Am. J. Respir. Cell Mol. Biol., Vol 11, No. 4, Oct 1994, 405-415.
Distribution of surfactant protein A in rat lung
IR Doyle, HA Barr and TE Nicholas
Department of Human Physiology, School of Medicine, Flinders University of South Australia, Adelaide.
Although surfactant protein A (SP-A) is an integral component of alveolar
surfactant, its relative abundance in lamellar bodies, regarded as the
intracellular storage organelles for surfactant, remains contentious. We
have previously shown that lamellar bodies, isolated from rat lung by
upward flotation on a sucrose gradient, can be subfractionated into
classic-appearing lamellar bodies (Lb-A) and a vesicular fraction (Lb-B),
which we have speculated may be a second release form of surfactant. In the
present study, we have used two- dimensional protein electrophoresis and
immunochemical analysis to clarify the origin and the composition of these
two subcellular fractions. In addition, we have examined the hypothesis
that the secretion of SP-A and surfactant phospholipids occurs by
independent pathways by examining the distribution of SP-A, total protein,
and disaturated phospholipids (DSP) in the tubular myelin-rich (Alv-1) and
tubular myelin-poor (Alv-2) fractions separated from lavaged material and
in Lb-A and Lb-B isolated from both lung homogenate and purified alveolar
type II cells. Our findings indicate that Lb-B is derived from type II
cells, although they do not indicate whether it is a secretory form of
surfactant, a reuptake vesicle, or a mixture of both. We found that the
lung has a large tissue pool of immunoreactive SP-A. The %SP- A/DSP of
total lamellar bodies isolated from type II cells was 0.96 +/- 0.1 (mean
+/- SE), intermediate between that in Lb-A (1.67 +/- 0.13) and in Lb-B
(0.65 +/- 0.04). In contrast, the %SP-A/DSP was 11.16 +/- 0.84 in whole
lung homogenate and 13.14 +/- 1.71 in whole type II cells. In the alveolar
compartment, the %SP-A/DSP was 17.38 +/- 3.40 in Alv-1, 6.34 +/- 0.31 in
Alv-2, and 10.49 +/- 1.43 in macrophages, values an order of magnitude
greater than found with the lamellar bodies. Our results indicate that only
a relatively small portion of alveolar SP-A is derived from lamellar
bodies, and we suggest that secretion of SP-A and DSP occurs via
independent pathways.
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Copyright © 1994 American Thoracic Society.
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