Inner retinal oxygen delivery and metabolism under normoxia and hypoxia in rat

Oxygen profiles and oxygen consumption in the isolated mouse retina

The retina has a large demand for oxygen, but there is only limited information on differences between oxygen utilization (QO2) in the inner and outer retina, and limited data on mouse, which has become a prevalent animal model. This study utilized the isolated mouse retina, which allowed more detailed spatial analysis of QO2 than other methods. Oxygen sensitive microelectrodes were used to obtain profiles of oxygen tension across the isolated mouse retina, and mathematical models of retinal oxygen diffusion with four and five layers were fitted to the data to obtain values for QO2 of the outer retina (QOR) and inner retina (QIR). The boundaries between layers were free parameters in these models. The five-layer model resulted in lower error between the model and data, and agreed better with known anatomy. The three layers for the outer retina occupied half of the retina, as in prior work on rat, cat, and monkey, and the inner half of the retina could be divided into two layers, in which the one closer to the vitreous (layer 5) had much lower QO2 than the more distal inner retina (layer 4). QIR in darkness was 3.9 ml O2-100 g-1-min-1, similar to the value for intact cat retina, and did not change during light. QOR in darkness was 2.4 ml O2-100 g-1-min-1, lower than previous values in cat and rat, possibly because of damage to photoreceptors during isolation. There was a tendency for QOR to be lower in light, but it was not significant in this preparation.

Alterations in retinal pulse wave velocity under experimental ocular hypertension

Impairments of blood flow and autoregulation have been implicated in diabetic retinopathy and glaucoma. Thus, identifying biomarkers of retinal vascular compliance and regulatory capacity is of potential value for understanding the pathophysiology and evaluating onset or progression of disease. Pulse wave velocity (PWV) represents the speed of the pulse-propagated pressure wave within blood vessels and has shown promise as a marker of vascular compliance. The purpose of the current study was to report a method for comprehensive assessment of retinal PWV based on spectral analysis of pulsatile intravascular intensity waveforms and determine alterations due to experimental ocular hypertension. Retinal PWV was linearly related to vessel diameter. Increased retinal PWV was associated with elevated intraocular pressure. Retinal PWV has the potential to serve as a vasoregulation biomarker for investigating in animal models vascular factors that contribute to the development of retinal diseases.

Impairments of retinal hemodynamics and oxygen metrics in ocular hypertension-induced ischemia-reperfusion

Ischemia-reperfusion (I/R) is an established model for retinal neurodegeneration. However, there is limited knowledge of retinal physiological metrics and their relationships to retinal function and morphology in the I/R model. The purpose of the study was to test the hypotheses that retinal hemodynamic and oxygen metrics are impaired and associated with visual dysfunction, retinal thinning, and retinal ganglion cell (RGC) loss due to I/R injury. Intraocular pressure (IOP) was increased in one eye of 10 rats for 90 min followed by reperfusion. Fellow eyes served as controls. After one week of reperfusion, multimodal imaging was performed to quantify total retinal blood flow (TRBF) and retinal vascular oxygen contents. Retinal oxygen delivery (DO2) and metabolism (MO2) were calculated. Pattern-evoked electroretinography (PERG) and optical coherence tomography were performed to measure RGC function and retinal thicknesses, respectively. RGCs were counted from retina whole mounts. After one week of reperfusion, TRBF was lower in study eyes than in control eyes (p < 0.0003). Similarly, DO2 and MO2 were reduced in study eyes compared to control eyes (p < 0.003). PERG amplitude, TRT, IRT, ORT, and RGCs were also lower in study eyes (p ≤ 0.01). DO2 and MO2 were correlated with PERG amplitude, TRT, IRT, and ORT (r ≥ 0.6, p ≤ 0.005). The findings improve knowledge of physiological metrics affected by I/R injury and have the potential for identifying biomarkers of injury and outcomes for evaluating experimental treatments.

Retinal oxygen kinetics imaging and analysis (ROKIA) based on the integration and fusion of structural-functional imaging

The retina is one of the most metabolically active tissues in the body. The dysfunction of oxygen kinetics in the retina is closely related to the disease and has important clinical value. Dynamic imaging and comprehensive analyses of oxygen kinetics in the retina depend on the fusion of structural and functional imaging and high spatiotemporal resolution. But it's currently not clinically available, particularly via a single imaging device. Therefore, this work aims to develop a retinal oxygen kinetics imaging and analysis (ROKIA) technology by integrating dual-wavelength imaging with laser speckle contrast imaging modalities, which achieves structural and functional analysis with high spatial resolution and dynamic measurement, taking both external and lumen vessel diameters into account. The ROKIA systematically evaluated eight vascular metrics, four blood flow metrics, and fifteen oxygenation metrics. The single device scheme overcomes the incompatibility of optical design, harmonizes the field of view and resolution of different modalities, and reduces the difficulty of registration and image processing algorithms. More importantly, many of the metrics (such as oxygen delivery, oxygen metabolism, vessel wall thickness, etc.) derived from the fusion of structural and functional information, are unique to ROKIA. The oxygen kinetic analysis technology proposed in this paper, to our knowledge, is the first demonstration of the vascular metrics, blood flow metrics, and oxygenation metrics via a single system, which will potentially become a powerful tool for disease diagnosis and clinical research.

Retinal Oxygen Delivery and Metabolism Response to Hyperoxia During Bilateral Common Carotid Artery Occlusion in Rats

Purpose

The purpose of the current study was to test the hypothesis that responses of total retinal blood flow (TRBF), inner retinal oxygen delivery (DO₂), metabolism (MO₂), and extraction fraction (OEF) to hyperoxia are higher after minutes of bilateral common carotid artery occlusion (BCCAO) as compared to days of BCCAO.

Methods

Twenty-eight rats were subjected to BCCAO for 30 minutes (n = 12), 1 day (n = 8), or 3 days (n = 8). Eight of the 12 rats were also evaluated at baseline, prior to BCCAO. During room air breathing (RA) and 100% O₂ inspiration (hyperoxia), blood flow and phosphorescence lifetime imaging were performed to measure TRBF and vascular O₂ contents, respectively. DO₂, MO₂, and OEF were calculated from these measurements.

