Systemic Hyperoxia Research Articles

Angiogenesis and vasculogenesis: Inducing the growth of new blood vessels and wound healing by stimulation of bone marrow–derived progenitor cell mobilization and homing

During embryonic development, the vasculature is among the first organs to form and is in charge of maintaining metabolic homeostasis by supplying oxygen and nutrients and removing waste products. As one would expect, blood vessels are critical not only for organ growth in the embryo but also for repair of wounded tissue in the adult. An imbalance in angiogenesis (a time-honored term that globally refers to the growth of new blood vessels) contributes to the pathogenesis of numerous malignant, inflammatory, ischemic, infectious, immune, and wound-healing disorders. This review focuses on the central role of the growth of new blood vessels in ischemic and diabetic wound healing and defines the most current nomenclature that describes the neovascularization process in wounds. There are now two well-defined, distinct, yet interrelated processes for the formation of postnatal new blood vessels, angiogenesis, and vasculogenesis. Reviewed are recent new data on vasculogenesis that promise to advance the field of wound healing.

RGC Sensitivity to Mild Systemic Hypoxia

Systemic hyperoxia, hypercapnia, and hypoxia are known to alter retinal perfusion. The effects such experimentally induced systemic blood gas perturbations have on inner neuroretinal function in humans were examined. Twenty healthy adults participated in each of three test sessions. The effects of breathing pure oxygen (O2), carbogen, or a hypoxic gas on the pattern electroretinogram (pERG) were investigated. The stimulus consisted of high-contrast, black-and-white, 48-min arc checks reversing at 1 Hz. pERGs were recorded with a Dawson-Trick-Litzkow electrode at the end of 5 minutes of breathing room air, 5 minutes of breathing the test gas, immediately after the flow of gas was stopped, and 10 minutes after the flow of gas was stopped. Amplitudes and implicit times of the major positive (P50) and negative (N95) components of the pERG were not altered during the pure O2 and carbogen breathing sessions. Although the amplitude and implicit time of P50 were not modified significantly with systemic hypoxia, they were depressed and delayed, respectively, for N95. Inner neuroretinal function remained unchanged during increased blood O2 and carbon dioxide levels known to alter retinal blood flow, but it was altered during decreased blood O2 levels. Overall, these results indicate that the generators of P50 are resistant to systemic hyperoxia, hypercapnia/hyperoxia, and hypoxia. They further indicate that the generators of N95, namely the retinal ganglion cells, are particularly sensitive to transient, mild systemic hypoxia.

Automatic Retinal Oximetry

To measure hemoglobin oxygen saturation (SO(2)) in retinal vessels and to test the reproducibility and sensitivity of an automatic spectrophotometric oximeter. Specialized software automatically identifies the retinal blood vessels on fundus images, which are obtained with four different wavelengths of light. The software calculates optical density ratios (ODRs) for each vessel. The reproducibility was evaluated by analyzing five repeated measurements of the same vessels. A linear relationship between SO(2) and ODR was assumed and a linear model derived. After calibration, reproducibility and sensitivity were calculated in terms of SO(2). Systemic hyperoxia (n = 16) was induced in healthy volunteers by changing the O(2) concentration in inhaled air from 21% to 100%. The automatic software enhanced reproducibility, and the mean SD for repeated measurements was 3.7% for arterioles and 5.3% venules, in terms of percentage of SO(2) (five repeats, 10 individuals). The model derived for calibration was SO(2) = 125 - 142 . ODR. The arterial SO(2) measured 96% +/- 9% (mean +/- SD) during normoxia and 101% +/- 8% during hyperoxia (n = 16). The difference between normoxia and hyperoxia was significant (P = 0.0027, paired t-test). Corresponding numbers for venules were 55% +/- 14% and 78% +/- 15% (P < 0.0001). SO(2) is displayed as a pseudocolor map drawn on fundus images. The retinal oximeter is reliable, easy to use, and sensitive to changes in SO(2) when concentration of O(2) in inhaled air is changed.

