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OCR for page 83
4
Markers of Altered Structure or Function
WHOLE LUNG
The simplest methods for assessing al-
terations of lung structure are examina-
tion with the naked eye and whole-tissue
examination with a dissecting microscope.
Of course, access to relevant materials
in vivo is problematic.
Events on the pleural surface can be used
to establish that lung disease is present
(Spencer, 1977~. For example, pleural
fibrosis and fibrotic adhesions between
the visceral and parietal pleurae are clear
indicators of exposure to toxic gases or
asbestos or of multiple infections (Spen-
cer, 1977~. Those alterations can be seen
when the chest cavity is opened, and fi-
brotic pleural plaques can be seen im-
mediately or in wet tissues viewed with
dissecting microscopy.
. ~. .. ..
The mechanisms
that mecilate the pathogenesis of pleural
fibrogenesis are not at all clear (Bignon
et al., 1983~. It is conceivable that in-
flammatory cells provoked to migrate into
the pleural cavity produce factors that
mediate pleural inflammation and later
fibrosis. Thus, analysis of pleural fluids
and cells could yield an important marker
of impending disease, just as bronchoscopy
has provided a window on the cells and flu-
ids of the airways and parenchyma.
The whole lung can be sampled with vari
83
ous techniques to establish the burden
of inhaled particles. Lungs collected
at autopsy provide exceptionally useful
material, because anatomic regions can
be sampled and quantitative values derived
(Abraham, 1978; Churg and Wright, 1983~.
Determination of particle types and their
correlation with the presence of lung dis-
ease have been valuable in increasing the
understanding of etiology (Abraham,
1978~. Whether such studies will allow
for the development of markers of pulmonary
disease can be determined only when the
nature of the lung burden in occupational
and environmental settings is known.
_ _
. . . .
AIRWAYS
A variety of inhaled agents, including
oxidant gases and pathogenic microbes,
can cause alterations of the large and
small airways. The cells of the airways
are relatively accessible with bronchos-
copy, brushing, and biopsy and therefore
have great potential as markers of exposure
· -
anc injury.
In studying a nasal, tracheal, or bron-
chial biopsy, it is necessary to establish
normal values and appearances of such en-
tities as ciliary beat frequency, cell
size, physiologic ion concentration,
and structural features as determined by
OCR for page 84
84
light and electron microscopy. However,
the most useful markers are likely to be
those related to the basic mechanisms by
which airway epithelial cells respond to
exposure.
The tracheobronchial lining consists
of a pseudostratified epithelium that
contains a diverse population of cell
types. The ciliated cell is one of the major
cell types and is probably a nonprolifera-
tive, terminally differentiated cell.
The conciliated population consists of
various secretory cells (mucous, Clara,
and serous cells, depending on species)
and a nonsecretory cell (the basal cell).
The Clara, mucous, and basal cells can
undergo cell division. Evidence is emerg-
ing that the mucous and Clara cells can
differentiate into ciliated cells, but
the function of the basal cell is not yet
established. The cells in the tracheobron-
chial epithelium can undergo differentia-
tion during vitamin A deficiency and after
toxic or mechanical injury. Understanding
the mechanisms of differentiation could
aid in providing markers of exposure and
injury. The availability of biochemical
markers of the various differentiated
phenotypes is essential because it pro-
vides a direct indication of cellular dam-
age. Mucous glycoproteins are used as
markers for alterations of mucous cells;
specific histochemical staining tech-
niques, as well as biochemical analysis,
have been used to identify and characterize
secretory cell products (Rearick et al.,
1987~. For ciliated cells, the presence
of dynein appears to be a good chemical
marker of structural effects. A low-molec-
ular-weight protein identified in Clara
cells appears to be a Clara cell-specific
secretory product that can function as
a biochemical marker of altered function
(Patton et al., 1986~. A biochemical mark-
er specific for basal cells has yet to be
discovered.