Results

After 30 minutes or 3 days of BCCAO, TRBF did not differ between RA and hyperoxia conditions (P ≥ 0.14) but decreased under hyperoxia after 1 day (P = 0.01). Compared to RA, DO₂ and MO₂ were increased under hyperoxia after 30 minutes of BCCAO (P ≤ 0.02). Additionally, MO₂ was decreased under hyperoxia after 1 day of BCCAO (P = 0.04). OEF was decreased under hyperoxia compared to RA (P < 0.001). Under hyperoxia, TRBF and DO₂ were reduced after all BCCAO durations compared to baseline (P ≤ 0.04), whereas MO₂ did not differ from baseline after 30 minutes of BCCAO (P = 1.00).

Conclusions

The findings indicate that hyperoxia introduced minutes after ischemia can reduce DO₂ impairments and potentially return MO₂ to approximately normal values. This information contributes to the knowledge of the effect of supplemental oxygen intervention on TRBF, DO₂, MO₂, and OEF outcomes after variable durations of ischemia.

Functional imaging of human retina using integrated multispectral and laser speckle contrast imaging

A novel integration of retinal multispectral imaging (MSI), retinal oximetry and laser speckle contrast imaging (LSCI) is presented for functional imaging of retinal blood vessels that could potentially allow early detection or monitoring of functional changes. We designed and built a cost-effective, scalable, retinal imaging instrument that integrates structural and functional retinal imaging techniques, including MSI, retinal oximetry and LSCI. Color fundus imaging was performed with 470 nm, 550 nm and 600 nm wavelength light emitting diode (LED) illumination. Retinal oximetry was performed using 550 nm and 600 nm LED illumination. LSCI of blood flow was performed using 850 nm laser diode illumination at 82 frames per second. LSCI can visualize retinal and choroidal vasculature without requiring exogenous contrast agents and can provide time-resolved information on blood flow, generating a cardiac pulse waveform from retinal vasculature. The technology can rapidly acquire structural MSI images, retinal oximetry and LSCI blood flow information in a simplified clinical workflow without requiring patients to move between instruments. Results from multiple modalities can be combined and registered to provide structural as well as functional information on the retina. These advances can reduce barriers for clinical adoption, accelerating research using MSI, retinal oximetry and LSCI of blood flow for diagnosis, monitoring and elucidating disease pathogenesis.

Assessment of retinal oxygen metabolism, visual function, thickness and degeneration markers after variable ischemia/reperfusion in rats

After total retinal ischemia induced experimentally by ophthalmic vessel occlusion followed by reperfusion, studies have reported alterations in retinal oxygen metabolism (MO₂), delivery (DO₂), and extraction fraction (OEF), as well as visual dysfunction and cell loss. In the current study, under variable durations of ischemia/reperfusion, changes in these oxygen metrics, visual function, retinal thickness, and degeneration markers (gliosis and apoptosis) were assessed and related. Additionally, the prognostic value of MO₂ for predicting visual function and retinal thickness outcomes was reported. Sixty-one rats were divided into 5 groups of ischemia duration (0 [sham], 60, 90, 120, or 180 min) and 2 reperfusion durations (1 h, 7 days). Phosphorescence lifetime and blood flow imaging, electroretinography, and optical coherence tomography were performed. MO₂ reduction was related to visual dysfunction, retinal thinning, increased gliosis and apoptosis after 7-days reperfusion. Impairment in MO₂ after 1-h reperfusion predicted visual function and retinal thickness outcomes after 7-days reperfusion. Since MO₂ can be measured in humans, findings from analogous studies may find value in the clinical setting.

Assessment of inner retinal oxygen metrics and thickness in a mouse model of inherited retinal degeneration

The retinal degeneration 1 (rd1) mouse is a well-established model of inherited retinal degeneration, displaying photoreceptor degeneration and retinal vasculature damage. The purpose of the current study was to determine alterations in the rate of oxygen delivery from retinal circulation (DO₂), the rate of oxygen extraction from the retinal circulation for metabolism (MO₂), and oxygen extraction fraction (OEF) in rd1 mice. The study was performed in a total of 18 wild type (WT) and 10 rd1 mice at both 3-weeks and 12-weeks of age. Retinal arterial and venous oxygen contents (O_2A and O_2V) were measured using phosphorescence lifetime imaging. Total retinal blood flow (TRBF) was determined by fluorescence and red-free imaging. DO₂ and MO₂ were determined as TRBF × O_2A and TRBF × (O_2A-O_2V), respectively. OEF was calculated as MO₂/DO₂. The thickness of individual retinal layers was measured from histology sections and inner retina (IR) and total retina (TR) thickness were calculated. TRBF, DO₂ and MO₂ were lower in rd1 mice compared to WT mice (P ≤ 0.001), whereas OEF was not significantly different between rd1 and WT mice (P = 0.4). TRBF and DO₂ were lower at 3-weeks of age compared to 12-weeks of age (P ≤ 0.01), while MO₂ was not significantly different between age groups (P = 0.4) and OEF was higher at 3-weeks of age compared to 12-weeks of age (P = 0.003). Additionally, the outer and inner retinal cell layer thicknesses were decreased in rd1 mice at 12-weeks of age compared to both age-matched WT mice and rd1 mice at 3-weeks of age (P ≤ 0.02). MO₂ was directly correlated with both IR and TR thickness (R ≥ 0.50; P ≤ 0.03, N = 20). The findings indicate that the rate oxygen is supplied by the retinal circulation is decreased and the reduction in oxygen extracted for metabolism is related to retinal cell layer thinning in rd1 mice.

