Physical properties of stimulated and unstimulated tears

It has long been assumed that unstimulated tears are more thoroughly equili brated with epithelial secretions than stimulated tears, since they are in contact with tarsal, bulbar and corneal surfaces for longer, It was also be lieved from results with model solutions that soluble mucin is responsible for the observed surface tension and viscosity of tears. If longer contact means more mucin is dissolved in the aqueous tears, then the surface activi ty (surface tension lowered by mucin) and viscosity (raised by mucin) of te ars should therefore be enhanced in unstimulated over stimulated tears. Pools of stimulated acid minimally-stimulated tears were collected from a g roup of healthy adult volunteers by glass capillary. Viscosities were measu red in the Contraves Low Shear 30 rheometer over the range of shear rates 0 -130 sec(-1). Surface tension was measured in the collection capillaries by a micro-technique, before and after refrigerated storage. Both surface ten sion and viscosity were determined for a variety of tear proteins and mucin s, No significant difference was Found between the viscosity/shear rate plots of stimulated and unstimulated tear samples. The viscosities of solutions o f individual tear proteins were low, except for the combination of lysozyme acid secretory IgA. Surface tensions were also similar in both cases, and unchanged by storage at room temperature or refrigeration, indicating no si gnificant loss of surface-active material by adsorption on the capillary wa lls. Results with model mucin solutions gave a variety of results indicatin g either little surface activity or losses due to wall adsorption, Tear pro teins. individually or in combination, did not tower surface tension to the level of tears. Tear viscosity seems not to depend on the level of dissolved mucins. This s uggests either that a constant level of these is picked up even by short-te rm contact with ocular surfaces, or that viscosity arises from currently un known materials which vary little :vith tear flow rate. This type of shear- dependent viscosity is most easily simulated in model solutions with polyio nic linear macromolecules. including mucins. The contribution of individual proteins to overall viscosity is small, but combinations including lysozym e show tear-like characteristics. and map indicate that proteins whose conc entration is relatively independent of tear flow rate combine with other te ar components (possibly including mucins or lipids) to produce their full e ffect on tear viscosity. The surface tension results suggest that mucins ar e not of primary importance. Theories of tear film structure and performanc e need revision. (C) 1999 Academic Press.