Assessment of ascorbate ocular disposition in the conscious rabbit: An approach using the microdialysis technique

Abstract

Purpose. This study was performed to evaluate the transport kinetics of ascorbate in aqueous humor of conscious rabbits. Methods. Following the development of a spectrophotometric assay for ascorbate in serum, aqueous humor and microdialysate, and preliminary studies of ascorbate systemic disposition in the rabbit, microdialysis probes were placed into the anterior chamber of one eye, and the posterior chamber of the contralateral eye, of four New Zealand white rabbits. After a one-month recovery period, conscious rabbits were placed in restraining devices, and marginal ear veins were cannulated for repeat blood sampling and ascorbate administration. A tracer i.v. bolus of 14 C-ascorbate, followed by stepwise increasing i.v. infusions of unlabelled ascorbate, was administered. Estimates of basal ascorbate transport into aqueous were determined by analysis of tracer 14 C-ascorbate in microdialysis probe effluent and serum. Kinetic modeling was employed to assess ascorbate disposition during infusion. Results. Systemic disposition of exogenously administered ascorbate was well characterized by a two-compartment model. Kinetic modeling returned physiologically realistic volumes for the posterior chamber, and reliable estimates governing ascorbate flux into, between, and from the posterior and anterior chambers. Conclusions. In vivo assessment of ascorbate kinetics in aqueous humor and blood of the rabbit was facilitated by the microdialysis technique. Contrary to reports in the literature, ascorbate saturable uptake from blood to aqueous was not observed at physiologic blood concentrations (~11 to 30 mg/L).