Mechanisms of O2 transport in andean dogs

MECHANISMS

Abstract.

Using previously

OF 0, TRANSPORT

IN ANDEAN

I I dogs native

inserted catheters.

to high altitude

DOGS’

(7.5 23 kg but.)

studied standing and unsedated m Cerro de Pasco. Peru at 4350 meters. Hemoglobin

(Hb).

(Hct).

obtained

and

0,

and CO,

mixed

venous

contents, blond

P,,:. P,.,,? and pH

samples.

Blood

the left ventricle and cardiac output (0) Moderately different 3X

higher

than

that

values found

for

Hb

were measured

pressures

were

was determined and

Hct

in sea level dogs:

were the

Hg, respectively. Cardiac

hypertension

pulmonary

found

P,,,

in these dogs.

in the

Andedn

pressure at 4350

Hb-Oz

Marked

output

hyperventilation

was normal

162_t39

was observed in the presence of normal

increased pulmonary

and

m (458

mm Hg)

mm

was no Hg

at

the partial

air were lower than at sea level: X4.3 and 56.4 mm

was observed (Pq.,,,.

(average

arterial

artery

aninity

dogs was 31.6

Pa,,, and Pi,,? were 55.5 and 32.9 mm Hg while the Saol and S\,,,

50.7”,,, respectively. normal.

in the

by dye dilution.

C and pH of 7.4. Because of the low barometric

pressures of oxygen in inspired and in alveolar

in simultaneously

measured

were

hematocrit

25.6 mm Hg),

ml;min;kg).

left ventricular

Moderate end diastolic

were 79.5 and

however. pulmonary

pH

was

arterial

pressure suggesting

vascular resistance. Hypoxia

Altitude Cardiac Dog

output

Oxygen

( C‘mi.s/rrmi/iuris)

Hb 0,

affinity

Hb -0,

dissociation

curve

transport

Pulmonary

clrculatlon

Pulmonary

hypertension

P,,,

Adaptation to an environment of chronic hypoxia requires a fairly complex combination of physiological mechanisms which tend to assure an adequate supply of oxygen to the different tissues despite the lower partial pressure of oxygen in inspired air. Some of these adaptative mechanisms are common to most mammalian species, for example increased hemoglobin concentration. while others, such as changes in hemoglobin oxygen affinity. appear to be present only in some species (Banchero et ul., 1971; Lenfant rr al., 1969). Acc~rprrtl

’

for

Supported

puh/iur/im

16 Jo~~utrty~~ 1975.

by U.S. Public Health

Service Grant

# HL 361

14317.

N. BANCHERO,

362

J. CRUZ

AND J. BUSTINZA

Information on the influence of chronic hypoxia on the cardiovascular and respiratory systems comes mainly from studies conducted on human volunteers native to high altitudes. Considerably less has been done in animals native to high altitude. these animals

The reasons for this appear to be related and in conducting studies in laboratories

