|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online October 7, 2008
Journal of Experimental Biology 211, 3323-3332 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018887
Blood flow and metabolic regulation in seal muscle during apnea
1 Center for Marine Biotechnology and Biomedicine, Scripps Institution of
Oceanography, University of California San Diego, La Jolla, CA 92093,
USA
2 Department of Biochemistry and Molecular Medicine, University of California
Davis, Davis, CA 95616, USA
3 GE Medical Systems, Fremont, CA 94539, USA
* Author for correspondence (e-mail: TJue{at}ucdavis.edu)
Accepted 19 August 2008
 |
Summary |
|---|
 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Key words: Doppler, muscle, myoglobin, NMR, oxygen, hemodynamics
|
INTRODUCTION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
). Both the breath holds and the post-apneic ventilatory
periods (eupneas) occur during slow-wave sleep
(Castellini et al., 1994;
Le Boeuf et al., 2000). These
two factors, apneic duration and a sleeping, non-mobile subject, make sleep
apnea in the elephant seal a model in which it is feasible to conduct detailed
physiological and biochemical investigations.
Physiological and metabolic responses during these sleep apneas contrast
with those observed during forced submersions of seals. First, during sleep
apnea, a mild bradycardia of 40–50 beats min–1
maintains cardiac output at levels characteristic of similarly sized mammals
at rest (Andrews et al., 1997;
Ponganis et al., 2006). Such
moderate bradycardias are also characteristic of many dives
(Andrews et al., 1997). Second,
mean apneic muscle blood flow (MBF), as measured by laser-Doppler flowmetry
(LDF), declines to only 50% of the eupneic level
(Ponganis et al., 2006).
Third, blood lactate concentrations do not change
(Castellini et al., 1986) even
as both arterial and venous oxygen partial pressure
(PO2) decline to 15–20 mmHg (1 mmHg=0.133
kPa) by the end of long apneas (Stockard
et al., 2007). By contrast, during forced submersions, the animals
exhibit severe bradycardia and peripheral vasoconstriction, which isolate
muscle and most organs from the circulation and preserve blood O2
for the brain and heart (Blix et al.,
1983; Elsner et al.,
1964; Scholander,
1940; Zapol et al.,
1979). Under the extreme physiological conditions during forced
submersions, muscle metabolism must presumably rely on the large Mb-bound
O2 store, creatine kinase reaction and glycolysis to generate ATP
(Scholander, 1940;
Scholander et al., 1942;
Stephenson and Jones,
1992).
The high Mb concentrations in seal muscle have classically been considered
to provide an O2 store
(Scholander, 1940;
Scholander et al., 1942).
Recently, however, studies have suggested that the Mb in seal muscle serves
primarily to facilitate diffusion of O2 and that blood-to-muscle
O2 transfer accounts for the less than expected Mb desaturation
detected with near-infrared spectroscopy (NIRS) during dives
(Guyton et al., 1995). Since
NIRS cannot distinguish Mb and Hb signals, questions still remain about the
role of Mb in regulating O2 consumption during a breath hold. In
addition, on the basis of measurements of Mb translational diffusion
coefficients, Mb does not contribute significantly to the O2 flux
in myocyte and perfused rat myocardium
(Lin et al., 2007a;
Lin et al., 2007b;
Papadopoulos et al., 2001).
Therefore, further investigations of seal muscle are necessary in order to
better define the muscle metabolic responses and the role(s) of the
exceptionally high Mb concentrations in these divers.
In contrast to NIRS, 1H NMR techniques can discriminate the
deoxy-Mb and deoxy-Hb proximal histidyl N
H signals in
vivo. These signals reflect the change in tissue and vascular
PO2. As PO2
falls, the proximal histidyl NH signal of deoxy-Mb and
deoxy-Hb signal intensity increases
(Kreutzer et al., 1992;
Ponganis et al., 2002;
Tran et al., 1999). These
peaks yield a quantitative measurement of dynamic changes in the intracellular
and vascular oxygenation during a breath hold
(Chung et al., 2005;
Jue, 2004;
Kreutzer and Jue, 1995;
Kreutzer et al., 1998;
Tran et al., 1999). In
addition, NMR techniques allow the calculation of Mb translational diffusion
coefficients (Lin et al.,
2007a; Lin et al.,
2007b). The present study has applied NMR spectroscopy in
combination with LDF to evaluate cellular metabolic responses of seal muscle
to the progressive ischemia and hypoxemia of sleep apnea.