Oxygen Distribution and Consumption in the Developing Rat Retina

To determine the intraretinal oxygen distribution and oxygen consumption in the rat before eye opening and maturation of the retina. Oxygen-sensitive microelectrodes were used to measure the oxygen tension as a function of depth through the retina in anesthetized Sprague-Dawley rats at postnatal day (P)15. Measurements were made under air-breathing conditions and at increasing levels of systemic oxygenation (20%, 40%, 60%, 80%, and 100% oxygen) under light-adapted conditions. Oxygen consumption in the outer retina and in the predominantly avascular region of the inner retina was assessed by fitting the oxygen profiles to a mathematical model of oxygen consumption. The retinas were processed for histology and compared with retinas from mature animals. Under normal conditions, the intraretinal oxygen distribution at P15 was significantly different in some respects from that in mature animals. Oxygen consumption analysis indicated an average outer retinal oxygen consumption rate of 103 +/- 15 nL/min per cm2 and an inner retinal oxygen consumption rate of 42.4 +/- 11.7 nL/min per cm2. Inner retinal oxygen consumption was significantly (P < 0.001) lower than that previously measured in mature rats, but outer retinal oxygen consumption was similar. Systemic hyperoxia increased the oxygen level throughout the retina, but choroidal PO2 in particular remained significantly lower than in adult rats (P < 0.001). At P15 there were marked differences in the relative thickness of some retinal layers when compared with adult rats. In particular the inner and outer nuclear layers were much thicker at P15, the outer segments of the photoreceptors and the inner and outer plexiform layers were not fully developed. At P15, before eye opening, the oxygen consumption of the inner retina is lower than in mature retinas, presumably reflecting the immaturity of the retina in such young animals so soon after their first exposure to light stimuli.

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.

Systemic Hyperoxia and Retinal Vasomotor Responses

Several studies have investigated the changes in retinal vessel diameter during physiological stress or pathologic conditions. These studies were principally based on individual fundus photographs and as such did not allow the evaluation of vessel dynamics over time. The research objective was to detail the time course and amplitude changes in the diameter of arteries and veins across all retinal quadrants, during and after hyperoxic vascular stress. The dynamics of changes in retinal vessel diameter were quantified with a retinal vessel analyzer, which digitizes fundus images in real time and simultaneously quantifies vessel diameter. The arterial and venous diameters within one disc diameter of the optic nerve head in each quadrant were studied. Twenty young adults participated in this study in which the vessel diameters were measured during successive phases of breathing either room air or pure oxygen. The oxygen saturation level (SaO(2)), end-tidal carbon dioxide (EtCO(2)), pulse rate (PR), respiratory rate (RR), and blood pressure (BP) were also monitored throughout testing. Breathing 100% O(2) caused an increase in SaO(2) and a decrease in the EtCO(2). All other systemic parameters measured (PR, RR, BP, and ocular perfusion pressure [OPP]) remained unchanged. However, the retinal veins and arteries constricted by approximately 14% and approximately 9% respectively, in all retinal quadrants. After experimental hyperoxia, inhalation of room air was associated with a progressive increase in the caliber of vessels toward their pretest size. The amplitude and overall profile of vessel reactivity to and recovery from hyperoxia was the same across retinal quadrants. These data indicate that, during systemic hyperoxic stress, the retinal vessels change in caliber uniformly across retinal quadrants in healthy young adults. This type of physiological vascular provocation could be used to investigate the quality of vascular regulation during aging and in vascular diseases of the eye.