Results of in viva and in vitro studies
indicate that differentiation of tracheo-
bronchial epithelial cells is a multistep
process (Jetten et al., 1986) and has sev-
eral characteristics in common with epi-
dermal differentiation. Like epidermal
cells, tracheobronchial epithelial cells
undergo cornification. Cornification
AL4RKERS IN PULMONARY TOXICOLOGY
involves the deposition of a layer of
cross-linked protein beneath the plasma
membrane. The extensive cross-linking
between proteins is catalyzed by the enzyme
transglutaminase. Biochemical and im-
munologic analyses have identified the
tracheal transglutaminase as type I (epi-
dermal) transglutaminase. Differentia-
tion of tracheobronchial cells is accom-
panied by an increase in the activity of
transglutaminase by a factor of 20-30
(Jetten and Shirley, 1986~. Immunohis-
tochemical staining of tracheas from vita-
min A-deficient hamsters with a monoclonal
antibody against type I transglutaminase
indicated that in vivo synthesis of this
enzyme is associated with differentiation
of tracheobronchial cells (Jetten and
Shirley, 1986~.
Squamous cell differentiation is accom-
panied by an increase in cholesterol sul-
fate (Rearick and Jetten, 1986~. The in-
crease appears to be due to an increase in
the enzyme cholesterol sulfotransferase.
The high correlation between the expres-
sion of that enzyme and increases in cor-
nification indicate that these chemical
changes play an important role in differen-
tiation. Such changes could be early in-
dicators of pathologic changes in easily
accessible airway lining cells.
.
Recently, a cDNA library-a library
of complementary DNA produced from an RNA
template by action of RNA-dependent DNA
polymerase (reverse transcriptase)-
was established from rabbit tracheal
epithelial cells that had undergone squa-
mous cell differentiation (Smite and
Jetten, in press). Two cDNAs that are
abundant in squamous cells were isolated:
SQ 10 and SQ 37, which identify RNAs of 1.0
and 1.25 kb, respectively. The two RNAs
appear to be squamous cell-specific and
can function as markers for the squamous
cell phenotype.
Intermediate filaments have been used
in various systems as markers for specific
cell types. It has been shown that Clara
cells express a keratin profile charac-
teristic of simple epithelium, whereas
basal cells express a keratin pattern char-
acteristic of stratified epithelium, such
as that consisting of epidermal keratino-
cytes. Squamous differentiation in vivo,
OCR for page 85
ALTERED STRUCTURE OR FUNCTION
as well as in vitro, is accompanied by qual-
itative and quantitative changes in kera-
tin expression (Huang et al., 1986~. Those
findings suggest that keratin expression
can be specific for particular cell types
of the tracheobronchial epithelium and
that changes in keratin expression can
indicate particular pathologic altera-
tions, such as metaplasia.
PARENCHYMA
It is in the alveoli that toxic particles
and gases exert a fibrogenic influence
on the lung interstitial cells that produce
connective tissue. The basic cellular
and biochemical mechanisms through which
inhaled materials cause lung fibrosis are
not well established, and it is difficult
to ascribe the function of markers to a
poorly understood pathologic process.
However, animal models of asbestos- and
silica-induced disease do offer some po-
tential for elucidation ot pathogenes~s
and thus provide markers of exposure, in-
jury, and disease progression. Detection
and analysis of inhaled particles at the
alveolar level have been discussed above.
The following discusses early cellular
responses and methods proposed to study
them.
Morphometric Markers of Injury from
Exposure to Inhaled Agents
Proliferation of epithelial cells,
fibroblasts, and macrophages can be meas-
ured in viva with light and electron mi-
croscopy techniques (Evans, 1982~. The
very earliest responses of the various
pulmonary cell types are likely to be pro-
liferative if injury has taken place. For
example, both oxidant gases (Evans, 1982)
and chrysotile asbestos fibers (Brody and
Overby, 1989) cause a rapid incorporation
of tritiated thymidine into nuclei of bron-
chiolar and alveolar cells. That reflects
a repair phase, and the degree and extent
of the incorporation can be used as a meas-
ure of lung injury. Inhaled chrysotile
asbestos and instilled crocidolite asbes-
tos cause proliferation of epithelial
cells, fibroblasts, and interstitial and
alveolar macrophages (Adamson and Bowden,
85
1986; Brody and Overby, 1989). It is reas-
onable to speculate that this early pro-
liferative event could serve as a marker
of exposure to a toxic gas or particle.