Indeed, MbO2 desaturates approximately 20% from its control
level during apnea and resaturates rapidly at the beginning of eupnea. The Mb
and MBF kinetics correlate and suggest a local vasoconstriction mechanism.
Moreover, the translational diffusion measurements demonstrate that Mb has a
predominant role in facilitating O2 diffusion under all
physiological conditions. Mb releases part of its O2 store at the
beginning of apnea. The blood-to-cell O2 gradient narrows as apnea
progresses and implicates a relative reduction in energy metabolism during
late apnea in comparison with eupnea and the start of apnea. The present study
establishes the experimental basis to study the metabolic adaptations
associated with sleep apnea in seals
(Ponganis et al., 2002).
|
MATERIALS AND METHODS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Two eight-month old seals (70 and 62 kg in body mass) were used for the NMR studies. They were transported by van to the University of California San Francisco (UCSF), CA, USA. En route, they were maintained at the University of California Santa Cruz (UCSC) Long Marine Laboratory, Santa Cruz, CA, USA. The 70 kg seal was also used in the blood flow studies.
This research protocol was approved by the UCSD, UCSC and UCSF Animal Subjects Committees and was conducted under a marine mammal permit (#732-1487).
Blood flow
The LDF measurements were conducted as described previously
(Ponganis et al., 2006).
Briefly, after intramuscular ketamine (2 mg kg–1) sedation, a
fiberoptic LDF probe (Transonics N Type probe, ALF 21 flowmeter, Ithaca, NY,
USA) was placed percutaneously under local anesthesia (2% xylocaine,
intradermal) into the longissimus dorsi muscle (5 cm depth). The probe was
secured to the skin with a neoprene patch and epoxy glue. After recovery from
anesthesia (4 h), the unrestrained 70 kg seal exhibited spontaneous,
4–10 min apneas during periods in which the seal appeared somnolent and
exhibited the occasional facial twitching characteristic of sleep apnea
(Castellini et al., 1994).
Blood flow was recorded on a personal computer at 100 Hz frequency with
Axotape (Axon Instruments, Foster City, CA, USA) software. Visual or auditory
monitoring of respirations allowed event marking on the recording system.
Instantaneous beat-to-beat heart rate (fH) and flow were
calculated and verified with Acqnowledge software (Biopac Systems, Santa
Barbara, CA, USA). Blood flow profiles were constructed on Origin 4.1. Apneic
and eupneic MBFs were defined as the flow rate measured during the entire
apneic or eupneic period.
The flow probe was removed after ketamine sedation. Prophylactic cefalexin
was administered intravenously (1 g) on the day of the study and for two days
afterwards (250 mg per orem, three times per day). Zero flow calibration of
the probe was conducted by inserting the probe into the longissimus dorsi
muscle of an elephant seal carcass and resulted in a value of 0.03±0.02
perfusion units (mean ± s.d.). Percutaneous needle muscle biopsy
samples were also obtained after ketamine sedation
(Ponganis et al., 1993b).
In vivo NMR
In vivo NMR measurements were performed on a 1 m bore diameter GE
Signa scanner (GE Medical Systems, Fremont, CA, USA) at 1.5 T using the same
seal used in the MBF study. In San Diego, the seal was trained to enter a
fiberglass tube, which was fitted to the magnet bore and in which the
unrestrained seal exhibited spontaneous sleep apnea episodes. After the seal's
entry into the tube at UCSF, the tube containing the seal was placed into the
scanner for the study. During the NMR experiments, an experienced observer
monitored the seal's breathing pattern and signaled to the investigators in
the adjacent control room when the seal took a breath. The breathing pattern
was noted and then cross-referenced to a timer as well as to the NMR signal
acquisition data block.
