Oxygen transfer during aerobic exercise in a varanid lizard Varanus mertensi is limited by the circulation
1 Department of Zoology, La Trobe University, Melbourne, Victoria, 3086,
Australia
2 School of Biological Sciences, Northern Territory University, Darwin, NT
0909, Australia
* Author for correspondence (e-mail: p.frappell{at}latrobe.edu.au)
Accepted 30 May 2002
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Summary |
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Key words: exercise, ventilation, oxygen consumption, oxygen transport, blood gas, breathing pattern, reptile, Varanus mertensi, lung diffusion, P50, oxygen affinity
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Introduction |
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A number of studies have searched for the component(s) that limit oxygen
transfer and hence aerobic metabolism in exercising varanid lizards, a group
that is considered to have one of the highest aerobic scopes amongst reptiles
(Gleeson et al., 1980;
Mitchell et al.,
1981a
,b
).
Almost all of these studies have been restricted to one species, Varanus
exanthematicus. It would appear that this species effectively ventilates
its lungs to match the increase in metabolic rate achieved during sustained
locomotion, at levels sufficient to achieve maximal rates of oxygen uptake
(Hopkins et al., 1995
;
Wang et al., 1997
). Indeed,
ventilation has been shown to increase proportionally more than metabolic rate
(Mitchell et al., 1981b
;
Wang et al., 1997
), giving
rise to an elevated alveolar partial pressure of oxygen
(PAO2)
(Mitchell et al., 1981b
). This
suggests that aerobic activity in varanids is not limited by ventilation, as
originally suggested in the axial constraint hypothesis proposed by Carrier
(1987b
). In contrast to the
elevated PAO2, arterial
PO2 (PaO2)
remains somewhat constant (Mitchell et
al., 1981b
; Hopkins et al.,
1995
), implying that the observed alveolararterial
PO2 difference observed during exercise is
potentially limited by pulmonary diffusion, the inability to increase
diffusing capacity during exercise, ventilation/perfusion inequalities and/or
cardiac shunting (Mitchell et al.,
1981b
; Hopkins et al.,
1995
).
It has been suggested that the greater factorial scope in V.
exanthematicus, compared to other lizards such as Iguana iguana,
is potentially due to a greater scope in both cardiac output and oxygen
extraction (Gleeson et al.,
1980; Bennett,
1994
). The iguana, at moderate to high speeds, is also unable to
match the increased metabolic rate with adequate ventilation; as speed
increases ventilation decreases (Carrier,
1987a
; Wang et al.,
1997
) and this decrease is accompanied with a decrease in
O2
(Wang et al., 1997
). Such a
finding adds support to the axial constraint hypothesis, at least for the
iguana.
Despite the sustained interest in the determinants of aerobic scope in
varanids, no study has yet measured all the O2 transfer components
in a single species. Further, the studies by Mitchell et al.
(1981a,b
)
used arterial PCO2
(PaCO2) and pulmonary gas exchange rates to
calculate ventilation and lung PO2 from
well-known alveolar gas equations. Such calculations assume that only
ventilatory changes affect PaCO2.
This study therefore analyses O2 transfer during sustainable treadmill exercise in a semi-aquatic varanid lizard, Varanus mertensi, to broaden our understanding of O2 transfer beyond one varanid species. The use of a treadmill as opposed to a water channel is justified as this species is just as home on land as it is in water, and the use of a treadmill permits comparison with the results of previous studies.
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Materials and methods |
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Protocol
Animals were initially acclimated to the experimental temperature (35
°C, body temperature, Tb, checked with an infrared
thermometer) for at least 12 h before being run on a treadmill, set at speeds
increasing from 0.2 to 0.42 m s-1, and wearing a lightweight
plastic mask that encompassed the head and enclosed the mouth and nostrils. A
bias flow of air (2800 ml min-1 STPD, standard temperature and
pressure, dry) was drawn through the mask, dried and analysed for the rate of
oxygen consumption,
O2, as outlined
below. Prodding of the tail ensured that the animals kept pace with the
treadmill. The speed at which the maximum level of
O2
(
O2max) was
sustained for 2 min and showed no further increase with increasing speed was
established as the speed to be used for subsequent experimental runs (0.33 m
s-1).
On a separate occasion, animals were fitted with Y-connector masks and ventilation and metabolism determined by the methods described below. The lizards were again run on a treadmill at 0.33 m s-1 until exhaustion. These measurements were used to ensure that the subsequent surgery and placement of arterial and venous catheters did not affect the performance of the lizards.
Following the placement of catheters (see below), the animals were again fitted with Y-connector masks and made to run on the treadmill (0.33 m s-1). This time ventilation and metabolism, end tidal gases (see below) and various blood parameters (see below) were measured at specified time intervals: pre-exercise (i.e. the animal masked and resting quietly on the treadmill for at least 30 min), during exercise at 1 min and 5 min, exhaustion (i.e. the animal no longer able to maintain pace), and after 2 min recovery.