Response of retinal blood flow to systemic hyperoxia in smokers and nonsmokers

We investigated the influence of chronic smoking on ocular vascular reactivity during breathing of 100% oxygen. Retinal vascular reactivity was tested during inhalation of 100% oxygen over 10 min. The observer-masked two-cohort study was performed in 24 healthy male volunteers (12 smokers and 12 nonsmokers) using the Zeiss Retinal Vessel Analyzer and laser Doppler velocimetry. From these parameters retinal blood flow was calculated. Hyperoxia significantly decreased arterial (smokers: p<0.001 vs baseline; nonsmokers: p=0.003 vs baseline) and venous (smokers: p<0.001 vs baseline; nonsmokers: p<0.001 vs baseline) diameters. This decrease was significantly more pronounced in smokers (arterial diameter: p<0.001, venous diameter: p=0.003). Hyperoxia decreased venous blood flow velocity (smokers: p=0.02 vs baseline; nonsmokers: p<0.001 vs baseline) to a comparable degree (p=0.51). The two groups showed a comparable decrease in retinal blood flow during hyperoxia (smokers: p<0.001 vs baseline; nonsmokers: p<0.001 vs baseline; p=0.76 between groups). The decrease of PCO(2) during inhalation of 100% oxygen was significantly more pronounced in smokers than in nonsmokers (p=0.038). The present study indicates an abnormal retinal vascular response to hyperoxia in smokers. Further studies are needed to identify possible neural or humoral factors involved in this shifted vasoconstrictory status in smokers.

Intraretinal oxygenation and oxygen consumption in the rabbit during systemic hyperoxia.

To determine the ability of the retina and choroid in the avascular region of the rabbit retina to regulate intraretinal oxygen levels during periods of acute systemic hyperoxia. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of depth through the retina and choroid in anesthetized rabbits before and after stepwise incremental increases in inspired oxygen level. The extent of any oxygen increases throughout the retina and choroid was determined, and retinal oxygen consumption was determined by analyzing intraretinal oxygen distribution. Increases in systemic arterial oxygen in the rabbit resulted in significant increases in oxygen tension in the choroid and in all retinal layers. There was no apparent regulation of choroidal oxygen tension, as seen in the avascular retina of the guinea pig. Neither was there any increase in oxygen consumption within the retina, which has been shown to dampen the extent of raised oxygen levels in the inner retina in other mammals. Consequently, all retinal layers in the avascular area of the rabbit retina were exposed to high oxygen levels during systemic hyperoxia. The magnitude of the oxygen changes in all retinal layers was consistent with that in the preretinal vitreous. Unlike other mammals studied to date, the rabbit does not possess any regulatory mechanisms for controlling the intraretinal oxygen environment during acute increases in systemic arterial oxygen levels. This may well account for the reported vulnerability of the rabbit retina to toxic damage during extended periods of systemic hyperoxia.

Effect of acetazolamide on the optic disc oxygenation in miniature pigs.

The purpose of our study was to evaluate the variations of the optic disc PO (2) during normoxia and hyperoxia (100 % O (2)), before and after intravenous administration of acetazolamide. PO (2) measurements were obtained at intervascular areas of the optic disc in 11 anaesthetized miniature pigs using oxygen-sensitive microelectrodes introduced through the vitreous cavity by a micromanipulator. PO (2) was measured continuously during 10 minutes under systemic normoxia and systemic hyperoxia. Oxygen measurements were repeated under these conditions after intravenous injection of acetazolamide (bolus of 500 mg) in 8 animals. In systemic hyperoxia, the optic disc PO (2) increased moderately (DeltaPO (2) = 4.7 +/- 2.5 mmHg; p < 0.001; n = 11) in parallel with systemic PaO (2). Acetazolamide led to a slow and progressive increase in the optic disc PO (2) (DeltaPO (2) = 2.1 +/- 1.7 mmHg; p > 0.1; n = 8 after 10 min, while DeltaPO (2) = 4.3 +/- 3.2 mmHg; p < 0.05; n = 8 after 30 min), in parallel with a slow and progressive increase in systemic PaCO (2). The optic disc PO (2) increased much more significantly after injection of acetazolamide under systemic hyperoxia (DeltaPO (2) = 13.3 +/- 3.1 mmHg; p < 0.001; n = 8). Systemic hyperoxia alone is not sufficient to increase substantially the optic disc PO (2) in miniature pigs due to a vasoconstrictor effect. Intravenous injection of acetazolamide can increase the optic disc PO (2) progressively, due to a vasodilatory effect of elevated systemic PaCO (2). The association of acetazolamide injection with systemic hyperoxia can further improve the oxygenation of the optic disc.

