Unsteady transport and hydration dynamics in the in vivo cornea

The unsteady response of the rabbit cornea to the normal periodic variations in tear tonicity which accompany the sleep-wake cycle is examined quantitatively in terms of a physical description of corneal mechanics and transport. Two different sets of experimental epithelial and endothelial flow conductivities and reflection coefficients are used, and the effect of variations in epithelial solute permeability and sodium pump rate is examined. The use of a set of experimental corneal parameters chosen earlier provides good agreement between calculated and observed in vivo corneal thickness dynamics when the tear tonicity is within the physiologic range. The factors affecting the time-course of corneal thickness dynamics are discussed, including the osmometric quality of the corneal stroma, the role of the epithelial sodium pump, the flow resistance of the limiting corneal layers, and cyclic changes in aqueous tonicity. The unsteady solutions presented here are related to the steady-state solutions given in earlier papers through the concept of the time-average steady state. Any realistic description of the normal in vivo cornea must recognize its unsteady character and the potential for transepithelial flow. On the average, the hypertonicity of the tears relative to the stromal fluid can be sufficient to account for rabbit corneal deturgescence. The absence of endothelial "pumping" from the in vivo rabbit cornea cannot be proven; neither is there any certain need to postulate such transport in the normal state.