Morphometry is a sensitive quantitative
tool that can be of great value in evaluat-
ing changes in lung tissue caused by toxic
substances. Morphometry is the quantifi-
cation of three-dimensional structure.
Values are derived from an analysis of two-
dimensional profiles based on microscopic
and other imaging techniques (Weibel,
1979~. The advantages of using morphometry
to study lung structure include objective
measurement of changes in lung structure,
identification of subtle changes in tissue
structure caused by different agents or
by progressive exposure to the same agent,
and selective measurement of changes in
specific tissue compartments. The en-
hanced sensitivity of detecting a change
in structure by using a morphometric analy-
sis is particularly beneficial when tissue
changes due to an experimental treatment
are probable, but not obvious. An impor-
tant application of morphometry involves
studies in which subjective grading stand-
ards suggest that changes have taken
olace, but are not conclusive with respect
to the relative toxicity of drugs, chemi-
cals, or pollutants at specific exposures
or doses. Morphometry eliminates the sub-
jective bias that occurs with many grading
techniques used to measure structural
changes. It also reduces or eliminates
variations in grading between and within
observers.
The types of morphometric information
that can be obtained in a study of lung par-
enchyma depend on the resolution used.
Light microscopy provides adequate reso-
lution to determine alveolar surface area,
proportional volumes of air and tissue,
and total number of alveoli. Light micros-
copy is rapid, is accurate, and can be used
with large numbers of specimens. Use of
the mean linear intercept (MLI) makes it
possible to measure alveolar surface den-
sity. MLI measurements have been used to
study postnatal lung growth, the normal
adult human lung, and lungs with emphysema
(Barry and Crapo, 1985~. Electron micros-
copy is better for determining the volumes,
thicknesses, and surface densities of
OCR for page 86
86
specific alveolar compartments. Electron
microscopy is required for adequate reso-
lution of alveolar tissue into its compo-
nents, including type I and type II epi-
thelium, cellular and noncellular inter-
stitium, endothelium, and inflammatory
cells. Subcellular components-such as
the nucleus, cytoplasm, and cytoplasmic
organelles-in specific cell types can
be measured with electron microscopy.
Numbers of particles, cells, and subcel-
lular organelles in the alveolar region
can be determined morphometrically. The
total number of cells in the lung and the
distribution of cells among the various
major types of alveolar cells have been
determined in humans and several species
of laboratory animals. The distribution
of cell types in the alveolar interstitium
has also been estimated (Barry and Crapo,
1985~.
The purpose and types of questions to
be answered will determine whether light
microscopy alone or with electron micros-
copy is needed. The advantages of light
microscopy include rapidity, accuracy,
and low cost. Its disadvantages include
the need to determine a correction factor
for tissue shrinkage due to paraffin em-
bedding of tissues (although other materi-
als that are now available, such as plas-
tic, minimize shrinkage) and restrictions
as to the types of alveolar structures that
can be easily and reliably measured-air-
space, tissue, and capillary volumes;
alveolar surface area; mean free path
lengths; and numbers of alveoli. Electron
microscopy has the advantages of providing
quantitative information on specific
alveolar compartments and requiring no
correction factors, because tissues can
be embedded in plastic that allows only
negligible shrinkage, as noted above.
Its disadvantages include higher costs
and requirements of substantially greater
labor and time.
Morphometry has been used most commonly
to study toxic agents that cause a given
kind of injury throughout the entire lung;
a random lung sample can be assumed to be
representative of the injury. Damage
caused by high concentrations of oxygen,
a gas that is highly diffusible in lung
tissue, is a good example of that type of
MARKERS IN PULMONARY TOXICOLOGY
injury. Some inhaled toxic agents selec-
tively damage the terminal bronchioles
and the adjacent alveoli. Those agents
include ozone and nitrogen dioxide, which
occur as air pollutants in relatively low
concentrations, but are more reactive than
oxygen. Particles can also selectively
injure the small airways in proximal al-
veolar tissue. The alveoli adjacent to
terminal bronchioles are the site of the
lesion of centrilobular emphysema, which
is at least in part caused by the particles
and gases in cigarette smoke.
