Surfactant - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 788

of tubules, is formed from secreted lamellar bodies. 9 Tubular myelin is a precursor

of the interfacial film.
Phase 1 is characterized by unpacking of lamellar bodies, their transformation
into tubular myelin, and the transfer of molecules toward the interface. Phase 1 is
affected by the local physicochemical conditions (hydration and clcium levels, pH,
etc.).

13.2.2 Alveolar Liquid Film Homeostasis

Alveolar epithelium (99% of the lung internal surface area) produces a liquid film
and maintains its homeostasis for effective gas exchange. The fluid balance results
from osmotic gradients created by active solute transport. In lungs, the alveolar
epithelium maintains air spaces free of excess liquid via Na + influx through highly
selective, apical epithelial Na + channels in response to an electrochemical gradient
created by basolateral Na + -K + ATPases. The resulting osmotic gradient leads to
fluid reabsorption from alveolar to interstitial spaces.

13.2.2.1 Ion Carriers of Type-1 Pneumocyte

Type-1 and -2 alveolar epithelial cells, or pneumocyte-1 and -2, that cover the
alveolar wall possess many types of ion channels. Alveolar epithelial type-1
cells (length m) have epithelial sodium, cyclic nucleotide-gated, and

potassium channels, as well as cAMP-stimulated cystic fibrosis transmembrane
conductance regulators [ 1604 ]. Potassium channels prevent cell depolarization
during Cl

∼ 50-100

efflux and Na + influx. Chloride uptake across CFTR contributes to fluid
absorption. In addition, pneumocyte-1 expresses aquaporin-5.
Pneumocyte-1 has the highest known osmotic water permeability of any mam-
malian cell type. The function of ENaC channel can be modulated by catechola-
mines. Alveolar type-1 cells indeed possess dopamine type-1 and -2 receptors that
can raise ENaC activity [ 1604 ]. Moreover, they also contain

−

γ -aminobutyric acid

receptor, a ligand-gated chloride channel.
Both type-1 and -2 pneumocytes have subunit A1 of Na + -K + ATPase, but only
type-1 pneumocyte possesses subunit A2 [ 1604 ]. Using this difference with associ-
ated distinct antagonist sensitivity, it can be demonstrated that type-1 pneumocyte
is responsible for at least 60% of the total alveolar fluid clearance in rats.

9 Surfactant components are packed by type-2 pneumocytes in lamellar bodies, although certain
components such as SPa can also be secreted separately from lamellar bodies.