Retinal Oxygen Delivery, Metabolism, and Extraction Fraction during Long-Term Bilateral Common Carotid Artery Occlusion in Rats

Retinal functional, biochemical, and anatomical changes have been previously reported in long-term experimental permanent bilateral common carotid artery occlusion (BCCAO). The purpose of the current study was to investigate progressive reductions in retinal oxygen metabolism (MO₂) due to inadequate compensation by oxygen delivery (DO₂) and extraction fraction (OEF) after BCCAO. Twenty-nine rats were subjected to BCCAO and were imaged after 3 hours, 3 days, 7 days, or 14 days. Six rats underwent a sham procedure. Phosphorescence lifetime and blood flow imaging were performed in both eyes to measure retinal oxygen contents and total retinal blood flow, respectively. DO₂, MO₂, and OEF were calculated from these measurements. Compared to the sham group, DO₂ and MO₂ were reduced after all BCCAO durations. OEF was increased after 3 hours and 3 days of BCCAO, but was not different from the sham group after 7 and 14 days. Between 3 and 7 days of BCCAO, DO₂ increased, OEF decreased, and there was no significant difference in MO₂. These findings may be useful to understand the pathophysiology of retinal ischemia.

Adaptive optics two-photon microscopy enables near-diffraction-limited and functional retinal imaging in vivo

In vivo fundus imaging offers non-invasive access to neuron structures and biochemical processes in the retina. However, optical aberrations of the eye degrade the imaging resolution and prevent visualization of subcellular retinal structures. We developed an adaptive optics two-photon excitation fluorescence microscopy (AO-TPEFM) system to correct ocular aberrations based on a nonlinear fluorescent guide star and achieved subcellular resolution for in vivo fluorescence imaging of the mouse retina. With accurate wavefront sensing and rapid aberration correction, AO-TPEFM permits structural and functional imaging of the mouse retina with submicron resolution. Specifically, simultaneous functional calcium imaging of neuronal somas and dendrites was demonstrated. Moreover, the time-lapse morphological alteration and dynamics of microglia were characterized in a mouse model of retinal disorder. In addition, precise laser axotomy was achieved, and degeneration of retinal nerve fibres was studied. This high-resolution AO-TPEFM is a promising tool for non-invasive retinal imaging and can facilitate the understanding of a variety of eye diseases as well as neurodegenerative disorders in the central nervous system.

Relation of Retinal Oxygen Measures to Electrophysiology and Survival Indicators after Permanent, Incomplete Ischemia in Rats

Studies in experimental ischemia models by permanent bilateral common carotid artery occlusion (BCCAO) have reported reduced retinal electrophysiological function, coupled with inner retinal degeneration and gliosis. In the current study, we tested the hypothesis that long-term (up to 14 days) BCCAO impairs oxygen delivery (DO₂), which affects oxygen metabolism (MO₂) and extraction fraction (OEF), electrophysiological function, morphology, and biochemical pathways. Twenty-one rats underwent BCCAO (N = 12) or sham surgery (N = 9) and were evaluated in separate groups after 3, 7, or 14 days. Electroretinography (ERG), optical coherence tomography, blood flow and vascular oxygen tension imaging, and morphological and biochemical evaluations were performed in both eyes. Reduced ERG b-wave amplitudes and delayed implicit times were reported at 3, 7, and 14 days following BCCAO. Total retinal blood flow, MO₂, and DO₂ were reduced in all BCCAO groups. OEF was increased in both 3- and 7-day groups, while no significant difference was observed in OEF at 14 days compared to the sham group. At 14 days following BCCAO, total and inner retinal layer thickness was reduced, while the outer nuclear layer thickness and gliosis were increased. There was an increase in nuclei containing fragmented DNA at 3 days following BCCAO. The compensatory elevation in OEF following BCCAO did not meet the tissue demand, resulting in the subsequent reduction of MO₂. The associations between retinal MO₂, DO₂, and retinal function were shown to be significant in the sequelae of persistent ischemia. In sum, measurements of DO₂, MO₂, and OEF may become useful for characterizing salvageable tissue in vision-threatening pathologies.

Alterations in Retinal Oxygen Delivery, Metabolism, and Extraction Fraction During Bilateral Common Carotid Artery Occlusion in Rats

Purpose

The purpose of the current study was to investigate alterations in retinal oxygen delivery, metabolism, and extraction fraction and elucidate their relationships in an experimental model of retinal ischemia.

Methods

We subjected 14 rats to permanent bilateral common carotid artery occlusion using clamp or suture ligation, or they underwent sham procedure. Within 30 minutes of the procedure, phosphorescence lifetime imaging was performed to measure retinal vascular oxygen tension and derive arterial and venous oxygen contents, and arteriovenous oxygen content difference. Fluorescent microsphere and red-free retinal imaging were performed to measure total retinal blood flow. Retinal oxygen delivery rate (DO₂), oxygen metabolism rate (MO₂), and oxygen extraction fraction (OEF) were calculated.

Results

DO₂ and MO₂ were lower in ligation and clamp groups compared to the sham group, and also lower in the ligation group compared to the clamp group (P ≤ 0.05). OEF was higher in the ligation group compared to clamp and sham groups (P ≤ 0.03). The relationships of MO₂ and OEF with DO₂ were mathematically modeled by exponential functions. With moderate DO₂ reductions, OEF increased while MO₂ minimally decreased. Under severe DO₂ reductions, OEF reached a maximum value and subsequently MO₂ decreased with DO₂.

Conclusions

The findings improve knowledge of mechanisms that can maintain MO₂ and may clarify the pathophysiology of retinal ischemic injury.

Retinal vessel oxygen saturation and vessel diameter in healthy individuals during high-altitude exposure

PURPOSE

To assess changes of retinal vessel oxygen saturation and vessel diameter in healthy individuals during high-altitude exposure.

METHODS

Retinal oxygen saturation and vessel diameter were obtained at sea level (SL, 40 m) and high altitude (HA, 3681 m) on 17 healthy individuals from Beijing (six males, 28.06 ± 8.06 years) using Oxymap T1 and then compared with 21 residents from Yushu (10 males, 28.63 ± 6.00 years). Systemic and ocular parameters were also measured before and after high-altitude exposure. Data were presented as mean ± SD and analysed using paired and independent Student t-test with significance accepted at p < 0.05.

RESULTS

Short-term high-altitude exposure of Beijing Group significantly affected all the systemic and ocular parameters, as well as retinal oxygen saturation and vessel diameter ranging from overall quadrant to different quadrants, other than retinal venous oxygen saturation and retinal arterial diameter. However, these changes were not evident in those permanently living at HA. Pearson's correlation analysis revealed correlations between retinal oxygen saturation and systemic and ocular parameters (all p < 0.05). The multivariate linear regression analysis indicated that retinal arterial oxygen saturation was significantly associated with arterial peripheral arterial oxygen saturation (SpO₂ ) and subfoveal choroidal thickness.