Neuroretinal function during systemic hyperoxia and hypercapnia in humans.

Breathing pure oxygen (O2) or carbogen is known to have differential effects on the retinal and choroidal blood flow. Our objective was to evaluate the effects these hemodynamic changes have on various retinal neurons receiving their vital nutrients from these two vascular beds. To that effect, we recorded the photopic flash electroretinogram (fERG) and oscillatory potentials (OP's) in man. Eighteen adults participated in two test sessions to examine the effects of breathing pure O2 or carbogen on the fERG's and OP's. The retinal potentials were recorded at the end of each of the following breathing phases: (1) room air for 5 min, (2) pure O2 or carbogen for 5 min, (3) immediately after the flow of gas was stopped, and (4) 10 min after the flow of gas was stopped. The heart rate, respiratory rate, oxygen saturation (Sao2), and end-tidal carbon dioxide (Etco2) were monitored. The blood pressure and intraocular pressure were measured to derive the ocular perfusion pressure. Breathing pure O2 increased Sao2, decreased heart rate and Etco2, but did not alter respiratory rate and ocular perfusion pressure. Breathing carbogen increased Sao2, Etco2, and ocular perfusion pressure, decreased respiratory rate but did not alter heart rate. The fERG's and OP's were not detrimentally affected by breathing either pure oxygen or carbogen. Only OP4 was delayed at the end of testing in the O2 session. Our results show that the neural generators of the photopic fERG's and OP's in man are largely unaltered by the degree of systemic hyperoxia and hypercapnia induced and their reported effects on retinal and choroidal hemodynamics. These results, combined with earlier studies showing that some components of the scotopic fERG's and OP's were altered during similar testing conditions, suggest that the photopic system is more resistant than the scotopic system to altered ocular hemodynamics.

Neuronal activity-induced changes of local cerebral microvascular blood oxygenation in the rat: effect of systemic hyperoxia or hypoxia

To investigate the effect of resting blood oxygen concentration on the hemodynamic response to functional brain activation, we compared activation-induced changes in hemoglobin oxygenation during normoxia with systemic hyperoxia or mild hypoxia. Hemoglobin oxygenation changes were measured by microfiber optical spectroscopy (500–590 nm) in response to physiological whisker barrel cortex activation by whole whisker pad deflection (4 s, 4 Hz) in α-chloralose/urethane anesthetised male Wistar rats. During systemic hyperoxia ( n=6), the stimulation-induced hyperoxygenation response was decreased and prolonged, whereas during mild systemic hypoxia ( n=7) the peak response was significantly increased followed by a faster return to baseline. During normoxia, a poststimulation under- (oxy-hemoglobin) and overshoot (deoxy-hemoglobin) was observed, which disappeared during systemic hyperoxia and was pronounced during systemic hypoxia. Although averaging out below statistical significance when combining all animals, during mild systemic hypoxia a very small early increase of deoxy-Hb at the beginning of the stimulation period was conspicuous more often than during normoxic or even hyperoxic conditions. This small early increase of deoxy-Hb never preceded the onset of the oxy-Hb response, and was not accompanied by a concomitant decrease in oxy-Hb. Hyperoxia or hypoxia did not affect the induced neuronal responses. Our findings support the concept that the hemodynamic response is regulated according to the metabolic demand of oxygen within the activated tissue.

A multi-layer model of retinal oxygen supply and consumption helps explain the muted rise in inner retinal Po2 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.