Ventilation and metabolism
Individual masks were constructed each time an animal was run by placing
the main stem of a Y-connector in the mouth of the animal and sealing the nose
and mouth with a quick-setting rubber (Impregum, ESPE). Plastic tubing was
connected to the two arms of the Y-connector. At one end a pump drew air (2800
ml min-1 STPD) under negative pressure through the mask and a
pneumotachometer connected at the other end. Flow was measured using a
differential pressure transducer (PT5, Grass Instruments, ±5 cm
H2O), integrated and recorded digitally (50 Hz; DT2801-A,
DataTranslations and ASYST, MacMillan Software) as volume. The bias flow from
the pump was electronically offset to zero, thereby allowing the recording of
tidal volume and breathing pattern. After passing through the pneumotachometer
the gas was passed through a drying column (Drierite) and analysed for the
fractional concentrations of O2 (S3-AII dual channel O2
analyser, Ametek) and CO2 (CD-3 CO2 analyser, Ametek),
FO2 and FCO2,
respectively. The outputs from the analysers were recorded digitally (50 Hz)
and used to calculate the overall rates of oxygen consumption
(O2) and carbon
dioxide production
(
CO2) by taking
into account the respiratory exchange ratio (R), as outlined in
Frappell et al. (1992
). In
brief,
![]() | (1) |
![]() | (2) |
The breathing pattern was analysed in terms of tidal volume
(VT), expiratory, inspiratory, inspiratory pause and total
times (TE, TI, TP
and TTOT), frequency
(f=1/TTOTx60) and minute ventilation
(E=VTxf)
in a similar approach to that detailed in Frappell et al.
(1992
). On average, 25
consecutive breaths were analysed. Volumes are expressed at BTPS
(Tb, barometric pressure, saturated).
End tidal gases
A 14-gauge needle was inserted from the centre of the two arms into the
main stem of the Y-connector. This was connected directly to the other channel
of the O2 analyser and the CO2 analyser with a short
length of small-bore tubing (PE, i.d. 1.00 mm, o.d. 1.50 mm). Gas was
subsampled through this circuit at a flow (=1.2 s) that permitted the
determination of end tidal PO2 and
PCO2 (PAO2 and
PACO2, respectively) [i.e. alveolar; strictly
speaking the surface of the lung wall is increased by a honeycomb-like
(faviform) system of partitions, which bear a matrix of capillaries on both
surfaces. The individual chambers form the faveoli
(Perry and Duncker, 1978
)],
determined as fractional concentration Fx
(PB-PH2O(35)), where PB is
barometric pressure and PH2O(35) is partial
pressure of water vapour at 35°C. This gas then rejoined the main flow of
air before it passed through the drying column.
For three animals the CO2 analyser was placed in the subsampling
circuit and used to determine end tidal PCO2
(PACO2); for the other three animals the
CO2 analyser was used to directly determine
CO2.
Heart rate
Heart rate (fH) was determined from an electrocardiogram (e.c.g.)
measured with two small needle electrodes placed on the dorsal service and
positioned diagonally across the heart. The signal was appropriately amplified
(7P4F, Polygraph, Grass Instruments).
Catheterisation
Animals were anaesthetised with an i.v. dose of ketamine administered
via the ventral coccygeal vein at a dose of 80-100 mg
kg-1. After induction of anaesthesia, a small mid-ventral incision
(approximately 2 cm) was made in the neck. The common jugular vein was
occlusively cannulated using a PE catheter (i.d. 0.58 mm, o.d. 0.96 mm). The
tip of the catheter was directed past the brachiocephalic vein to rest in the
anterior vena cava and was checked by measuring the length of the catheter on
removal. The internal carotid artery was occlusively cannulated with PE
catheter (i.d. 0.58 mm, o.d. 0.96 mm) and the tip advanced to rest as close to
the heart as possible, presumably in the brachiocephalic artery or aorta. The
catheters were sutured in place, looped loosely to avoid tension, led to the
exterior through a small hole on the animal's dorsal surface just above the
shoulders, filled with heparinized saline and sealed. The wound was closed
with sutures. The surgical procedure lasted no more than 40 min and the
animals were allowed to recover for at least 24 h prior to further
testing.
Blood variables
Small blood samples (250-300 µl) were taken at designated times from
both the artery (modifier, a) and the vein (mixed venous blood, modifier,
) and stored anaerobically on ice. The partial
pressures of blood gases PO2,
PCO2 and the pH were measured at 35°C (BMS
MK2, Radiometer). The electrodes were calibrated before and after each
measurement. PO2 and
PCO2 were measured over 3 min and regressed
back to time zero; pH was measured on incremental volumes of blood until the
variation between successive measurements was less than 0.005 units. The
oxygen content, CO2, of each blood sample was
determined on a 15µl subsample of blood using a galvanic cell (Oxygen
Content Analyser, OxyCon). Hematocrit, hemoglobin concentration [Hb]
(Boehringer Mannheim kit no. 124729) and lactate concentration (Sigma kit no.
826) were also measured for each sample.
On another occasion a 2 µl blood sample was taken and oxygen equilibrium
curves determined on a modified Hem-O-Scan (Aminco Instruments). The method
followed is largely that presented by Holland et al.
