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1 Laboratory of Respiration Physiology, Carlos Chagas Filho Biophysics Institute, Federal University of Rio de Janeiro, Ilha do Fundão, 21949 - 900 Rio de Janeiro; and 2 Department of Pathology, University of São Paulo, 01246 - 000 São Paulo, Brazil
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The dynamic mechanical properties of lung tissue and its contents of collagen and elastic fibers were studied in strips prepared from mice instilled intratracheally with saline (C) or silica [15 (S15) and 30 days (S30) after instillation]. Resistance, elastance, and hysteresivity were studied during oscillations at different frequencies on S15 and S30. Elastance increased from C to silica groups but was similar between S15 and S30. Resistance was augmented from C to S15 and S30 and was greater in S30 than in S15 at higher frequencies. Hysteresivity was higher in S30 than in C and S15. Silica groups presented a greater amount of collagen than did C. Elastic fiber content increased progressively along time. This increment was related to the higher amount of oxytalan fibers at 15 and 30 days, whereas elaunin and fully developed elastic fibers were augmented only at 30 days. Silicosis led not only to pulmonary fibrosis but also to fibroelastosis, thus assigning a major role to the elastic system in the silicotic lung.
pulmonary fibrosis; tissue mechanics; mouse
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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SILICA IS A NATURALLY OCCURRING mineral oxide dust that exists as a crystal structure of silicon dioxide (SiO2) (24, 32). Once silica has entered the lungs, much of it cannot be removed, and the insult continues even after a single administration of the agent (29). Although silicosis has been studied intensely, little is known about the crucial cellular mechanisms that initiate and drive the process of inflammation and fibrogenesis (24). Mice have been extensively used in research on pulmonary disease, but little information is presently available concerning their pulmonary tissue mechanics (3, 26, 33, 34).
The mechanical properties of pulmonary parenchyma are major determinants of lung physiological function (1, 2, 8-10, 15). Elastic and collagen fibers are the main structural components of pulmonary connective tissue matrix, but their elastic properties are essentially different (20). They form a continuous network throughout the lung that provides the forces necessary for passive expiration. Because the extracellular matrix (ECM) is considerably altered in interstitial pulmonary fibrosis, knowledge of the composition and distribution of the ECM at different stages of the disease may offer further understanding to the pathogenesis of silicosis (27). An overall increase of collagen expression in silicosis has been well documented (18). However, the functional significance of increased elastin production in such disorder is unknown (17). Presently, little is known about the mechanical interactions between ECM elements and how they influence pulmonary mechanics, especially tissue hysteretic properties (35).
The present work examines mice tissue mechanics, independent of surface forces, and the connective tissue matrix, allowing the determination of possible correlation between matrix changes and tissue mechanics before and at two different occasions after silica exposure.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Twenty-three male BALB/c mice (25-30 g body wt) were lightly anesthetized with inhaled sevoflurane and randomly divided into two groups. In the control group (C, n = 11), the animals received 50 µl of sterile saline solution (0.9% NaCl) directly into the dissected trachea, through an insulin syringe, with a 25-gauge needle, at a 45° angle. In the silica group (S, n = 12), the animals underwent the same experimental procedure but with an intratracheal injection of 20 mg of silica crystals (SiO2, particle size: 80% between 1 and 5 µm; Sigma Chemical, St. Louis, MO) suspended in saline solution (total volume = 50 µl). After instillation, mice skin was sutured with a cotton line.
Measurements in both C and S animals were done 15 and 30 days after instillation. The C group was considered as one (n = 11, 6 studied after 15 days, and 5 after 30 days, no statistically significant difference was found between the 2 groups), and the S group was divided into animals studied 15 (S15, n = 6) and 30 days (S30, n = 6) after instillation.
Tissue preparation.
