AJP - Lung Cellular and Molecular Physiology, Vol 268, Issue 5 772-L780, Copyright © 1995 by American Physiological Society
Degradation of surfactant lipids and surfactant protein A by alveolar macrophages in vitro
J. R. Wright and D. C. Youmans
Cardiovascular Research Institute, University of California, San Francisco 94143, USA.
Pulmonary surfactant is synthesized and secreted into the airspaces by the
alveolar type II cell. After it is secreted, surfactant undergoes a series
of poorly understood transformations resulting in formation of a surface
tension-reducing surface at the air-liquid interface. The by-products of
the surface film and/or other products of surfactant metabolism are
eventually cleared from the alveolar space. Both the alveolar type II cell
and the macrophage are thought to be involved in surfactant clearance and
have been shown to internalize surfactant lipid in vitro. The goal of the
current investigation was to characterize further and to quantitate the
role of the macrophage in surfactant clearance by investigating the uptake
and metabolism of surfactant lipids and surfactant protein A (SP-A) by
macrophages in vitro. SP-A enhanced the uptake of lipids by macrophages in
a time-, temperature-, and concentration-dependent manner. In contrast,
neither of the collagen-like proteins SP-D or C1q enhanced the uptake.
Phosphatidylcholine was rapidly degraded by macrophages and the degradation
occurred both in the presence and absence of SP-A. In addition, macrophages
degrade SP-A by a process that is time- and temperature-dependent. These
results and calculations of uptake and degradation rates suggest that
macrophages may contribute significantly to the process of surfactant
clearance.