Rigorous morphometric study of a select
region of the lung, such as the terminal
bronchioles or the adjacent alveoli, re-
quires sampling techniques that are de-
signed to assess structures that are local-
ized to specific regions of the lung. Com-
mon morphometric formulas and basic sam-
pling procedures can be applied, but the
methods of tissue selection and data anal-
yses must be adapted. Morphometric studies
of the effects of toxic substances on ter-
minal bronchioles and adjacent alveoli
have been few, because of the difficulties
involved in dealing with samples from spe-
cific regions. The appearance of a few
studies in recent years directed at the
terminal bronchioles or the proximal al-
veoli indicates increasing awareness of
this site of toxic injury and the develop-
ment of the necessary morphometric tech-
niques (Barry and Crapo, 1985; Chang et
al., 1988~.
PULMONARY VASCULATURE
To assess the development of structural
markers of pulmonary vascular change, one
must understand the normal architecture
of the arterial and venous circulation.
Within the pulmonary circulation are four
structural types of pulmonary artery in
a progression from hilum to periphery-
elastic, muscular, partially muscular,
and nonmuscular (Reid, 1979; Meyrick and
Reid, 1983~. Elastic arteries, by defini-
tion, contain more than five elastic lam-
inae, including the internal and external
elastic laminae; muscular arteries con-
tain two to five elastic laminae, partially
muscular arteries have muscle in only part
of the wall; and nonmuscular arteries have
OCR for page 87
ALTERED STRUCTURE OR FUNCTION
no muscle in the wall and are structurally
similar to alveolar capillaries (Meyrick
and Reid, 1979a). The four types can be
related to external diameter: arteries
greater than 2,000 fig in external diameter
are elastic, those from 150 to 2,000 Am are
muscular, and those less than 150 ,um are
muscular, partially muscular, or
nonmuscular.
In addition to the conventional arteries
that run and divide with the airways, there
is a population of supernumerary arteries
(short arterial branches) in the lung that
do not (Elliott and Reid, 1965~. The super-
numerary arteries from the axial pathway
are more numerous than the conventional
arteries and carry approximately 30% of
the blood volume. It is likely, but not
certain, that the supernumerary and con-
ventional arteries respond similarly to
· .
vascu ar Injury.
Although not described in detail here,
the pulmonary veins can be divided into
structural regions similar to those of
the arteries. In many cases, the pulmonary
veins are easily distinguished from pul-
monary arteries on the basis of their posi-
tion in the lung. For example, the walls
of the veins are less muscular than those
of arteries for a given diameter, and the
muscular coat of the bongs is not bounded
on its luminal aspect by an internal elas-
tic lamina (Hislop and Reid, 1973~.
Two potential outcomes of exposure of
the lung to toxic chemicals are progressive
restructuring of the lung vasculature and
chronic pulmonary hypertension. One of
the few well-documented examples of that
result in humans is the outbreak of chronic
pulmonary hypertension in Europe associ-
ated with ingestion of the drug, Aminorex.
The paucity of documented examples of chem-
ically induced, chronic vascular injury
in the lung other than that incident raises
a question as to the importance of the phe-
nomenon in humane. Chronicpulmonaryhy-
pertension is difficult to diagnose and
might be associated with other maladies,
so it could be more common that it seems
to be. Indeed, animal studies have reveal-
ed that chronic exposure to low doses of
several agents that injure endothelium
can cause progressive injury to the lung
vasculature and pulmonary hypertension.
87
Moreover, the position of the lung in the
circulation renders it the first vascular
bed to encounter toxic metabolites of chem-
icals that are produced by the liver and
enter into the venous circulation. The
pulmonary vasculature should therefore
be considered as a target for chemical
insult and chronic injury.