1H (63.86 MHz) NMR signal acquisition utilized a body coil
transmit/surface coil (13 cm diameter) receive configuration
(Ponganis et al., 2002). The
receive coil was positioned over the region of the longissimus dorsi muscle
group. Magnetic field shimming was achieved using a three-point Dixon method,
yielding a water line-width of approximately 40 Hz
(Glover and Schneider, 1991).
A modified-DANTE pulse sequence excited the deoxy-Mb His-F8
NH signals, approximately 4.6 kHz from the water resonance
(Morris and Freeman, 1978).
Each spectrum required 300 transients and corresponded to a total acquisition
time of 60 s.
31P NMR data were collected from a second seal of similar age but only 62 kg in body mass. 31P (25.85 MHz) signal acquisition utilized a conforming flexible coil, which wrapped around the dorsum of the lumbar region. A 50 mm slice was selected and then excited with a self-refocused 45 deg. radio frequency pulse. The effective echo time was set at 2.5 ms. The other acquisition parameters were as follows: spectral width, 2.5 kHz; data points, 2048; acquisition time, 820 ms; recycle time, 2 s. Each 31P NMR spectrum consisted of 25 transients and required a total acquisition time of 70 s. All spectra were apodized with a 15 Hz exponential function and referenced to phosphocreatine (PCr) as 0 p.p.m.
Because the 31P peak line-widths during eupnea and apnea did not vary, the analysis set the most intense PCr peak during eupnea as 100%. The relative change in PCr and ATP concentration during the course of the experiment, as reflected in the signal intensity of PCr at 0 p.p.m. and βATP at –16 p.p.m., used this value as a basis to determine statistical significance.
Data were exported from the Signa system to a Sun Sparc2 workstation and processed using Omega 6.0 software (GE Medical Systems). All spectra were zero filled to 2 k and apodized using a Gaussian-exponential function with a line-broadening of 50 Hz. All spectra were baseline corrected and referenced to water at 4.65 p.p.m.
Mb signal calibration
Analysis of the muscle biopsy sample from the longissimus dorsi muscle
yielded a Mb concentration of 4.5 g 100 g–1 muscle
(Reynafarje, 1963;
Ponganis et al., 2002). The
deoxy-Mb signal was calibrated against the reference signal from a phantom
containing a 100 ml solution of 2 mmol l–1 metHb as
previously described (Tran et al.,
1999). No T1-based saturation factor
correction was necessary as the T1s of both the Mb and Hb
signals were sufficiently rapid to permit full recovery within the recycle
time. Multi-slice images of the seal revealed the surface coil detectable
tissue volume and an approximately 3 cm-thick fat layer. Digitally removing
the fat contribution from the image yielded an estimate of the muscle volume.
The muscle volume, the signal intensity of the phantom metHb solution and the
experimentally determined Mb concentration of seal muscle are necessary to
determine the percentage change in the deoxy-Mb and deoxy-Hb signal intensity
during apnea. As the image analysis overestimates the muscle volume, the
percentage change in MbO2 and HbO2 desaturation
represents an underestimated value.
Tissue temperature
The temperature-dependent chemical shift of the proximal histidyl
NH signal of deoxy-Mb and deoxy-Hb provided a basis to
estimate tissue temperature, using a calibration curve derived from the
measurement of chemical shift of the NH signal of pure
deoxy-Mb and deoxy-Hb in solution vs 1/T
(Ponganis et al., 2002). The
calculation used 14 chemical shift measurements of the deoxy-Mb signal after
the intensity had reached a steady state in two representative apnea
periods.
Mb diffusion in seal muscle
Measurements of Mb translational diffusion were conducted on a muscle
biopsy sample obtained from a juvenile elephant seal. The sample was kept on
ice and shipped overnight to University of California Davis. The NMR analysis
used portions of the sample immersed in isotonic saline. Upon completion of
the measurements, an optical assay of the isotonic solution determined if any
Mb had leaked from the tissue (Masuda et
al., 2008). None was detected.