(1988), where gases of
appropriate PO2 are pulsed into the chamber and
the percentage saturation determined. Curves were constructed from 10-15
points at 35°C and at CO2 tensions of 39 and 79 mmHg (1
mmHg=133.3 Pa); the gases mixed by a pump (Wosthoff). Each point was digitised
and the P50 determined from the linear section of the Hill
plot in the middle range of saturations. The Bohr factor was determined as
(
logP50/
pH), pH being determined from blood
tonometered at the PCO2 values used in the
Hem-O-Scan.
Resting lung volume and pulmonary diffusing capacity
Resting lung volume and pulmonary diffusing capacity were determined for
six animals after quietly resting for 30 min and during exercise using helium
dilution and carbon monoxide clearance techniques. The lizards were fitted
with a mouthpiece as previously described. The mask was directly attached
via a three-way stopcock to a closed circuit of known volume
(VSYS) that consisted of flexible tubing, helium and carbon monoxide
analysers (CO and He Analysers, Morgan, UK), a collapsible reservoir and a
pump that ensured a continual flow (approximately 1000 ml min-1)
through the circuit. The stopcock was initially open to enable the lizard to
breathe room air. The circuit contained a test gas that comprised carbon
monoxide (CO, 0.28%), helium (He, 0.138%) and air (the balance). During an end
inspiratory pause, and with constant levels of He and CO in the circuit, the
stopcock was turned so that the lungs were connected to the closed circuit.
The breathing of the animal effected mixing of the test gas and alveolar gas.
During this time the analysers continually monitored the levels of He and CO.
The stopcock was opened to room air once the value of He reached a new steady
state (Fig. 1).
|
As the solubility of He in blood is very low (0.0095 ml ml-1
atm-1) (Muysers and Smidt,
1969) the volume of the lung (VL) and circuit combined
(VSYS+VL) can then be determined from VSYS and the
known fractional concentrations of He initially and finally contained in the
circuit (FIHe - FEHe) as previously
described by Glass et al.
(1981
):
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The determination of carbon monoxide diffusing capacity of the lungs
(DLCO) was based largely on that described in Depledge
(1985) and Crawford et al.
(1976
). During the rebreathing
of the test gas the fraction of CO continually declined and
DLCO could be determined by the equation:
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Right-to-left shunt fraction
The existence of a right-to-left (RL) cardiopulmonary shunt was
determined from measurements of arterial O2 content
CaO2, venous O2 content
CO2 and
PAO2 during 100 % oxygen breathing for three
lizards resting on a treadmill. The total amount of oxygen leaving the
cardiopulmonary system is given by total cardiac output
(
tot)xCaO2,
which must equal the sum of the amounts of oxygen in the shunted blood, shunt
blood flow
(
shunt)xC
O2,
and the pulmonary end-capillary blood,
(
tot
shunt)xCc'O2,
where Cc'O2 is the O2 content
of end-capillary blood. This relationship can be rearranged to yield the
RL shunt fraction:
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Substitution of Cc'O2 and
CaO2 into the above equation yields:
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Calculated variables
A number of variables were derived.
Total cardiac output
(tot) was determined from
rearrangement of the Fick equation to:
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Alveolar ventilation (A) was
determined from the alveolar gas equation:
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Data collection and statistics
All data signals were fed into an A/D board (DT2801/A, Data Translations,
MA, USA) collecting at a speed of 50 Hz using ASYST (Macmillen Software,
Keithley Instruments, NY, USA) and stored on computer for later analysis. Data
are presented as means ± 1 S.D. Differences between treatments and time
intervals were analysed using repeated-measures ANOVA. Post-hoc
modified two-tailed t-tests were used to assess differences between
appropriate comparisons using the Bonferroni method. A significant difference
was defined as P<0.05.
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Results |
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Ventilation and breathing pattern before, during and after
locomotion
The breathing pattern at rest was typical for a lizard, i.e. evenly spaced
single breaths, each interrupted by an end-inspiratory pause
(Fig. 3). During sustained
locomotion there was a substantial reduction in TP (and a
slight shortening of TI and TE, though
this was not significant for TI)
(Fig. 3; Tables
1,
2).
E increased
(Table 2;
Fig. 4), aided by increases in
both VT and f
(Fig. 3B;
Table 2), in direct proportion
to
O2
(Fig. 4). Therefore, the air
convection requirement for oxygen,
E/
O2,
was maintained constant (Table
2; Fig. 4). A
similar situation existed for
E/
CO2
(Table 2). In the 2 min
following exercise, VT and f remained unchanged;
as a result
E was
maintained at the exercise level and, as a consequence of the tendency for
O2 to decline,
the
E/
O2
ratio tended to increase, though not significantly
(Fig. 4).
E/
CO2
was maintained at the same level as observed during exercise
(Table 2).
