Functional
pulmonary
and structural adaptation of the yak
circulation to residence at high altitude
ANTHONY
G. DURMOWICZ,
STEPHEN
HOFMEISTER,
T. K. KADYRALIEV,
ALMAS
A. ALDASHEV,
AND KURT
R. STENMARK
Cardiovascular-Pulmonary
Research Laboratory and Department
of Pediatrics, University of Colorado
Health Sciences Center and Children’s Hospital, Denver, Colorado 80262; and the Kyrghyz Institute of
Cardiology, Bishkek 720361, Republic of Kyrghyzstan
ment of severe pulmonary hypertension and car pulmoDURMOWICZ,ANTHONY G., STEPHENHOFMEISTER,T. K.
KADYRALIEV, ALMAS A. ALDASHEV,ANDKURT R. STENMARK. nale (brisket disease) on exposure to a chronic hypoxic
Functional and structural adaptation
of the yak pulmonary
cirenvironment (4,19,22). Domestic cattle (Bos taurus) naculation to residence at high altitude. J. Appl. Physiol. 74(5):
tive to high altitudes and their offspring have a low inci2276-2285,
199X-The
high-altitude
(HA) native yak (Bos
dence of brisket disease (2%), whereas herds recently
grunniens)
has successfully adapted to chronic hypoxia (CH)
transported from sea level have incidences as high as
despite being in the same genus as domestic cows, which are
JO-5075, a finding strongly suggestive of genetic selection
known for their great hypoxic pulmonary
vasoconstrictor
reof resistant animals (6, 21).
sponses (HPVRs), muscular pulmonary
arteries, and developThe native high-altitude yak (Bos grunniens) has sucment of severe pulmonary hypertension
on exposure to CH. To
cessfully adapted over many generations to the chronic
determine possible mechanisms by which the pulmonary
circulation may adapt to CH, yak pulmonary
vascular reactivity to
hypoxia of high altitude despite belonging to the genus
both vasoconstrictor
and vasodilator
stimuli and yak pulmoBos in which the closely related domestic bovine
nary artery structure were assessed. Hypoxia caused a small
members are known for their large hypoxic pulmonary
but significant
HPVR, and norepinephrine
infusion caused a vasoconstrictor responses, muscular pulmonary arteries,
greater rise in pulmonary arterial pressure (Ppa) than did hypand the development of severe pulmonary hypertension
oxia. Acetylcholine,
an endothelium-dependent
vasodilator,
(brisket disease) when exposed to chronic hypoxia. The
had no effect on Ppa but lowered pulmonary
resistance (Rp) by
factor(s)
responsible for the remarkable ability of the yak
causing an increase in cardiac output. Sodium nitroprusside,
an
to
not
only
live but to work as a beast of burden at very
endothelium-independent
vasodilator, decreased both Ppa and
high altitudes is unknown. However, the maintenance of
Rp significantly.
Yak small pulmonary
arteries had a 4.1 t
0.1% medial t.hickness, with vessels 400 pm devoid of smooth
both low pulmonary arterial pressures and thin-walled
muscle. Yak pulmonary
artery endothelial
cells were much
pulmonary arteries appears to contribute. Previous studlonger, wider, and rounder in appearance than those of domesies have demonstrated that yaks in the London Zoo have
tic cows. Thus the yak has successfully adapted to HA condisimilar pulmonary pressures to those residing at high
tions by maintaining
both a blunted HPVR and thin-walled
altitude in Nepal (3) and that one adult female yak pospulmonary
vessels. Differences
in both endothelial
cell morsessedthin-walled pulmonary arteries (10). However, no
phology and response to acetylcholine
between the yak and
data exist on the assessment of pulmonary vascular reacthose reported in the domestic cow suggest the adaptation
to
tivity and structure in the same set of animals.
