Purpose. Ascorbate concentration in ocular tissues of diurnal species is high relative to that in plasma. Aqueous humor ascorbic acid (AA) is only slightly elevated in nocturnal animals, but little is known about retinal levels. This study investigates the distribution AA in the retina, vitreous and aqueous humor of the cat. Methods. Double-barreled polarographic AA electrodes were used to simultaneously measure local voltages and AA levels. Local ERGs were recorded from the voltage barrel of the microelectrode. AA levels within the vitreous, inner retina, subretinal space and choroid were recorded during 2 minute IV infusions of 20% ascorbic acid, 20% glucose and insulin (600 mU/cc). The effects of acidosis during inspiration of 95% O2/5% CO2 and hyperoxia during 100% O2 were also investigated. AA concentrations found in vivo were compared with AA concentrations of extracted blood, vitreous and aqueous samples determined by HPLC. Results. Average AA concentrations in the vitreous, aqueous and blood were 1.53 ± 0.41 (n=6), 1.55 ± 0.48 (n=6), and 0.82 ± 0.01 (n=4) mg/dl, respectively. In most cases, the average retinal AA concentration, measured with the microelectrode, was slightly higher than that in the vitreous. There was usually little variation of AA with retinal depth, but levels occasionally increased towards the RPE. The microelectrodes clearly responded well to AA, as evidenced by increased electrode currents in the choroid and outer retina during AA infusions. The AA infusions also caused smaller and slower changes in inner retinal AA, and no detectable change in vitreal AA. There was no change in AA during hyperoxia or acidosis produced with hypercapnia, or in response to infusions of glucose or insulin. The AA levels in the retina remained unchanged during the 4s and 30s light stimuli at rod saturation, and the infusion of AA had no effect on the b-wave and c-wave amplitudes of the ERG. Conclusions. These results indicate that in a nocturnal animal, retinal as well as vitreous and aqueous AA is ordinarily low. The pattern of changes in retinal AA during I.V. infusions indicates that when blood AA is high, AA can be transported into the retina from the choroid but not from the retinal circulation.
|Original language||English (US)|
|Journal||Investigative Ophthalmology and Visual Science|
|State||Published - Feb 15 1996|
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