. Taken together, the immunohistological results revealed different expression patterns of the three Kv11 channels in the mouse retina supposing distinct physiological roles.     

Developmental expression of GLUT2 in the rat retina

We previously demonstrated that GLUT2, a facilitated-diffusion glucose transporter isoform known to play critical roles in the regulation of systemic blood glucose level, is present at the apical ends of Müller cells in the rat retina. As a means of elucidating the ontogeny and possible role(s) of GLUT2 in the developing retina, this study examined its expression at various stages of retinal development by immunofluorescence staining using GLUT2-specific antibody. Evidence of GLUT2 expression first appeared at embryonic day 14 (E14) as linear staining along the boundary between the inner and outer layers of the optic cup, with this staining pattern being present throughout subsequent embryonic and neonatal stages. After the development of photoreceptor cell inner and outer segments (i.e., photoreceptor layer), GLUT2 immunoreactivity was localized along the boundary between the outer nuclear layer and photoreceptor layer. Localization of GLUT2 expression and the timing of its appearance, which coincided with the formation of choriocapillaries, together suggest that GLUT2 is involved in the anterior transport of glucose supplied by choroidal circulation from the early stages of retinal development.     

Expression pattern of Glycoconjugates in Rat Retina as Analysed by Lectin Histochemistry

The present study sought to characterize the expression and distribution of complex glycoconjugates in the rat retina by lectin histochemistry, using a panel of 21 different lectins with different carbohydrate specificities. Paraffin sections of Carnoy-fixed Sprague–Dawley rat eyes were stained with various biotinylated lectins, followed by the streptavidin-peroxidase and glucose oxidase–diaminobenzidine–nickel staining procedures. The results showed that the retinal pigment epithelium was stained intensely with LCA, Jacalin, WFA, S-WGA, PWA, DSA, UEA-I, LTA and PHA-E, suggesting that this epithelium contained glycoconjugates with α-Man, α-Glc, α-Gal/GalNAc, β-GalNAc, α-Fuc, NeuAc and other oligosaccharide residues. The outer and inner segments of the photoreceptor layer showed different lectin binding affinities. The outer segments reacted with S-WGA and GS-II, whereas the inner segments reacted with UEA-II, UEA-I, LTA and MAA, suggesting that the inner segments contained glycoconjugates rich in α-Fuc and NeuAc(α2,3)Gal residues. PNA labelled specifically the cones and could be used as a specific marker for these photoreceptors. RCA-I, WFA, S-WGA, DSA, MAA and PHA-E reacted with both the outer and inner plexiform layers. On the other hand, UEA-I and LTA specifically labelled the outer plexiform layer, while PNA labelled the inner plexiform layer. The retinal microglial cells were labelled specifically by GS-I-B₄ and SNA. Interestingly, we also observed that WFA bound specifically to Müller cells and could be used as a novel marker for this retinal glial cell. The capillaries and larger vessels in the retina and choriocapillaris reacted intensely with GS-I-B₄, RCA-I, S-WGA, PWA, DSA and PHA-E. No significant differences in lectin binding were observed in the microvessels at these two sites. In summary, the present study demonstrated the expression patterns of glycoconjugates in the rat retina and that certain lectins could be used as histochemical markers for specific structural and cellular components of the rat retina.     

The localization of glutathione peroxidase in the photoreceptor cells and the retinal pigment epithelial cells of Wistar and Royal College of Surgeons dystrophic rats

INTRODUCTION: If degenerating photoreceptor outer segments not phagocytized by RPE cells in the retina of Royal College Surgeons (RCS) rats were to undergo peroxidation, the distribution of glutathione peroxidase (GSH-PO) in the mitochondria or cytoplasm of the retina might be altered. We evaluated the immunocytochemical localization of GSH-PO to identify subcellular organelles in sections of the retinas of RCS rats. METHODS: Immunoblot analysis confirmed the presence of GSH-PO molecules in the retinas of RCS and Wistar rats aged 3 weeks. Sections were reacted with the F(ab) fragment of anti-rat alphaGSH-PO and then examined by laser scanning microscopy (LSM) and transmission electron microscopy (TEM). RESULTS: The size of the GSH-PO molecule in the retina was about 21 KD in the mitochondria and 23 KD in the cytosol in both strains of rats. LSM revealed fluorescent granules in the photoreceptor inner segments of the Wistar rats, and immunohistochemical TEM revealed GSH-PO in the mitochondria of their photoreceptor inner segments and retinal pigment epithelial (RPE) cells. In the RCS rats, the degenerating photoreceptor outer segments were clearly seen to be positive for anti-GSH-PO by conventional light microscopy (CLM). However, the photoreceptor inner segments of the RCS rats were negative for staining with anti-GSH-PO by LSM, and no GSH-PO could be detected in the mitochondria of the photoreceptor inner segments or RPE cells by immuno-TEM. CONCLUSION: Degeneration of the photoreceptor outer segments induced mitochondrial damage in the photoreceptor inner segments, and as a result GSH-PO shifted from the photoreceptor inner segments to the degenerating outer segments.     