to difficulties in procuring usually not well equipped

for large animal experimentation. We have conducted a study on the oxygen transport mechanisms in normal Andean dogs at an altitude of 4,350 m in the new laboratory facilities of the High Altitude Research Institute of the Universidad Peruana Cayetano Heredia, in Cerro de Pasco, Peru. Materials and methods Eleven mongrel dogs, weighing from 7.5 to 23.0 kg, natives of Cerro de Pasco, Peru (4,350 m), were studied at their place of birth at a mean barometric pressure of 458 mm Hg. The animals were obtained from a local dog pound. The study was conducted with the dogs awake and unsedated, at rest and in the standing position. Two days prior to the experiment, under general anesthesia (30 mg/kg sodium pentobarbital) and using aseptic technics, two No. 6F Rothene catheters were inserted. One catheter was introduced in the left carotid artery via a small cutdown in the neck. The second catheter was inserted percutaneously or through a cutdown in the right external jugular vein, and advanced, with the aid of pressure recordings into the pulmonary artery. Blood samples were obtained anaerobically in heparinized glass syringes from the carotid and pulmonary arteries via the previously inserted catheters. Partial pressures of oxygen and carbon dioxide, and pH were measured at 38 “C using the appropriate electrodes (Radiometer, London Co.). Oxygen and carbon dioxide contents were determined in the Van Slyke manometric apparatus, which was also used to determine oxygen capacity. Hemoglobin concentration, arterial and mixed venous oxygen saturation were then calculated from these data. Hematocrit was determined using the microcapillary method. No correction for trapped plasma was made. Mean corpuscular hemoglobin concentration was calculated by dividing the hemoglobin concentration by the hematocrit. The body temperature of the dog was measured during the experiment using a rectal thermistor probe (Yellow Springs Instruments, Model 43TA). The mean alveolar oxygen pressure was calculated at body temperature using the alveolar gas equation (Rahn and Fenn, 1955) assuming a respiratory quotient of 0.9. In viva hemoglobinoxygen dissociation curves were constructed using arterial and mixed venous 0, saturations and simultaneous partial pressures of oxygen in arterial and mixed venous blood, corrected to 38 “C and pH of 7.4. Also, a Calcomp plot of the mean hemoglobinoxygen dissociation curve was obtained on a double logarithmic scale based on the calculation of the K values by the following equation (Battaglia et ul., 1968; Hellegers et al., 1959).

OXYGEN TRANSPORT IN ANDEAN

log PO:=

K, -K,(7.4-pH)+K,

DOGS

363

log &

using a model 6400 Control Data Corp. Digital Computer. K,, K, and K, are constants. and S is saturation. K, is the logarithm of P,,, K, is the Bohr constant and K,= l!n. where n is Hill’s constant. Blood pressures were recorded via the 3 indwelling catheters using Statham strain gauge transducers (Model P23Db) and a Gilson direct write recorder (Model M8PM). The zero reference level was taken at the uppermost point of the humerous (greater tubercle). Cardiac output (0) was measured by the indicator dilution technic using indocyanine green (CardiogreenR). Injections were made in the pulmonary artery while continuous systemic arterial blood sampling was done through a Waters densitometer (Model XC-250) with a Harvard infusion-withdrawal pump (Model 904). These curves were plotted on a Varian X-Y plotter. Systemic arterial and mixed venous oxygen transport were calculated as a product of Q and arterial or mixed venous oxygen content. respectively. Results HEMOGLOBIN

AND HEMATOCRIT( table

I)

Average values for hemoglobin and hematocrit were higher than those of normal sea level dogs and agreed with values reported by Rotta (1938) in Andean dogs at 4540 m. The average mean corpuscular hemoglobin concentration ([Hb]/Hct) was 33.4” o, while the relationship between hemoglobin concentration and hematocrit is given by the equation [Hb]=0.367 Hct- 1.62 (r=0.94).

TABLE I Hematological data in Andean

dogs at 4.350 m

Hb

Hct

MCHC

(g.100 ml)

(“J

(g:lOO ml)

16.5 k2.2

49.2 * 5.5

33.4 i I.5

averageiI SD. MCHC = mean corpuscular

BLOOD GASES AND PH (table

hemoglobin

concentration.

2)

Cerro de Pasco, Peru has an average barometric pressure of 458 mm Hg, and the average calculated PloL and PA,? values for these dogs were 84.3 and 56.4 mm Hg, respectively. As a consequence OF this, and as seen in fig. 1, the partial pressure of oxygen measured at different sites in the bloodstream were lower than

N. BANCHERO.

364

J. (‘RUZ

TABLE Blood gas data

b,,

%k

(mm Hg)

(“,I

C% (Vol ‘I,,)

AND J. BUSTINZA

Z

m Andean

dogs at 4.350 m

pHa

HCO,

(mM L) 7.31 I

25.6

i4.5

+ _ 2.7

il.5

& ,047

_t4.4

i2.I

PVC, Cm; Hg)

%,> (“J

CV,,

pH\

(Vol ‘1,)

PV(0 (mm Hg)

( C)

19.5

55.5

II.3

7.338

f 2.6

i_ .037

50.7

32.9

+7.5

* 5.4 Average

17.5

k

3 I .9

14.3

Body temp.

