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First published online July 20, 2007
Journal of Experimental Biology 210, 2607-2617 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.008078
Blood oxygen depletion during rest-associated apneas of northern elephant seals (Mirounga angustirostris)
1 Center for Marine Biotechnology and Biomedicine, Scripps Institution of
Oceanography, University of California, San Diego, CA 92093-0204,
USA
2 US Naval Medical Center, Balboa Hospital, San Diego, CA 92134,
USA
3 Limnological Institute, Siberian Division of Russian Academy of Sciences,
Irkutsk, Russia
* Author for correspondence (e-mail: pponganis{at}ucsd.edu)
Accepted 17 May 2007
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Summary |
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Key words: blood gases, oxygen, PO2, elephant seal, apnea
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Introduction |
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; Blackwell and Le Boeuf,
1993; Kenny,
1979). In northern elephant seals (Mirounga
angustirostris), such breath-holds can last as long as 25 min, with mean
durations of 7–10 min in animals ranging from older weanlings to adults
(Blackwell and Le Boeuf, 1993).
These sleep apneas are considered to conserve energy and water during the
on-shore fast (Ortiz et al.,
1978), and it has been documented that the seals spend
60–70% of time during sleep in apnea
(Blackwell and Le Boeuf, 1993).
It is presumed that significant aerobic metabolism continues during these
apneas because of (1) the lack of lactate accumulation
(Castellini, 1994;
Castellini et al., 1986) and
(2) the maintenance of heart rate, cardiac output and presumed perfusion of
splanchnic organs during sleep apnea
(Ponganis et al., 2006).
Continued aerobic metabolism during these prolonged breath-holds would be
dependent on significant depletion of a large blood O2 store, a
rapid blood O2 depletion rate (relative to forced submersion) and
extreme hypoxemic tolerance. The latter is essential in order to achieve a low
enough oxygen partial pressure (PO2) to allow
depletion of the blood O2 store.
We investigated the change in blood gases and the depletion of blood
O2 during sleep apneas of young elephant seals in order to
determine (1) the magnitude of the pre-apneic blood O2 store, (2)
the percentage of this store consumed during a breath-hold, and its
contribution to total metabolic rate during the apnea, (3) the rate of apneic
blood O2 utilization and (4) the degree of hypoxemia that occurs
during apnea. We hypothesized that the blood O2 store would be
significantly depleted during rest apneas, the blood O2 depletion
rate would be greater than that during forced submersion, and the hypoxemic
tolerance of the seals would allow them to tolerate
PO2 values less than 25–30 mmHg [a
threshold at which humans lose consciousness
(Ferretti et al., 1991;
Ferrigno and Lundgren, 2003)],
thereby enabling them to extract more oxygen from blood.
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Materials and methods |
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Catheterizations and instrumentation
Catheterization of the brachial or femoral artery, extradural vein (EDV),
hepatic sinus (HS), and thoracic vena cava (TVC)–heart–pulmonary
artery (PA) were accomplished as previously described
(Ponganis et al., 2006). A
given seal was instrumented either with arterial and EDV catheters, or with a
suite of venous/cardiac catheters. Catheters were inserted percutaneously into
seals under general 1–2% isoflurane–O2 anesthesia after
mask induction and intubation. Catheter position was confirmed by
characteristic pressure wave forms (Hewlett Packard 78302A/78205D pressure
monitor; Palo Alto, CA, USA) and fluoroscopy before being secured with
neoprene, VelcroTM and Loctite 401 glue (Loctite Corporation, Rocky Hill,
CT, USA).
Transthoracic surface electrocardiogram (ECG) electrodes were affixed with rubber and Loctite glue and attached to an ECG/impedance monitor (RespI/ECG; UFI, Morro Bay, CA, USA). After extubation, seals were placed unrestrained in an open-framework PVC cage, which allowed easy access to catheters and ECG cables under a draped blind. The animals were left undisturbed, usually with hose water running over the bottom of the cage. The ECG and thoracic impedance (transduced from the ECG electrodes) were recorded to computer continuously at 100 Hz and 25 Hz sampling rates, respectively, with a BIOPAC Systems Model MP100 analog-to-digital interface, using AcqKnowledge software (BIOPAC Systems, Goleta, CA, USA). Blood sampling times were marked in the computer record.