An Avance 400 MHz Bruker spectrometer (Billerica, MA, USA) measured the
translational diffusion with a 10 mm microimaging gradient probe. The
1H 90 deg. pulse was 17.5 µs. A modified Stejskal–Tanner
pulsed-field gradient spin echo (PGSE) sequence followed the Val E11 resonance
of MbCO and MbO2 at –2.4 p.p.m. and –2.8 p.p.m.,
respectively (Price, 1997;
Stejskal and Tanner, 1965).
The gradient field strength ranged from 0 to 95 G [gauss cm–1
(1 gauss=104 tesla)]. A typical spectrum required 1024 scans and
used the following signal acquisition parameters: spectral width, 8192 Hz;
data points, 4096; acquisition time, 255 ms. The H2O line served as
the spectral reference, 4.75 p.p.m. at 25°C relative to
sodium-3-(trimethylsilyl)propionate-2,2,3,3-d4 at 0 p.p.m.
Diffusion measurements in seal muscle tissue experiments utilized a modified PGSE sequence in which a modified 1331 binomial pulse sequence replaced the hard 90 deg. and 180 deg. pulses in order to suppress the H2O line and excite the MbO2 Val E11 resonance at –2.8 p.p.m. For the diffusion measurements, the acquisition parameters included the following: acquisition time, 64 ms; spectral width, 8012 Hz; data points, 1024. A typical spectrum took 24,576 scans, requiring approximately 30 min of signal accumulation. The free induction decays were zero filled to 4 k and multiplied by an exponential-Gaussian window function.
Previous reports have detailed the diffusion analysis based on a modified
Bloch equation (Lin et al.,
2007a; Lin et al.,
2007b):
 | (1) |
is the magnetogyric ratio. The first three terms on the right
correspond to the general Bloch equation; the last term,
=
, reflects the effect of
anisotropic diffusion (Torrey,
1956). Its solution, based on the NMR diffusion experiment,
requires measurements of the echo amplitude S(G) as a
function of the applied field-gradient G(t)
(Price, 1997). For isotropic
diffusion, the solution reduces to the following equation
(Nicolay et al., 2001):
 | (2) |
=0 [S(0)],
signal intensity in an applied field G [S(G)], the
applied field-gradient G, echo time , transverse relaxation time
T2, diffusion coefficient D, gradient pulse width
, and gradient pulse interval 
.
The following equation is then used to assess the relative contribution of
free O2 and Mb to the intracellular flux:
 | (3) |
flux density from Mb,
O2 flux density from free O2, K0 = Krogh's
diffusion constant for free O2, DMb = Mb
diffusion coefficient, CMb = Mb concentration,
P50 = PO2 that half
saturates Mb, which reflects the O2 binding affinity of Mb. When
,
Mb-facilitated O2 and free O2 contribute equally to the
O2 flux. The PO2 corresponding to
the condition
is defined as the equipoise diffusion PO2.
Statistical analysis
Statistical analysis used the Sigma Plot/Sigma Stat program (Systat
Software, Point Richmond, CA, USA) and expressed the data as mean values
± standard deviation. Linear least-squares regression analysis of the
individual data points determined the slopes, intercepts and correlation
coefficients. Statistical significance was determined by two-tailed Student's
t-test, P<0.05. All regression analyses used primary data
points.
|
RESULTS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
|
A 90% reduction in MBF during apnea requires a t1/2 of 1.9±1.2 min. At the start of the apnea to eupnea transition, the initial MBF rises with a t1/2 of 0.2±0.04 min. Within the first minute, the MBF reaches a mean value of 83±13% of the preceding eupnea MBF. Table 1 summarizes the physiological data.
|
Fig. 2 shows the
1H NMR spectra acquired during an 8 min episode of eupnea to apnea.
