It is proposed to study the chloride conductance pathway in exorbital lacrimal gland apical plasma membranes. This project will combine methods for purifying plasma membranes and for measuring transport in isolated membrane vesicles. It represents a new approach to studying the transport processes involved in formation of the aqueous component of the tear film. The tear film is necessary for the quality of the visual image and for the protection of the ocular surface. Dry eye states are common vision-threatening disorders, and deficiencies of the aqueous component, which is produced by the lacrimal glands, have been implicated in a large proportion of cases. Since very little is known about the mechanisms of formation of the aqueous component, it is not yet feasible to attempt to restore normal lacrimal function. Consequently, current dry eye treatments are palliative replacements with artificial tears. General models predict that exocrine water secretion is an osmotic phenomenon driven by an active transcellular flux of chloride ions. A chloride conductance pathway in the acinar cell apical plasma membrane is believed to be an important rate-limiting step. Current methods for studying lacrimal secretion utilize intact tissue preparations, in which it is impossible to measure transport properties of the apical membranes. Therefore, the predicted apical chloride conductance pathway has not yet been characterized, there is no information about how it might be controlled, and there is no way of knowing whether it might be defective in dry eye states. Centrifugation and polymer phase-partitioning methods are now available for isolating apical membranes from rat exorbital gland. The membranes are in the form of sealed vesicles, and they are already known to retain certain transport systems of the intact cell membrane. They will make it possible to use tracer uptake and spectrophotometric techniques to study apical chloride transport directly and to determine whether it is activated when fluid secretion has been stimulated by carbachol. The general approach has already been highly productive in studies of absorptive epithelia, and the transport methodologies are well-established. From this starting point, it will become possible to design detailed studies on ionic mechanisms driving lacrimal fluid secretion, on the control of these mechanisms, and on their possible deficiencies in dry eye states.