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Oxygen transport before, during and after locomotion
The Hb affinity in V. mertensi
(P50=49.3±4.4 mmHg, 35 °C,
PCO2=39 mmHg, pH approx. 7.5), together with
the Bohr factor (-0.30±0.048), were similar to values previously
published in varanid species; the P50 for V.
exanthematicus is 47 mmHg at 35 °C, pH 7.5, Bohr factor -0.30
(Wood et al., 1977), and for
V. gouldii P50 is 48 mmHg at 35 °C, pH 7.3
(Bennett, 1973a
). Despite a
small increase in PAO2 and
PaO2 (Tables
1 and
2), the alveolararterial
PO2 difference
(PAO2PaO2)
remained unchanged during exercise from its value at rest (approx. 20 mmHg)
and decreased slightly during the first few minutes of recovery
(Fig. 4). Arterial
O2 content, CaO2, remained constant
at all levels of
O2, though due
to the decrease in
P
O2 the venous
O2 content,
C
O2, dropped
substantially (Tables 1 and
2). As expected, given no
change in
E/
O2,
PaCO2 remained constant and no different from
PACO2 at all levels of
O2 (Table
1 and
2). Despite a tendency for
blood pH to decrease with exercise and during recovery this was not
significant. Lactate concentration [La] increased approx. 2.5-fold during and
after exercise (Tables 1 and
2). Hct and [Hb] remained
constant throughout exercise and in the recovery phase; the value of Hct was
identical to that reported for V. gouldii
(Bennett, 1973a
).
Systemic cardiac output and right-to-left shunt fraction
The associated increase in arterialvenous O2 content
difference
(CaO2CO2)
with exercise was not enough to offset any increases in cardiac output, hence
tot increased (Tables
1 and
2). The increase in
tot was achieved through a
corresponding increase in fH, and stroke volume remained unaltered
(approximately 1.72 ml kg-1). Both
tot and fH
remained high during the first 2 min of recovery. At rest V. mertensi
possessed a considerable right-to-left shunt
(Table 1), but unfortunately no
information was obtained on the shunt during exercise.
Lung volume and DLCO before and during locomotion
Locomotion had no effect on lung volume (VL) or the pulmonary
diffusing capacity for carbon monoxide (DLCO) (Table
1 and
2).
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Discussion |
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The [La] values reported here (8.5 mmol l-1) are similar to
levels found in V. exanthematicus (7.2 mmol l-1) at
maximal sustainable speed (Gleeson et al.,
1980). Both these values are much lower than the values
(approximately 28 mmol l-1) associated with high and probably
unsustainable levels of activity in that species
(Mitchell et al., 1981a
).
Obviously, accumulation of [La] implies that anaerobic metabolism is
supplementing aerobic energy production. During intensive bouts of activity,
anaerobic glycolysis in reptiles can account for most of the ATP produced,
with aerobic metabolism playing a minor role
(Bennett, 1994
). The reader is
referred to the reviews by Gleeson
(1991
,
1996
); also see
(Bennett, 1994
) for a detailed
account of anaerobic metabolism during intense activity in reptiles and the
aerobic payback that accompanies recovery.
Of particular interest in this study, given the recent findings that gular
movements may augment ventilation in V. exanthematicus
(Owerkowicz et al., 1999), are
the similar values in
O2 achieved for
lizards exercised with a mask encompassing the head or while fitted with a
mouthpiece. In the study by Owerkowicz et al.
(1999
), lizards in which the
mouth was propped open were unable to reach the level of
O2max achieved
when exercised with their mouth closed and therefore capable of gular pumping.
The implication from that study was that gular pumping was required to achieve
appropriate levels of
E
necessary to support the high levels of
O2 associated
with exercise. If the presence of a functional gular pump is an attribute of
all varanids during exercise, then it may be concluded that the mouthpiece
used in the present experiments did not impair the gular pump. However, we did
not measure buccal cavity pressure to determine if gular pumping was
occurring. On the other hand, Schultz et al.
(1999
) studied four species of
varanids (V. spenceri, V. gouldii, V. panoptes and V.
mertensi) and showed that, whereas at rest all species relied on nasal
breathing, when exercised they did so with their mouths open to various
degrees, which could account for 11-49 % of the expired gases. This finding
tends to suggest that gular-assisted ventilation is not obligatory and/or
widespread amongst the varanids. Obviously, further studies are required to
assess the degree to which gular pumping is a characteristic of varanids.
The finding that varanids breathe through their mouths during exercise
raises some concerns about the levels of ventilation reported in previous
studies, where ventilation was measured only through the nostrils. Carrier
(1987a) assessed inspiratory
airflows through the nostrils with a thermistor, and Hopkins et al.
(1995
) and Wang et al.
(1997
) measured airflow
through a pneumotacograph attached to the nostrils. Hopkins and coworkers
checked for mouth breathing at rest in an attempt to validate this approach
and found none; perhaps not surprising, given the findings of Schultz et al.
(1999
).
Is ventilation constrained by locomotion?
It has been hypothesised that mechanical interference exists between
locomotor and ventilatory function in lizards, as the intercostal muscles are
required for both locomotion (lateral flexion of the trunk) and ventilation
(expansion of the thorax) (Carrier,
1987a,
b
,
1989
,
1990
). Accordingly, the aerobic
needs of an exercising lizard cannot be met because of the locomotory
constraints on lung ventilation. Indeed, Wang et al.