Modulation of the scotopic electroretinogram and oscillatory potentials with systemic hyperoxia and hypercapnia in humans

Purpose. Systemic hyperoxia reduces blood flow to the retina while systemic hypercapnia has the opposite effect. However, the effect this modification in blood flow has on neuroretinal function in humans has not been documented yet. The purpose of this study was to evaluate the effect of pure oxygen and carbogen breathing on scotopic electroretinograms (ERGs) and oscillatory potentials (OPs) in humans. Methods. Thirty-five healthy adults volunteered for this study. The ERGs and OPs were recorded: 1) during room air breathing, 2) after a period of pure oxygen (O 2) or carbogen breathing, 3) in room air just after the flow of gas was interrupted, and 4) 10 minutes after the end of the gas administration. Results. The amplitude and latency of the a- and b-waves were not altered during systemic hyperoxia. The amplitude of OP3 increased during hyperoxia while the amplitude of the other OPs was not altered. The latency of all OPs remained stable throughout the O 2 session. Ten minutes after the end of pure O 2 breathing, the a- and b-wave latencies were delayed. No change was found in the amplitude of the a-wave during the carbogen session that increased the end-tidal carbon dioxide by 7.1%, whereas the b-wave was reduced ten minutes after the end of carbogen breathing. The amplitude of OP5 was reduced during carbogen breathing, as well as 10 minutes later. The amplitude of all other OPs, as well as the latencies of all ERG and OP components remained stable throughout the carbogen session. Conclusions. Breathing pure O 2 or carbogen did not compromise retinal function in any major way, likely due to adequate retinal and choroidal regulatory mechanisms. Further investigations are required to better delineate the impact and temporal characteristics of such physiological challenges on retinal function.

Measurements in the peripheral retina using LDF and laser interferometry are mainly influenced by the choroidal circulation

Purpose. Two laser based methods for the assessment of ocular hemodynamics in humans have been investigated: laser Doppler flowmetry (LDF) and laser interferometric measurement of fundus pulsation amplitude (FPA). When the laser with either of the two methods is focused onto the fovea it is obvious that only choroidal blood flow contributes to the signals. When the laser is, however, directed to other parts of the retina the situation is more complex. Whereas the retina shows a pronounced vasoconstrictor response to systemic hyperoxia the effect in the choroid is small. We therefore investigated the effect of 100% O 2 breathing on results as obtained with the above mentioned techniques at different fundus locations. Methods. Twelve healthy subjects were included. Four 15-minutes 100% O 2 breathing periods were scheduled for each subject. During two of these breathing periods LDF was performed at the fovea (ChBFf) and at a fundus location approximately 7.5 degrees nasally to the fovea (ChBFp), respectively. During the other two periods FPA was assessed at the same fundus locations (FPAf, FPAp). Results. ChBFf tended to decrease during 100% oxygen breathing (6 ± 4%), but this effect was not significant. The decrease in ChBFp (10 ± 4%), was comparable. FPAf (10 ± 2%; P < 0.001) and FPAp (13 ± 2%; P < 0.001) decreased significantly during systemic hyperoxia, but again there was no difference in the response obtained at the two fundus locations. Conclusion. When LDF and FPA are applied at the peripheral retina the obtained signal is mainly influenced by the choroidal circulation.

Tissue oxygen tension and blood-flow changes in rat incisor pulp with graded systemic hyperoxia

The role of oxygen in the regulation of the pulpal microcirculation is unknown. This investigation is aimed to measure tissue oxygen tension and blood-flow changes in the pulp of rat lower incisors during graded systemic hyperoxia, and to determine the response of the pulpal vasculature to various oxygen tensions. Twenty-four Sprague–Dawley rats were anaesthetized and artificially ventilated with the appropriate gas mixture. Recessed oxygen-sensitive microelectrodes were used to measure pulpal tissue oxygen tension via a small access cavity filled with saline on the labial surface of the incisor. A laser Doppler flowmeter was used to record pulpal blood-flow. Inspired oxygen was increased stepwise from 20 to 100% in 20% steps. Systemic blood-gas concentrations were measured at each step. Systemic arterial oxygen tension at 100% oxygen ventilation reached 481.2±30.7% of the baseline at 20% oxygen breathing ( n=21). Pulpal tissue oxygen tension did not change significantly whereas pulpal blood-flow fell dose-dependently to 74.6±5.0% at 100% oxygen ventilation ( n=21). Systemic hyperoxia, therefore, induces a significant reduction in pulpal blood-flow whereas pulpal tissue oxygen tension remains relatively stable, indicating an oxygen-dependent local regulatory mechanism.