The animals were sedated with diazepam (1 mg ip) and anesthetized with
pentobarbital sodium (20 mg/kg ip), and a snugly fitting cannula
(jelco, 20 gauge) was introduced into the trachea. The anterior chest
wall was surgically removed, and the animals were mechanically
ventilated with 100% O2 at a frequency of 1.5 Hz for 20 min to degas the lung. The tracheal cannula was then clamped, and the
lungs were removed en bloc and rinsed in a modified Krebs-Henseleit (K-H) solution containing (in mM) 118.4 NaCl, 4.7 KCl, 1.2 K3PO4, 25 NaHCO3, 2.5 CaCl2 · H2O, 0.6 MgSO4 · H2O, and 11.1 glucose, at
pH = 7.40 and 6°C. A 3 × 3 × 10-mm strip of
subpleural parenchyma was cut from the periphery of each left lung.
Lung strips were weighed, and their unloaded resting lengths
(L0) were determined with a caliper. Lung strip
volume (vol) was measured by simple densitometry as vol = 
F/
, where F is the total change in force before and after
strip immersion in K-H solution, and is the mass density of the
tissue (1.06 g/cm3). The strips were kept in a
recirculating bath of iced K-H solution that was continuously bubbled
with a mixture of 95% O2 and 5% CO2.
Apparatus.
Parenchymal strips were suspended vertically in a K-H organ bath
maintained at 37°C and continuously bubbled with 95%
O2-5% CO2. Metal clips made of 0.5-mm-thick
music wire were glued to both ends of the tissue strip with
cyanoacrylate. One clip was attached to a force transducer (FT03, Grass
Instruments, Quincy, MA), whereas the other one was fastened to a
vertical rod. This fiberglass stick was connected to the cone of a
woofer, which was driven by the amplified sinusoidal signal of a
waveform generator (3312A function generator, Hewlett Packard,
Beaverton, OR). A sidearm of the rod was linked to a second force
transducer (FT03, Grass Instruments) by means of a silver spring of
known Young's modulus, thus allowing the measurement of displacement.
Length and force output signals were conditioned, fed through
eight-pole Bessel filters (902LPF frequency devices, Bessel, Haverhill,
MA), analog-to-digital converted (DT2801A, Data Translation, Marlboro, MA), and stored on a computer. All data were collected with the use of
LABDAT software (RHT-InfoData, Montreal, Quebec). The frequency response of the system was dynamically studied by using calibrated silver springs with different elastic Young's modulus. Neither amplitude dependence (<0.1% change in stiffness) nor phase changes with frequency were detected in the range of 0.01 to 14 Hz. The hysteresivity (
) of the system was independent of frequency and had
a value <0.003.
Preconditioning. Cross-sectional unstressed area (A0) of the strip was determined from volume and unstressed length, according to A0 = vol/L0. Basal force (FB) for a stress of 10 g/cm2 was calculated as FB (g) = 10 (g/cm2) · A0 (cm2) and adjusted by vertical displacement of the force transducer (22, 35). The displacement signal was then set to zero. Once FB and displacement signals were adjusted, the length between bindings (LB) was measured by means of a precision caliper. Instantaneous length during oscillation around LB was determined by adding the value of LB to the measured value of displacement at any time.
Each parenchymal strip was preconditioned by sinusoidally oscillating the tissue during 30 min (frequency = 1 Hz, amplitude large enough to reach a maximal stress of 20 g/cm2). Thereafter, the amplitude was adjusted to 5% L0, the strips were unloaded to the FB, and the oscillation was maintained for another 30 min or until a stable length-force loop was reached.Measurements of parenchymal mechanics.
To calculate tissue resistance (R), elastance (E), and ,
force-length curves were analyzed (14). Instantaneous
average cross-sectional area (Ai) was determined
as Ai = vol/Li
(cm2), where Li is instantaneous
length. Instantaneous stress (
i) was calculated by
dividing force (F; in g) by Ai, i.e.,
i = F/Ai.
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is the angular
frequency [ = 2
f (rad/s), where f is
frequency], and 
is the normalized strain or peak-to-peak change
in length divided by LB. Tissue dynamic E was
determined as
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i is the peak-to-peak change in force
and 
is the phase lag between force and displacement { = sin
1
[4 · H/( · i · )]}.