HA may include changes not only in the amount of pulmonary
Inherent differences in the pulmonary vacular smooth
vascular smooth muscle but in endothelial
cell function and
structure as well.
muscle itself, such as a blunted contractile response to
hypoxia or an absolute lack of smooth muscle in the pulmonary arteries, could account for the maintenance of
pulm onary hypertension;
chron ic hypoxia; Has grunniens; vascular reactivity; vasodilators
low pulmonary arterial pressures and thin-walled pulmonary arteries in high-altitude-adapted species. Alternatively, differences in the function of the pulmonary vascular endothelium, which plays a major role in the regulation of pulmonary vascular tone and pressure, could
THE MAINTENANCE
OF low pulmonary arterial pressures
at high altitude as an adaptation to chronic hypoxia oc- contribute to a species’ ability to adapt to a high-altitude
curs over many generations and appears to be genetically
environment. We hypothesized that adaptation to a
high-altitude environment results in significant changes
transmitted within populations. A slow evolutionary process is suggested by the relatively high pulmonary pres- in endothelial cell function. Thus, in an effort to examine
sures in human populations that have recently moved to the role of the pulmonary vascular endothelium in the
altitude (6) compared with populations that have lived at adaptatation of the yak’s pulmonary arteries to a chronic
altitude for thousands of years (5, 8, 13). Although the hypoxic environment, the present study was undertaken
mechanism(s) by which adaptation to chronic hypoxia
to assesspulmonary pressures, pulmonary vascular reacoccurs remains unknown, the fact that the mechanism is tivity, and pulmonary vascular structure in the native
high-altitude bovine species Bos grunniens. Pulmonary
genetically transmitted is best illustrated in cattle that
are either “susceptible” or “resistant” to the developvasoconstriction was measured by the use of both alveo2276
0161-7567/93
$2.00
Copyright
G 1993 the American
Physiological
Society
YAK PIJLMONARY
ADAPTATION
lar hypoxia and the a-agonist norepinephrine. Both endothelium-dependent and endothelium-independent vasodilators were infused in the pulmonary arteries under
baseline conditions and under conditions of increased
tone (hypoxia) to help assessthe relative role of the endothelium in determining vascular reactivity in the high-altitude yak. In addition, morphometric analysis of yak
pulmonary arteries was done to determine the thickness
of the muscular media of small pulmonary arteries, and
endothelial cell surface structure and size were assessed
in both the yak and domestic calf by scanning electron
microscopy.
MATERIALS
AND
METHODS
Animals. Three juvenile (5-7 mo old) active and
healthy appearing male yaks weighing -100 kg each
were used in this study. These animals were born and
lived in a semi-wild yak herd at 3,200 m altitude in the
Talas region of the Republic of Kyrghyzstan. The yaks
were transported to the Kyrghyz Institute of Cardiology
in the city of Bishkek (previously Frunze), Republic of
Kyrghyzstan (altitude 730 m) and were studied in Bishkek ~12 h after their arrival. The care and treatment of
the study animals were in full compliance with the
Kyrghyz and Soviet National Animal Care Committee
policies.
Hemodynamic studies. All physiological studies were
performed in unanesthetized animals in the left lateral
recumbent position. Although initially the animals had
to be physically restrained, after a period of acclimatization to their surroundings, they remained quiet when
held by the investigators. A dual-thermistor 7-Fr SwanGanz catheter (Nova Dual Therm, Berlin, NJ) and a separate pulmonary artery catheter for drug infusion (PE160, Clay Adams, Parsippany, NJ) were placed percutaneously via the external jugular vein by the Seldinger
technique with the use of local lidocaine anesthesia.
Correct placement was determined by pressure wave tracing. An aortic catheter (PE-200, Clay Adams, Parsippany, NJ) was introduced via the left saphenous artery
that had been surgically exposed after local lidocaine anesthesia. Pressures were measured with disposable transducers (Sorenson Transpac, North Chicago, IL) and continuously recorded on a two-channel recorder (model
402, Soltec, Sun Valley, CA) with zero pressure set 4-5
cm above floor level to approximate the level of the heart
with the animal in left lateral recumbancy. Cardiac output was measured by the thermodilution method by use
of the dual-thermistor pulmonary artery catheter with a
lo-ml injectate volume of normal saline and a T-110-D
cardiac output computer (Nova Medical Specialties,
Berlin, NJ). Measurements were made at least three
times in each animal under both basal and experimental
conditions. Total pulmonary and systemic arterial resistances were calculated by dividing mean pressure values
by cardiac output and were expressed as millimeters of
mercury per liter per minute.
