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Angelina Duan, Phillip A Bedggood, Bang V Bui, Andrew B Metha; Imaging localized neurovascular coupling in the normal human retinal microvasculature. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2605.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify changes in the diameter in retinal vessels of < 28 µm diameter in response to local flicker stimulation.
The foveal microvascular network was imaged in 3 healthy subjects with a flood-based adaptive optics ophthalmoscope. Each 1.25° imaged region was selected so as to contain a “major” arteriole or venule (~18-28 µm diameter) and associated branches of varying size. Image sequences were collected a) Following 30 seconds of fixation on a static, dimly illuminated grid; b) Following 30 seconds of fixation during which the 593 ± 25 nm (FWHM) imaging light was flickered at 8 Hz on a low power setting (5 µW, cf. 0.5 mW for imaging) for 30 seconds. Five image sequences were collected in each area (200 fps, 80 frames), separated by a resting period of 2 minutes. Acquired sequences were flat-fielded, co-registered and averaged. Vessel diameter was measured semi-automatically, using custom Matlab software: Straight-line regions 4-60 μm in length were traced manually and used to generate intensity cross-sections spaced 0.5 µm apart. In each cross-section, vessel edges were located by finding a maximum in the local image intensity gradient (Prewitt filter). Vessel segments ranging from 6.5 - 28 µm in diameter were measured.
For the venules, functional dilatation was measurable in segments as small as 13 µm (mean ± SEM = 13.2 ± 2.9%). Local differences in the response were observed along the same venule, e.g. ranging from 5.8 ± 1.1% to 13.4 ± 3.4%. For the arterioles, functional constriction was observed. As with the venules, vessel diameter changes appeared to be localized along the vessel length, ranging from -2.4 ± 1.4% to -13.3 ± 1.4%. The largest constrictions observed were immediately upstream of branch points, which may corroborate evidence of pericyte-mediated neurovascular coupling in peri-capillary vessels reported recently in the literature.
The diameters of the smallest arterioles and venules in the retina can be modulated in response to functional stimulation. Venules were observed to dilate, while arterioles constricted. The highly focal nature of the response, together with the large magnitude compared with that previously measured in larger vessels, suggest an active local mechanism.
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