CONCLUSION

Short-term exposure to HA induces retinal microcirculation disturbance and auto-regulatory response in healthy individuals, which is probably attributed to arterial SpO₂ and endothelial dysfunction under hypoxic conditions.

Retinal oxygen saturation as a non-invasive estimate for mixed venous oxygen saturation and cardiac output

PURPOSE

To investigate the correlation between retinal vessel oxygen saturation and mixed venous oxygen saturation (SvO_2-mixed ) and cardiac output (CO).

METHODS

Retinal arterial (SaO_2-retinal ) and venous (SvO_2-retinal ) oxygen saturation were measured non-invasively with dual-wavelength retinal oximetry in subjects receiving invasive measurements of SvO_2-mixed and CO through right heart catheterization. Correlations were analysed using Spearman's rank correlation coefficients and linear regression models.

RESULTS

Fourteen patients (median age 62.7 years, range: 21-77) were included in the analysis. When adjusted for age, SvO_2-retinal showed a positive correlation with SvO_2-mixed (β = 0.80, p = 0.003). Retinal arteriovenous oxygen saturation difference was significantly correlated with the inverse of CO (Spearman's ρ = 0.59, p = 0.026).

CONCLUSION

This pilot study provides proof of concept for the use of retinal oximetry as a non-invasive tool to assess systemic cardiovascular function.

Two-photon phosphorescence lifetime microscopy of retinal capillary plexus oxygenation in mice

Impaired oxygen delivery and/or consumption in the retinal tissue underlies the pathophysiology of many retinal diseases. However, the essential tools for measuring oxygen concentration in retinal capillaries and studying oxygen transport to retinal tissue are still lacking. We show that two-photon phosphorescence lifetime microscopy can be used to map absolute partial pressures of oxygen (pO2) in the retinal capillary plexus. Measurements were performed at various retinal depths in anesthetized mice under systemic normoxic and hyperoxic conditions. We used a newly developed two-photon phosphorescent oxygen probe, based on a two-photon absorbing platinum tetraphthalimidoporphyrin, and commercially available optics without correction for optical aberrations of the eye. The transverse and axial distances within the tissue volume were calibrated using a model of the eye's optical system. We believe this is the first demonstration of in vivo depth-resolved imaging of pO2 in retinal capillaries. Application of this method has the potential to advance our understanding of oxygen delivery on the microvascular scale and help elucidate mechanisms underlying various retinal diseases.

Imaging of oxygen and hypoxia in cell and tissue samples

Molecular oxygen (O2) is a key player in cell mitochondrial function, redox balance and oxidative stress, normal tissue function and many common disease states. Various chemical, physical and biological methods have been proposed for measurement, real-time monitoring and imaging of O2 concentration, state of decreased O2 (hypoxia) and related parameters in cells and tissue. Here, we review the established and emerging optical microscopy techniques allowing to visualize O2 levels in cells and tissue samples, mostly under in vitro and ex vivo, but also under in vivo settings. Particular examples include fluorescent hypoxia stains, fluorescent protein reporter systems, phosphorescent probes and nanosensors of different types. These techniques allow high-resolution mapping of O2 gradients in live or post-mortem tissue, in 2D or 3D, qualitatively or quantitatively. They enable control and monitoring of oxygenation conditions and their correlation with other biomarkers of cell and tissue function. Comparison of these techniques and corresponding imaging setups, their analytical capabilities and typical applications are given.

A Model for Graded Retinal Ischemia in Rats

Purpose

Retinal ischemic injury depends on grade and duration of an ischemic insult. We developed a method to induce ischemic injury in rats permitting: (1) Variable grades of retinal blood flow (F) reduction, (2) controllable duration of F reduction, (3) injury without collateral neural damage, and (4) optical measurements of F and O₂-related factors: O₂ delivery (DO₂), O₂ extraction fraction (OEF), and metabolic rate of O₂ (MO₂).

Methods

In five anesthetized rats the left common carotid artery (CA) was ligated and the right CA was exposed. A variable clamp having a backstop and a rod mounted on a micromanipulator straddled the right CA. Advancing the rod with the micromanipulator produced graded compressions of the CA. F and O₂-related factors were measured with established optical techniques.

Results

Four to seven grades of F for at least 10 minutes were achieved per rat. F decreased only with compressions of over 60%. DO₂ changed in proportion to F, particularly at low F. As F decreased, OEF initially changed little, but then rose steeply to its maximum of 1 when F was approximately 4 μL/min. MO₂ was stable with reduced F until OEF maximized, after which it decreased progressively.

Conclusions

This model in rats permits acute, graded inner retinal ischemia that is reversible after prescribed durations, does not otherwise injure the eye and allows optical measurement of important physiologic factors during ischemia.

Translational Relevance

This model will allow improved understanding of retinal ischemic injury and enable better management of this common, sight-threatening affliction.

A method for volumetric retinal tissue oxygen tension imaging

PURPOSE

Inadequate retinal oxygenation occurs in many vision-threatening retinal diseases, including diabetic retinopathy, retinal vascular occlusions, and age-related macular degeneration. Therefore, techniques that assess retinal oxygenation are necessary to understand retinal physiology in health and disease. The purpose of the current study is to report a method for the three-dimensional (3D) imaging of retinal tissue oxygen tension (tPO₂) in rats.

METHODS

Imaging was performed in Long Evans pigmented rats under systemic normoxia (N = 6) or hypoxia (N = 3). A vertical laser line was horizontally scanned on the retina and a series of optical section phase-delayed phosphorescence images were acquired. From these images, phosphorescence volumes at each phase delay were constructed and a 3D retinal tPO₂ volume was generated. Retinal tPO₂ volumes were quantitatively analyzed by generating retinal depth profiles of mean tPO₂ (M_tPO2) and the spatial variation of tPO₂ (SV_tPO2). The effects of systemic condition (normoxia/hypoxia) and retinal depth on M_tPO2 and SV_tPO2 were determined by mixed linear model.