With apnea, Mb releases O2, as reflected by the emergence of the
proximal histidyl NH signal of deoxy-Mb at 76.0±0.09
p.p.m. The signal increases to a steady-state level within 4 min
(Fig. 2a–h). With the
onset of eupnea after this short breath hold, the deoxy-Mb signal intensity
decreases rapidly. Mb resaturates to its resting level within 1 min
(Fig. 2n–o). An upfield
peak also appears at 72–73 p.p.m., corresponding to the signal from the
β subunits of deoxy-Hb (Tran et al.,
1999). The deoxy-Hb signal continues to increase during apnea,
even after Mb desaturation level has reached a steady-state level.
|
Fig. 3 traces the kinetics of Mb desaturation and resaturation during two apnea to eupnea cycles. During eupnea, 1H NMR detects neither the deoxy-Mb nor the deoxy-Hb signal. At the onset of apnea, the deoxy-Mb signal appears as MbO2 desaturates. MbO2 releases about 20% of its oxygen store within 4 min. Mb maintains this desaturation level for rest of the apnea period. At the start of eupnea, both the deoxy-Mb and deoxy-Hb signals disappear as MBF returns to 83±5% of its eupnea level within 1 min to restore the vascular O2 supply. Correspondingly, MbO2 saturation recovers to about 95% of its resting level within 1 min. However, full recovery appears to require about 6 min.
|
).
Despite the rise and fall in MbO2 saturation, which reflects the cellular PO2, the 31P NMR spectra show no perturbation. Fig. 4 shows a typical bank of 31P NMR spectra during eupnea to apnea cycles. Fig. 4a,b shows the 31P spectra acquired during eupnea. Fig. 4c–f shows the corresponding signals during apnea. Fig. 4g–h shows the spectra during the second eupnea cycle. Under all conditions, the 31P spectra remain unperturbed. The inorganic phosphate (Pi) chemical shift does not change, reflecting an unperturbed pH.
|
|
|
|
|
DISCUSSION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
; Grinnell et al.,
1942; Zapol et al.,
1979). The inhomogeneous flow distribution during apnea agrees
with the previously observed mosaic pattern of oxygenated/deoxygenated muscle
(Scholander, 1940;
Scholander et al., 1942;
Blix et al., 1983). During the
eupneic intervals between sleep apneas, MBF fluctuates; this is probably
related to variations in respiratory rate and associated changes in heart rate
and cardiac output during eupnea (Ponganis
et al., 2006).
Oxygen supply
Synchronous with the fall in MBF, the O2 level in the cell
decreases rapidly to a reduced steady-state level, as reflected in the
deoxy-Mb proximal histidyl NH signal. In the control eupneic
state, the undetectable deoxy-Mb signal indicates a cellular
PO2 well above the Mb P50.
Given the signal-to-noise ratio of the deoxy-Mb signal, NMR can detect the
deoxy-Mb proximal histidyl NH signal reflecting 10%
MbO2 desaturation. The resting eupneic state
PO2 must saturate at least 90% MbO2,
equivalent to above 18 mmHg based on the reported P50 of 2
mmHg at 35°C (Schenkman et al.,
1997). As the respiratory rate starts to decrease during late
eupnea, MbO2 also begins to desaturate. Within 4 min of apnea, Mb
has desaturated from the resting 3.8 mmol l–1 to 3.0 mmol
l–1 MbO2. Between 4 and 8 min of apnea, the
MbO2 does not desaturate further, indicating a constant cellular
PO2. During apnea, the
PO2 has fallen to approximately 6 mmHg,
corresponding to 80% MbO2 saturation.
The returning oxygen at the transition from apnea to eupnea resaturates Mb more rapidly than the Mb desaturation during apnea. Within 1 min of the first breath, the deoxy-Mb signal diminishes significantly. MBF returns to 83±5% of the eupnea level whereas MbO2 saturation recovers from about 80% to 90%. Full recovery of MbO2 saturation to the control level appears to require an additional 5 min. The recovery of blood flow and cellular O2 appears to follow a similar time course, although the slight mismatch at the initial phase of eupnea suggests a time lag in the appearance of a shortfall in O2 delivery and a decrease in the intracellular O2 level. Moreover, the recovery profile suggests a biphasic kinetics, which future experiments with improved time resolution of the deoxy-Mb detection must confirm.
