Maturational changes in extracellular matrix and lung tissue mechanics.

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; approximately 3 wk), and adult (A; approximately 8 wk). Strips were suspended in an organ bath, and resistance (R), elastance (E), and hysteresivity (eta) 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 (L(V)/Pr(alv)), and the arithmetic mean thickness of alveolar septae (T(a)) were calculated by morphometry. Tissue was also stained for alpha-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 eta was significantly smaller than in Y or A strips. Changes in these parameters with ACh were greater in B strips. T(a), %ASMA, and HA were greatest in B strips, whereas %Col and L(V)/Pr(alv) were least. There were significant positive correlations between R and E vs. T(a) and between percent change in R and eta post-ACh vs. T(a) and vs. %ASMA, and significant negative correlations between R and E vs. %Col and vs. L(V)/Pr(alv) 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.