Hyperoxia stimulates endothelin-1 secretion from endothelial cells; modulation by captopril and nifedipine

PURPOSE. Retinopathy of prematurity (ROP) is a vasoproliferative condition that can result in severe visual impairment and blindness in preterm babies. Two conditions seen very early in radioimmunoassay (ROP) are vasoconstriction and vaso-obliteration. A potent vasoconstrictor secreted by endothelial cells is endothelin-1 (ET-1). Premature birth results in a relative systemic hyperoxia, compared to the in utero oxygen milieu. We tested the hypothesis that hyperoxia increases ET-1 expression as a possible mechanism for vasoconstriction in the retinal vasculature. METHODS. Bovine retinal endothelial cells and adrenal capillary endothelial cells were isolated and maintained in culture. Cells were exposed to control or hyperoxic culture conditions for 24 h, with and without addition of captopril and nifedipine. Media was collected and assayed for ET-1 by ROP. In addition, cell counts and secreted LDH assays were performed. RESULTS. Conditioned media from cultured bovine retinal and adrenal endothelial cells exposed to hyperoxic culture conditions for 24 h were found to have higher levels of ET-1 than conditioned media from normoxic control cells. Captopril (10 -6 M and 10 -4 M) and nifedipine (10 -6 M and 10 -4 M) inhibited ET-1 release from hyperoxia-exposed endothelial cells. Under normoxic conditions, ET-1 release was inhibited by 10 -4 M captopril or 10 -4 M nifedipine. CONCLUSIONS. These results demonstrate that (1) hyperoxia stimulates in vitro ET-1 secretion in bovine retinal and adrenal capillary endothelial cells, and (2) captopril and nifedipine downregulate ET-1 secretion under normoxic and hyperoxic culture conditions, in a dose-dependent fashion. We speculate that ET-1 may be involved in retinal vessel vasoconstriction seen early in the development of ROP. Further, ACE inhibitors and calcium-channel blocking agents, such as captopril and nifedipine, may provide an avenue for blocking vasoconstriction in ROP.

Systemic Hyperoxia Decreases Vascular Endothelial Growth Factor Gene Expression in Ischemic Primate Retina

To determine whether systemic hyperoxia can reverse retinal hypoxia and decrease vascular endothelial growth factor (VEGF) gene expression in ischemic nonhuman primate retina. Six eyes of 3 cynomolgus monkeys were studied. Retinal ischemia was induced via laser vein occlusion and confirmed with fluorescein angiography. Animals were randomly assigned to treatment with either 21% or 100% inhaled oxygen. Arterial PO2 was monitored while systemic acid-base status was maintained. An oxygen microelectrode on a micromanipulator was used to measure preretinal oxygen concentrations in ischemic and nonischemic retina in situ. RNA was isolated from fresh whole retinas, and VEGF messenger RNA levels were quantified with Northern blotting. The preretinal PO2 in ischemic retina was less than the PO2 in nonischemic retina in animals breathing 21% oxygen (intervascular zone PO2, 14.3+/-0.53 vs 21.8+/-0.55 mm Hg; P=.002). After 8 hours of systemic hyperoxia (arterial PO2, 512+/-18 mm Hg), the preretinal PO2 in ischemic retina increased to 166.2+/-15.6 mm Hg (21.8% oxygen) and retinal VEGF messenger RNA levels were reduced by an average of 55%. These data demonstrate that systemic hyperoxia can lower retinal VEGF gene expression and reoxygenate ischemic adult primate retina.

Effects of systemic hyperoxia on eustachian tube ventilatory function.