The NMR spectra also show a signal from the β proximal histidyl
NH of deoxy-Hb, which appears on the shoulder of the
deoxy-Mb peak. As blood flow decreases, HbO2 saturation decreases,
consistent with declining vascular PO2. During
eupnea, the deoxy-Hb signal disappears accordingly.
Muscle temperature
The chemical shift of the deoxy-Mb signal reflects the temperature of the
cellular environment, as calibrated in the study of isolated elephant seal Mb
(Ponganis et al., 2002).
During apnea, the intracellular temperature remains at 35.1±0.4°C.
The deoxy-Hb signal also indicates a similar temperature. The results of the
present study agree with pulmonary artery blood measurements during similar
spontaneous apneas in young elephant seals, which also show an intravascular
temperature of 36–37°C (Ponganis
et al., 2006). These muscle temperatures are 1–2°C less
than deep muscle temperatures in Weddell seals at rest but are consistent with
lower temperatures in peripheral muscle and tissues of both humans and marine
mammals (Irving and Hart,
1957; Ponganis et al.,
1993b; Saltin et al.,
1968; Sessler,
2000).
O2 control of cellular respiration
Under resting eupneic conditions, the vascular O2 supplies the
cell with sufficient O2 to saturate at least 90% of the
MbO2; otherwise, the NMR spectra would reveal a proximal histidyl
NH signal of deoxy-Mb. Throughout the entire apnea period,
the cell operates under a PO2 that saturates Mb
to approximately 80%. However, no sign of hypoxemia appears. The
31P PCr and ATP signals remain constant throughout the entire
eupnea to apnea cycles. The Pi chemical shift remains unaltered,
reflecting a constant pH and rate of glycolysis. Indeed, sleep apnea
experiments in seals have not detected any lactate washout
(Castellini et al., 1994). All
the metabolic indices do not reveal any tissue hypoxemia during apnea, despite
the drop in cellular PO2.
However, the rate of oxygen uptake
(
O2) should
remain at least constant during apnea, as the major energy expenditure
incurred by muscle movement does not occur. If
O2 remains
constant, a decreasing vascular and intracellular O2 supply that
maintains the same
O2 does not
indicate that the O2 level controls respiration during apnea.
O2 could
potentially decrease as the O2 delivery continues to fall during
apnea. However, the intracellular PO2 has
reached a reduced steady-state level within 4 min, while MBF continues to
decline. A constant intracellular PO2 in the
face of a falling
O2 and MBF also
argues against the simplistic notion of the O2 level controlling
O2
(Chung et al., 2005;
Jue, 2004).
Mb contribution to O2 transport
The regulation of
O2 depends upon
the free O2 and Mb-facilitated O2 flux in the cell.
According to the Mb facilitated diffusion theory, as intracellular
PO2 falls, the oxygen-carrying capacity of Mb
will enhance its flux contribution over free O2. So, even if the
O2 level falls, the overall O2 flux may still meet the
O2 demand if the
Mb-facilitated contribution to O2 diffusion increases. This idea
predicates usually on the hypothesis of a partially saturated Mb in resting
muscle and requires a sufficiently rapid translational diffusion of Mb in the
cell. Certainly, Mb does not appear significantly desaturated in the normoxic
state in seal muscle as there is no sign of any deoxy-Mb signal. Moreover,
recent studies have determined the Mb translational diffusion coefficient of
4.24x10–7 cm2 s–1 in rat
heart, which yields an equipoise PO2 of 1.77
mmHg (Lin et al., 2007b).
Below a PO2 of 1.77 mmHg, the Mb-dependent
contribution to the O2 flux begins to dominate. In steady-state
normoxic rat muscle, Mb doesn't appear to play a significant role in
transporting cytoplasmic O2 in the steady state
(Lin et al., 2007a;
Papadopoulos et al.,
2001).