The seal was allowed to recover from anesthesia for at least 6 h before blood samples were drawn at the resumption of prolonged (>3 min), spontaneous apneas. Blood was drawn intermittently during eupneas and serially during apneas. At the end of the study, catheters and ECG electrodes were removed after 0.5 mg kg–1 ketamine intravenous sedation. Prophylactic cephalexin was administered intravenously (1 g every 6 h) while the seals were instrumented, and then orally (250 mg, three times per day) for three days afterwards.
Blood sampling, [hemoglobin], hematocrit and blood volume assays
Duplicate 1-ml blood samples were drawn into pre-heparinized blood gas
syringes (MarquestTM GaslyteR; Marquest Medical Products,
Englewood, CO, USA) with extra 0.05 ml heparin (10 000 U
ml–1) in the hub. One sample was used for blood gas analysis;
the duplicate for hematocrit (Hct) and hemoglobin concentration ([Hb])
determinations. Blood analyses for PO2,
PCO2, pH and oxygen saturation
(sO2%) were performed at 37°C with an i-STAT Portable Clinical
Blood Analyzer (i-STAT Corporation, East Windsor, NJ, USA) using G3+
cartridges (Abbott Laboratories, Abbott Park, IL, USA). [Hb] was determined by
a cyanmethemoglobin colorimetric assay, using either the Sigma #525 Total
Hemoglobin Test Kit (Sigma Diagnostics, St Louis, MO, USA) or the Teco
Diagnostics (Anaheim, CA, USA) total hemoglobin kit. Hematocrit was determined
by microcentrifugation. Blood oxygen contents presented in this paper were
calculated by assuming 1.34 ml O2 g–1 Hb and
multiplying by the percent oxygen saturation from the i-STAT analyzer and the
[Hb] result from the assay (Qvist et al.,
1986). Accuracy of this calculation was assessed in one experiment
in which blood oxygen content measurements
(Tucker, 1967) were made with
the use of a borrowed Tucker chamber and oxygen electrode (Strathkelvin
Instruments, Motherwell, UK: Tucker chamber, model 1302 electrode, and 781
meter).
Plasma volume (VP) was determined using the Evans blue
dye dilution technique (Castellini et al.,
1987) either during anesthesia or at the end of an experiment (4
ml of a 10 mg ml–1 dye stock solution were injected). Samples
were drawn at 5, 10, 15, 20 and 30 min after injection. Absorbances were read
at 624 nm and compared to a standard curve made in the individual seal's blank
plasma (collected prior to injection of the dye). To calculate blood volume
(VB), the highest Hct recorded during an apnea with a
given seal was used in the formula
VB=VP/(1–Hct)
(Ponganis et al., 1993).
Data analysis
Data from eupneic and apneic periods were sorted according to the
respiratory impedance trace on the continuous computer record. An apnea was
defined as any breath-hold longer than 3 min duration. In order to conduct
analyses over a wide range of apneic durations, data from a given blood
sampling site were pooled from all animals and analyzed together.
Venous samples were categorized into three compartments for comparison: pulmonary artery (PA), central venous (CV: right atrium (RA), thoracic vena cava, hepatic sinus) or extradural vein (EDV).
Data were graphed and analyzed with Excel (Microsoft), Origin (Microcal Software, Northampton, MA, USA) and SPSS (Chicago, IL, USA) software. All means are ± 1 s.d. Statistical significance was assumed at P<0.05. Because the output of the blood gas analyzer was in mmHg, those units will be presented in tables and curve-fitting equations throughout this paper. In the graphs, however, dual axes for both mmHg and kPa will be shown (1 mmHg=0.133 kPa).
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Calculated vs measured O2 content
A linear regression analysis was performed on calculated vs
measured O2 content values of 24 blood samples from one seal (W1).
The resulting equation was: y=1.003x+1.1 (where y
is calculated O2 content and x is measured O2
content); r2=0.90; P<0.0001.
