HUMAN AIRWAY ION-TRANSPORT .1.

The proper homeostasis of the liquids that line the surfaces of the lung is vitally important for lung defense. During the past 20 years, data derived primarily from animal studies have indicated that ion transport by the pulmonary epithelium regulates the volume and the composition of these surface liquids. The role of pulmonary epithelial ion-transport systems in this activity can be viewed at two levels: (1) the requirement to regulate surface liquid volumes and composition relevant to the local environment, e.g., adjusting the height and composition of surface liquids to permit efficient function of the mucociliary clearance system in the airways; and (2) the requirement of the pulmonary epithelium to coordinate intrapulmonary surface liquid flow (i.e., so-called ''axial'' flow) between different pulmonary regions, e.g, alveolar and airway surfaces. Because the role of pulmonary active ion transport in experimental animal models has been reviewed elsewhere (1-5), and because of the rapidly increasing knowledge of human airway ion-transport mechanisms in health and disease, this review will focus on data derived from human subjects.