The animals were exposed to hypoxic conditions by
blending oxygen and nitrogen in a loo-liter meteorological balloon to achieve the desired oxygen concentration
as determined by a Fyrite 0, concentration test kit (Bacharach, Pittsburgh, PA). Administration of the hypoxic
TO HIGH
2277
ALTITUDE
gas was accomplished with a custom-made cattle face
mask equipped with one-way valves to prevent rebreathing expired gases.Arterial blood gas determinations (Radiometer, Copenhagen, Denmark) were made under normoxie and hypoxic conditions to document pH and arterial PO, and Pco,.
Measurements of heart rate,
pulmonary and systemic arterial blood pressures, and
cardiac output were made in room air and while the animals were breathing a hypoxic gas mixture (fractional
concentration of inspired 0, = 0.11-0.13). The animals
breathed the hypoxic gas mixture continuously for -8O90 min, which was the length of time required to measure
and record pulmonary arterial pressures and cardiac
outputs both at baseline and during drug infusions. Approximately 5-7 min were allowed between each drug
dose for a steady state to be achieved.
Acetylcholine and sodium nitroprusside (Sigma Chemical, St. Louis, MO) were dissolved in 0.9% NaCl at concentrations of 300 pg/ml and were administered by continuous infusion by utilizing Harvard infusion pumps at
rates of 1, 3, 6, and 9 pg kg-l. min-’ in the pulmonary
artery during both room air and hypoxic conditions. Data
were collected after 5 min of drug infusion, at which time
hemodynamic measurements appeared stable. Drug
doses were administered serially without interruption
beginning with the smallest dose. Two animals received
the acetylcholine infusion first followed by nitroprusside,
and one animal received the nitroprusside infusion followed by acetylcholine. Sufficient time (lo-15 min) was
allowed between drugs for arterial pressures and heart
rates to return to predrug values.
Norepinephrine was administered undiluted by continuous infusion directly in the pulmonary artery after recovery from both acetylcholine and nitroprusside at a
rate of 20 pg. kg-‘. min-’ for a period of 3 min under
normoxic conditions, with data collected during the period of maximum norepinephrine effect.
Histology and morphological analysis. After completion
of the hemodynamic studies, the yaks were anticoagulated by administration of 10,000 U of intravenous porcine heparin, anesthetized with 1.2 g of intravenous pentobarbital sodium, and killed by exsanguination. The
heart and lungs were removed en bloc. The left caudal
lobe was dissected free and prepared for histological examination as reported previously (16). Briefly, the lung
lobe was perfusion fixed via the left caudal lobe bronchus
(25-30 cmH,O) and left lobar pulmonary artery (at the
measured in vivo mean pulmonary arterial pressure)
with 10% phosphate-buffered formaldehyde, pH 7.40.
After 20-24 h of fixation, the lobar pulmonary artery was
perfused and lobar bronchus was lavaged with 70% ethanol to remove the formaldehyde, and the lung was then
stored in fresh 70% ethanol at room temperature until
the time of histological processing. At least six tissue
blocks were taken randomly from the left caudal lobe of
each lung, embedded in paraffin, cut into 4-pm-thick sections, and stained with hematoxylin
and eosin,
trichrome, or Verhoeff van Gieson (VVG) stains by use of
standard histological techniques. The percent medial
thickness (medial wall thickness/external vessel diameter X 100) of 250 small pulmonary arteries (100-300 pm
in diameter) was calculated for each animal. The external vessel diameter was taken as the mean of two meal
2278
YAK
PULMONARY
ADAPTATION
surements made at right angles to each other, and medial
thickness
was estimated as the mean of four measurements around the circumference
of each vessel (17). The
number of complete elastic lamellae present in elastic
pulmonary
arteries 4,000 pm in diameter was counted.
Lung and conducting pulmonary
artery sections from an
age-matched Hereford calf that had lived at 3,000 m altitude in South Park, CO and suffered from brisket disease
were prepared in a similar fashion and served as a comparison altitude-sensitive
bovine species (see Figs. lB,
ZB, and 3B).
Right ventricular
mass was determined
as a criterion
for sustained
pulmonary
arterial hypertension
during
life. The heart was dissected free from the great vessels,
and the atria were removed. The weights of the heart,
right ventricular
free wall (RV), and left ventricle plus
intraventricular
septum (LV + S) were obtained after
careful removal of fat and cardiac valves. From these
data the RWLV + S and right ventricular-to-total
ventricular weight (RWT)
ratios were calculated.
