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Meakins-Christie Laboratories, Royal Victoria Hospital, McGill University, Montreal, Quebec, Canada H2X 2P2
The viscoelastic properties of the pulmonary parenchyma change
rapidly postparturition. We compared changes in mechanical properties
with changes in tissue composition of rat lung parenchymal strips in
three groups of Sprague-Dawley rats: baby (B; 10-14 days), young
(Y; ~3 wk), and adult (A; ~8 wk). Strips were suspended in an organ
bath, and resistance (R), elastance (E), and hysteresivity (
) were
calculated during sinusoidal oscillations before and after the addition
of acetylcholine (ACh) (10
3 M). Strips were then fixed in
formalin, and sections were stained with hematoxylin and eosin,
Verhoff's elastic stain, or Van Gieson's picric acid-fuchsin stain
for collagen. The volume proportion of collagen (%Col), the length
density of elastic fibers
(LV/Pralv), and the arithmetic mean
thickness of alveolar septae (Ta) were calculated by
morphometry. Tissue was also stained for
-smooth muscle actin
(ASMA), and the volume proportion of ASMA (%ASMA) was calculated.
Hyaluronic acid (HA) was quantitated by radioimmunoassay in separate
strips. R and E in B strips were significantly higher, whereas
was
significantly smaller than in Y or A strips. Changes in these
parameters with ACh were greater in B strips. Ta, %ASMA, and HA were greatest in B strips, whereas %Col and
LV/Pralv were least. There were
significant positive correlations between R and E vs. Ta
and between percent change in R and
post-ACh vs. Ta and
vs. %ASMA, and significant negative correlations between R and E vs.
%Col and vs. LV/Pralv and percent
increase in all three mechanical parameters post-ACh vs. %Col. These
data suggest that the relatively high stiffness, R, and contractile
responsiveness of parenchymal tissues observed in newborns are not
directly attributable to the amount of collagen and elastic fibers in
the tissue, but rather they are related to the thickened alveolar wall
and the relatively greater percent of contractile cells.
viscoelasticity; collagen; elastic fibers; hyaluronic acid; contractile cells
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