(1997
) found no change in
tidal volume during exercise in V. exanthematicus but a two- to
fourfold increase immediately following exercise. An increase in
E did occur, however,
during exercise as a result of changes in f. The present study with
V. mertensi also finds an increase in
E during exercise, the
result of an increase in VT and f (primarily
through a shortening in TP), and this increase is retained
in the recovery period immediately following exercise. As previously
mentioned, costal ventilation during exercise was most likely constrained in
V. exanthematicus and gular pumping, by circumventing the constraint,
contributed to the overall increase in ventilation and hence
O2
(Owerkowicz et al., 1999
).
Gular pumping is not used in Iguana iguana and ventilation decreases
at speeds greater than a very slow walk
(Wang et al., 1997
;
Owerkowicz et al., 1999
).
While costal ventilation may be inhibited in some lizards during exercise,
the undulatory movement of the body trunk that accompanies locomotion may be
of benefit to gas exchange. The highly compliant nature of the respiratory
system in lizards (Perry and Duncker,
1978; Milsom,
1989
) provides minimal constraint to changes in body posture and
is prone to distortion. Any distortion of the lungs that occurs could enhance
the effectiveness of gas exchange by improving gas mixing. However, in an
experiment with V. gouldii, where individuals were hyperventilated to
lower the PaCO2 levels below the apneic
threshold, no change was detected in the time course of arterial blood gas
levels with or without lateral passive body movements
(Frappell and Mortola, 1998
).
This suggested that lateral chest wall movements neither hindered nor
facilitated gas exchange.
Does ventilation limit
O2 during exercise?
In the study of Wang et al.
(1997), the air convection
requirement
(
E/
O2)
in V. exanthematicus approximately doubled during exercise at
aerobically sustainable speeds, and on recovery returned to pre-exercise
levels, indicating a substantial hyperventilation with exercise. In contrast,
the present study finds that the increase in
E during exercise in V.
mertensi adequately meets the increase in
O2, hence
maintaining the
E/
O2
ratio constant between exercise and pre-exercise. While the study of Bennett
(1973b
) reports a relative
hypoventilation for V. gouldii immediately after activity, the
results should be viewed with some degree of scepticism, as activity was
achieved following electrical stimulation of restrained lizards.
While not directly measuring ventilation, Mitchell and coworkers
(Mitchell et al., 1981b)
indicated, from measurements of PaCO2 and the
alveolar gas equation, that during exercise V. exanthematicus
increased
A relatively more than
O2, giving rise
to an increase in PAO2 (again calculated, not
measured). Likewise, Hopkins et al.
(1995
) report a small
hyperventilation in V. exanthematicus, though the associated increase
in PaO2 was not significant. The present study
also revealed a small increase in PAO2 and
PaO2 with exercise and recovery in V.
mertensi. In the absence of any change in
E/
O2,
the increase in PAO2 suggests a proportionally
greater increase in alveolar ventilation (i.e. relative decrease in dead
space). At rest,
A accounts for
approx. 70 % of
E, whereas
during exercise and recovery this increased to approx. 95 % (Tables
1 and
2). In summary, the available
evidence suggests that, for varanids exercised at sustainable speeds,
E increases sufficiently
during exercise to meet the increased
O2, and the
small increase in PAO2 observed most likely
reflects a proportionally greater increase in
A.
Does O2 transfer across the lung limit
O2 during exercise?
At rest, a substantial alveolararterial
PO2
(PAO2-PaO2)
difference was observed in V. mertensi (approx. 21 mmHg), as has been
previously reported for V. exanthematicus (approx. 14 mmHg,
Mitchell et al., 1981b;
approx. 27 mmHg, Hopkins et al.,
1995
). During exercise the increase in
PAO2 and PaO2 in
both species (this study; Hopkins et al.,
1995
) maintains
PAO2-PaO2,
whereas Mitchell et al.
(1981b
) reported an increase
in PAO2-PaO2
during exercise to values similar to that found by Hopkins and coworkers. The
level of
PAO2-PaO2 is an
indication of the efficacy of alveolararterial O2 transfer
and could result from right-to-left (RL) shunts, diffusion limitation
or ventilationperfusion inhomogeneity.
At rest reptiles may possess RL shunts. A substantial RL
shunt of between 16 % (Berger and Heisler,
1977) and 6 % (Hopkins et al.,
1995
) has been reported in V. exanthematicus at rest,
though the lower value reported by Hopkins is for an intrapulmonary shunt
only, as their measurements excluded intracardiac shunting. The present study
reports a RL shunt in V. merstensi of approx. 17 % at rest.
Studies on pulmonary gas transfer must, strictly speaking, be based on blood
gases sampled from the pulmonary veins
(Wang et al., 1998
). In the
present study our measure of PaO2 could
potentially include intracardiac shunts, which conceptually are identical to
that of intrapulmonary shunts. However, based on studies on V.
niloticus (Millard and Johansen,
1974
; Ishimatsu et al.,
1988
) and V. exanthematicus
(Burggren and Johansen, 1982
),
it would appear that in varanids RL intracardiac shunting is much
reduced or absent.
During exercise V. exanthematicus decreased the RL
intrapulmonary shunt from about 6 % to 2 %
(Hopkins et al., 1995).