Distribution of mitochondria within Muller cells – II. Post-natal development of the rabbit retinal periphery in vivo and in vitro: dependence on oxygen supply

The occurrence and localization of mitochondria within glial (Muller) cells and neurons of the peripheral (avascular) rabbit retina was studied electron microscopically and by immunocytochemical demonstration of the mitochondrial enzyme GABA transaminase (GABA-T). Post-natal development in vivo was compared with development of organ cultures from neonatal rabbit retinae, grown over 2 weeks in vitro. The adult pattern of mitochondrial localization (restriction to the sclerad end of the cells) was observed from the beginning of enzyme expression at early post-natal stages. However, when neonatal retinal pieces were grown in vitro with their vitread surface exposed to the air, their Muller cells contained mitochondria along most of their length. When functionally developed retinae from postnatal day 14 were explanted in vitro, they retained their sclerad mitochondrial distribution for almost 24 h but thereafter the inner portions of their cytoplasm became occupied by mitochondria within a few hours. This was achieved mainly by mitochondrial migration rather than by formation of new mitochondria because it was not prevented by cycloheximide-induced inhibition of protein synthesis. These data support the following hypotheses: (1) the mitochondrial distribution in Muller cells is determined by the local cytoplasmic O2 pressure (pO2), (2) existing mitochondria move towards cytoplasmic regions of sufficient pO2 by rather rapid migration and (3) the start of this migration is delayed by almost 24 h due to the action of as yet unknown control mechanisms. In contrast, the mitochondrial content of retinal ganglion and amacrine cells in the vitread retinal layers was virtually independent of the source and level of oxygen supply.     

Immunocytochemical localization of vitamin A in the retina and pineal organ of the frog, Rana esculenta

Vitamin A immunoreactive sites were studied in the retina and pineal organ of the frog, Rana esculenta, by the peroxidase antiperoxidase, avidin-biotinperoxidase and immunogold methods. In dark-adapted material, strong immunoreaction was found in the outer and inner segments of the photoreceptor cells of both retina and pineal organ, as well as in the pigment epithelium, retinal Müller cells and pineal ependymal cells. In light-adapted retina, cones and green (blue-sensitive) rods were immunopositive. At the electron microscopic level, immunogold particles were found on the membranes of the photoreceptor outer segments as well as on the membranes of the endoplasmic reticulum and mitochondria. Individual retinal photorecptor cells exhibited strong immunoreaction in the distal portion of the inner segment, the ciliary connecting piece and the electron-dense material covering the outer segment. In the pigment epithelium, the immunolabeling varied in intensity in the basal and apical cytoplasm and phagocytosed outer segments. The immunocytochemical results indicate that retinoids (retinal, retinol and possibly retinoic acid) are present not only in the photoreceptor cells of the retina but also in those of the pineal organ. The light-dependent differences in the immunoreactivity of vitamin A underlines its essential role in the visual cycle of the photopigments. Our results suggest that the pineal ependyma plays a role comparable to that of the Müller cells and pigment epithelium of the retina with regard to the transport and storage of vitamin A. The presence of a retinoid in nuclei, mitochondria and cytoplasmic membranes suggests an additional role of vitamin A in other metabolic processes.     

Oxygen distribution and consumption in the macaque retina

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. P(O2) was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O(2) microelectrodes. Oxygen consumption (Q(O2)) of the photoreceptors was extracted by fitting a steady-state diffusion model to P(O2) measurements. In the perifovea, the P(O2) was 48 +/- 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 +/- 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The P(O2) in the inner half of the retina in darkness was 17.9 +/- 7.8 mmHg. When averaged over the outer retina, photoreceptor Q(O2) (called Q(av)) was 4.6 +/- 2.3 ml O(2).100 g(-1).min(-1) under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Q(av) to 72 +/- 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O(2) from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Q(O2) of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Q(O2) with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.     

Distribution of activities of aspartate aminotransferase isoenzymes and malate dehydrogenase in guinea pig retinal layers

Distributions of activity of the cytosolic (cAAT) and mitochondrial (mAAT) isoenzymes of aspartate aminotransferase and of malate dehydrogenase (MDH) were determined in guinea pig retinal layers. The distribution of total AAT activity (tAAT = cAAT + mAAT) and of mAAT activity correlated well (r = 0.88-0.91) with the distribution of MDH activity. mAAT activity was highest in the inner segments of the photoreceptors; there was a greater than twelve-fold difference between activity in that layer and in the inner retinal layers. cAAT activity was also highest in the inner segments, but the difference between the activity in the inner segments and the other layers was not nearly as great as with mAAT. cAAT activity was also relatively high in the outer nuclear layer, outer plexiform layer, and part of the inner plexiform layer. The high activity of cAAT, mAAT, and MDH in the inner segments indicates that all of these enzymes are involved in metabolic reactions related to energy production and/or to photoreceptive processes in the outer segments and, therefore, that the enzymes are probably involved in energy-related metabolism at synapses. However, other functions, including those related to neurotransmission, are not excluded.     

Ageing transition of superoxide dismutase in rat retina

To examine the relationship between retinal ageing and superoxide dismutase, the distribution and expression of the dismutase was studied in the retina of 2-year-old Sprague--Dawley albino rats with immunohistochemistry and immunochemical quantitative analysis. Eight-week-old Sprague--Dawley albino rats were used as controls. In 2-year-old rats, manganese superoxide dismutase (Mn-SOD) immunoreactivities in the photoreceptor inner segments, the outer nuclear layer and the inner plexiform layer were stronger than those in 8-week-old rats. Copper--zinc superoxide dismutase (CuZn-SOD) immunoreactivities in the outer nuclear layer and inner plexiform layer of 2-year-old rats were stronger than those in 8-week-old rats. Faint CuZn-SOD immunoreactivity became visible in the photoreceptor inner segments of 2-year-old rats, whereas no CuZn-SOD immunoreactivity was observed in 8-week-old rats. Our immunochemical quantitative analysis also showed an increase in the immunoreactivities of superoxide dismutases in the sensory retina with age. The transition of the dismutases may have some relationship with retinal ageing. © 1998 Chapman & Hall