Comprehensive studies of the structure
of the pulmonary vessels have used autopsy
specimens of pulmonary arteries or veins
which were filled with barium gelatin be-
fore airway inflation (Reid, 1979; Meyrick
and Reid, 1983~. The technique allows easy
identification of small arteries and veins
and full distention of the vascular bed.
Full and reproducible distention of the
pulmonary circulation allows the use of
morphometric techniques; the severity
of changes can be assessed, and hyperten-
sive lungs can be compared with normal
lungs. The same techniques can be used in
lung biopsy tissue (Rabinovitch et al.,
1984), although the more subtle and earlier
changes of vascular injury might be
harder to identify in biopsy tissue.
This section outlines how the pulmonary
vasculature responds to changes in hemody-
namic behavior, to direct injury, and to
environmental changes, such as altera-
tions in ambient oxygen.
Results of pathologic studies of clini-
cal and experimental lung samples have
indicated some ways in which the pulmonary
vasculature can respond to injury and to
changesinpulmonaryhemodynamics. Patho-
logic markers or structural alterations
that occur in the pulmonary vessels are
usually associated with the development
of chronic pulmonary hypertension-e.".,
intimal hyperplasia, extension of muscle
into smaller and more peripheral arteries
than normal, increase in medial thickness
of normally muscular arteries, reduction
in peripheral arterial volume (seen either
as a reduction in number of arteries or as
a narrowing of intra-acinar arterial lu-
minal diameter), recanalization of block-
ed arteries, fibrinoid necrosis, dilata-
tion, and plexiform lesions (Wagenvoort
and Wagenvoort, 1977; Harris and Heath,
1986~. A single lung biopsy or autopsy
specimen might reveal the entire range
of morphologic markers of vascular injury
or only a few, depending on the stimulus
OCR for page 88
88
and the severity of the injury. For exam-
ple, the finding of plexiform lesions or
fibrinoid necrosis is associated with end-
stage pulmonary vascular disease (Heath
and Edwards, 1958; Harris and Heath,
1986~; this seemingly restricted response
could reflect the paucity of cell types
normally encountered in the walls of lung
vessels.
In Vivo Observations of Pulmonary
Vasculature
Disease of the pulmonary vasculature
can be examined in patients both by radiog-
raphy and by angiography. The radiograph-
ic appearance of edema marks pulmonary
endothelial injury and is dealt with ear-
lier in this report. Angiographic radi-
opaque mass marks thrombus formation and
vascular occlusion. Rate and abruptness
of narrowing and loss of vascular volume
can be assessed in pulmonary arterial
wedge angiograms; the latter markers have
been described in association with chronic
pulmonary hypertension secondary to con-
genital heart defects (Rabinovitch et al.,
1981).
Gross Examination of Pulmonary
- Vasculature
The lung is supplied by two vascular
beds: the bronchial, which supplies oxy-
genated blood to the connective tissue
around large arteries, veins, and airways;
and the pulmonary, which carries venous
blood to the lung capillaries for oxygena-
tion. The pulmonary circulation accounts
for more than 95% of the blood volume in
the lung. Arterial and venous sides of the
pulmonary circulation are easily iden-
tified on gross examination or cut lung
surfaces, because the arteries run cen-
trally in the acinus (terminal bronchioli
and its branches) and the veins at the edge
of the acinus. Gross examination of lung
slices also allows detection of thrombi,
areas of atelectasis, and emphysema, and
careful dissection along arterial and
venous pathways can provide evidence of
congenital defects in the pulmonary
vasculature.
MARKERS IN PULMONARY TOMCOLOGY
Light Microscopic Examination of
Pulmonary Arteries
Heath and Edwards (1958) suggested a
grading system for the progression of the
structural changes of chronic pulmonary
hypertension. Grade I represented medial
hypertrophy; Grade II, intimal hyper-
plasia; Grade III, luminal occlusion by
intimal hyperplasia; Grade IV, arterial
dilatation; Grade V, angiomatoid lesions;
Grade VI, fibrinoid necrosis. They con-
sidered Grades I-III reversible markers
of vascular injury. The grades are readily
recognizable in autopsy and biopsy tissue,
but only recently have structural markers
been correlated with hemodynamic behavior
of the lungs.
