Effects of light and darkness on oxygen distribution and consumption in the cat retina.

Abstract

These experiments were done to investigate the effects of light and darkness on the oxygenation of the retina in anesthetized cats. Measurements were made with double-barreled oxygen microelectrodes capable of recording both oxygen tension (P0 2 ) and local voltages . Diffuse white illumination presented to a dark-adapted retina led to an increase in P02 of up to 30 mmHg in the outer half of the retina . Changes were maximal at^-75% depth, corresponding to the outer nuclear layer . No change or decrease in P0 2 was observed in the inner retina. Light-evoked increases in outer retinal P02 were graded with the duration and strength of illumination, and were maximal in response to 60 s of illumination at rod saturation . For these stimuli, the increase at the onset of illumination was slower (average half-time, 12 .2 s) than the recovery at the end of illumination (average half-time, 5 .9 s), but for stimuli above rod saturation, P02 recovered much more slowly . The profile of P02 was measured during electrode penetration and withdrawal and during light and dark adaptation . Dark-adapted profiles were characterized by a minimum P0 2 of nearly 0 mmHg at depths of 65-85%, and a steep gradient from the minimum to the choroid . During light adaptation at rod saturation, P0 2 was elevated in the outer half of the retina and the minimum was eliminated . Fits of the profiles to a one-dimensional model of oxygen diffusion indicated that light reduced the oxygen consumption of the outer retina to^-50% of its darkadapted value .

Magnetic resonance imaging of tissue and vascular layers in the cat retina

“…Cognizant of the partial-volume effect, we estimated the MRI-derived neural retinal thickness to be ϳ239 m, in reasonable agreement with published histological data (neural retina: 150 -250 m) (13,32,34,35) but slightly larger than our histology (neural retina: 184 Ϯ 33 m). There are many factors that could account for the difference between MRI and histology data.…”

“…Electroretinography (11,12) is widely used to study retinal electrical activity and function. Oxygenation and oxygenation changes in the retina have been studied using oxygen electrodes (13,14), phosphores-cence (15), and, more recently, intrinsic optical imaging technique (16 -18). Several methods also exist for measuring blood flow in the retina, including fluorescein angiography (19,20) which can measure retinal circulation and indocyanine-green angiography (21,22) which can measure choroidal circulation in the fovea.…”

Why photoreceptors die (and why they don't)

“…In a dark-adapted rod, illumination has surprisingly little effect on energy consumption, since the increase in utilization of GTP by the cascade (for production of T a -GTP and cGMP by the guanylyl cyclase) is more than compensated by the closing of the cGMP-gated channels in the outer segment, which reduces Na þ influx and decreases the activity of the Na þ /K þ ATPase. (84,85) Since however bright light drives the PDE much faster than is necessary to keep all of the channels closed, it is quite likely that translocation of transducin and arrestin can reduce energy consumption by the cascade with little or no re-opening of the channels. This could produce a significant decrease in total energy expenditure by the cell in bright light.…”

Oxygen consumption in the isolated toad retina

“…Linsenmeier, 1986), monkey (Stefansson, Wolbarsht and Landers, 1983;Ahmed et al, 1993), rabbit (Ames et al, 1992) and rat (Medrano and Fox, 1993). By employing measurements of intraretinal oxygen tension (PO2) in the cat, the values of QO2 for the photoreceptors were derived from a diffusion model of the outer retina (Linsenmeier, 1986;Haugh, Linsenmeier and Goldstick, 1990). High oxygen consumption was localized to the photoreceptor inner segments (Haugh et al, 1990) where mitochondria are located (Sjostrand, 1953;Robertis, 1956 ;Cohen and Noell, 1965).…”