Blood gases, O2 content, pH and [Hb] during eupnea
During the eupneic periods between rest-associated apneas, the blood gas
values were widely variable; ranges are presented in
Table 3. Variability was
evident both within and between individual seals. Ranges of venous
PO2 and PCO2
were quite similar among the three venous compartments (PA, CV and EDV).
Overall, the venous PO2 varied between 34 and
71 mmHg, while the arterial spanned 40 to 108 mmHg; venous
PCO2 ranged from 45 to 64 mmHg and arterial
from 35 to 56 mmHg. Ranges of pH, [Hb] and O2 content were similar
for both arterial and venous blood during eupnea. Mean eupneic Hct of venous
and arterial blood among seven seals was 53±5.0% (N=60).
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Venous PO2 values in the EDV, CV and PA each declined exponentially, while O2 contents decreased linearly in these three compartments (Fig. 2). The data from the different venous compartments were also pooled to construct curve fits for a combined venous response (black lines in Fig. 2). On average, the initial oxygen tension in the venous system was 59 mmHg and decreased to 21 mmHg after 9 min of apnea (Fig. 2A). The lowest PO2 recorded was 15 mmHg after a 7.8 min apnea. Venous O2 content at the onset of apnea was 26.0 ml dl–1 on average and decreased linearly, at a rate of 2.0 ml dl–1 min–1, to 7.8 ml dl–1 by 9 min (Fig. 2B). The lowest venous O2 content recorded was 4.8 ml dl–1 after a 7.1 min apnea.
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PCO2, pH, [Hb] and Hct during apnea
Changes in arterial and venous PCO2, pH,
[Hb] and Hct during apnea are shown in Fig.
4. During apnea, mean PCO2 rose
gradually from initial values of 49 (arterial) and 52 (venous) mmHg to 63 mmHg
by 9 min. pH was identical in both systems and, on average, decreased less
than 0.05 units during a 9 min apnea. [Hb] and Hct were scattered but showed a
slowly increasing trend with apnea duration. Arterial and venous [Hb] in all
animals ranged between 19 and 28 g dl–1, while Hct ranged
between 47 and 67%.
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), seals in this size range do successfully complete these
trips (Hindell et al., 1999;
Le Boeuf et al., 1994;
McConnell et al., 2002;
Thorson and Le Boeuf, 1994).
In addition, their Hct, [Hb], VB and heart rate responses
were in the same ranges as those of healthy, young elephant seals both in the
wild and in captivity (Andrews et al.,
1997; Castellini et al.,
1994a; Castellini et al.,
1994b; Falabella et al.,
1999; Ponganis et al.,
2006; Simpson et al.,
1970; Thorson and Le Boeuf,
1994). Therefore, as already reviewed by Ponganis et al.
(Ponganis et al., 2006), we do
not think that the data collected in this study were compromised by the
rehabilitated status of these seals.
It should be noted that mass-specific VP and
VB increase during the first year of life in seals. These
increases are probably secondary to hematopoietic processes and to changes in
body mass/composition. Plasma volume and VB are 45 ml
kg–1 and 110 ml kg–1, respectively, in
northern elephant seal pups (Castellini and
Castellini, 1993; Castellini
et al., 1987), 74 ml kg–1 (estimated from mean
Hct and VB data) and 175 ml kg–1,
respectively, in 90 kg, 8-month-old juvenile elephant seals on return from
their first trip to sea (Thorson and Le
Boeuf, 1994), and 63–98 ml kg–1 (estimated
from individual Hct and VB data) and 175–256 ml
kg–1 in captive 6-month to 3-year-old juvenile elephant seals
(Simpson et al., 1970). The
values of the seals in this study are closest to those of the young juveniles
in the latter two studies.
Blood sampling and data collection during an experiment began after the
seal resumed spontaneous, long (>3 min) apneas, and at least 6 h
post-anesthesia. By this time, the heart rate had also resumed normal sinus
arrhythmic patterns, equivalent to those recorded from seals more than 24 h
post-anesthesia (P.J.P. and T.K.S., unpublished data)
(Castellini et al., 1994b).
For these reasons, and because of the fast wash-out of isoflurane in pinnipeds
(Gales, 1998), it was
concluded that there was no residual effect of anesthesia on the data.