The , which is an empirically determined variable that quantifies
the dependence of dissipative processes on elastic processes
(5), was calculated as = tan.
Morphometric analysis.
At the end of the experiments, the organ bath was removed, and the
parenchymal strips were frozen at the tension maintained during the
experiment by rapid immersion in liquid nitrogen. Frozen strips were
fixed in Carnoy's solution (ethanol-chloroform-acetic acid, 70:20:10)
at 
70°C for 24 h. Solutions with progressively increasing
concentrations of ethanol at 20°C were then substituted for
Carnoy's until 100% ethanol was reached. The tissue was maintained at
20°C for 4 h, warmed to 4°C for 12 h, and then allowed
to reach and remain at room temperature for 2 h (25).
After fixation, the tissue was embedded in paraffin.
Four-micrometer-thick slices were obtained by means of a microtome.
They were stained with hematoxylin-eosin.
Statistical analysis.
The normality of the data (Kolmogorov-Smirnov test) and the homogeneity
of variances (Levene median test with Lilliefor's correction) were
tested. In all cases, both conditions were satisfied, and thus one-way
ANOVA for repeated measurements was used to determine the effect of
frequency on E, R, and in each group. One-way ANOVA was used to
compare mechanical (E, R, ) and morphological data among C, S15, and
S30. In both cases, if multiple comparisons were required,
Student-Newman-Keuls test was applied.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The effects of different frequencies on E, R, and are shown in
Fig. 1. R had a negative and E a positive
dependence on frequency in all groups, whereas remained unchanged
in C and S15 and increased in S30 at 0.3, 1, and 3 Hz.
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Values of R and E were higher in S groups than in C mice, independent
of the frequency studied (Fig. 1). Moreover, S30 showed higher R values
(in relation to C) at 1 and 3 Hz (+86 and +89%, respectively) than did
S15 (+40 and +41%, respectively). The was higher in S30 at 0.3 (+30%), 1 (+48%), and 3 Hz (+53%) than in S15 (+9, 0, and +1.7%, respectively).
Table 1 shows the results of the
morphometric analysis. All groups showed similar anatomic composition.
Total cell content and the amount of polymorphonuclear cells increased
progressively from C to S30, whereas mononuclear cells decreased
progressively from C to S30. S groups presented granulomatous nodules
(Fig. 2).
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The S groups had increased collagen fiber content, with no difference
found between S15 and S30 (Fig. 3). ES
content increased progressively from C to S30. This increment was
related to the higher amount of Oxy fibers at S15 and S30, whereas Ela
and FDEF increased only at S30 (Fig. 3).
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The correlations between mechanical and morphological data are shown in
Fig. 4. R values at 1 Hz were correlated
with total cell count, ES, and Oxy fiber contents (Fig. 4,
A-C, respectively). The values at 1 Hz
were correlated with total cell count, ES, Oxy, and Ela + FDEF
contents (Fig. 4, D-G, respectively). R and E were correlated with changes in the collagen fiber content (Fig. 4,
H and I, respectively).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Although human silicosis develops over years, animal studies have shown that even a single silica exposure can lead to pulmonary functional and morphological changes, providing an experimental model of silicosis (11, 16-18, 29, 30, 32). On the other hand, it is well established that, once the silica particles have entered the lungs, the insult is progressive (29). The histological findings of the present work are in accordance with these concepts.
The present data show progressive functional and histological changes
in a murine model of silicosis. The intratracheal injection of silica
particles produced changes in tissue mechanics that were characterized
by increased E and R. Additionally, at 30 days, there was also an
increment in and a further rise in R.