Scanning electron microscopy. Yak and cow lungs to be
examined by scanning
electron
microscopy
were obtained from 26 male yaks and 10 domestic male cows -1
yr old. The lungs were perfusion fixed via the lobar pulmonary artery at a pressure of 25-30 mmHg with a 2.5%
buffered glutaraldehyde
solution. Small intralobar
pulmonary arteries 300-500 pm in diameter were then dissected free from surrounding
lung tissue and cut longitudinally. The vessels were dehydrated
in increasing concentrations
of ethanol,
air-dried,
and covered
with
palladium, and the pulmonary artery endothelial surface
was examined with a Philips PSEN-500
scanning electron microscope
(12).
Statistical
analysis. Dose-response
curves were analyzed by analysis of variance (ANOVA).
Comparisons
between doses were made by Fisher’s protected least-significant difference test at the 95% significance level. Blood
gas values in room air and hypoxic conditions were compared with a paired Student’s t test. P < 0.05 was considered significant.
Data are presented as means t SE.
RESULTS
Hemodynamic
studies. During
room air breathing,
mean pulmonary
arterial pressures in the yak (Table 1)
were slightly lower than either the 29 mmHg reported for
yearling steers at 1,600 m or the 28 mmHg reported for
cattle ~1 yr old at sea level (7, 24). However, cardiac
output values of 11.3 l/min in the loo-kg yaks (Table 1)
were similar to values of 11.3 1 min-’ 100 kg body wt-’
in the steers (7). The administration of hypoxic gas decreased arterial PO, but did not change arterial PCO, or
pH (Table 1). Pulmonary arterial pressure (+42%) and
cardiac output (+ZZ%) were both noted to increase with
hypoxic exposure (Table 1). The increase in pulmonary
resistance, however, was only 17%. Systemic blood pressure was unchanged. Total systemic resistance decreased
by 22%. Norepinephrine infused at a dose of 20 pg kg-’
min-’ in the pulmonary artery under room air conditions
to induce vasoconstriction increased pulmonary arterial
pressure and decreased cardiac output and stroke volume (Table 1). Total pulmonary resistance more than
l
l
l
l
TO
HIGH
ALTITUDE
1. Measurements of hemodynamic and blood gas
responses to vasoconstrictors
TABLE
Room
Air
HR, beats/min
CO, l/min
SV, ml
Ppa, mmHg
Psys, mmHg
TPR, mmHg
TSR, mmHg.
PH,
Paoz, Torr
Paco2, Torr
l
1-l
1-l
l
l
min
min
9328
11.3kO.7
122?6
24+_1
17124
2.220.2
15.92 1.7
7.5520.02
90211
32k1.5
Hypoxia
(11-13s
10727
13.820.7’
129212
34?1*
16824
2.6+0.2*
13.0+1.4*
7.55kO.01
42+3*
3020.2
0,)
Norepinephrine
min-‘)
(20 pg. kg-’
l
181+53*t
6.2+0.9*t
34t6*t
44+5*-f
283&17*t
7.l+l.l*t
48.1+5.5*t
Values are means + SE for heart rate (HR),
cardiac output
(CO),
stroke volume
(SV), mean pulmonary
arterial
pressure
(Ppa),
mean
systemic
arterial
pressure
(Psys), total pulmonary
resistance
(TPR),
total systemic
resistance
(TSR),
arterial
O2 and CO, partial pressures
and arterial
pH (pH,)
in blood sampled
from left
( Pao2 and Pac,J,
saphenous
artery.
Hemodynamic
and blood gas measurements
in native high-altitude
yaks breathing
room air at 730 m altitude
are compared with values from same animals
breathing
either
11-13s
0, or
room air during an infusion
of norepinephrine
in main pulmonary
artery. * Significantly
different
compared
with room air (P < 0.05, ANOVA for hemodynamics,
paired Student’s
t test for blood gas values).
t Significantly
different
compared
with hypoxia
(P < 0.05, ANOVA).
tripled. Systemic blood pressure, heart rate, and total
systemic resistance increased.
Infusion of acetylcholine in the yak pulmonary artery
under room air conditions had no effect on mean pulmonary or systemic blood pressures at any infusion rate
studied (Table 2). However, cardiac output increased at
the higher infusion rates, resulting in decreases in total
pulmonary and systemic resistances. Under hypoxic conditions, acetylcholine infusion did not change pulmonary
arterial pressure. Cardiac output did increase, however,
resulting in a decrease in calculated pulmonary resistance (Table 2). A decrease in systemic resistance was
also observed. Heart rate increased.