Further, many reptiles develop a significant LR shunt during exercise
(Hicks and Krosniunas, 1996
),
though in V. exanthematicus this was shown not to occur
(Hopkins et al., 1995
). A
reduction in the RL shunt may improve
PaO2, depending on
O2 and blood
O2 carrying capacity. If the O2 levels of the pulmonary
veins are on the flat portion of the oxygen dissociation curve then
CaO2 and PaO2
will be affected only marginally (Wang and
Hicks, 1996
). Further, changes in
CaO2-C
O2
can compensate for changes in shunt fraction to produce the same
PaO2 (see Equation 9 used for determining
shunt/
tot).
Compared with those of mammals, reptilian lungs have a lower surface area
for diffusion and a blood gas barrier of increased and variable thickness
(Perry, 1983;
Perry et al., 1994
). From the
relationship between
O2 and
DLO2
[
O2=DLO2x(PAO2-PaO2)],
the alveolararterial difference (for reptiles,
falveolarpulmonary vein difference) can be determined. While reptiles
have a lower metabolic rate than mammals, DLO2
is even lower, and as a result the equivalent
PAO2-PaO2 will
be higher in reptiles than mammals (Glass,
1991
). The present study measured DLCO
(DLO=1.23DLCO) in V. mertensi
and found at rest a value about double that previously reported in V.
exanthematicus at 35°C (Glass et
al., 1981
). In V. exanthematicus it was concluded that
diffusion limitation did not contribute significantly to the
PAO2-PaO2 at
rest (Hopkins et al., 1995
)
and presumably, given a higher DLCO, the same also applies
to V. mertensi. During exercise, however, Hopkins and coworkers found
that diffusion limitation accounts for a substantial part of the
PAO2-PaO2
difference in V. exanthematicus, a situation often reported in
mammals (see Powell,
1994
).
The overall ventilation perfusion ratio for the lung was 0.71 at rest and
increasing to 2.13 during exercise in V. exanthematicus, similar to
that estimated
{E/
L=
A/[
totx(1-
shunt/
tot)],
where
L is the blood flow through the
lung} for V. mertensi (approx. 0.8 and 2.1, respectively). With
exercise this reflects, in both species, a greater increase in ventilation
than perfusion. Alveolararterial PO2
difference is also enhanced by spatial ventilationperfusion
inhomogeneity, which during exercise increased in V. exanthematicus
(Hopkins et al., 1995
). A
similar situation occurs in exercising mammals, though the mechanisms
responsible for the development of ventilationperfusion inhomogeneity
during exercise remain unclear (Wagner et
al., 1986
; Schaffartzik et
al., 1992
; Hopkins et al.,
1994
). Together, the available information suggests that pulmonary
gas exchange in reptiles is no more limiting in varanids than it is in
mammals.
Does oxygen transport limit
O2 during exercise?
Oxygen delivery to the tissues depends on both
tot and
CaO2-C
O2
(oxygen extraction). The reliance on both
tot and oxygen extraction
by the tissues during exercise is fairly common amongst vertebrates
(Gleeson et al., 1980
) and
occurred in V. mertensi; the increase in
O2 during
exercise is achieved through increases in both O2 extraction and
cardiac output (Fig. 5). As
PAO2 and PaO2
are maintained or slightly increased during exercise, high values of arterial
saturation were preserved (Fig.
5). Likewise, V. exanthematicus had a high
CaO2 which, in turn, permitted a greater
CaO2-C
O2
during sustainable levels of exercise
(Gleeson et al., 1980
). Wood
et al. (1977
) also note that
the arterialvenous O2 content difference is large in V.
exanthematicus compared with most other groups of reptiles, and that this
was a result of low shunt flows and corresponding high values of arterial
saturation (PaO2=94 mmHg, at 35°C), hence
high CaO2.
|
The ability to maximise oxygen extraction during exercise is assisted by a
distinct Bohr shift in the hemoglobin saturation curve that further favours
unloading of O2, and a greater increase in
CaO2-CO2
than would otherwise be achieved (Fig.
5). Previously, Gleeson et al.
(1980
) noted that both V.
exanthematicus and I. iguana decreased mixed venous blood
O2 content to similar levels; the difference in
CaO2-C
O2
was a reflection of the varanid's greater CaO2.
Further, in that study, while the factorial scope in cardiac output was
greater in V. exanthematicus, both the varanid and iguana reached
maximum
tot at intermediate
levels of
O2.
This implies that without further increases in O2 extraction,
transport by the circulation could be limiting. It is also interesting to note
that although the air convection requirement
(
E/
O2)
remained constant, the blood convection requirement
(
tot/
O2)
decreased with exercise in V. mertensi, V. exanthematicus
(Gleeson et al., 1980
;
Wang et al., 1997
) and I.
iguana (Gleeson et al.,
1980
). A decreasing blood convection requirement during exercise
and an already large arterialvenous O2 content difference
suggests that there exists limited capacity for further increases in
O2 transport by the circulatory system. More recently, Farmer and
Hicks (2000
) suggested that in
I. iguana both the circulatory and ventilatory systems imposed limits
to O2 transfer at anything faster than a slow walk. However, in the
iguana maximum blood flow and ventilation occur during recovery
(Wang et al., 1997
;
Farmer and Hicks, 2000
). This
is in contrast to varanids, where ventilation and cardiac output reach a
maximum during exercise (Gleeson et al.,
1980
; Wang et al.,
1997
; this study).