RESULTS

Each 3D retinal tPO₂ volume was approximately 500 × 750 × 200 μm (horizontal × vertical × depth) and consisted of 45 en face tPO₂ images through the retinal depth. M_tPO2 at the chorioretinal interface was significantly correlated with systemic arterial oxygen tension (P = 0.007; N = 9). There were significant effects of both systemic condition and retinal depth on M_tPO2 and SV_tPO2, such that both were lower under hypoxia than normoxia and higher in the outer retina than inner retina (P < 0.001).

CONCLUSION

For the first time, 3D imaging of retinal tPO₂ was demonstrated, with potential future application for assessment of physiological alterations in animal models of retinal diseases.

A Method for Combined Retinal Vascular and Tissue Oxygen Tension Imaging

The retina requires adequate oxygenation to maintain cellular metabolism and visual function. Inner retinal oxygen metabolism is directly related to retinal vascular oxygen tension (PO₂) and inner retinal oxygen extraction fraction (OEF), whereas outer retinal oxygen consumption (QO₂) relies on oxygen availability by the choroid and is contingent upon retinal tissue oxygen tension (tPO₂) gradients across the retinal depth. Thus far, these oxygenation and metabolic parameters have been measured independently by different techniques in separate animals, precluding a comprehensive and correlative assessment of retinal oxygenation and metabolism dynamics. The purpose of the current study is to report an innovative optical system for dual oxyphor phosphorescence lifetime imaging to near-simultaneously measure retinal vascular PO₂ and tPO₂ in rats. The use of a new oxyphor with different spectral characteristics allowed differentiation of phosphorescence signals from the retinal vasculature and tissue. Concurrent measurements of retinal arterial and venous PO₂, tPO₂ through the retinal depth, inner retinal OEF, and outer retinal QO₂ were demonstrated, permitting a correlative assessment of retinal oxygenation and metabolism. Future application of this method can be used to investigate the relations among retinal oxygen content, extraction and metabolism under pathologic conditions and thus advance knowledge of retinal hypoxia pathophysiology.

Inner Retinal Oxygen Delivery, Metabolism, and Extraction Fraction in Ins2Akita Diabetic Mice

Purpose

Retinal nonperfusion and hypoxia are important factors in human diabetic retinopathy, and these presumably inhibit energy production and lead to cell death. The purpose of this study was to elucidate the effect of diabetes on inner retinal oxygen delivery and metabolism in a mouse model of diabetes.

Methods

Phosphorescence lifetime and blood flow imaging were performed in spontaneously diabetic Ins2^Akita (n = 22) and nondiabetic (n = 22) mice at 12 and 24 weeks of age to measure retinal arterial (O_2A) and venous (O_2V) oxygen contents and total retinal blood flow (F). Inner retinal oxygen delivery (DO₂) and metabolism (MO₂) were calculated as F ∗ O_2A and F ∗ (O_2A − O_2V), respectively. Oxygen extraction fraction (OEF), which equals MO₂/DO₂, was calculated.

Results

DO₂ at 12 weeks were 112 ± 40 and 97 ± 29 nL O₂/min in nondiabetic and diabetic mice, respectively (NS), and 148 ± 31 and 85 ± 37 nL O₂/min at 24 weeks, respectively (P < 0.001). MO₂ were 65 ± 31 and 66 ± 27 nL O₂/min in nondiabetic and diabetic mice at 12 weeks, respectively, and 79 ± 14 and 54 ± 28 nL O₂/min at 24 weeks, respectively (main effects = NS). At 12 weeks OEF were 0.57 ± 0.17 and 0.67 ± 0.09 in nondiabetic and diabetic mice, respectively, and 0.54 ± 0.07 and 0.63 ± 0.08 at 24 weeks, respectively (main effect of diabetes: P < 0.01).

Conclusions

Inner retinal MO₂ was maintained in diabetic Akita mice indicating that elevation of the OEF adequately compensated for reduced DO₂ and prevented oxidative metabolism from being limited by hypoxia.

The Effects of Diabetic Retinopathy Stage and Light Flicker on Inner Retinal Oxygen Extraction Fraction

Purpose

We determined the effects of light flicker and diabetic retinopathy (DR) stage on retinal vascular diameter (D), oxygen saturation (SO2), and inner retinal oxygen extraction fraction (OEF).

Methods

Subjects were categorized as nondiabetic control (NC, n = 42), diabetic with no clinical DR (NDR; n = 32), nonproliferative DR (NPDR; n = 42), or proliferative DR (PDR; n = 14). Our customized optical imaging system simultaneously measured arterial and venous D (DA, DV) and SO2 (SO2A, SO2V) before and during light flicker. Inner retinal OEF was derived from SO2 values. Light flicker-induced ratios of metrics (DAR, DVR, SO2AR, SO2VR, OEFR) were calculated.

Results

Arterial D was larger in NPDR compared to NC (P = 0.01) and PDR (P = 0.002), whereas DV was similar among groups (P ≥ 0.16). Light flicker increased DA and DV (P ≤ 0.004), but DAR and DVR were similar among groups (P ≥ 0.09). Arterial SO2 was higher in all groups compared to NC (P ≤ 0.02) and higher in PDR compared to NDR and NPDR (P<0.001). Arterial SO2 did not change with light flicker (P ≥ 0.1). Venous SO2 was higher in NPDR and PDR compared to NC and NDR (P ≤ 0.02). Light flicker increased SO2V in NC, NDR, and PDR (P ≤ 0.003), and SO2VR was lower in NPDR compared to NC and NDR (P ≤ 0.05). Inner retinal OEF was lower in NPDR compared to NDR and PDR (P ≤ 0.02). Light flicker decreased OEF (P ≤ 0.03), but OEFR was greater in NPDR compared to NC and NDR (P ≤ 0.03).

Conclusions

The findings of alterations in retinal D, SO2, OEF, and their light flicker-induced responses at stages of DR may be useful to elucidate the pathophysiology of DR.

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.