Rabinovitch and colleagues ( 1984) re-
fined the grading system of Heath and
Edwards, particularly the early rever-
sible changes, by applying morphometric
techniques to the lungs of patients with
congenital heart defects associated with
high flow. Additional structural markers
of chronic pulmonary hypertension were
identified with that system and graded
A, B or C. The quantitative techniques are
well described for structural changes in
distended and nondistended arteries in
autopsy and biopsy tissue (Hislop and
Reid, 1973; Reid, 1979; Rabinovitch et
al., 1984~. Grade A structural changes
(extensions of muscle into smaller and
more peripheral arteries than normal) are
found in patients with an increase in only
pulmonary blood flow; Grade B changes
(Grade A changes plus increased medial
thickness), when both pulmonary arterial
pressure and blood flow are increased;
and Grade C changes (Grade B changes plus
reduction in peripheral arterial volume),
when pulmonary vascular resistance is
increased.
Those correlations can be extended by
examining animal models of chronic pul-
monary hypertension: rats exposed to hy-
poxia and rats given Crotalaria spectabilis
seeds (which contain monocrotaline). The
same structural changes (Grades A, B. and
C) can be identified and correlated with
hemodynamic behavior (Meyrick and Reid,
1978, 1 979b; Rabinovitch et al., 1979;
Meyrick et al., 1980~. Such studies have
shown that the two models of pulmonary
OCR for page 89
ALTERED STRUCTURE OR FUNCTION
hypertension lead to the same structural
markers of pulmonary hypertension, but
at different rates (Meyrick and Reid,
1983~. The structural changes after hypox-
ia occur faster than those after adminis-
tration of Crotalaria. The severity of each
structural marker also differs between
the two models (Meyrick and Reid, 1983~.
For example, loss in peripheral arterial
volume after administration of Crotalaria
is more striking than that after hypoxia,
and muscle extension is more severe after
hypoxia. Pulmonary arterial pressure also
occurs faster after hypoxia. Correlation
of the structural changes with the hemody-
namic behavior in both models reveals that
the best structural correlate of increased
pulmonary arterial pressure is extension
of muscle into small arteries; for Crotalar-
ia administration, it is increased medial
thickness.
The data also suggest that the speed of
the changes can depend on the stimulus for
the development of chronic pulmonary
hypertension. With hypoxia, one of the
mechanisms involved is almost certainly
hypoxic vasoconstriction; with Crotalaria
administration, the changes are thought
to be secondary to endothelial damage,
and the role of vasoconstriction is less
certain. Additional evidence that en-
dothelial damage can contribute to the
development of pulmonary hypertension
was provided by Jones and colleagues (Jones
et al., 1984) who used rats exposed to
hyperopic conditions. Recent data on sheep
given repeated infusions of endotoxin
(Meyrick and Brigham, 1986) and sheep
subjected to continuous air embolization
(Meyrick et al.. 1987: Perkett et al.,
1988) suggest that inflammation of the
lung can be a trigger. The data suggest
that there are several stimuli of develop-
ment of chronic pulmonary hypertension
and that the structural markers of pul-
monary hypertension might not be identical
in each type of hypertension. Development
of chronic pulmonary hypertension is
likely to be seen in association warn
hypoxia, hyperopia, prolonged vasocon-
striction, chronic or repeated lung in-
flammation, and endothelial injury.
The variation in severity and appearance
of structural markers of chronic pulmonary
89
hypertension and the range of initiating
stimuli are borne out in the clinical set-
ting (Reid, 1979; Meyrick and Reid, 1983~.
For example, natives of high-altitude
environments and children with persistent
fetal circulation (pulmonary hypertension
following right-to-left blood shunting)
have marked muscle extension, and patients
with chronic bronchitis and emphysema,
cystic fibrosis, and primary pulmonary
hypertension have a less severe change.