In conclusion, we have examined O2 transfer in the varanid,
V. mertensi, during maximal levels of sustained aerobic exercise.
Ventilation and O2 transfer across the lung would appear to be
adequate to meet the aerobic needs of V. mertensi during exercise.
This is also the case in mammals, where the lungs are built with a significant
excess structural capacity (Hoppeler and
Weibel, 1998). On the other hand, at
O2max the
circulatory system in mammals operates at or close to the upper limit of
structural capacity for O2 transport
(Hoppeler and Weibel, 1998
).
Together with an already large arterialvenous O2 content
difference in V. mertensi, a decreasing blood convection requirement
with exercise suggests a physiological limit occurring with the transport of
oxygen by the circulatory system.
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Acknowledgments |
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References |
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Bennett, A. F. (1973a). Blood physiology and oxygen transport during activity in two lizards, Varanus gouldii and Sauromalus hispidus. Comp. Biochem. Physiol. 46A,673 -690.
Bennett, A. F. (1973b). Ventilation in two species of lizards during rest and activity. Comp. Biochem. Physiol. 46A,653 -671.
Bennett, A. F. (1994). Exercise performance of reptiles. In Advances in Veterinary Science and Comparative Medicine: Comparative Vertebrate Exercise Physiology, vol38B (ed. J. H. Jones), pp.113 -138. Academic Press: San Diego.
Berger, P. J. and Heisler, N. (1977). Estimation of shunting, systemic and pulmonary output of the heart, and regional blood flow distribution in unanaesthetised lizards (Varanus exanthematicus) by injection of radioactively labelled microspheres. J. Exp. Biol. 71,111 -122.[Abstract]
Burggren, W. and Johansen, K. (1982). Ventricular haemodynamics in the monitor lizard Varanus exanthematicus: pulmonary and systemic pressure separation. J. Exp. Biol. 96,343 -354.
Carrier, D. R. (1987a). Lung ventilation during walking and running in four species of lizards. Exp. Biol. 47,33 -42.[Medline]
Carrier, D. R. (1987b). The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. Paleobiol. 13,326 -341.
Carrier, D. R. (1989). Ventilatory action of the hypaxial muscles of the lizard Iguana Iguana: a function of slow muscle. J. Exp. Biol. 143,435 -457.[Abstract]
Carrier, D. R. (1990). Activity of the hypaxial muscles during walking in the lizard Iguana Iguana. J. Exp. Biol. 152,453 -470.[Abstract]
Christian, K. A. and Conley, K. E. (1994). Activity and resting metabolism of varanid lizards compared with `typical' lizards. Aust. J. Zool. 42,185 -193.
Crawford, E. C., Jr, Gatz, R. N., Magnuzzen, H., Perry, S. F. and Piiper, J. (1976). Lung volumes, pulmonary blood flow and carbon monoxide diffusing capacity of turtles. J. Comp. Physiol. B 107,169 -178.
Depledge, M. H. (1985). Respiration and lung function in the mouse, Mus musculus (with a note on mass exponents and respiratory variables). Respir. Physiol. 60, 83-94.[Medline]
Farmer, C. G. and Hicks, J. W. (2000).
Circulatory impairment induced by exercise in the lizard Iguana iguana.J. Exp. Biol. 203,2691
-2697.
Frappell, P. B., Lanthier, C., Baudinette, R. V. and Mortola, J.
P. (1992). Metabolism and ventilation in acute hypoxia: a
comparative analysis in small mammalian species. Am. J.
Physiol. 262,R1040
-R1046.
Frappell, P. B. and Mortola, J. P. (1998).
Passive body movement and gas exchange in the frilled lizard
(Chlamydosaurus kingii) and goanna (Varanus gouldii). J.
Exp. Biol. 201,2307
-2311.
Glass, M. L. (1991). Pulmonary diffusion capacity of ectothermic vertebrates. In The Vertebrate Gas Transport Cascade (ed. J. E. P. W. Bicudo), pp.154 -161. Boca Raton, Florida: CRC Press.
Glass, M. L., Johansen, K. and Abe, A. S. (1981). Pulmonary diffusing capacity in reptiles (relations to temperature and O2-uptake). J. Comp. Physiol. B 142,509 -514.
Gleeson, T. T. (1991). Paterns of metabolic recovery from exercise in amphibians and reptiles. J. Exp. Biol. 160,187 -207.
Gleeson, T. T. (1996). Post-exercise lactate metabolism: a comparative review of sites, pathways, and regulation. Annu. Rev. Physiol. 58,565 -581.[Medline]
Gleeson, T. T., Mitchell, G. S. and Bennett, A. F. (1980). Cardiovascular responses to graded activity in the lizards Varanus and Iguana. Am. J. Physiol. 239,R174 -179.[Medline]
Hicks, J. W. and Krosniunas, E. (1996). Physiological states and intracardiac shunting in non-crocodilian reptiles. Exp. Biol. Online 1,3 .
