Effect of Silicone Oil Components on Corneal Endothelial Passive Permeability and Ion Transport

Abstract

Silicone oil currently employed as a retinal tamponade is a complex mixture of monomeric to long-chain polymers, and different components may be responsible for corneal endothelial or retinal toxicity. This study on the corneal endothelium had two aims. First, to identify which specific component of a linear series of low-molecular-weight compounds caused the greater permeability increase, because when given collectively in earlier experiments they increased endothelial passive permeability. Passive permeability changes were determined by in vitro measurements of inulin and dextran permeability following 1 week of in vivo exposure to oil containing different additives. Permeability was found to be increased after three of four linear compounds varying in length from 1 to 10 silicone atoms (MDM [octamethyltrisiloxane] to MD₁₀M [hexacosa-methyldodecasiloxane]) were added to purified silicone oil. The linear series offers no individual candidate for maximal effect, suggesting that there is either an interactive or concentration-dependent effect when these compounds appear together at high concentration in an oil compared to being present individually. The second aim was to determine if previous observations on permeability and overt corneal changes using various additives could be explained by differences between passive permeability increases alone versus passive permeability increases coupled with ion transport inhibition. This was achieved by measuring unidirectional bicarbonate fluxes in vitro after 7 days of in vivo exposure to oil plus additive. Passive bicarbonate flux was increased after exposure to a catalyst, a representative of the linear series (MD₁₀M), and a cyclic series of compounds. Only the cyclic series also decreased active ion transport. A direct interaction between these additives and corneal components may offer a better explanation for the differential effects of additives on corneal opacity rather than effects on permeability and/or ion transport.