Postocclusive reactive hyperemia occurs in the rat retinal circulation but not in the choroid

was used to paralyze the animals and prevent eye movement.20, 21 End-tidal CO2 (Surgivet
18 capnometer), arterial oxygen saturation (mouseOx oximeter), heart rate (mouseOx oximeter),
19 and rectal temperature were maintained within normal physiological ranges. … Purpose: To test the hypothesis that retinal blood flow has a postocclusive reactive hyperemia response modulated by occlusion duration and metabolic activity and that choroidal blood flow does not. Methods: Anesthetized and paralyzed rats (n=34) were studied. Retinal and choroidal blood flow was measured by laser speckle imaging and laser Doppler flowmetry, respectively. Blood-oxygenation-level-dependent functional magnetic resonance imaging (BOLD fMRI) was used to measure changes in relative blood oxygenation of the retinal and choroidal circulations. Transient carotid occlusion was elicited with a hydraulic occluder on the common carotid artery. Several occlusion durations were tested during dark, constant light and flicker light conditions to modulate metabolic demand. The hyperemia response magnitude was quantified by integrating the area above the blood flow baseline for the 3 minutes after release of the occlusion. Results: Systemic arterial pressure (108.2±1.4mmHg) was unaffected by the carotid occlusions and was similar among animals and conditions. Retinal blood flow had a reactive hyperemia but choroidal blood flow did not (e.g. 14±2%•sec versus 0.5±4%•sec after 60sec occlusion). The hyperemia magnitude increased as a non-linear function of occlusion duration and reached a plateau at occlusion durations <60sec. The hyperemia magnitude was not altered by different lighting conditions at occlusion durations of 15 and 60sec. BOLD fMRI results were similar to the laser-based blood flow measurements. Conclusions: The results indicate that metabolic local control has a negligible role in choroidal blood flow regulation and only partially accounts for the blood flow behavior in the retinal circulation.

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