Quantification of erythrocyte flow in the choroid of the albino rat

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 ocular 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. With the Sprague-Dawley rats under urethan anesthesia, rhodamine-labeled liposomes were injected intravenously and served as a plasma marker. Rat RBC were labeled ex vivo with 1,1'-dioctadecyl-3,3,3',3'-tetramethylin docarbocyanine perchlorate and then infused intravenously. The flow of the fluorescent RBC through 69 choroidal vessels with diameters between 12 and 52 pm in six rats was recorded on videotape, and the images were used to determine vascular diameter, RBC flux and velocity, and microvessel hematocrit (Hct). RBC flux and RBC velocity were positively correlated with vessel diameter (r = 0.67, P < 0.001 and r = 0.29, P = 0.016, respectively). Microvascular Hct ranged between 4 and 32% (8 and 67% of systemic Hct) and was negatively correlated with diameter (r = -0.28, P = 0.018). The relationships of RBC flux and RBC velocity with vessel diameter were the same as found in other tissues. However, in other vascular beds, microvascular Hct and diameter are positively correlated. Because microvascular Hct is a determinant of relative viscosity and oxygen delivery, this relatively high Hct in small choroidal vessels could have significant implications for local blood flow and oxygen transport.