Reduction in peripheral arterial volume
is striking in patients with primary pul-
monary hypertension, but blood volume is
within normal limits in cases of persistent
pulmonary hypertension. Plexiform
lesions are found in patients with primary
pulmonary hypertension and in older pa-
tients with congenital heart disease, but
have not been seen in the hypoxic forms of
chronic pulmonary hypertension.
Not only the pulmonary arterial circula-
tion is limited in its response to injury,
but also the veins. On the venous side of
the circulation, "arterialization" of
the veins occurs if pressure in the veins
is chronically increased. The structural
markers are a medial coat and an internal
elastic lamina in the venous walls. Those
changes are often accompanied by intimal
fibrosis (Harris and Heath, 1986~. An
increase in pressure also leads to the
development of the structural markers of
pulmonary hypertension on the arterial
side of the circulation.
Electron Microscopic Examination of
Pulmonary Arteries
Electron microscopy shows the major cell
types in the walls of the pulmonary cir-
culation to be endothelial cells, smooth
muscle cells and their precursors and fi-
broblasts. In experimental models of
chronic pulmonary hypertension, such as
the rats exposed to hypoxia (Meyrick and
Reid, 1978, 1 979b) or given Crotalaria
spectabilis (Meyrick et al., 1980), it has
been shown that the precursor smooth muscle
cells undergo hypertrophy and, at least
in hypoxic animals, cell division; that
accounts for the appearance of muscle
in the normally nonmuscular intra-acinar
pulmonary arteries (Meyrick and Reid,
OCR for page 90
go
1983~. Encroachment of the new muscle
cells and endothelial hypertrophy con-
tribute to the reduction in peripheral
arterial volume (Meyrick and Reid, 1983~.
Ultrastructural studies also show that
smooth muscle cell hypertrophy and altera-
tions in connective tissue synthesis and
accumulation contribute to the increased
medial thickness seen in preacinar arter-
ies (Meyrick and Reid, 1980~.
Future Directions
Few reports have suggested alterations
in bloodborne mediators in patients with
chronic pulmonary hypertension although
Geggel and associates have suggested that
measurements are possible. Increases in
ristocetin cofactor activity relative
to plasma von Willebrand factor have been
reported in patients with primary pulmon-
ary hypertension (Geggel et al., 1987~.
Similarly, patients with primary pulmon-
ary hypertension and monocrotaline-treat-
ed rats with increased pulmonary arterial
pressure have increased plasma copper
concentrations (Ganey and Roth, 1987~.
The origin of the latter change is unclear,
but such observations deserve further
attention with regard to development of
markers of chronic pulmonary vascular
changes. Advancements in this direction
will be of obvious importance to patients,
especially if changes can be detected early
in the development of the disease.
MARKERS IN PULMONARY TOXICOLOGY
Results of recent biochemical and molec-
ular studies of chronic pulmonary hyper-
tension have indicated that the develop-
ment of the structural changes might depend
on the stimulus. For example, in the mono-
crotaline model of pulmonary hypertension
in rats, an increase in elastase activity
in the preacinar arteries resulted in thin-
ning and fragmentation of the internal
elastic lamina of the thickened walls
(Todorovich et al., 1986~. In the hypoxic
neonatal cow, the increased thickness of
the walls of the preacinar arteries includ-
ed proliferation of adventitial fibro-
blasts invoked by a growth factor released
from the medial muscle cells (Mecham et
al., 1987~. In hypoxic rats, treatment
with a latherogen (beta-aminopropio-
nitrile) or with an inhibitor of collagen
production (cis-4-hydroxy-L-proline)
caused attenuation of the structural mark-
ers of pulmonary hypertension examined.
as well as partial protection against the
increase in pulmonary arterial pressure
(Kerr et al., 1984, 1987~. Thus, markers
of pulmonary vascular disease are likely
to be diverse and to depend on the initial
stimulus of hypertension. The introduc-
tion of such techniques as in situ hybridi-
zation is likely to advance our understand-
ing of these alterations at the cellular
and molecular level and to yield markers
useful in the early detection of chronic
pulmonary hypertension.
Representative terms from entire chapter:
chronic pulmonary
