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Tess Kornfield, Eric A Newman; Retinal Blood Flow is Differentially Regulated within the Trilaminar Capillary Network. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4334.
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© ARVO (1962-2015); The Authors (2016-present)
Blood flow in the retinal vasculature is closely tied to neuronal activity. Though large arterioles are known to affect blood flow, the role of capillaries in actively regulating local blood flow is currently under debate. Pericytes, contractile cells that wrap around capillaries, may actively control capillary diameter, but results from brain studies are conflicted and in vivo retinal studies have not been conducted. Moreover, blood flow regulation within the three distinct capillary layers of the retina has not been characterized. The goal of this study was to determine which vessels in the retina actively regulate blood flow and to investigate the role of capillaries in controlling blood flow.
We characterized light-evoked diameter and flux changes throughout the retinal vascular network in the in vivo rat retina. The retina was stimulated by a 5Hz diffuse flickering white light. Vessel diameter was measured with confocal line scans across the blood vessel and flux was calculated by counting the number of fluorescently labeled red blood cells passing through the line scan per second. Post-experiment immunohistochemistry was performed to localize pericytes.
The largest light-evoked dilations were in the primary (7.8 ± 0.7%; mean ± SEM) and secondary (5.2 ± 1.0%) arterioles, although capillaries and primary venules also dilated (2.2 ± 0.1% and 1.1 ± 0.2%, respectively). The magnitude of capillary dilations was not correlated with pericyte proximity. All blood vessels displayed light-evoked flux increases. These flux increases were most heterogeneous in capillaries. Strikingly, capillaries in the intermediate capillary layer, which lies at the outer border of the inner plexiform layer, displayed large, slowly developing dilations that were not seen in the other two capillary layers. Flux increases in the intermediate capillary layer were larger than increases in either the superficial or deep layers.
Due to their large dilations, we conclude that primary and secondary arterioles drive neuronally induced blood flow increases in the rat retinal vascular network. Our data suggest that pericytes do not actively control capillary diameter in vivo, although the differential dilation and flux responses within the trilaminar capillary network hints at active regulation. Additional work is required to address this conflict.
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