Longitudinal Gradients of Oxygen Tension in the Retinal Vasculature of Rat

Abstract

Purpose: : Oxygen transport in the retinal vasculature plays a crucial role in maintaining normal retinal metabolic function. Therefore, study of changes in oxygen tension (PO₂) along retinal blood vessels is helpful to gain fundamental knowledge of retinal physiology. The purpose of this study is to report measurements of PO₂ longitudinal gradients in retinal capillaries, arteries and veins in the rat, using our previously established phosphorescence lifetime imaging technique.

Methods: : Phosphorescence lifetime imaging was performed in 12 rats under normoxia. A narrow vertical laser line was scanned horizontally across the retina in small steps after injection of an oxygen-sensitive molecular probe and three-dimensional retinal vascular PO₂ maps were constructed. Retinal blood vessels were segmented semi-automatically and vessel centerlines were identified using distance transformations. PO₂ was quantified within cross sectional areas along each blood vessel path. An average PO₂ measurement in each blood vessel was calculated. Linear regression analysis was used to determine PO₂ gradients in retinal capillaries, arteries and veins. Statistical significance was accepted at p < 0.05.

Results: : Systemic arterial PO₂ was 100 ± 22 mmHg (mean ± SD, N=12). PO₂ measurements in retinal capillaries, arteries and veins were 32 ± 6 mmHg (N = 8), 42 ± 9 mmHg (N = 12) and 28 ± 6 mmHg (N =12), respectively. Statistically significant negative PO₂ longitudinal gradients (decrease from upstream to downstream) were found in 6 of 8 retinal capillaries and 5 of 12 retinal arteries. Mean PO₂ longitudinal gradients in these retinal capillaries and arteries were -5.26 ± 1.35 mmHg/100µm (N = 6) and -1.47 ± 0.64 mmHg/100µm (N = 5), respectively. Statistically significant positive PO₂ longitudinal gradients (increase from upstream to downstream) were found in 7 of 12 retinal veins with a mean PO₂ longitudinal gradient of 0.48 ± 0.35 mmHg/100µm (N = 7).

Conclusions: : Quantitative measurements of PO₂ longitudinal gradients in retinal blood vessels of the rat demonstrated retinal tissue oxygen extraction occurs predominantly in the capillaries. This method has the potential to help elucidate retinal capillary oxygen transport in health and disease.

Keywords: retina • oxygen • imaging/image analysis: non-clinical