A RAT MODEL FOR SICKLE CELL-MEDIATED VASOOCCLUSION IN RETINA

Occlusions of the retinal vasculature are the initiating event in sick le cell retinopathy. In order to understand the mechanism(s) of sickle cell-mediated occlusion, a rat model was developed. Red blood cells ( RBCs) from patients homozygous for hemoglobin (Hb) S (SS) or double he terozygous for Hb S and Hb C (SC) were separated on Percoll-Larex cont inuous density gradients, labeled with fluorescein isothiocyanate (FIT C), and delivered via the left ventricle to anesthetized ventilated ra ts. Blood gas levels were altered by changing inspired gas and monitor ed via a femoral arterial catheter. After the RBCs circulated for 5 mi n, animals were perfused with heparinized saline, the eyes enucleated, and the retinas removed and processed by our ADPase flatmount techniq ue. The retinal vasculature was visualized under dark-field illuminati on and the FITC-RBCs visualized by fluorescence microscopy. Greater nu mbers of high-density SS cells (SS4, which consist of dense, dehydrate d discocytes and irreversible sickled cells) were retained in the norm al rat retinal vasculature than normal-density SS cells (SS2, which ha ve the same density as normal AA cells, but consist of reticulocytes a nd young cells). Retention of SS4 cells was inversely dependent on the arterial oxygen tension. Most SS4s were retained in capillaries, but a few were observed within precapillary arterioles. The retained RBCs occupied the full lumenal diameter of vessels in most cases. In contra st, very few RBCs from SC donors (normal or high density) were retaine d in the normal retinal vasculature and retention did not increase sig nificantly with hypoxia. This model demonstrates that high-density SS cells, which include irreversibly sickled cells, are retained in norma l rat retinal vessels and that the number retained is oxygen dependent . Furthermore, it appears that trapping, not adhesion, is responsible for retention of RBCs in the normal retinal vasculature because there was preferential retention of SS4 cells, which are known to have lower adherence propensity, and the retained RBCs blocked the full diameter of the vessel. These results also demonstrate that the mechanism of v ascular obstruction by SS and SC RBCs is different because low retenti on of SC cells was observed. The well-known propensity of SC patients to have retinal abnormalities must involve extraerythrocytic factors l ike increased hematocrit, induction of adhesive molecules and integrin s, etc. (C) 1996 Academic Press, Inc.