Retinal and Choroidal Vasoreactivity to Altered PaCO2 in Rat Measured With a Modified Microsphere Technique

Abstract

 

Purpose:

 

Rats are increasingly used in ophthalmic research. However, little is known about metabolic regulation of ocular blood flow (BF) in the rat eye. The purpose of this study was to measure vasoreactivity in both retina and choroid of the rat eye in response to experimentally altered partial arterial pressure of CO₂ (PaCO₂).

 

Methods:

 

Adult male Brown-Norway rats under isofluorane anesthesia were mechanically ventilated. PaCO₂ was monitored continuously by recording end tidal carbon dioxide level. The arterial oxygen partial pressure remained above 100 mmHg. Both femoral arteries and a femoral vein were cannulated for mean arterial blood pressure monitoring, blood sampling and drug administration, respectively. The intraocular pressure in both eyes was manometrically controlled at 20 mmHg. The retinal and choroidal BF was simultaneously measured using a modified microsphere method by cardiac injection of a mixture containing 3.75 million of 8 µm, and 0.5 million of 10 µm microspheres; each size having a distinct color. In one experiment (n=10), BF was first measured under normocapnia (PaCO₂ = 35 mmHg) and then under hypocapnia (PaCO₂ = 20-25 mmHg) using a second injection of differently colored spheres. In another experiment (n=7), BF was measured under hypercapnia (PaCO₂ = 45-50 mmHg).

 

Results:

 

The table shows the mean arterial pressure, PaCO₂, retinal and choroidal BFs during each experimental condition.The differences of mean BF across all the three PaCO₂ groups were highly significant for both retina and choroid (ANOVA: P < 0.0001 and P = 0.01, respectively). The mean BF under hypocapnia was significantly lower than normocapnia in both retina and choroid (P<0.02). The mean BF under hypercapnia was significantly higher than normocapnia in retina (P=0.001) but not in choroid (P=0.62).  

 

Conclusions:

 

This study demonstrated that the dual-size and dual-dose microspheres mixture can be used to measure retinal and choroidal BF reliably and simultaneously in rats. Similar to most other species, hypocapnia decreases both the retinal and choroidal BF. Hypercapnia, however, increases BF in the retina but not in the choroid in rats.

 

Keywords: blood supply • retina • choroid