3x.9 f .9

k4.3

1 SD.

Data given at body temperature. SOY

Fig. I. Average

those

differences

in partial

pressure of 0, between inspired in Andean dogs at 4,350 m.

air and

mixed

venous

blood

found at sea level. The average Pa,, (55.5 mm Hg) was similar to the found by Vogel et ul. (1974) in the native man at the same elevation PaoL although other investigators have found lower values in man (Torrance rt (11.. 1970/1971). The values for PacoL were low (average 25.6 mm Hg) suggesting marked hyperventilation in these dogs. The mean arterial pH at the animals’ body temperature was 7.371, but when corrected to 37 ‘C, the pH was 7.399. The arterial [HCO;] was accordingly low.

0XYC;EN

As seen in fig. 2. the mean and 29.4 mm Hg humans native to No differences in and Cain, 1966)

TRANSPORT

P,,

IN ANDEAN

365

DOc;S

in these dogs was 31.6 mm Hg at 38

C. pH 7.4,

at 37 C, pH 7.4, which is similar to the P,, (37 ‘C, pH 7.4) in Cerro de Pasco ( Lenfant rr al., 1969; Torrance et (II., 19701197 I ). Hb-Oz affinity were found between dogs at sea level (Rossing and Andean dogs (fig. 3). Because Andean dogs had a lower

hemoglobin concentration, and therefore I lower 0, capacity than the Andean man (Torrance ef trl.. 1970; 197 1), the Ca,, at ;I given O2 saturation level was lower. 1.0

0.6

-0.6

1.4

1.6

LOG P 02 Fig. 2. Regression plotted

line of Hb 0,

on logarithmic

by interpolation

(log

dissoctation

curve in the dog at pH

I =0)

7.4 and

c)S’,, confidence

scales. The shaded area represents

is 3 I.6 mm Hg. The values for K ,, K2 and

temperature

limits. The K,

are

38

C

Pj,, obtained

1.5. 0.34. and 0.36,

respectively.

/.-~

_._.-.-.-

_..’

. ..A .’

,r;’ 0..

,,.’ I

’

pH 1.4

4 35-c

i

.’

_._.

i

l

NORYALDOGAT SEA LEVEL

ri

.

3 I

.’

1.1 0 0

.x

.x 50

100

PO (mmHg) 2

Fig.

3. Values

for

blood

0,

saturation

and

P,,? plotted

on

a

Hb 0,

coordinate

average curve for normal dogs at sea level k also shown.

system.

The

366

N. BANCHERO. J. CRUZ AND J. BUSTINZA TABLE 3 Circulatory d (ml/min/kg) 162

dogs at 4.350 m

TC,, (ml/‘min,‘kg)

(7iyimin/kg)

+39

(Ca,, - Cv,JiCa,, (I’<,)

28.7

18.4

36.3

* 8.2

f 6.6

k 9.2

1 SD.

Average ) Q =cardiac oxygen

data in Andean

output.

transport.

CARDIAC OUTPUT AND

(Ca,,

TaoA = systemic

oxygen

- CV,:)./Ca,,

= Coefficient

transport.

BLOOD O2 TRANSPORT (table

of 0,

TV,, = venous utilization.

3)

The arterial-venous oxygen content difference was 6.44 vol”/,, which is significantly larger than the a-v O2 difference in the resting Andean man (Banchero er al., 1966; Torrance ef ul., 1970/1971; Vogel et ~1.. 1974). The coefficient of O2 et al., utilization in these dogs was 36.3% us, 25.4 ‘:b in the Andean man (Torrance 1970/1971). Because of the greater 0, extraction, the average PV o, in these dogs was 32.9 mm Hg, and the average SVoL (50.7”/;,) was lower than in man (55.4’Y0). This value is within the normal The cardiac output (Q) was 162f 39 ml/min/kg. range in sea level dogs awake or anesthetized. Systemic arterial and mixed venous 0,

transport

in these dogs were 28.7 and 18.4 ml/min/kg,

respectively.