Nitroprusside infusion into the pulmonary artery
under room air conditions did not change mean pulmonary arterial pressure or cardiac output, but there was a
small (22%) decrease in calculated pulmonary resistance
at the highest dose (Table 3). Systemic resistance also
decreased by 22% at the highest infusion rate. Under hypoxic conditions, nitroprusside infusion decreased mean
pulmonary arterial pressure but cardiac output did not
change. Pulmonary resistance decreased by 37% (Table
3). Systemic pressure also decreased, and systemic resistance decreased 33%.
Heart weight determinations. The weight of RV was
130 t 6 g, and that of LV + S was 327 t 16 g. The calculated RV/LV+S was 0.40 t 0.02. RVIT was 0.29 t 0.02.
Histology and morphometric analysis. Histological analysis of trichrome- and VVG-stained sections of yak lung
demonstrated the small pulmonary arteries (100-300
pm) to be extremely thin walled with a single layer of
smooth muscle and little extracellular matrix (Fig. 1A).
In comparison, lung sections from an age-matched calf
that had died of brisket disease while living near South
Park, CO (altitude 3,000 m) showed the small pulmonary
arteries (100-300 pm) to be very thick walled, characterized by both an increase in medial smooth muscle mass
and extracellular matrix deposition (Fig. 1B).
YAK
TABLE
PULMONARY
ADAPTATION
TO
2279
ALTITUDE
2. Hemodynamic measurements related to acetylcholine infusion
Acetylcholine
HR, beatdmin
Ppa, mmHg
Psys, mmHg
CO, l/min
TPR, mmHgTSR, mmHg
1-l min
1-l . min
l
l
Values are means
0.05, ANOVA).
Room air
Hypoxia
9029
2522
17424
10.8-tO.8
2.4&O. 1
16.821.4
96+0
34&Z
16727
13.42 1.0
2.6kO.l
13.2kl.O
k SE. * Significantly
different
Dose, pg kg-’
l
l
min-’
3
1
0
Room air
Hypoxia
99k3
24+2
163*7
12.6k0.7
1.9kO.l”
13.2+0.9*
125k3*
33+2
167?7
13.9kl.O
2.3kO.l’
12.6kO.9
compared
with
6
Room air
Hypoxia
baseline
values
9
Room air
136+5*
3422
151k13
16.4+0.8*
2.0+0.0*
10.1?0.6*
131+10
25k3
151212
15.320.9*
1.6kO.l”
10.5+1.0*
155k23
23+3
149klO
16.9+1.1*
1.5?0.0*
9.6kO.9’
(0 infusion
rate)
under
Hypoxia
Room air
Hypoxia
147+7*
33+3
146+13
18.0*0.6*
1.7kO.l”
8.9+0.5*
165+39*
23+2
146k 10
16.6t0.6*
1.420.1*
9.4?0.4*
173+10*
33&Z
139+6
17.3?0.6*
1.8&0.1*
8.5+0.3*
room
air and hypoxia
conditions
(P <
virtually no smooth muscle in the small pulmonary arterioles cl00 pm in diameter. The pressures, resistances,
and vessel structure found in the yak are similar to those
found in humans who have adapted to high altitude over
thousands of years (8, 9) but are in contrast to those of
the closely related domestic cow that typically doubles its
pulmonary arterial pressure with hypoxia, has pulmonary arteries with a thick muscular coat (Fig. lB), and
has smooth muscle present in pulmonary arterioles as
small as 20 pm (1, 16,22; Fig. 2B). In addition, the native
high-altitude yak has no evidence of chronically increased pulmonary arterial pressures as evidenced by low
RV/LV + S and RV/T ratios and the maintenance of
thin-walled muscular and elastic pulmonary arteries. In
fact, RV/LV + S and RV/T in the yak were approximately equal to those of the closely related domestic cow
residing in Denver (RV/LV + S of 0.47, RV/T of 0.26)
and much less than those of cattle living at high altitudes
Morphometric analysis of 159 yak pulmonary arterioles from 100 to 300 pm in diameter showed a medial
thickness of 5.4 t 0.2 pm, which was 4.1 t 0.1% of the
total vessel diameter. In addition, pulmonary arteries
1100 pm in diameter had no vascular smooth muscle
present. In contrast to yak nonmuscular pulmonary arterioles, size- and location-matched
(bronchoalveolar
junction) pulmonary arterioles from a calf with brisket
disease had at least two layers of smooth muscle in the
vessel media (Fig. 2, A and B).
