Observation and quantification of erythrocyte flow in the choroid of the albino rat
Purpose. Choroidal blood flow is one of the highest in the body on a global volume basis. Little is known, however, about flow through individual vessels, which has important consequences for blood delivery and oxygen transport. The purpose of this study was to use a new epifluorescent technique to view, record and quantify erythrocyte (RBC) flow in individual rat choroidal vessels through the intact sclera. Methods. Five Sprague-Dawley rats were used in this study. This strain was used because its choroidat vasculature is similar to human, the eye is small enough to permit observation on a microscope stage, the sclera is thin and avascular, and there is no pigment in the choroid. These factors permit the vessels to be viewed through the sclera using epifluorescence. Rhodamine-labelled liposomes were injected IV and served as a plasma marker in the blood vessels. Rat RBCs were labelled ex-vivo with 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (Dil) and then infused IV. The flow of the fluorescent RBCs through 59 choroidal vessels with diameters between 14 and 60 μm was recorded on video tape, and the images were used to determine vascular diameter, RBC flux and velocity, and microvessel hematocrit (Hct). Results. RBC flux was positively correlated with vessel diameter (r=0.68; p<0.001), but RBC velocity was not (r=0.23; p>0.05). Microvascular Hct ranged between 10% to 60% of systemic Hct and was negatively correlated with diameter (r=0.39; p=0.002). Conclusions. Flow of RBCs in small choroidal vessels was quantified for the first time in vivo using a new technique. The relationships of RBC flux and RBC velocity with vessel diameter were the same as found in other tissues. However, in other vascular beds local Hct and diameter are positively correlated. Since local Hct is one determinant of relative viscosity, the high Hct may increase flow resistance in small vessels of the choroid compared to other tissues.
Braun, RD; Dewhirst, MW; Papahadjopoulos, D; Hatchell, DL
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