Aqueous humour formation

Abstract

The effects of a number of metabolic inhibitors on aqueous formation in rabbits are described: These are discussed in the light of experimental findings and explanatory hypotheses about the process of aqueous humour formation which are briefly reviewed. The evidence for active sodium transport is examined and it is shown that support for this hypothesis may arise from experiments using isolated ciliary epithelial tissue in vitro. Aqueous formation is inhibited by fluoroacetate, dinitrophenol and ouabain whilst the composition of the aqueous undergoes little change. The composition may, however, readily by changed by alterations of plasma osmolarity, and it is suggested that the rate of secretion depends upon: Active Na transport is oxygen dependent and subject to inhibition by citrate cycle inhibitors such as malonate and fluoroacetate and by ouabain which inhibits Na+ - K+ - Mg-activated ATP-ase. Histochemical and biochemical studies on ciliary epithelium suggest that the enzymes succinate dehydrogenase, Na-K-Mg-activated ATP-ase and cytochrome oxidase occur predominantly in the non-pigmented cell layer of the epithelium. It is postulated that the systems: citrate cycle, oxidative phosphorylation, ATP synthesis furnish the substrate for Na-K-Mg-activated ATP-ase which is responsible for extruding sodium from the epithelial cell into the posterior chamber. This system appears co-terminous with the non-pigmented epithelium. Glycolysis rates in ciliary epithelium are high, but are not efficient as sources of energy for Na transport. The histochemistry suggests that glycolysis may be associated particularly with the ciliary pigment epithelium. The effects on aqueous secretion of a number of other substances (aldosterone antagonists, agents with selective toxicity to retinal pigment epithelium) are described and discussed, and a conjectural account is given of the metabolism of the ciliary epithelium. In the final section an hypothesis is advanced which attempts to take account of the role of the ciliary pigment epithelium in aqueous production. It is thought that this layer may function as an electrically charged membrane, the charge being maintained from cell metabolism and acting as a ‘permselective filter’ influencing the final result of the active ‘Na-pump’ in the non-pigmented cell layer. In an appendix a description is given of a procedure used to determine the surface area of the ciliary epithelium in rabbits.