Scleral permeability to hydrocortisone and mannitol in the albino rabbit eye

To further explore barrier properties of the sclera, the diffusion of 3H-hydrocortisone and 14C-mannitol was measured across isolated rabbit scleral membrane. In vitro permeability studies were performed using side by side diffusion cells. Bicarbonated Ringer Solution with oxidized glutathione (GBR) at pH 7.4 was the perfusion medium, and the temperature was kept at 37 degrees C. Diffusion of hydrocortisone through the cornea was also measured to compare scleral and corneal permeation. Scleral permeability was found to be five times greater than corneal permeability. Drug analyses were performed by radionuclide counting (LSC), and permeability coefficients were obtained. In vitro metabolism of hydrocortisone was examined by incubation of tissue in hydrocortisone solution in GBR for 5 hours and 37 degrees C. Permeability coefficients of hydrocortisone diffusion through the sclera were also obtained at 25 degrees C, 15 degrees C, and 5 degrees C. Activation energy of scleral transport of hydrocortisone was calculated from an Arrhenius plot. The low activation energy suggests an aqueous pore pathway unlike permeation of the drug across the cornea which uses a transcellular pathway.