In this study, we assumed that a seal was asleep during breath-holds if we
observed closed eyes, occasional facial twitching and lack of body movement.
These signs are indicative of sleep apnea, as documented by
electroencephalography (EEG) (Castellini
et al., 1994a). However, because we had no direct EEG evidence of
sleep state in our seals, we have referred to these breath-holds as
`rest-associated' apneas.
In order to have sufficient data to profile a given parameter over a range of apneic durations, it was necessary to pool the data from different breath-holds and from different animals for analysis. This was due to (1) catheter malfunctions, (2) the amount of time required to clear dead space, draw samples and flush lines, and (3) the inability to predict when a given apnea would start and end. A univariate analysis of covariance test (SPSS) of O2 contents revealed that there were no significant differences among either venous or arterial samples when the interaction between individual seal and time into apnea was tested (venous, F=1.615, P=0.101; arterial, F=2.058, P=0.096). Nevertheless, it is acknowledged, and should be noted by the reader, that this pooling approach is limited in that it does not allow evaluation of differences between individual seals or within apneas of different durations. However, given the technical difficulties of successful blood sample collection in these studies, this approach was necessary.
Lastly, we concluded that it was appropriate to calculate O2
contents from the [Hb] of a given sample and its percent saturation given by
the i-STAT analyzer, based on the regression analysis of calculated
vs empirical results from one experiment
(y=1.003x+1.1, r2=0.90,
P<0.0001, N=24). The use of calculated values has been
accepted in a previous study (Qvist et
al., 1986) and was necessitated in the present study by the
unavailability of the borrowed Tucker chamber and oxygen electrode in
remaining experiments.
Eupnea
Due to the characteristic irregular, intermittent breathing pattern of
phocid seals (Bartholomew,
1954; Blackwell and Le Boeuf,
1993; Harrison and Kooyman,
1968; Kenny,
1979), the eupneic blood values recorded from the elephant seals
in this study were variable, as found in previous studies (Tables
3,
4). The data in
Table 3 demonstrate that
similar ranges and variation existed within individuals. In general, arterial
PO2 values were lower, and
PCO2 values higher, than those of terrestrial
animals at rest (Taylor et al.,
1987) and were within the same range as eupneic values of actively
diving seals (Kooyman et al.,
1980; Qvist et al.,
1986). Differences in respiratory rates, tidal volumes,
ventilation/perfusion matching in the lungs and splenic contraction may
contribute to such variation.
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The blood O2 store
The magnitude of the available blood O2 store is dependent on
blood volume, Hb content and the pre-apneic Hb saturation. The circulating Hb
content and the Hct used in the calculation of VB from
VP are affected by the degree of splenic contraction
(Ponganis et al., 1993;
Qvist et al., 1986). In
determining VB from VP, we used the
highest Hct measured during the rest apneas of a given seal, an approach
(Ponganis et al., 1993) that
was confirmed with labeled red blood cell and plasma volume determinations in
Weddell seals (Hurford et al.,
1996). This technique may result in an underestimation of
VB and the blood O2 store if splenic
contraction is incomplete during rest apneas. However, Hcts used in the
VB calculation (Table
1) were >55% in five of seven seals and 
60% in four seals.
Given the similarity between VB results in this study and
values in the literature as noted above
(Simpson et al., 1970;
Thorson and Le Boeuf, 1994),
we believe this approach yields reasonable estimates of VB
in the seals.
Calculation of the available pre-apneic blood O2 stores assumed
blood volume fractions of 0.33 arterial and 0.67 venous and that all blood
O2 was available for consumption. The mean arterial and venous
pre-apneic O2 contents (y-intercepts in
Fig. 3) were 27.2 and 26.0 ml
O2 dl–1, respectively. With these assumptions and
variables, the mean mass-specific blood O2 store prior to
rest-associated apnea would be 52 ml O2 kg–1. In
contrast to this value for rest apnea, it is quite possible that the available
blood O2 store prior to a dive may be higher, since ventilatory and
heart rates are higher during surface intervals between dives than they are
during rest apneas (Andrews et al.,
2000; Andrews et al.,
1997). These differences could result in higher
PO2 values, lower
PCO2 values, higher Hb saturation and greater
O2 contents for a given [Hb] in both the arterial and venous
reservoirs prior to a dive. For example, in free-diving Weddell seals, the
highest arterial PO2s recorded were always from
the short rest period prior to a dive and were higher than mean resting values
(Kooyman et al., 1980;
Qvist et al., 1986). The
highest pre-apneic arterial and venous O2 contents in this study
were 35.0 and 32.9 ml O2 dl–1, respectively. These
were from blood samples that were 97% and 90% saturated. These levels, which
are approaching the maximum oxygen contents possible for a [Hb] near 27 g
dl–1, would result in a pre-dive blood O2 store of
66 ml O2 kg–1.