BLOOD PRESSURES AND PULMONARY VASCULAR RESISTANCE (table

The Andean hypertension

4)

dog, as other mammals at high altitude, exhibits moderate pulmonary with pulmonary arterial pressure values similar to those observed in

man native to Morococha (4,540 m) (Banchero ef al., 1966). However, the variability of measurements of blood pressure in the dog was more marked than in man. This variability can be explained in part by the difficulty of keeping an awake and unsedated

dog calm

and

in basal

conditions

for long

periods

of time.

TABLE 4 Blood pressures and pulmonary resistance L.V. S/D

Pulmonary artery S

Mean

D

P.V.R. (mm Hgjmljminjkg)

(mm Hg)

(mm Hg) 38 *8 Average

24

I5 +7 +

+7

141/l &

17i5

0. I56 2.080

1 SD.

P.V.R. = Pulmonary vascular resistance=( PPA- PLVED)/Q. L.V. S/D= Left ventricular systolic/diastolic pressure.

Because

OXYGEN

of the movement level and

TRANSPORT

of the animal,

the relative

position

vertical

IN ANDEAN

fluctuations

of the tricuspid

367

DOGS

valve

between (point

the zero

reference

of hydrostatic

in-

difference in the dog) (Banchero er al., 1967: Guyton and Greganti, 1956) occurred. The left ventricular end diastolic pressure was entirely normal, suggesting that the pulmonary arterial hypertension was due to increased pulmonary vascular resistance probably at precapillary level. This has been observed in other species including man (Banchero et al.. 1966, 1971). The value for left ventricular systolic pressure was somewhat elevated, especially in three dogs which had values above 150 mm Hg. This finding has also been observed in dogs exposed to acute hypoxia (Thilenius et al., 1964). Discussion Dogs, as other mammals, show increased hemoglobin concentration and hematocrit within days after exposure to hypoxia (Banchero et al., unpublished observations). In chronic hypoxia the increase in hemoglobin is absolute (higher total red cell mass). This is mainly due to enhanced erythropoeisis, which results from increased production of erythropoeitin. In the only hematological study on Andean dogs that we have been able to find, conducted by Rotta (1938) at 4,540 m, hemoglobin concentration and hematocrit were 16.0 g/100 ml and 52.1%, respectively. These values agree with our data at high altitude; however, Vogel et al. (1968) found considerably higher values for hemoglobin concentration and hematocrit in selected purebred beagles kept at 4,100 m for four months under carefully controlled dietary and environmental conditions. Dogs obtained in the Andes were of mixed breeds and their food intake and their housing before the experiment could not be controlled. It is not unusual to find considerable variability in the normal hematological data in dogs at sea level (Altman and Dittmer, 1966) which may reflect variations in one or more of the following: animal size, breed, nutrition and experimental conditions at the time the sample is obtained. We believe these factors are important in explaining the differences observed at high altitude among the individual animals. Andean dogs at 4,350 m showed marked pulmonary hyperventilation which had been compensated by a loss of plasma [HCO;] resulting in an average arterial pH of 7.399 at 37 “C. However, the average pHa value at body temperature was 7.371, which may lead to the assumption that these animals lived in a condition of chronic acidosis, which is not the case. An average pHa of 7.360 (range 7.31-7.42) was reported (Altman and Dittmer, 1961) in sea level dogs whose average body temperature was 38.9 “C. Often, however, awake dogs show high arterial pH and low Pa co> probably as a result of excitement. In vitro experiments have shown that in mammalian blood an increase of 1 “C is associated with a decrease of 0.014 pH unit (Rosenthal, 1948). The same relationship holds true in in uiuo experiments. In the basis of these experimental data and assuming a normal pH of 7.4 at 37 “C, the theoretical arterial pH for dogs at 38.9 “C should be 7.373, which compares well with the pH we actually measured.

