Influence on corneal permeability of bathing with solutions of differing reaction and tonicity.

THE commonest method of investigating the permeability of the cornea is to bring a solution of the test substance into contact with its epithelial surface and to measure how much penetrates into the aqueous humour or into the stroma of the cornea itself. Clearly, it is desirable to know what range of properties of the test solution may be tolerated without causing changes in the property that is being measured. A related question arises in the preparation of pharmaceutical agents for instilling into the eye where maximum action should be obtained without causing any damage to the tissues. One such property, about which disagreement exists, is the tonicity of the solution. Nakamura (1923) carried out experiments which suggested that both hypotonic and hypertonic solutions caused an increase in the permeability of the cornea as a whole to ionic substances. Potts (1953) reached similar conclusions and located the tonicity which gave the minimum epithelial permeability at the equivalent of 1 35 per cent. NaCl, which he inferred was the tonicity of tears. Many investigators, however, have used test solutions made isotonic with blood, that is the equivalent of 0 9 per cent. NaCl. The conclusions drawn by both Nakamura and Potts did not seem in every case to have a satisfactory experimental basis, and it was decided to re-investigate the problem by determining the penetration of 24Na into the eye from solutions of differing tonicity. The experiments described here show no evidence of an increase in epithelial permeability over the range of tonicity 0 9-10 per cent. NaCl. Hypotonic solutions did, however, cause the rise in permeability described by Nakamura and Potts. It was also found that an increase in the tonicity of the bathing solution in the neighbourhood of 0 9 per cent. NaCl caused an increase in the permeability of the corneal endothelium, that is, of the cellular layer covering the cornea on the side remote from the solution. Whether this increase was a result of damage to the layer or an active process was not determined. The reaction of the bathing solution is an influence which has remained virtually uninvestigated as far as ionized substances are concerned. For poorly dissociated compounds the changes in penetration have been cor-