March 2014
Volume 55, Issue 3
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Retina  |   March 2014
Inner Retinal Oxygen Delivery and Metabolism in Streptozotocin Diabetic Rats
Author Notes
  • Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois 
  • Correspondence: Mahnaz Shahidi, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, 1855 West Taylor Street, Chicago, IL 60612; mahnshah@uic.edu
Investigative Ophthalmology & Visual Science March 2014, Vol.55, 1588-1593. doi:10.1167/iovs.13-13537
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      Justin Wanek, Pang-yu Teng, Norman P. Blair, Mahnaz Shahidi; Inner Retinal Oxygen Delivery and Metabolism in Streptozotocin Diabetic Rats. Invest. Ophthalmol. Vis. Sci. 2014;55(3):1588-1593. doi: 10.1167/iovs.13-13537.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

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/4wk; n = 10) and 6 (STZ/6wk; 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/4wk, STZ/6wk, 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/4wk, and STZ/6wk rats, respectively (P = 0.95). MO2_IR measurements were 516 ± 175, 444 ± 103, and 496 ± 84 nL O2/min in control, STZ/4wk, and STZ/6wk rats, respectively (P = 0.37).

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

Introduction
Diabetic retinopathy (DR) is a significant cause of blindness in developed countries. 1 3 It is thought that in some stages of DR, the ability of the retinal vasculature to deliver oxygen and of the retinal tissue to consume oxygen may be impaired. Retinal blood flow, as a marker of oxygen delivery, has been studied extensively in DR subjects using a variety of techniques such as fluorescein angiography, 47 laser Doppler velocimetry, 811 and blue field entoptic phenomenon. 12,13 Findings of these studies have been variable, with reports of reduced, unchanged, or elevated blood flow. 1418 Due to limited technologies for measuring inner retinal oxygen consumption, abnormalities in retinal vascular oxygenation have been sought as a surrogate for impaired oxygen metabolism. In subjects with DR, retinal venous oxygen saturation (SO2) measured by oximetry was found to be increased 19,20 while arterial SO2 was reported to be higher than 20 or similar 19 to values in healthy subjects. Combined blood flow and oximetry can be used to estimate inner retinal oxygen metabolism (MO2_IR). However, this combination has been used in only one study, and that was of neurogenic optic atrophy. 21 Therefore, in humans, the effect of diabetes on MO2_IR remains unknown. Knowledge of alterations in inner retinal oxygen delivery (DO2_IR) and MO2_IR would be useful to better understand DR pathophysiology and potentially for development of therapeutic interventions. 
The streptozotocin (STZ) diabetic rat develops retinal pathological alterations that resemble those observed in early human DR. 22 In early experimental diabetes (within 6 weeks), blood–retinal barrier breakdown, 2326 leukostasis, 23,27,28 upregulation of endothelin-1, 29 abnormal retinal blood flow, 3034 and reduced retinal arterial wall oxygen tension (PO2) 35 have been reported in STZ rats, suggesting the possibility of alterations in DO2_IR. However, significant reductions in tissue PO2 may not occur even if oxygen delivery is reduced to some extent. In fact, indicators of hypoxia measured by pimonidazole 36 and hypoxia-inducible factor (HIF) levels 37 were not abnormal in excised retinal tissue of early STZ rats, though upregulation of vascular endothelial growth factor (VEGF) 24,38,39 has been reported. In addition to oxygen availability, retinal neural activity is also a determinant of the rate at which the retina consumes oxygen for energy generation. In early STZ rodents, neural changes including increased apoptosis of retinal ganglion cells, 4043 thinning of the inner plexiform layer, 44 and reduction of amacrine cells 45 have been observed before the appearance of vascular cell changes, 22 suggesting that there may be alterations in MO2_IR. However, to date, measurements of neither oxygen delivery nor oxygen metabolism in the inner retina in living STZ rats have been published. The purpose of this study was to quantitatively assess DO2_IR and MO2_IR in STZ rats with our previously established optical imaging method. 46,47  
Methods
Animals
Male Long Evans pigmented rats (Charles River Laboratories, Wilmington, MA) were treated in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Diabetes was induced by intravenous injection of STZ (60 mg/kg) in citrate buffer. Nonfasting blood glucose levels were measured in STZ rats weekly and immediately before imaging with the use of a commercially available blood glucometer (FreeStyle Lite; Abbott, Alameda, CA) to confirm continued hyperglycemia. Imaging was performed either 4 weeks (STZ/4wk; n = 10) or 6 weeks (STZ/6wk; n = 10) after administration of STZ. Imaging in rats with longer duration of diabetes was precluded due to cataract formation. 
Prior to imaging, rats were anesthetized with intraperitoneal injections of ketamine (100 mg/kg) and xylazine (5 mg/kg), with additional injections given to sustain anesthesia as necessary. To ensure normal systemic blood gas levels, rats were mechanically ventilated with room air (or room air and supplemental oxygen) using a small-animal ventilator (Harvard Apparatus, Inc., South Natick, MA) connected to an endotracheal tube. The femoral artery was cannulated, and a catheter was attached to draw blood and to measure the animal's physiological status with a pressure transducer. Systemic arterial oxygen tension (PaO2), carbon dioxide tension (PaCO2), and pH were measured immediately prior to imaging from arterial blood using a blood gas analyzer (Radiometer, Westlake, OH) 5 to 10 minutes after initiation of ventilation. Blood pressure (BP) and heart rate (HR) were monitored continuously with a data acquisition system (Biopac Systems, Goleta, CA) linked to the pressure transducer. Continuous BP and HR measurements obtained during imaging were averaged to derive a representative BP and HR value. Hemoglobin concentration (HgB) was also measured with a hematology system (Siemens, Tarrytown, NY) from arterial blood. 
Rats were placed in an animal holder with a copper tubing water heater to maintain the body temperature at 37°C. Pupils were dilated with 2.5% phenylephrine and 1% tropicamide. A glass cover slip with 1% hydroxypropyl methylcellulose was applied to the cornea to eliminate its refractive power and prevent dehydration. For retinal vascular PO2 imaging, an oxygen-sensitive molecular probe, Pd-porphine (Frontier Scientific, Logan, UT), was dissolved (12 mg/mL) in bovine serum albumin solution (60 mg/mL) and administered through the femoral arterial catheter (20 mg/kg). For retinal blood velocity imaging, 2-μm polystyrene fluorescent microspheres (Invitrogen, Grand Island, NY) were injected through the catheter. For vascular caliber measurement, red-free retinal imaging was performed; and in three STZ rats with low-quality red-free images, fluorescein angiography (FA) was performed by intravascular injection of 10% fluorescein sodium (5 mg/kg, AK-FLUOR; Akorn, Decatur, IL) for improved visualization of vessel diameter. Overall, the duration of the vascular PO2 and blood flow imaging session was approximately 1 hour. 
Oxygen Tension Imaging
Retinal vascular PO2 measurements were obtained using our optical section phosphorescence lifetime imaging system. 48,49 Briefly, a laser line was projected on the retina after intravenous injection of the Pd-porphine probe. Due to the angle between the excitation laser beam and imaging path, an optical section phosphorescence image was acquired in which the retinal vessels were depth-resolved from the underlying choroid. Phosp