The present experiment demonstrated similar mechanical behavior along
frequencies, as reported in the literature for other species
(5). Nagase et al. (26), using alveolar
capsules to assess tissue mechanical properties, found a
positive-frequency dependence of in mice. However, their data were
obtained with the whole lung. The present study is the first analysis
of mice tissue mechanical properties by oscillation of lung parenchymal strips, where the influence of kinetics of surface-active molecule absorption-desorption to the surface film and of
recruitment-derecruitment is not present. As a result, a direct
analysis of the role of fiber-fiber networking within the connective
tissue matrix on tissue mechanical properties is ensured (5,
35).
The changes in tissue mechanics were accompanied by progressive polymorphonuclear infiltration in lung parenchyma and deposition of collagen and elastic fibers. The time course of the kinetics of silica-induced pulmonary inflammation and fibroelastosis indicates a continuous transition among C, S15, and S30 groups, which is in accordance with previous works (3, 29, 30). The cellularity changes are in agreement with the known pulmonary inflammatory response of this disease (16, 23, 34).
Our data showed that collagen content in lungs from silica-exposed mice was significantly higher than that of C animals at 15 days and remained at the same level at 30 days (Fig. 3). Reiser et al. (29) reported similar findings in rats studied until 12 mo after silica administration. Conversely, some authors verified a progressive increment in collagen content (3, 28). Callis et al. (3) assessed collagen deposition in six different mice strains and observed variability in response to silica among them but did not report the specific response of BALB/c mice along time. In another study, Ramos et al. (28) showed increased collagen content and biosynthesis, but there was great variability among rats in relation to collagenolytic activity, which could alter the individual final collagen content. In addition, a species difference may not allow comparison of their results to ours.
The present work also disclosed a progressive increase in the fiber content of the ES, which was related to a progressive increment in Oxy fiber content and to a higher amount of Ela and FDEF at 30 days (Fig. 3). Although there are a number of studies describing collagen changes after silica exposure, the ES is scantily dealt with (17). Hitherto, there has been no report on the behavior of each component of the ES in silicosis. The ES is composed of Oxy, Ela, and FDEF, defined according to increasing amounts of elastin and fibril orientation (21). The fact that there is a differential distribution of Oxy, Ela, and FDEF in tissues suggests that the difference in quality of elasticity shown by the slender Oxy fibers compared with FDEF adds versatility to the ES (21). The elastic properties of elastic fibers depend on their amorphous component, i.e., elastin, and the microfibrils are reported as unimportant in this respect (21, 31). The Oxy fibers, composed solely of microfibrils without elastin, do not elongate under mechanical stress. It has been postulated that these fibers act to prevent overstretching of the tissue. Ela fibers, which contain both microfibril bundles and a small content of amorphous material, are expected to display elastic properties that are intermediate between those of elastic and Oxy fibers (21). However, some authors demonstrated an important relationship between elastin and collagen in the stress-strain behavior of arteries, suggesting that an interweaving of collagen and elastic fibers permits the elastic fibers to sustain a higher stress (31).
Compared with surface film or contractile elements, little is known about the mechanisms that might govern the rheology of intraparenchymal connective tissues and fiber networks contained therein. Some authors favor the view that dissipation in connective tissues may originate at the microstructural level, whereas others observe that elasticity and hysteresis of the connective tissue network would appear to be more a property of the fiber matrix than of the material from which the fiber is constituted (5). In such systems, the dissipation may be governed by contact phenomena between stress-bearing connective tissue fibers, such as contact (coulomb) friction (5, 35). Additionally, energy dissipation could occur at the molecular level within elastin or collagen fibers, it could occur by shearing of the glycosaminoglycan ground substance between fibers, or it could occur at surfaces of direct fiber-fiber sliding contact (20).
Recently, Yuan et al. (35), measuring the dynamic
properties of viable and nonviable lung parenchymal strips, compared
the contributions of cellular elements and fiber network with the macroscopic mechanical properties. These authors found that tissue mechanics were dominated by the connective-tissue fiber network, whereas interstitial cells played a less significant role
(35). In the present model of silicosis, R and showed
a positive correlation with the content of polymorpho- and mononuclear
cells (Fig. 4, A and D, respectively). Our study
does not allow the distinction between the effects of inflammation or
the inflammatory cells themselves on R and .