The initial arterial and venous O2 contents used in the calculation of the pre-apneic blood O2 store differed by only 1.2 ml O2 dl–1. Because of this small difference, the magnitude of the blood O2 store is not affected by the degree of splenic contraction or relaxation at the start of an apnea. Even if 50% of the venous blood volume (66 ml blood kg–1) were stored with an arterial O2 content in the spleen, this would add less than 1 ml O2 kg–1 to the calculated blood O2 store in this study. Therefore, in this situation, the contraction state of the spleen has little effect on the total blood O2 store.
Apnea
The apneic profiles of PO2, O2
content, pH, PCO2, [Hb] and Hct (Figs
1,
2,
3,
4) describe the general pattern
of change in these parameters among several seals in relation to time into
apnea. Although the rate of change in these parameters cannot be calculated
for an individual seal or apnea, the data nevertheless provide several
valuable observations.
First, arterial PO2 values are
indistinguishable from venous data after the first minute of apnea. In
addition, initial venous O2 content is nearly as high as the
arterial value prior to the start of the breath-hold. These data confirm
Scholander's prediction (Scholander,
1940) that the venous blood is highly saturated in seals, and
reinforce the concept of a venous O2 reservoir
(Elsner, 1969;
Elsner et al., 1964). It would
appear that a small apneic lung volume (due to exhalation prior to the
breath-hold) and probable lung atelectasis minimize any contribution of the
lung to the total body O2 store during these breath-holds.
Second, Elsner and colleagues' (Elsner,
1969; Elsner et al.,
1964) observation that hepatic sinus O2 contents were
greater than aortic values during forced submersions is also supported by the
general patterns of decline in PO2 and
O2 content at the EDV, CV and PA sites
(Fig. 2) and by the two paired
analyses (HS vs PA; EDV vs PA) conducted in the current
study. Although limited by sample size, these findings support the concept
that depletion of lung O2 and mixture of less oxygenated blood from
the brain and heart with blood from the hepatic sinus will result in a lower
O2 content in the pulmonary artery and aorta during apnea.
Third, the relatively high initial Hb concentrations and O2
contents during apneas in this study, and the small change in [Hb] throughout
apneas, are similar to previous findings during sleep apnea
(Castellini et al., 1986) and
support the concept that short eupneic intervals do not provide sufficient
time for the large spleen of a seal (Bryden
and Lim, 1969; Ponganis et
al., 1993; Qvist et al.,
1986) to fully re-expand and significantly lower hematocrit and
[Hb] between apneic periods. Castellini
(Castellini, 1994) noted that
Hct remained elevated during sequential apnea–eupnea cycles (as were
characteristic of the seals in our study), and that it was necessary for the
seal to be awake and breathing for several minutes after these cycles before
Hct would decrease significantly. Two other studies have documented that
splenic re-expansion times observed after dives (6–9 min half-times)
(Hurford et al., 1996) and
forced submersions (18–22 min for complete re-expansion)
(Thornton et al., 2001) are
long in comparison with eupneic intervals. The eupneic phases in our study
generally lasted only a few minutes, with much of that time characterized by
intermittent, slow breaths and usually with the seal still apparently asleep.
The relatively high [Hb]s throughout apnea–eupnea cycles during both
diving activity (Castellini et al.,
1988) and sleep apnea suggest that the spleen is predominantly
contracted during these activities, with only minor variation in the degree of
expansion or contraction during eupnea and apnea, respectively. In addition,
as already discussed, the magnitude of the blood O2 store in these
seals is not significantly affected by the degree of splenic contraction or
relaxation. However, it should be noted that any further contraction of an
incompletely contracted spleen during apnea would presumably contribute to the
maintenance of hepatic sinus O2 content during apnea.