Methylene blue prevents retinal damage in an experimental model of ischemic proliferative retinopathy

Perinatal asphyxia induces retinal lesions, generating ischemic proliferative retinopathy, which may result in blindness. Previously, we showed that the nitrergic system was involved in the physiopathology of perinatal asphyxia. Here we analyze the application of methylene blue, a well-known soluble guanylate cyclase inhibitor, as a therapeutic strategy to prevent retinopathy. Male rats (n = 28 per group) were treated in different ways: 1) control group comprised born-to-term animals; 2) methylene blue group comprised animals born from pregnant rats treated with methylene blue (2 mg/kg) 30 and 5 min before delivery; 3) perinatal asphyxia (PA) group comprised rats exposed to perinatal asphyxia (20 min at 37°C); and 4) methylene blue-PA group comprised animals born from pregnant rats treated with methylene blue (2 mg/kg) 30 and 5 min before delivery, and then the pups were subjected to PA as above. For molecular studies, mRNA was obtained at different times after asphyxia, and tissue was collected at 30 days for morphological and biochemical analysis. Perinatal asphyxia produced significant gliosis, angiogenesis, and thickening of the inner retina. Methylene blue treatment reduced these parameters. Perinatal asphyxia resulted in a significant elevation of the nitrergic system as shown by NO synthase (NOS) activity assays, Western blotting, and (immuno)histochemistry for the neuronal isoform of NOS and NADPH-diaphorase activity. All these parameters were also normalized by the treatment. In addition, methylene blue induced the upregulation of the anti-angiogenic peptide, pigment epithelium-derived factor. Application of methylene blue reduced morphological and biochemical parameters of retinopathy. This finding suggests the use of methylene blue as a new treatment to prevent or decrease retinal damage in the context of ischemic proliferative retinopathy.

Relationship between retinal blood flow and arterial oxygen

KEY POINTS

Vascular reactivity, the response of the vessels to a vasoactive stimulus such as hypoxia and hyperoxia, can be used to assess the vascular range of adjustment in which the vessels are able to compensate for changes in PO2. Previous studies in the retina have not accurately quantified retinal vascular responses and precisely targeted multiple PaO2 stimuli at the same time as controlling the level of carbon dioxide, thus precluding them from modelling the relationship between retinal blood flow and oxygen. The present study modelled the relationship between retinal blood flow and PaO2, showing them to be a combined linear and hyperbolic function. This model demonstrates that the resting tonus of the vessels is at the mid-point and that they have great vascular range of adjustment, compensating for decreases in oxygen above a PETCO2 of 32-37 mmHg but being limited below this threshold. Retinal blood flow (RBF) increases in response to a reduction in oxygen (hypoxia) but decreases in response to increased oxygen (hyperoxia). However, the relationship between blood flow and the arterial partial pressure of oxygen has not been quantified and modelled in the retina, particularly in the vascular reserve and resting tonus of the vessels. The present study aimed to determine the limitations of the retinal vasculature by modelling the relationship between RBF and oxygen. Retinal vascular responses were measured in 13 subjects for eight different blood gas conditions, with the end-tidal partial pressure of oxygen (PETCO2) ranging from 40-500 mmHg. Retinal vascular response measurements were repeated twice; using the Canon laser blood flowmeter (Canon Inc., Tokyo, Japan) during the first visit and using Doppler spectral domain optical coherence tomography during the second visit. We determined that the relationship between RBF and PaO2 can be modelled as a combination of hyperbolic and linear functions. We concluded that RBF compensated for decreases in arterial oxygen content for all stages of hypoxia used in the present study but can no longer compensate below a PETCO2 of 32-37 mmHg. These vessels have a great vascular range of adjustment, increasing diameter (8.5% arteriolar and 21% total venous area) with hypoxia (40 mmHg P ETC O2; P < 0.001) and decreasing diameter (6.9% arteriolar and 23% total venous area) with hyperoxia (500 mmHg PETCO2; P < 0.001) to the same extent. This indicates that the resting tonus is near the mid-point of the adjustment ranges at resting PaO2 where sensitivity is maximum.

Relation of retinal blood flow and retinal oxygen extraction during stimulation with diffuse luminance flicker

Cerebral and retinal blood flow are dependent on local neuronal activity. Several studies quantified the increase in cerebral blood flow and oxygen consumption during activity. In the present study we investigated the relation between changes in retinal blood flow and oxygen extraction during stimulation with diffuse luminance flicker and the influence of breathing gas mixtures with different fractions of O₂ (FiO₂; 100% 15% and 12%). Twenty-four healthy subjects were included. Retinal blood flow was studied by combining measurement of vessel diameters using the Dynamic Vessel Analyser with measurements of blood velocity using laser Doppler velocimetry. Oxygen saturation was measured using spectroscopic reflectometry and oxygen extraction was calculated. Flicker stimulation increased retinal blood flow (57.7 ± 17.8%) and oxygen extraction (34.6 ± 24.1%; p < 0.001 each). During 100% oxygen breathing the response of retinal blood flow and oxygen extraction was increased (p < 0.01 each). By contrast, breathing gas mixtures with 12% and 15% FiO₂ did not alter flicker–induced retinal haemodynamic changes. The present study indicates that at a comparable increase in blood flow the increase in oxygen extraction in the retina is larger than in the brain. During systemic hyperoxia the blood flow and oxygen extraction responses to neural stimulation are augmented. The underlying mechanism is unknown.

Inner retinal oxygen metabolism in the 50/10 oxygen-induced retinopathy model

Retinopathy of prematurity (ROP) represents a major cause of childhood vision loss worldwide. The 50/10 oxygen-induced retinopathy (OIR) model mimics the findings of ROP, including peripheral vascular attenuation and neovascularization. The oxygen metabolism of the inner retina has not been previously explored in this model. Using visible-light optical coherence tomography (vis-OCT), we measured the oxygen saturation of hemoglobin and blood flow within inner retinal vessels, enabling us to compute the inner retinal oxygen delivery (irDO₂) and metabolic rate of oxygen (irMRO₂). We compared these measurements between age-matched room-air controls and rats with 50/10 OIR on postnatal day 18. To account for a 61% decrease in the irDO₂ in the OIR group, we found an overall statistically significant decrease in retinal vascular density affecting the superficial and deep retinal vascular capillary networks in rats with OIR compared to controls. Furthermore, matching the reduced irDO₂, we found a 59% decrease in irMRO₂, which we correlated with a statistically significant reduction in retinal thickness in the OIR group, suggesting that the decreased irMRO₂ was due to decreased neuronal oxygen utilization. By exploring these biological and metabolic changes in great detail, our study provides an improved understanding of the pathophysiology of OIR model.