The numbers of elastic lamellae from size-matched
elastic conducting pulmonary arteries of the yak and pulmonary hypertensive calf were compared as a way of
looking at the structural adaptive response of elastic pulmonary arteries to chronic hypoxia and increased pulmonary pressure. The yak elastic pulmonary artery had 1718 lamellae, whereas the hypertensive calf elastic pulmonary artery had 18-20 lamellae even though the wall of
the pulmonary hypertensive conducting artery was over
three times as thick as that of the yak pulmonary artery
(Fig. 3, A and B).
Scanning electron micrographs of yak and cow small
pulmonary artery endothelial surfaces showed that yak
pulmonary artery endothelial cells appeared to be larger,
wider, and more rounded than those of a cow (Fig. 4, A
and B).
(2, 7, 11, 16).
The adaptive mechanism(s) responsible for the diminished hypoxic pulmonary vasoconstrictor response in
high-altitude-adapted animals is unknown, although the
fact that it is genetically transmitted has been best described in bovine species. Will et al. (22) and Weir et al.
(19) demonstrated that offspring of cattle that were determined to be either susceptible or resistant to the development of brisket disease retained their respective susceptible or resistant trait for at least two generations.
Also, Cruz et al. (4) performed cattle embryo experiments in which susceptible animal embryos that were
transplanted into resistant female cows retained the susceptible trait postnatally. The yak’s small hypoxic pulmonary vasoconstrictor response also appears to be trans-
DISCUSSION
The present study indicates that the native high-altitude juvenile yak has a minimal increase in pulmonary
arterial pressure and resistance in response to alveolar
hypoxia administered at an altitude of 730 m. Furthermore, morphological analysis demonstrates that these
animals have a thin pulmonary vascular medial layer and
TABLE
HIGH
‘1
3. Hemodynamic measurements related to sodium nwoprusslae
quslon
‘1-r
Sodium
0
HR, beatdmin
Ppa, mmHg
Psys, mmHg
CO, l/min
TPR, mmHg=
TSR, mmHg
l
1-l. min
1-l min
l
Values are means
0.05, ANOVA).
1
Room air
Hypoxia
Room air
97215
2221
172k7
12.1+_0.9
2.0~0.2
14.821.1
115klO
3522
172k5
12.020.8
3.OkO.l
14.8kO.9
101?14
2221
16924
12.OkO.8
1.920.1
14.7kl.l
t SE. * Significantly
Nitroprusside
different
compared
l
Dose, pg kg-’
l
l
min-’
3
Hypoxia
123-tl9
3222
168-t3
10.620.7
3.0&O. 1
16.120.9
with
baseline
Room air
115213
21+2
156k4
11.9kO.7
1.8kO.l
13.720.9
values
6
Hypoxia
123k19
3123
155?3*
12.4k0.8
2.5kO.l”
12.8kO.9
(0 infusion
9
Room air
126kll
Zl-tZ
15425*
12.821.0
1.720.1”
12.5kO.8
rate)
under
Hypoxia
Room air
Hypoxia
147212
27?1*
141+1*
13.6k0.8
2.0+0.1*
10.6+0.6*
137+7*
2021
150+6*
13.020.5
1.6_+0.0*
11.6?0.3*
149-t 14
25+1*
129&4*
13.2kO.5
1.9kO.l”
9.9?0.3*
room
air and hypoxic
conditions
(P <
2280
YAK PULMONARY
ADAPTATION
TO HIGH
ALTITUDE
FIG. 1. A: photomicrograph
of small pulmonary artery (185 pm diam) from native high-altitude 6-mo-old yak with
its accompanying airway. Mean pulmonary arterial pressure while breathing room air at 750 m altitude was 24 mmHg.
Note thin vessel wall with little smooth muscle or extracellular matrix [Verhoeff van Gieson stain (VVG), X100]. B:
photomicrograph
of size-matched (210~pm diam) small pulmonary artery from 6-mo-old calf that died of brisket
disease at altitude of 3,000 m. Note increases in medial thickness and elastin deposition and presence of distinct
adventitial layer compared with yak vessel (VVG, X100).