Lastly, the minimal change in pH throughout these apneas is presumably
secondary to the modest increase in PCO2
(Fig. 4) and is consistent with
the lack of lactate accumulation previously documented during sleep apnea of
seals (Castellini, 1994;
Castellini et al., 1986).
Given the low range of arterial PO2 during
these apneas, the lack of evidence for significant glycolysis again emphasizes
the ability of these animals to continue to function in what would be
considered extreme hypoxemia in other mammals.
Depletion of blood O2 stores
During apnea, O2 contents declined linearly in both the arterial
and venous compartments, at rates of 2.3 and 2.0 ml O2
dl–1 min–1, respectively (Figs
1,
2). These values represent
overall depletion rates, not instantaneous blood O2 consumption
rates throughout the apnea. The depletion rates found during rest apnea in
this study are greater than those found during forced submersions of similarly
sized seals, or during free dives of larger seals. For example, Scholander
found that during forced submersions of grey seals (Halichoerus
grypus) and hooded seals (Cystophora cristata), arterial oxygen
was depleted at a constant rate of 1.3 and 1.6 ml O2
dl–1 min–1, respectively
(Scholander, 1940). Analyses
of forced submersion data revealed arterial O2 depletion rates of
1–2 ml O2 dl–1 min–1 in
harbor seals (Kerem and Elsner,
1973), and venous and arterial O2 depletion rates of
1.0 and 1.3 ml O2 dl–1 min–1 in
elephant seals (Elsner et al.,
1964; Elsner,
1969). In the only study that measured O2 content in
free-diving seals (Weddell seals), Qvist et al. found that arterial
O2 content decreased by 0.8 ml O2 dl–1
min–1 (Qvist et al.,
1986). In comparison with the data from forced submersions, the
rapid rates of blood O2 depletion observed in the current study are
consistent with higher cardiac outputs during rest apneas
(Ponganis et al., 2006) than
during forced submersions (Blix et al.,
1983). This is consistent with the classic concept that heart rate
and organ perfusion control the rate of blood O2 depletion during
apnea.
With a mean pre-apneic blood O2 store of 52 ml O2
kg–1 (calculated using the intercepts of arterial and venous
O2 content regressions in Fig.
3B), the mean blood oxygen depletion rates in
Fig. 3B would result in
consumption of 56% of the initial blood O2 store by the end of a
typical 7-min apnea. This depletion of blood O2 would contribute
4.2 ml O2 kg–1 min–1 to total
body metabolic rate during apnea. This estimated blood O2
contribution to apneic metabolic rate appears reasonable in that oxygen
consumption measured in 10–22-month-old juvenile northern elephant seals
resting at the surface in a laboratory tank was 4.46±0.3 ml
O2 kg–1 min–1
(Webb et al., 1998). In
addition, Thorson and Le Boeuf found that oxygen consumption, averaged over
both the apneic and eupneic intervals in young elephant seals, ranged from 2
to 6 ml O2 kg–1 min–1, with a
value near 3 ml O2 kg–1 min–1 for
a 7 min apnea (Thorson and Le Boeuf,
1994). This large blood O2 contribution to apneic
metabolic rate is again consistent with the maintenance of cardiac output
during rest-associated apneas (Ponganis et
al., 2006) and reinforces the hypothesis that most metabolic
processes continue aerobically during these breath-holds.
