Role of chloride channels in regulating the volume of acinar cells of the rabbit superior lacrimal gland.

PURPOSE: To characterize the outward chloride currents (Cl(OR)) in single acinar cells isolated from the rabbit superior lacrimal gland (RSLG) to investigate the hypothesis that Cl(OR) may have a role in regulating the volume of RSLG acini. METHODS: Cl(OR) was characterized by using patch-clamp electrophysiology. Confocal microscopy was used to measure intracellular calcium concentration ([Ca(2+)](i)) and cell volume. Cell volume was altered by superfusing the cells with a hyposmotic solution. RESULTS: The Cl(OR) current contributed 33% of total membrane conductance. With normal osmotic conditions, the Cl(OR) current was activated by [Ca(2+)](i) with an EC(50) of 10(-8) M. A decrease in intracellular pH from 7.4 to 6.8 totally inhibited Cl(OR) current activity. Continuous superfusion of hyposmotic solution caused (1) an increase in cell volume that peaked within 4 minutes and gradually returned to baseline levels after 12 minutes, (2) an increase in [Ca(2+)](i) that peaked between 6 and 8 minutes and gradually returned to baseline levels after 15 minutes, and (3) an increase the Cl(OR) current that peaked within 6 minutes after commencement of perfusion and quickly returned to baseline levels. CONCLUSIONS: The Cl(OR) current appears to be triggered by an increase in cell volume and then deactivates within the period of raised [Ca(2+)](i) during hyposmotic stress, suggesting that Cl(OR) may be an initiating event for volume homeostasis. This effect would be important during RSLG tear secretion, which usually involves cell volume changes and is accompanied by intracellular pH changes in the presence of the raised [Ca(2+)](i) to support secretion.

Implementation of evidence-based practice in optometry.

Evidence-based practice in health care requires that treatment decisions be supported by high-quality clinical evidence, preferably involving well-designed large-scale clinical trials of the various treatment options. How does this relate to everyday primary-care optometry? This article provides a background to the emergence of the 'evidence-based' movement and investigates areas where the efficacy of clinical optometry might be improved by an evidence-based approach. It is argued that evidence-based practice is appropriate for all aspects of optometry but it may be most useful in the selection of treatments with topical therapeutic agents and in optometry's growing public health role in the detection and management of sight-threatening disease.

The surface activity of purified ocular mucin at the air-liquid interface and interactions with meibomian lipids.

PURPOSE: Ocular mucins are thought to contribute to the stability of the tear film by reducing surface tension. The purpose of this study was to compare the effect of different mucins and hyaluronic acid (HA) alone and mixed with meibomian lipids on the surface pressure at an air-liquid interface. METHODS: A Langmuir trough and Wilhelmy balance were used to measure and compare the surface activity of bovine submaxillary gland mucin (BSM), purified BSM, purified bovine ocular mucin and HA, and mixtures of these with meibomian lipids, phosphatidylcholine, and phosphatidylglycerol. Their appearance at the surface of an air-buffer interface was examined using epifluorescence microscopy. RESULTS: Purified ocular mucin had no surface activity even at concentrations that were 100 times more than normally occur in tears. By contrast, commercial BSM caused changes to surface pressure that were concentration dependent. The surface pressure-area profiles showed surface activity with maximum surface pressures of 12.3-22.5 mN/m depending on the concentration. Purified BSM showed no surface activity at low concentrations, whereas higher concentrations reached a maximum surface pressure of 25 mN/m. HA showed no surface activity, at low or high concentrations. Epifluorescence showed that the mucins were located at the air-buffer interface and changed the appearance of lipid films. CONCLUSION: Purified bovine ocular mucin and HA have no surface activity. However, despite having no surface activity in their own right, ocular mucins are likely to be present at the surface of the tear film, where they cause an increase in surface pressure by causing a compression of the lipids (a reorganization of the lipids) and alter the viscoelastic properties at the surface.

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure.

PURPOSE: Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears. METHODS: A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears. RESULTS: All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pieq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pieq of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pieq was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pieq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred. CONCLUSION: Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

The effects of novel amphipathic block copolymers on stabilization of the rat tear film.

PURPOSE: To determine whether various novel amphipathic polymers could be used to stabilize the tear film of the rat. The rheologic properties of these polymers were examined to investigate whether particular structural or physical characteristics improve the stability of the tear film. METHODS: Amphipathic polymers or particular phospholipids were mixed with a test solution of tears and saline and applied to the clean, dry corneal surface of a rat. The specular reflection of the tear film was observed at high magnification and recorded. For each of the polymers or lipids, the effects on surface regularity and tear break-up time were compared. After the experiments, histologic sections of the tested eyes were prepared and examined for acute cytotoxic effects on the cornea and ocular conjunctiva. RESULTS: Tear film break-up time was markedly affected by differences in polymer structure. Copolymers consisting of separate hydrophobic and hydrophilic regions appeared to be the best stabilizers. No acute cytotoxic effects were observed in histologic sections of corneas to which the polymers had been applied. CONCLUSIONS: Amphipathic polymers can be designed to increase tear film stability. Increased tear film stability occurred more readily with copolymers, possibly through their interaction with both lipid and aqueous tear components.

A survey of pediatric visual assessment by optometrists in New South Wales.

BACKGROUND: The importance of early vision care in the prevention and treatment of amblyopia is clear from an abundance of literature on visual development. Optometrists possess the necessary skills to detect and manage amblyogenic factors and thus have an important role to play in paediatric visual assessment and management. The present study investigates the role that optometrists in New South Wales are taking in paediatric vision care and the methods and strategies used in this role. METHODS: Questionnaires were sent to 400 optometrists practising in New South Wales; 179 completed questionnaires were returned. RESULTS AND DISCUSSION: Our findings reveal a number of associations between practice characteristics (such as location) and the management of paediatric patients. In general, practitioners in rural locations, in full-time practice or self-employed were more likely to treat patients and less likely to refer them elsewhere than those in city locations, in part-time practice or employed by others. In addition, our findings suggest the possibility of a changing pattern of collaboration between optometrists and ophthalmologists in the management of paediatric patients.

The effect of N-acetylcysteine on the microscopic fluid dynamics of rat tears.

BACKGROUND: The classical view of the tear film is of a 10-micron film of aqueous tears, sandwiched between thin layers of lipid and mucus. This has been challenged recently by the revelation that the tear film may be considerably thicker than 10 microns and that dissolved mucus and protein may play a much more important role than simply promoting tear adherence. In particular, the primary role of mucus may be to form a structured aqueous gel that adheres closely to the corneal surface and evens out its irregularities, thus providing a high-quality optical surface. METHODS: We have used the robust tear film of the rat eye as an animal model to investigate the contribution of mucus and low-molecular-weight (LMW) proteins to tear film structure. The ocular surface was first exposed to saline, which washed away the tear film. Single drops of a tear/saline mixture, treated with various concentrations of the thiol-reducing agent N-acetylcysteine (NAC), were placed on the ocular surface and the resulting fluid behaviour was recorded with video-microscopy. RESULTS: At five per cent concentration, NAC appeared to degrade the gap-filling and anti-evaporative qualities of the tears, features that give the rat tear film its robust characteristics. Lower concentrations had no significant effect. DISCUSSION: In a previous publication, we showed that five per cent NAC alters the profile of LMW proteins in rat tears. The present observations suggest that the robust wetting properties of rat tears depend critically on their mucus and/or LMW protein content and possibly are related to the formation of an aqueous/mucous gel.