Permeability of the cornea and the blood-aqueous barrier to oxygen

Abstract

(1) A method suitable for the measurement of the concentration of oxygen in the aqueous humour by a polarographic technique is described. The sample for analysis is withdrawn into a micro-cell directly from the anterior chamber of the eye without protein precipitation or contact with air. (2) The permeability of the excised cornea to oxygen has been determined. At 4°C. the diffusion coefficient of the excised cornea was 0 018±0 0013 in eleven tests. Removal of the epithelial and endothelial layers did not affect this value significantly. Oxygen was found to move freely across the cornea in either direction, and its rate of transfer increased by 1 to 2 percent. per ° C. (3) In similar experiments with living animals, the oxygen tension in the aqueous humour on exposure of the anterior surface of the eye to an atmosphere of oxygen was analysed. The oxygen tension in the aqueous humourrose to 130 mm. Hg in conditions of steady state from an initial value of 45 mm. Hg. From these results and a knowledge of the oxygen uptake of the component layers of the cornea, it has been calculated that the oxygen diffusing into the cornea from the aqueous humour is inadequate to supply the respiratory requirements of the epithelial layer. (4) The oxygen tension in the aqueous humour of conscious rabbits before and after exposure to an atmosphere of 95 per cent. oxygen and 5 percent. carbon dioxide has been determined. The blood-aqueous barrier has been found to be freely permeable to oxygen, and equilibration between the blood and the aqueous humour to be attained within approximately 1 hr. In animals in which the arterial oxygen tension was over 600 mm. Hg, the tension in the aqueous humour rose to approximately 250 mm. Hg. Evidence is given that the oxygen tension in the aqueous humour is determined by blood having a composition approaching that of arterial blood. (5) Changes in blood flow through the ciliary processes, caused by the ligation of one common carotid artery or by cutting the cervical sympathetic, resulted in no marked change in the oxygen tension of the aqueous humour. In the sympathectomized animals the oxygen tension was equal in both eyes, while in animals with a ligation of the carotid artery the oxygen tension on the operated side was significantly below that on the contralateral side but still within the same range of values as in normal eyes.