Changes in extracellular matrix and tissue viscoelasticity in bleomycin-induced lung fibrosis - Temporal aspects

Bleomycin-induced lung fibrosis results in changes in tissue mechanical pro perties due to alterations in the extracellular matrix (ECM). How oscillato ry mechanics and changes in the matrix evolve over time has not been addres sed. Sprague-Dawley rats were instilled with bleomycin sulfate (BM) (1.5 U) intratracheally; control animals (C) received saline. At 7, 14 and 28 d af ter BM, parenchymal strips (7 x 2 x 2 mm) were obtained and strips suspende d in a Krebs-filled organ bath. One end of the strip was attached to a forc e (F) transducer and the other to a lever arm that effected sinusoidal leng th (L) oscillations. Strips were oscillated at varying amplitudes (1, 3, an d 10% of resting L) and frequencies (f = 0.3, 1, 3, and 10 Hz) at an operat ing stress of 2 kPa. Resistance (R) and elastance (E) were estimated by fit ting changes in F and L to the equation of motion. Hysteresivity (eta) was calculated as eta = (R/E) 2 pi f. Strips were then fixed for morphological study of collagen, elastic fibers, and the small proteoglycans (PGs), bigly can and fibromodulin (FM). R and E were significantly greater and eta signi ficantly less in BM versus C strips (p < 0.001). The increase in R and E pe aked at 14 d after BM; the decrement in eta was maximal at Day 7. Biglycan was increased in BM lung strips at all time points, FM and elastic fibers w ere increased at 14 and 28 d, and collagen was increased at 28 d only. Henc e, changes in mechanics were maximal before collagen content had increased. In addition, we demonstrated a significant correlation between biglycan an d all mechanical parameters. These data suggest that changes in PGs may be critical in determining changes in lung tissue viscoelastic behavior in thi s fibrosis model.