YAK PULMONARY
ADAPTATION
TO HIGH ALTITUDE
2281
FIG. 2. A: photomicrograph of pulmonary arteriole 26 pm in diameter present at bronchoalveolar junction (distal
respiratory bronchiole) in native high-altitude 6-mo-old yak [hematoxylin + eosin (H + E), x400]. B: photomicrograph
of 23-pm-diam pulmonary arteriole in 6-mo-old calf that died of brisket disease while living at 3,000 m altitude. Note
narrowed lumen and presence of 2 layers of medial smooth muscle in calf vessel compared with absolute lack of smooth
muscle in yak arteriole (H + E, x400).
mitted genetically. Anand et al. (3), in cross-breeding
studies of yaks and cows, have suggested that the genetic
transmission
of an attenuated hypoxic vasoconstrictor
response is via simple autosomal dominant inheritance.
In cattle resistant to the development of pulmonary
hypertension
and in the native high-altitude
yak, the
principal factor determining
altitude resistance appears
to be the maintenance of low pulmonary pressures with
hypoxia. This hypothesis is supported by the work of Ou
et al. (15), who have shown that rather than the actual
2282
YAK
PULMONARY
ADAPTATION
TO
HIGH
ALTITUDE
FIG. 3. A: photomicrograph
of wall of elastic pulmonary
artery 4,000 ym in diameter
in 6-mo-old
yak. Note presence
of -17-18
elastic lamellae
(VVG, X100). B: photomicrograph
of wall of elastic pulmonary
artery 4,000 Frn in diameter
from 6-mo-old
calf that died of brisket
disease. Note approximately
equal number
of elastic lamellae
as in size- and
age-matched
yak elastic pulmonary
artery despite greatly increased
thickness
of calf pulmonary
arterial
wall (VVG,
X100).
YAK PULMONARY
ADAPTATION
TO HIGH ALTITUDE
2283
FIG. 4. A: scanning electron photomicrograph
of endothelial
surface of yak distal pulmonary
artery demonstrating
long, wide, smooth-surfaced
endothelial
cells separated
by longitudinal
ridges (X360). B: scanning
electron photomicrograph
of pulmonary
artery
endothelial
surface from low-altitude
cow showing smaller,
more narrow
endothelial
cells densely packed together
(X360).
amount of pulmonary vascular smooth muscle present, it
is the existence of a mechanism to blunt the hypoxic pulmonary vasoconstrictor response to chronic hypoxia that
is the most important factor in the adaptation of a speties to high altitude. This is suggested by the ability of
the Madison strain of the Sprague-Dawley rat to survive
when exposed to chronic hypoxic conditions even though
it has a greater acute hypoxic pulmonary vasoconstrictor
response than the Hilltop strain, which shows signs of
chronic mountain sickness and has a high mortality on
exposure to high altitudes. The Madison strain’s hypoxic
pulmonary vasoconstrictor response is, unlike the Hilltop strain’s, attenuated on exposure to chronic hypoxia (15).
2284
YAK
PIJLMONARY
ADAPTATION
The findings presented here clearly demonstrate
that
the yak has a markedly attenuated hypoxic vasoconstrictor response compared with either a general domestic
or high-altitude-resistant
domestic
calf population
calves (16, 20, 23). Possible reasons for the blunting of
the hypoxic pulmonary
vasoconstrictor
response during
the process of adaptation to high altitude could include
cellular changes in the smooth muscle cell itself, such as
differences
in calcium flux, actin-myosin
linkages, or a
change in responsiveness
to vasodilator
and/or vasoconstrictor substances.
Indeed, pulmonary
hypertensive
rat
pulmonary
vascular smooth muscle has a decreased responsiveness
to nitroprusside
possibly due to desensitization of the soluble guanylate cyclase fraction in the
pulmonary
vascular smooth muscle cell (18). Alternatively, changes in endothelial
cell release of vasoactive
substances
could affect the underlying
smooth muscle,
resulting in blunting of the hypoxic pulmonary
vasoconstrictor response.