Holland, R. A. B., Rimes, A. F., Comis, A. and Tyndale-Biscoe, C. H. (1988). Oxygen carriage and carbonic anhydrase activity in the blood of a marsupial, the Tammar Wallaby (Macropus eugenii), during early development. Respir. Physiol. 73, 69-86.[Medline]
Hopkins, S. R., McKenzie, D. C., Schoene, R. B., Glenny, R. W.
and Robertson, H. T. (1994). Pulmonary gas exchange during
exercise in athletes I. Ventilation-perfusion mismatch and diffusion
limitation. J. Appl. Physiol.
77,912
-917.
Hopkins, S. R., Hicks, J. W., Cooper, T. K. and Powell, F.
L. (1995). Ventilation and pulmonary gas exchange during
exercise in the savannah monitor lizard (Varanus exanthematicus).
J. Exp. Biol. 198,1783
-1789.
Hoppeler, H. and Weibel, E. R. (1998). Limits
for oxygen and substrate transport in mammals. J. Exp.
Biol. 201,1051
-1064.
Ishimatsu, A., Hicks, J. W. and Heisler, N. (1988). Analysis of intracardiac shunting in the lizard, Varanus niloticus: a new model based on blood oxygen levels and microsphere distribution. Respir. Physiol. 71, 83-100[Medline]
Millard, R. W. and Johansen, K. (1974). Ventricular outflow dynamics in the lizard, Varanus niloticus: responses to hypoxia, hypercarpia and diving. J. Exp. Biol. 6,871 -880.
Milsom, W. K. (1989). Compatative aspects of vertebrate pulminary mechanics.. In Lung Biology in Health and Disease: Comparative Pulmonary Physiology, vol39 (ed. S. C. Wood), pp.587 -619. New York: Marcel Dekker.
Mitchell, G. S., Gleeson, T. T. and Bennett, A. F. (1981a). Ventilation and acidbase balance during graded activity in lizards. Am. J. Physiol. 240,R29 -R37.[Medline]
Mitchell, G. S., Gleeson, T. T. and Bennett, A. F. (1981b). Pulmonary oxygen transport during activity in lizards. Respir. Physiol. 43,365 -375.[Medline]
Muysers, K. and Smidt, U. (1969). Respirations-Massenspektrometric. Schattauer, Stuttgart New York.
Owerkowicz, T., Farmer, C. G., Hicks, J. W. and Brainerd, E.
L. (1999). Contribution of gular pumping to lung ventilation
in monitor lizards. Science
284,1661
-1663.
Perry, S. F. (1983). Reptilian Lungs. Functional Anatomy and Evolution. Springer-Verlag, Berlin.
Perry, S. F. and Duncker, H.-R. (1978). Lung structure, volume and static mechanics in five species of lizards. Respir. Physiol. 34,61 -81.[Medline]
Perry, S. F., Hein, J. and van Dieken, E. (1994). Gas exchange and morphometry of the lungs of the tokay, Gekko gekko L. (Reptilia: Squamata: Gekkoniodae). J. Comp. Physiol. B 164,206 -214.
Powell, F. L., Jr (1994). Respiratory gas exchange during exercise. In Advances in Veterinary Science and Comparative Medicine: Comparative Vertebrate Exercise Physiology, vol. 38A (ed. J. H. Jones), pp.253 -285. San Diego: Academic Press.
Schaffartzik, W., Poole, D. C., Derion, T., Tskukimoto, K. and
Hogan, M. C. (1992).
A/
distribution during heavy exercise and recovery in humans: implications for
pulmonary edema. J. Appl. Physiol.
72,1657
-1667.
Schultz, T. R., Christian, K. A. and Frappell, P. B. (1999). Do lizards breathe through their mouth while running? Exp. Biol. Online 4,39 -46.
Thompson, G. G. and Withers, P. C. (1997). Standard and maximal metabolic rates of goannas (Squamata: Varanidae). Physiol. Zool. 70,307 -323.[Medline]
Wagner, P. D., Gale, G. E., Moon, R. E., Torre-Bueno, J. R.,
Stolp, B. W. and Saltzman, H. A. (1986). Pulmonary gas
exchange in humans exercising at sea level and simulated altitude.
J. Appl. Physiol. 61,260
-270.
Wang, T. and Hicks, J. W. (1996). The
interaction of pulmonary ventilation and the right-left shunt on arterial
oxygen levels. J. Exp. Biol.
199,2121
-2129.
Wang, T., Carrier, D. R. and Hicks, J. W.
(1997). Ventilation and gas exchange in lizards during treadmill
exercise. J. Exp. Biol.
200,2629
-2639.
Wang, T., Smits, A. W. and Burggren, W. W. (1998). Pulmonary function in reptiles. In Biology of the Reptilia (ed. C. Gans and A. S. Gaunt), pp.297 -374. Ithaca: Society for the study of amphibians and reptiles.
Wood, S. C., Johansen, K. and Gatz, R. N. (1977). Pulmonary blood flow, ventilation/perfusion ratio, and oxygen transport in a varanid lizard. Am. J. Physiol. 233,R89 -R93.[Medline]