Middle ear negative pressure and effusions, decreased middle ear compliance, and abnormal tympanometry results have been described after diving on oxygen. Middle ear gas hyperoxia has been shown to down-regulate the eustachian tube ventilatory function (ETVF). The purpose of the present study was to investigate to what extent systemic hyperoxia in the face of air-equivalent middle ear gas composition might interfere with the ETVF. ETVF was investigated in four young adult female cynomolgus monkeys by the forced-response and inflation-deflation tests using air while the animals breathed either room air or 100% normobaric oxygen. Higher opening, closing, and steady-state pressures were observed under systemic hyperoxia. The percentage of the applied pressure equalized, and the maximal pressure change on a single swallow in the deflation test were both lower under hyperoxic conditions. The results show that systemic hyperoxia might impair ETVF. This observation adds to our understanding of the pathophysiology of middle ear dysfunction observed after diving on oxygen.

Oxygen consumption in the avascular guinea pig retina.

Oxygen consumption across the retina of a mammal with a naturally avascular retina has not previously been investigated. The oxygen consumption across the avascular retina of the guinea pig was measured in vivo by spatial analysis of the intraretinal oxygen profile. The avascular nature of the guinea pig retina allows the inner retina to be included in the analysis without disrupting the normal physiological state of the retina. Oxygen-sensitive microelectrodes (1-micron tip) were used to make high-resolution measurements of oxygen tension through the retina of anesthetized, mechanically ventilated guinea pigs (n = 10). Oxygen profiles were then analyzed in terms of oxygen tension as a function of distance from the choriocapillaris/Bruch's membrane, and the data were fitted to an established mathematical model of retinal oxygen consumption. The average oxygen consumption of the full thickness of the guinea pig retina was 1.1 +/- 0.09 ml O2.min-1 x 100 g-1 (n = 10). The average oxygen consumption of the outer half of the retina was 2.07 +/- 0.17 ml O2.min-1 x 100 g-1, while that of the inner half was only 0.12 +/- 0.04 ml O2.min-1 x 100 g-1. A localized region of high oxygen consumption was identified in the outer retina in every case, and this region accounted for an average of 93.9 +/- 2.0% of the total retinal oxygen consumption. Only 5.2 +/- 1.4% of the total oxygen consumption was attributable to the inner half of the retina. When choroidal oxygen tension was increased via a combination of systemic hyperoxia and hypercapnia, high oxygen levels could be sustained in all retinal layers. Under these conditions of an excess oxygen supply, the inner retina still consumed only 0.45 +/- 0.11 ml O2.min-1 x 100 g-1, which was 13.8 +/- 2.5% of the total retinal oxygen consumed. The relatively low oxygen consumption in the inner retina of the guinea pig may reflect an interesting adaptation to the constraints imposed by the absence of a retinal circulation in this species.

The Oxygen Distribution in the Prelaminar Optic Nerve Head of the Cat

Oxygen-sensitive microelectrodes were used to measure oxygen profiles from the prelaminar region of the cat optic nerve head. From the profiles, the oxygen distribution of this tissue was determined. Changes in oxygen levels in response to flicker stimulation were obtained. Also, the regulatory ability of this tissue was assessed by acutely increasing IOP and by presenting the cat with periods of hyperoxia. The cells in the optic nerve head were found to exist in the same oxygen environment as cells of the inner retina. Oxygen tensions usually varied between 0 and 40 mmHg, with the mean being between 10 and 20 mmHg. There was no difference in oxygen levels between central and peripheral parts of the optic nerve head, nor was there a difference as a function of depth. Different levels of light adaptation of the retina had no effect on oxygen levels in the optic nerve head, but flicker stimulation of the retina resulted in decreases in tissue oxygen tension. A small number of experiments in which the intraocular pressure was raised showed that oxygen levels in the optic nerve head were not drastically altered during acute decreases in perfusion pressure. Based on the size of changes in tissue oxygen tensions during hyperoxia, it appeared that blood vessels in the optic nerve head constricted in response to systemic hyperoxia.