Our data imply that endothelial regulation of pulmonary vascular tone may, indeed, be different in the yak
than in cattle. Although
the endothelium-independent
vasodilator
nitroprusside
lowered
pulmonary
arterial
pressure in both the hypoxic yak (present study) and calf
(14), the failure of acetylcholine
to lower pulmonary arterial pressure in the hypoxic yak was in striking contrast
to its marked pressure lowering effect in the closely related altitude-sensitive
hypoxic calf (14). This observation is consistent
with a different role for endotheliumdependent vasodilation
in these two members of the Bos
genus. Further
evidence that the pulmonary
endothelium, rather than smooth muscle, might be important
in
the differences
between yak and cow was the striking
difference
in endothelial
morphology
between the two
animal species. For example, one might speculate that
the larger, more metabolically
active yak endothelial
cells that possess an increased number of ribosomes and
have more prominent
mitochondrial
cristae than those
of domestic bulls (12) were somehow related both to low
resting pulmonary
vascular tone and lack of reactivity
with alveolar hypoxia.
Morphologically,
we have shown that yak pulmonary
arterial walls remain very thin despite their exposure to
chronic hypoxic conditions. The factor(s)
responsible for
either the maintenance of thin-walled
pulmonary vessels
or, alternatively,
the increase in wall thickness
in pulmonary hypertensive
animals is unknown.
Because the yak
lives under chronically
hypoxic conditions
and yet has
thin-walled
pulmonary
arteries,
data presented
here
would suggest that hypoxia per se is not the principal
stimulus for the increases in pulmonary
artery wall cell
proliferation
and extracellular
matrix production seen in
hypoxic pulmonary
hypertension.
Hemodynamic
forces
such as increased pressure and flow thus appear to play a
more important
role in the structural
changes (or lack
thereof)
seen in pulmonary
vasculature
exposed to
chronic hypoxia. The yak pulmonary
vasculature,
by
maintaining
low pulmonary
pressures
under hypoxic
conditions,
is not subjected to increased hemodynamic
forces and so may lack the stimulus for vascular cell proliferation and matrix protein production.
Interestingly,
despite the yak having a vessel wall
thickness
less than one-third
that of the domestic calf
TO
HIGH
ALTITIJDE
with brisket disease, the number of elastic lamellae in the
conducting
pulmonary
arteries of the yak and calf was
similar. This finding is consistent with those of Wolinsky
and Glagov (25), who reported the number of elastic lamellar units in the aorta was proportional
to the radius of
the vessel regardless of wall thickness
or animal species.
Thus, in the process of both adaptation
(yak) and extreme maladaptation
(calf with brisket disease) to the
chronic hypoxia of high altitude, the basic architecture
of
large pulmonary arteries is maintained even though pulmonary artery wall thickness
may change dramatically.
In summary, this study demonstrates
the exceptional
adaptation of the species Bos grunniens to high altitude
as demonstrated
by its greatly diminished hypoxic pulmonary vasoconstrictive
response, its thin-walled
pulmonary arteries with little smooth muscle, and its lack of
right ventricular
hypertrophy,
all of which are uncharacteristic for the closely related domestic cow member of
the genus Bos when exposed to high-altitude
environments (l-3, 11, 14, 16, 20, 21, 24). Differences
in both
endothelial cell morphology and response to the endothelium-dependent
vasodilator
acetylcholine
between the
high-altitude
yak and low-altitude
domestic calf suggest
the evolutionary
process of adaptation to chronically hypoxic environments
may include changes not only in the
amount of pulmonary
vascular smooth muscle present
but also in endothelial cell function and structure.
These
changes could subsequently
contribute to the differences
in pulmonary
vascular tone and reactivity that exist between high-altitude-adapted
and -sensitive populations.
We thank Dr. John T. Reeves for critically
reviewing
the manuscript
and the staff of the Kyrghyz
Institute
of Cardiology,
Bishkek,
KyrghyzStan, for both assistance
in the study and hospitality
during our stay.
This study was supported
in part by National
Heart,
Lung, and
Blood Institute
(NHLBI)
Grants
HL-14985
and HL-46481,
the NHLBI
IJSA:IJSSR International
Exchange
Program,
and a Career Investigator Award
from the American
Lung Association
(to K. R. Stenmark).
Address for reprint
requests: A. G. Durmowicz,
Cardiovascular-Pulmonary
Research
Lab, Box R-133, Univ. of Colorado
Health
Sciences
Center,
4200 E. Ninth Ave., Denver,
CO 80262.
Received
1 April
1992; accepted
in final
form
4 December
1992.
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