End of apnea and hypoxemic tolerance
Table 5 compares the results
from our study with previously published data from rest apneas, forced
submersions, and free-diving in different seal species. The lowest
PO2 values (18 mmHg arterial; 15 mmHg venous)
during rest apneas of these elephant seals are greater than those (10 mmHg
arterial and 2.5 mmHg venous) in extreme forced submersions of harbor seals
(Kerem and Elsner, 1973), but
are similar to arterial and end tidal values during rest apneas and free dives
of Weddell seals (Kooyman et al.,
1980; Ponganis et al.,
1993; Qvist et al.,
1986). A similar pattern holds for O2 content. The
lowest arterial PO2 values during
rest-associated apneas of elephant seals are also lower than the end-tidal
PO2 of climbers on Mt. Everest without
supplemental oxygen (35 mmHg) (West et
al., 1983), and lower than the 25 mmHg
PO2 threshold typically associated with loss of
consciousness in humans (Ferretti et al.,
1991; Ferrigno and Lundgren,
2003). Several mechanisms may contribute to such hypoxemic
tolerance. In addition to potential effects of hypercarbia on Hb-O2
binding and the magnitude of cerebral blood flow, one possible explanation is
the higher brain capillary density and shorter diffusion distances found in
elephant seals vs. terrestrial animals
(Kerem and Elsner, 1973).
Another possibility is that the brains of seals may contain high
concentrations of the recently discovered oxygen-binding protein neuroglobin
(Burmester et al., 2000),
although this has yet to be studied.
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Given that juvenile elephant seals spend up to 70% of their sleep in apnea
(Blackwell and Le Boeuf, 1993),
it appears that these animals spend a significant portion of their time in
what would be considered a hypoxemic state in other mammals. For example,
tolerance of these low oxygen tensions would make it possible for up to 88% of
the initial blood O2 store to be extracted during an 11-min apnea
(as exhibited by one of our seals). Yet, despite such low levels of oxygen
tension, which are necessary to allow dissociation of O2 from Hb
for utilization of the blood O2 store, there is no evidence of
significant glycolysis or lactate accumulation
(Castellini et al., 1986;
Castellini, 1994). The minimal
pH changes observed in our study, which were attributable to CO2
accumulation during apnea, support those previous findings and emphasize the
ability of these animals to maintain aerobic metabolic function, even at low
PO2 values.
Data from arterial blood samples drawn within the last minute of apnea were
plotted against apnea duration (Fig.
5) to discern if there were any critical parameters that would
trigger the end of apnea. The endpoint values were variable, both within and
among individuals, indicating that there were no definable thresholds
associated with the resumption of breathing. Qualitatively, end-of-apnea
PO2 and O2 contents were lower with
longer apneas, varying by as much as 22 mmHg or 18 ml dl–1,
respectively. End-of-apnea PCO2 showed an
increasing trend as apnea duration increased, varying between 49 and 70 mmHg,
while pH fluctuated by less than 0.1 pH unit. In previous studies
(Milsom et al., 1996;
Stephenson, 2005), it has been
suggested that CO2 may play a primary role in the drive to breathe,
but such investigations of respiratory control were beyond the scope of the
present project. Our results serve only to demonstrate that end-of-apnea
levels are variable in the blood and that there is no apparent threshold
associated with the end of apnea.
Conclusions
The rest-associated apneas of the juvenile elephant seals in this study
lasted between 3.1 and 10.9 min. The mean pre-apneic venous O2
content of 26.0 ml O2 dl–1 was almost equivalent
to the mean arterial value of 27.2 ml O2 dl–1.
During apnea, blood oxygen was depleted at an average rate of 2.3 ml
O2 dl–1 min–1 in the arterial
system and 2.0 ml O2 dl–1 min–1
in the venous system. During a typical 7-min breath-hold, blood O2
depletion was calculated to contribute 4.2 ml O2
kg–1 min–1 to total body metabolic rate
during apnea and to consume 56% of the initial blood O2 store (52
ml O2 kg–1). These O2 depletion rates
are approximately twice those observed during forced submersion and are
consistent with maintenance of cardiac output during rest-associated apneas.
Blood PO2 declined exponentially during apnea,
with arterial and venous values becoming similar after the first minute of the
breath-hold. The lowest PO2 values recorded
were 18 mmHg (arterial) and 15 mmHg (venous), demonstrating remarkable
hypoxemic tolerance in these seals even during rest-associated apneas. Despite
such low O2 tensions, changes in pH were consistent with increased
PCO2 but showed no evidence of a metabolic
acidosis and lactate accumulation. All these findings are consistent with the
maintenance of aerobic metabolism despite low
PO2 during these apneas. Such hypoxemic
tolerance is necessary in order to allow O2 dissociation from Hb
and utilization of the blood O2 store during apnea.
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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