Microdialysis measurement of ascorbic acid in rabbit vitreous after photodynamic reaction

A method for long-term intravitreous microdialysis was used to measure endo genous reduced ascorbic acid in the vitreous of rabbits by RPLC-ECD before and after exposure to intense visible light in the presence of fluorescein. Cellulose microdialysis probes were implanted into the vitreous humor of e ach eye and after stabilization ascorbic acid measurements were recorded ov er a 14 day period. Under this experimental condition, normal ascorbic acid concentrations in vitreous varied from 98.0 +/- 9.8 to 106.9 +/- 20.3 muM (mean +/- S.D.). The eyes received light irradiation (25 000 lux) for 2 hr and fluorescein was used as the photosensitizer once or twice. No immediate effects on ascorbic acid concentrations could be observed in the eyes irra diated twice without fluorescein Lv. injections and in the twice fluorescei n injected without irradiation. However, in the eyes irradiated once with f luorescein (30 mg kg(-1)), ascorbic acid concentration after irradiation si gnificantly decreased from day 2 and continued over a period of 10 days com pared with that before irradiation and maximal reduction was 32.6 % (P < 0. 005) on day 6 after irradiation. By day 13, the ascorbic acid concentration returned to control levels (P > 0.01), In the eyes irradiated twice with f luorescein injections, ascorbic acid concentration after irradiation decrea sed even more over the experimental period and the maximal reduction was 65 .5 % (P < 0.005) on day 5 after irradiation and did not recover over the ne xt 9 days. In the eyes irradiated twice with fluorescein injections plus ad ministration of ascorbic acid (150 mg kg(-1)) 30 min before irradiation, a significant increase (52.5 %) of ascorbic acid (P < 0.005) was found on day I and control levels of ascorbic acid were maintained from day 2 onward. T he protective role of ascorbic acid in the vitreous humor against photodyna mic. reaction is suggested, (C) 2001 Academic Press.