Evidence of carrier mediated transport of ascorbic acid through mammalian cornea

The purpose of the present study was to evaluate the transport of ascorbic acid, a water soluble molecule, through a predominantly lipophilic cornea. Thus in-vitro permeation of ascorbic acid from aqueous drops through freshly excised mammalian cornea was studied. Aqueous isotonic ophthalmic solutions of ascorbic acid of different concentrations (0.125% w/v to 2% w/v) (pH 5.4) were made. Further 1.0% w/v or 0.5% w/v ascorbic acid solution containing NaCl or dextrose as tonicity modifiers or Na(+)K(+)-ATPase inhibitors were also made. Permeation characteristics of drug were evaluated by putting 1 ml formulation on freshly excised cornea fixed between donor and receptor compartments of an all-glass modified Franz diffusion cell and measuring the drug permeated in the receptor by spectrophotometry at 265 nm, after 120 min. Statistical analysis was done by one-way analysis of variance (ANOVA) followed by Dunnett's test or paired t-test. Increase in drug concentration in the formulation resulted in an increase in the quantity permeated but after a certain level increase in permeation with increase in concentration was minimal. Aqueous drops made isotonic with dextrose showed decreased permeation through paired cornea compared with aqueous drops made isotonic with NaCl from 1% w/v ascorbic acid solution suggesting likely involvement of Na(+) co-transporter but there was decreased permeation through 0.5% w/v ascorbic acid solution made isotonic with NaCl as compared to solution made isotonic with dextrose. Further aqueous drops containing Na(+)K(+)-ATPase inhibitor {MAG-Mono Ammonium Glycyrrhizinate (25 μmol)} showed decreased corneal permeation from 0.5% w/v ascorbic acid solution but there was not significant decrease from 1% ascorbic acid solution since MAG is a competitive inhibitor of ascorbic acid. Aqueous drops containing Na(+)K(+)-ATPase inhibitor {MAG (50 μmol) or Ouabain (1 mmol)} showed decreased corneal permeation of ascorbic acid compared with control from 1% ascorbic acid solution confirming the involvement of Na(+) co-transporter.