368

N. RANCHERO.

J. (‘RUT

The mean Pa,,, value in the Andean by Vogel r/ al. (l-968) in dogs exposed

AND J. BUSI‘INZA

dogs was slightly lower than that reported to an elevation of 4100 m for four months.

This difference in Pa,,, is probably the result of a relative hypoventilation in the dogs native to the Andes, in comparison to the sojourning dog, as has been previously found to occur in man (Lefrancois rt NI., 1968: Weil rf (I/.. 1970). However. Lefranqois ct (I/. (1968) reported normal ventilatory response to pure oxygen in anesthetized Andean dogs. The Hb~~-O, affinity in the Andean dog was not different from the Hb-0, affinity in the sea level dog (Dill ef al.. 1932, Rossing and Cain. 1966). This is in sharp disagreement with data reported by Cropp and Gee (1972) in a group of anemic dogs who showed a considerable increase in 2,3-DPG in red blood cells and an associated decrease in HbpOz affinity in response to tissue hypoxia resulting from the anemia. The findings in the Andean dogs are also qualitatively different from observations in man at high altitude in whom a decrease in Hb-0, affinity with lower Pj, values has been measured (Lenfant c’t ul., 1969). It should be noted, however. that since dogs normally operate at body temperatures above 38 C and pH’s below 7.4 the actual in riro Hb-0, curve is to the right of the so called ‘normal’ curve at pH 7.4 and temperature of 37 C. We have recently found that the capillary bed of skeletal muscle in dogs from low altitudes changes substantially after a 3-week exposure to simulated altitude of 435 mm Hg (Banchero. 1975). Following exposure to hypoxia there was a reduction in the average muscle fiber diameter from 65 to 46 ~1with a concomitant increase in capillary density from 624 to 1262 cap;mm2. It has, furthermore. been observed that these Andean dogs have smaller skeletal muscle fibers with an average diameter of 27 ,U and an even richer capillary density (2016 capimm2) (Banchero rt a/.. unpublished observations). These findings seem to provide anatomical support for the large arteriovenous oxygen difference measured in the Andean dog at rest. The PV,,: in these high altitude dogs was similar to the PV,, measured in man at the same altitude but because dogs operate at a higher body temperature and with lower hemoglobin concentration their SVo and CVoL were lower than in man. We have measured an increase in myoglobin concentration and in mitochondrial protein content in skeletal muscle of dogs exposed to simulated altitude of 435 mm Hg for 3 weeks. These results are in agreement with previous studies conducted in the dog native to high altitude (Hurtado et u/., 1937; Reynafarje, 1962). Myoglobin is a simple heme protein capable of combining reversibly with O,, thus contributing to 0, delivery at tissue level. A higher mitochondrial protein content is believed to be associated with increased concentration of respiratory enzymes andior with an increased number of mitochondria. thus increasing the metabolic capability in muscle. The effects of acute hypoxia on the pulmonary circulation of sea level dogs have been extensively studied. However. information on the pulmonary circulation of dogs living in an environment of chronic hypoxia is scarce and when available (PCrezNtiiiez. 1965) is grossly incomplete. We have found moderate degree of pulmonary

OXYGEN

arterial

hypertension

resistance.

TRANSPORT

in the Andean

The pulmonary

IN ANDkAY

dog

hypertension

due

369

IXIGS

to increased

is similar

pulmonary

to that

observed

vascular

in man

and

other mammalian species who are known to adapt and thrive at high altitude. The cardiac output in these dogs was within the normal range for sea level dogs. This is also in agreement with data obtained in the Andean man (Banchero rt (II.. 1966; Vogel er al.. 1974). However. much greater than it is in man.

the individual

variability

in dogs

is

Acknowledgements

The authors gratefully acknowledge the assistance of Ms. Sheila Hackett, Messrs. Ralph Sokol, Pedro Vega-Centeno. Guillermo Hurtado and Santiago Torres. The facilities in Peru were partly supported by U.S. Army grant G-0012, from the Defense Research Office. References

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