The effect of intravitreal vascular endothelial growth factor on inner retinal oxygen delivery and metabolism in rats

Vascular endothelial growth factor (VEGF) is stimulated by hypoxia and plays an important role in pathologic vascular leakage and neovascularization. Increased VEGF may affect inner retinal oxygen delivery (DO2) and oxygen metabolism (MO2), however, quantitative information is lacking. We tested the hypotheses that VEGF increases DO2, but does not alter MO2. In 10 rats, VEGF was injected intravitreally into one eye, whereas balanced salt solution (BSS) was injected into the fellow eye, 24 h prior to imaging. Vessel diameters and blood velocities were determined by red-free and fluorescent microsphere imaging, respectively. Vascular PO2 values were derived by phosphorescence lifetime imaging of an intravascular oxyphor. Retinal blood flow, vascular oxygen content, DO2 and MO2 were calculated. Retinal arterial and venous diameters were larger in VEGF-injected eyes compared to control eyes (P < 0.03), however no significant difference was observed in blood velocity (P = 0.21). Thus, retinal blood flow was greater in VEGF-injected eyes (P = 0.007). Retinal vascular PO2 and oxygen content were similar between control and VEGF-injected eyes (P > 0.11), while the arteriovenous oxygen content difference was marginally lower in VEGF-injected eyes (P = 0.05). DO2 was 950 ± 340 and 1380 ± 650 nL O2/min in control and VEGF-injected eyes, respectively (P = 0.005). MO2 was 440 ± 150 and 490 ± 190 nL O2/min in control and VEGF-injected eyes, respectively (P = 0.31). Intravitreally administered VEGF did not alter MO2 but increased DO2, suggesting VEGF may play an offsetting role in conditions characterized by retinal hypoxia.

Retinal oxygen extraction in humans

Adequate function of the retina is dependent on proper oxygen supply. In humans, the inner retina is oxygenated via the retinal circulation. We present a method to calculate total retinal oxygen extraction based on measurement of total retinal blood flow using dual-beam bidirectional Doppler optical coherence tomography and measurement of oxygen saturation by spectrophotometry. These measurements were done on 8 healthy subjects while breathing ambient room air and 100% oxygen. Total retinal blood flow was 44.3 ± 9.0 μl/min during baseline and decreased to 18.7 ± 4.2 μl/min during 100% oxygen breathing (P < 0.001) resulting in a pronounced decrease in retinal oxygen extraction from 2.33 ± 0.51 μl(O₂)/min to 0.88 ± 0.14 μl(O₂)/min during breathing of 100% oxygen. The method presented in this paper may have significant potential to study oxygen metabolism in hypoxic retinal diseases such as diabetic retinopathy.

Cerebral metabolic rate of oxygen (CMRO2) assessed by combined Doppler and spectroscopic OCT

A method of measuring cortical oxygen metabolism in the mouse brain that uses independent quantitative measurements of three key parameters: cerebral blood flow (CBF), arteriovenous oxygen extraction (OE), and hemoglobin concentration ([HbT]) is presented. Measurements were performed using a single visible light spectral/Fourier domain OCT microscope, with Doppler and spectroscopic capabilities, through a thinned-skull cranial window in the mouse brain. Baseline metabolic measurements in mice are shown to be consistent with literature values. Oxygen consumption, as measured by this method, did not change substantially during minor changes either in the fraction of inspired oxygen (FiO2) or in the fraction of inspired carbon dioxide (FiCO2), in spite of larger variations in oxygen saturations. This set of experiments supports, but does not prove, the validity of the proposed method of measuring brain oxygen metabolism.

Visible light optical coherence tomography measures retinal oxygen metabolic response to systemic oxygenation

The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Using visible light optical coherence tomography (vis-OCT), we demonstrated accurate and robust measurement of retinal oxygen metabolic rate (rMRO₂) noninvasively in rat eyes. We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge. We found that both oxygen delivery, and rMRO₂ increased from the highly regulated retinal circulation (RC) under hypoxia, by 0.28 ± 0.08 μL min^-1 (p < 0.001), and 0.20 ± 0.04 μL min^-1 (p < 0.001) per 100 mmHg systemic pO₂ reduction, respectively. The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation. Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations. We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.

Molecular probes for imaging of hypoxia in the retina

Hypoxia has been associated with retinal diseases which lead the causes of irreversible vision loss, including diabetic retinopathy, retinopathy of prematurity, and age-related macular degeneration. Therefore, technologies for imaging hypoxia in the retina are needed for early disease detection, monitoring of disease progression, and assessment of therapeutic responses in the patient. Toward this goal, we developed two hypoxia-sensitive imaging agents based on nitroimidazoles which are capable of accumulating in hypoxic cells in vivo. 2-nitroimidazole or Pimonidazole was conjugated to fluorescent dyes to yield the imaging agents HYPOX-1 and HYPOX-2. Imaging agents were characterized in cell culture and animal models of retinal vascular diseases which exhibit hypoxia. Both HYPOX-1 and -2 were capable of detecting hypoxia in cell culture models with >10:1 signal-to-noise ratios without acute toxicity. Furthermore, intraocular administration of contrast agents in mouse models of retinal hypoxia enabled ex vivo detection of hypoxic tissue. These imaging agents are a promising step toward translation of hypoxia-sensitive molecular imaging agents in preclinical animal models and patients.

