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Sarah Walters, Christina Schwarz, Amber Walker, Louis DiVincenti, Jennifer J Hunter; Two-photon autofluorescence kinetics of macaque photoreceptors are slowed during systemic hypoxia. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1155.
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The kinetics of two-photon excited fluorescence (TPEF) from photoreceptors in response to visual stimulation are indicative of all-trans-retinol production and clearance and show promise as an objective, non-invasive measure of visual cycle function in both healthy and diseased retina. However, the ability of TPEF kinetics to detect functional change that may occur in disease or altered physiological state has not been established. As changes in oxygen supply play a role in many diseases leading to retinal degeneration, we induced systemic hypoxia in macaque as a model of altered physiological state and tracked the TPEF kinetics of photoreceptors using adaptive optics (AO) ophthalmoscopy.
Macaques were anesthetized with isoflurane, paralyzed, and ventilated with 100% O2 (pre-hypoxia). Repeatedly, systemic hypoxia was induced by ventilating with 10% O2 and 90% N2 for a period of 10-12 min, followed by recovery with 100% O2 for 10-15 min (post-hypoxia). Heart rate was recorded and venous blood samples provided oxygen saturation (SpO2) and partial pressure of oxygen (PO2). An AO scanning light ophthalmoscope was used to collect TPEF (ex: 730nm, 3.5mW; em: <550nm) for 2 min in a 2.4° square field of view from the photoreceptors of 3 macaques in 2 cone-dominated locations (~2° nasal and temporal). Measurements of TPEF after 10 min dark adaptation in each condition were repeated three times per location. The TPEF time courses were fit with a single exponential function.
After 10-12 min of hypoxia, PO2 and SpO2 decreased from >300 to ~35 mmHg and 100 to ~70%, respectively, while heart rate increased an average of 5.6±0.9%. Baseline TPEF time constants varied among animals from 16.9±0.3 to 33.5±1.2 s. Systemic hypoxia produced no significant change in the total TPEF increase (ANOVA, p=0.207); however, it significantly slowed TPEF response, increasing the time constant by 11±2% on average (ANOVA, p=0.002). TPEF responses were not significantly different pre- and post-hypoxia (paired t-test, p=0.133).
Systemically reduced oxygen supply slows the time course of TPEF in photoreceptors, yet the total TPEF increase is unaffected, possibly indicative of visual cycle slowing in response to hypoxia. This demonstration broadens the potential utility of two-photon AO ophthalmoscopy to detect changes in visual cycle function that occur with disease or altered physiological state.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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