Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 746

The mucus layer is needed for particle transport. Cilia beat depends mostly on
stimulation. The epithelial lining fluid is more stable than the overlying mucus
layer, the latter being replaced every 10 to 20 mn. Each layer has a corresponding
glandular apparatus: mucoid and serous secretions for the superficial and deep layer,
respectively. Cilia freely beat in deep sol layer, but their tips contact the overlying
gel layer. The mucus layer entraps all deposited particles that are transported by
coordinated ciliary movements. The superficial gel layer and deep fluid layer are
separated by surfactant.
Furthermore, airway surface liquid can be decomposed into 3 compartments
when the glycocalyx coating of the apical cell surface is added to the epithelial lining
fluid and the overlying mucus layer. Carbohydrate-rich glycocalyx contains intrinsic
glycolipids, glycoproteins, and proteoglycans as well as adsorbed glycoproteins and
proteoglycans.
The height of the airway surface liquid layer varies from about 3

m over goblet

or non-ciliated brush cells to 7 m over ciliated cells (layer height is only estimated

due to measurement disturbance) [ 1514 ].
The lubricating layer contains tethered mucins and other molecules such as cell
surface glycolipids, glycoproteins, and proteoglycans, whereas the mucus layer
consists of high-molecular-weight mucin dimers and trimers (length 0.5-20

m)

that interact with crosslinkers and innate defense proteins.

12.1.1 Control of the Quantity and Composition of the Airway

Surface Fluid

Mechanism of mucus transport thus depends on interaction between the near-
wall environment and overlying mucus, regulation of mucus hydration, and mucus
adhesion. Electrolyte transport through and between airway epithelial cells controls
the quantity and composition of the airway surface liquid.
Water transport across the apical plasma membrane of respiratory epithelial
cells, i.e., the hydration status of airway surface liquid, indeed relies on ion fluxes,
mainly chloride ion export through both cystic fibrosis transmembrane conductance
regulator (CFTR) and calcium-activated chloride channels (CaCC) as well as
sodium import through epithelial Na + channel (ENaC; Fig. 12.1 ).
Water flux across the wetted surface of the respiratory epithelium is controlled
not only by motions of Cl − across the apical membrane, but also K + flux across the
basolateral membrane. Both Cl − and K + conductances of airway epithelial cells are
influenced by calcium ions. Conversely, features of the extracellular fluid play a role
in calcium signaling [ 1515 ].
Two parallel routes — the trans- and paracellular paths — are used for the
transepithelial electrolyte transport that determine the quantity and composition
of the airway surface liquid and gel. The transcellular route depends on distinct
populations of active and passive ion channels, transporters, and pumps in apical
and basolateral membranes. Two main transcellular mechanisms of active ion
transport comprise ENaC-dependent Na + absorption and CFTR-dependent anion