Regulation of retinal oxygen metabolism in humans during graded hypoxia

Animal experiments indicate that the inner retina keeps its oxygen extraction constant despite systemic hypoxia. For the human retina no such data exist. In the present study we hypothesized that systemic hypoxia does not alter inner retinal oxygen extraction. To test this hypothesis we included 30 healthy male and female subjects aged between 18 and 35 years. All subjects were studied at baseline and during breathing 12% O₂ in 88% N₂ as well as breathing 15% O₂ in 85% N₂. Oxygen saturation in a retinal artery (SO₂art) and an adjacent retinal vein (SO₂vein) were measured using spectroscopic fundus reflectometry. Measurements of retinal venous blood velocity using bidirectional laser Doppler velocimetry and retinal venous diameters using a Retinal Vessel Analyzer (RVA) were combined to calculate retinal blood flow. Oxygen and carbon dioxide partial pressure were measured from earlobe arterialized capillary blood. Retinal blood flow was increased by 43.0 ± 23.2% (P < 0.001) and 30.0 ± 20.9% (P < 0.001) during 12% and 15% O₂ breathing, respectively. SO₂art as well as SO₂vein decreased during both 12% O₂ breathing (SO₂art: -11.2 ± 4.3%, P < 0.001; SO₂vein: -3.9 ± 8.5%, P = 0.012) and 15% O₂ breathing (SO₂art: -7.9 ± 3.6%, P < 0.001; SO₂vein: -4.0 ± 7.0%, P = 0.010). The arteriovenous oxygen difference decreased during both breathing periods (12% O2: -28.9 ± 18.7%; 15% O₂: -19.1 ± 16.7%, P < 0.001 each). Calculated oxygen extraction did, however, not change during our experiments (12% O₂: -2.8 ± 18.9%, P = 0.65; 15% O₂: 2.4 ± 15.8%, P = 0.26). Our results indicate that in healthy humans, oxygen extraction of the inner retina remains constant during systemic hypoxia.

Response of inner retinal oxygen extraction fraction to light flicker under normoxia and hypoxia in rat

PURPOSE

Oxygen extraction fraction (OEF), defined by the ratio of oxygen metabolism (MO2) to delivery (DO2), determines the level of compensation of MO2 by DO2. In the current study, we tested the hypothesis that inner retinal OEF remains unchanged during light flicker under systemic normoxia and hypoxia in rats due to the matching of MO2 and DO2.

METHODS

Retinal vascular oxygen tension (PO2) measurements were obtained in 10 rats by phosphorescence lifetime imaging. Inner retinal OEF was derived from vascular PO2 based on Fick's principle. Measurements were obtained before and during light flicker under systemic normoxia and hypoxia. The effects of light flicker and systemic oxygenation on retinal vascular PO2 and OEF were determined by ANOVA.

RESULTS

During light flicker, retinal venous PO2 decreased (P < 0.01, N = 10), while inner retinal OEF increased (P = 0.02). Under hypoxia, retinal arterial and venous PO2 decreased (P < 0.01), while OEF increased (P < 0.01). The interaction effect was not significant on OEF (P = 0.52), indicating the responses of OEF to light flicker were similar under normoxia and hypoxia. During light flicker, OEF increased from 0.46 ± 0.13 to 0.50 ± 0.11 under normoxia, while under hypoxia, OEF increased from 0.67 ± 0.16 to 0.74 ± 0.14.

CONCLUSIONS

Inner retinal OEF increased during light flicker, indicating the relative change in DO2 is less than that in MO2 in rats under systemic normoxia and hypoxia. Inner retinal OEF is a potentially useful parameter for assessment of the relative changes of MO2 and DO2 under physiologic and pathologic conditions.

Metabolic adaptation and neuroprotection differ in the retina and choroid in a piglet model of acute postnatal hypoxia

BACKGROUND

Hypoxic-ischemic insults to the neonatal brain may cause neurodevelopmental disorders. Vulnerability of different areas of the neural tissue to hypoxic-ischemic stress might be explained by either heterogeneous sensitivity to oxygen or neuroprotective capability. Our understanding of regional heterogeneity is still incomplete in terms of metabolic reconfiguration and/or activation of neuroprotective mechanisms.

METHODS

We studied, by western blotting, reverse-transcriptase PCR, and tandem mass spectrometry, the response of retina and choroid at protein, gene, and metabolic levels during hypoxia in a piglet model of acute postnatal hypoxia.

RESULTS

We evidenced a metabolic shift towards glycolysis in choroid after hypoxia while retina experienced a dramatic energy stress with decreased mitochondrial metabolites. Hypoxia-inducible transcription factor-1α (HIF-1α) was not stabilized in retina during hypoxia, supported by a deficient signaling from v-akt murine thymoma viral oncogene (AKT) and ERK1/2, and unchanged glutathione redox status. In retina, but not in choroid, phosphorylation of p65 (NF-κB) and increased transcription of target genes may have a major role during hypoxic stress.

CONCLUSION

We showed that the retina engages a distinct pattern of signaling and transcriptional events than observed in the choroid. Retina and choroid may reflect regional sensitivity to hypoxia. While prolonged and intense hypoxia may jeopardize retinal cell survival, choroid sets up a different pattern of response, which promotes adaptation to these adverse conditions.

Inner retinal oxygen delivery and metabolism in streptozotocin diabetic rats

PURPOSE

The purpose of the study is to report global measurements of inner retinal oxygen delivery (DO2_IR) and oxygen metabolism (MO2_IR) in streptozotocin (STZ) diabetic rats.

METHODS

Phosphorescence lifetime and blood flow imaging were performed in rats 4 (STZ/4 wk; n = 10) and 6 (STZ/6 wk; n = 10) weeks following injection of STZ to measure retinal arterial (O2A) and venous (O2V) oxygen contents and total retinal blood flow (F). DO2_IR and MO2_IR were calculated from measurements of F and O2A and of F and the arteriovenous oxygen content difference, respectively. Data in STZ rats were compared to those in healthy control rats (n = 10).

RESULTS

Measurements of O2A and O2V were not significantly different among STZ/4 wk, STZ/6 wk, and control rats (P ≥ 0.28). Likewise, F was similar among all groups of rats (P = 0.81). DO2_IR measurements were 941 ± 231, 956 ± 232, and 973 ± 243 nL O2/min in control, STZ/4 wk, and STZ/6 wk rats, respectively (P = 0.95). MO2_IR measurements were 516 ± 175, 444 ± 103, and 496 ± 84 nL O2/min in control, STZ/4 wk, and STZ/6 wk rats, respectively (P = 0.37).

CONCLUSIONS

Global inner retinal oxygen delivery and metabolism were not significantly impaired in STZ rats in early diabetes.