In our work, tissue E changes were correlated with the collagen system content (Fig. 4I). Yuan et al. (36) reported that collagen fibers contribute to tissue elasticity during normal breathing, which is in accordance with our findings but contradicts the accepted concept that collagen limits lung expansion at high volumes. Yuan et al. also found that the ES contributes to tissue elasticity. We found no correlation between E and the ES content when all points were considered. However, it should be emphasized that both collagen content and E increased from C to S15 and remained unaltered thereafter (Figs. 1 and 3), whereas the components of the ES increased progressively from C to S30 (Fig. 3). Hence, we decided to correlate E with elastic elements, taking two points in time only, i.e., C and S30, and found significant correlations between E vs. ES (P = 0.017, r = 0.66) and E vs. Oxy (P < 0.001, r = 0.80). At an earlier time (S15), only Oxy were correlated with E (P = 0.006, r = 0.73). It can be concluded that Oxy fibers play a role in determining E both at S15 and S30, whereas elastin comes into the picture later on during the remodeling of lung parenchyma in our experimental model of silicosis.
On the other hand, R was positively correlated with the amount of the
ES and Oxy fibers and with the collagen content (Fig. 4, B,
C, and H, respectively). Hence, tissue R seems to
be dependent on the ES as well as on the collagenous system. The higher
increase of R in S30 was accompanied by a greater content of ES fibers, whereas the collagenous system was similar between S15 and S30. These
data suggest a new role for the ES on the resistive component of tissue
mechanics. Unfortunately, the model used does not allow for an
individualized interpretation of viscoelastic component contribution.
The increased only on S30, which was mainly correlated with the
increased number of Ela and FDEF (Fig. 4G), but also with ES
and Oxy (Fig. 4, E and F, respectively). Moretto
et al. (22) studied the contribution of the connective
tissue matrix in determining tissue hysteretic properties using strips
exposed to elastase. They found no changes in as a result of
elastase exposure, despite significant changes in the other mechanical parameters, suggesting that does not depend on the amount of tissue
participating in expansion. However, the model used by these authors
leads to a progressive increase in L0 and to a
decrease in peak force. Additionally, they did not examine the role of the different types of elastic fibers. In the present work, the amount
of each connective tissue component changed according to the evolution
of the disease and consequent parenchymal remodeling.
In conclusion, silica exposure leads to increased tissue R, E and ,
which were correlated with tissue inflammation and remodeling, thus
affecting the coupling between elastic and dissipative processes within
the tissue. The data indicate that silica instillation did not lead to
pulmonary fibrous changes alone; instead, a well-defined fibroelastosis
was found, which assigns a major role to the ES in the silicotic lung.
Finally, our results disclose the observation that the relative amounts
of different fibers of the ECM vary disproportionately with the
duration of the disease, which may result in a varying mechanical
profile that should be interpreted cautiously.
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ACKNOWLEDGEMENTS |
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We are grateful to Antônio Carlos de Souza Quaresma for skillful technical assistance.
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FOOTNOTES |
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This study was supported by Programa de Núcleos de Excelência-Ministério de Ciência e Tecnologia, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro, Financiadora de Estudos e Projetos, Fundação Universitária José Bonifácio, and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior.
Address for reprint requests and other correspondence: W. A. Zin, Universidade Federal do Rio de Janeiro, Centro de Ciências da Saúde, Instituto de Biofísica Carlos Chagas Filho, Ilha do Fundão, 21949-900 Rio de Janeiro, RJ, Brazil (E-mail: wazin{at}biof.ufrj.br).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 6 October 2000; accepted in final form 25 October 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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), S15
(
), and S30 (
) groups (5 strips/group).
Resistance (R) values correlated with total cell count (A),
elastic fiber system (ES; B), and Oxy fiber content
(C). Hysteresivity (