Oxygen Metabolism of the Inner Retina in the 50/10 Rat Model of Retinopathy of Prematurity

Abstract

 

Purpose

 

To compare the inner retinal metabolic rate of oxygen (rMRO₂) in normal rats and rats with 50/10 oxygen-induced retinopathy (OIR) using visible-light optical coherence tomography (vis-OCT) at P18.

 

Methods

 

Beginning at birth, four Sprague-Dawley rat pups were exposed to alternating hyperoxia (50% O₂) and hypoxia (10% O₂) every 24 hours for 14 days (OIR group), while six rat pups were allowed to grow at room air (control group). On P14, the OIR pups were returned to room air. On P18, imaging was performed to measure the oxygen saturation of hemoglobin (sO₂) of the inner retinal arterioles and venules for both control and OIR groups [A, B]. These experiments used a vis-OCT system, which incorporated a supercontinuum source with wavelengths from 500 nm to 620 nm. A dual-circle scanning protocol implemented on the same OCT system enabled the measurement of vessel diameter, blood velocity, and volumetric blood flow [C, D]. Using the sO₂ measurements, we calculated the oxygen extraction fraction (OEF) of the inner retina. Combining the OEF with the total blood flow permitted the calculation of the rMRO₂. Following the imaging experiments on P18, the retinas were harvested, immunostained with Alexa Fluor 594 isolectin, and imaged using fluorescence microscopy [E]. We quantified the percentage of vaso-obliteration and counted the number of clock-hours with neovascularization in the OIR retinas. An unpaired Student’s t-test was used to compare the measurements between control and OIR groups.

 

Results

 

There was no significant difference in the OEF between the control and OIR groups (0.231 ± 0.037 vs. 0.262 ± 0.054; p=0.3550). The average total estimated blood flow was significantly lower in OIR versus control groups (2.74 ± 0.58 μl/min vs. 7.37 ± 2.96 μl/min; p=0.003). Additionally, rMRO₂ was significantly decreased in OIR versus control groups (128 ± 32 nl min-1 vs. 338 ± 138 nl min-1; p = 0.0108). For the OIR group, the average vasoobliteration was 10.92 ± 3.50% and the average number of clock hours with neovascularization was 4.25 ± 2.25 clock hours.

 

Conclusions

 

We observed a 59% reduction in the rMRO₂ in the OIR group as compared with that of control group, suggesting that the metabolism of the inner retina is markedly reduced at P18 in the 50/10 OIR rat model.