Velocity Measurements of Normal and Sickle Red Blood Cells in the Rat Retinal and Choroidal Vasculatures

The purpose of this study was to develop an in vivo, noninvasive method to assess the velocities of normal and sickle red blood cells (RBCs) in the retinal and choroidal vasculatures of rats. Human and rat RBCs were isolated from whole blood, labeled with fluorescein isothiocyanate (FITC), and administered intravenously to anesthetized rats. A Rodenstock scanning laser ophthalmoscope (SLO) was used to image the FITC-labeled RBCs as an NTSC video signal. Video sequences of RBC transit in the retinal (pigmented rats) and choroidal (albino rats) vessels were captured directly to digital format. Following in vivo angiography, the animals were sacrificed, the eyes enucleated, and retinas prepared by our adenosine diphosphatase vascular labeling technique for viewing by conventional optical microscopy. Although rat and normal human RBCs differ slightly in size, their velocities were similar in the retinal arteries and capillaries (within 4%). Velocities of RBCs from sickle cell patients (sRBCs) were slower by 12 and 9% in arteries and by 38 and 25% in capillaries, compared to rat and normal human RBCs, respectively. Compared to velocities in retinal capillaries, the velocities in choroidal capillaries were much slower for rat RBCs (77%), normal human RBCs (79%), and sRBCs (67%). In contrast to normal human RBCs, sRBCs were often retained transiently in retinal capillaries at preferred sites, but in choroidal capillaries large numbers of cells were retained for extended periods. SLO imaging of FITC-labeled RBCs in rat retina and choroid provided a reliable method for evaluating normal and abnormal hemodynamics.