In contrast to the 0.19 mmol l–1 Mb concentration in rat
heart, seal muscle contains a much higher concentration of Mb (4.5 g per 100 g
tissue or approximately 3.8 mmol l–1 cytoplasmic Mb). Despite
the 20x higher concentration in tissue, seal Mb still exhibits a
translational diffusion of 4.5x10–7 cm2
s–1 at 25°C. Given a free O2 diffusion
coefficient, K0, of 2.5x10–5 ml
O2 cm–1 min–1
atm–1, a P50 of 1.5 mmHg at 25°C and
a Mb concentration of 3.8 mmol l–1, the analysis yields an
equipoise PO2 of 67 mmHg. At physiological
temperature of 37°C, K0, P50 and
DMb will increase to lower the equipoise
PO2 (Lin et
al., 2007a).
During the intermittent breathing pattern of eupnea, venous
PO2 ranges between 34 and 71 mmHg
(Stockard et al., 2007). At
the start of apnea, average O2 tension is approximately 59 mmHg. By
9 min of apnea, the mean venous PO2 drops to 21
mmHg. Therefore, the calculated equipoise PO2
is still much greater than the venous PO2. The
eupnea to apnea transition does not correspond to any switch in intracellular
O2 transport as intracellular O2 level falls. Mb plays a
predominant role in transporting O2 under all physiological
conditions.
Transient vs steady-state levels of O2
However, steady-state bioenergetics analysis misses the transient response
that maintains the energetic homeostasis. At the start of apnea, Mb
desaturates within 2 min by 10% of its control value. Within 4 min, Mb
desaturates by 20% of the control value to the steady-state apneic value. Mb
remains at 80% oxygen saturation throughout apnea. Given the assumption of no
vascular O2 contribution and a Mb concentration of 3.8 mmol
l–1, Mb desaturates at 190 µmol l–1
min–1 or about 3 µmol l–1
s–1. The rate of Mb desaturation implies a resting muscle
O2 of at least 3
µmol l–1 s–1, consistent with the
observed values in mammalian muscle (Blei
et al., 1993; Chung et al.,
2005). This leads to approximately 2 ml O2
kg–1 muscle min–1 or 3 ml O2
kg–1 muscle min–1 (based on a 20%
desaturation of 4.5 g Mb 100 g–1 or 60 ml O2
kg–1 over 4 min). Indeed, the
O2 estimate
matches the lower range of resting
O2 observed in
the latissimus dorsi muscle of diving Weddell seals
(Guyton et al., 1995) and
suggests that Mb supplies the transient O2 need at the beginning of
apnea, when vascular O2 supply can no longer meet the change in
O2 demand (Castellini et al.,
1992).
Within 1 min of the apnea to eupnea transition, the vascular O2
restores Mb almost to the control saturation level at a rate of 760 µmol
l–1 min–1 or 13 µmol l–1
s–1, about 4.3x faster than the Mb desaturation rate at
the beginning of apnea. The difference in Mb desaturation and resaturation
kinetics parallels the change in blood flow. Blood flow falls to 30% of its
normoxic level within 5 min of apnea and recovers to its basal level within 1
min of eupnea. The oxygen restoration profile parallels the end tidal
PO2 response in free-diving Weddell seals
(Ponganis et al., 1993a).
If the MBF and
O2 always match,
as some researchers have asserted, then the eupneic and apneic
O2 must differ
by at least a factor of five as eupneic MBF
(MBFE)
2xMBFA
(Table 1) but
(vide infra). The relationship in the relative change in eupneic
vs apneic
O2 is as
follows:
 | (4) |
= ratio of eupneic to apneic
O2,
= ratio of eupneic to apneic O2 gradient,
(MBFE)/(MBFA) = ratio of eupneic to apneic MBF. The
rapid MBF kinetics, as reflected in the MbO2 resaturation, should
match the rising
O2. However,
during post-exercise, when the
O2 falls,
studies have also observed a rapid MbO2 resaturation kinetics
(Chung et al, 2006). A rapid
resaturation kinetics with either rising or falling
O2 argues
against a tight match between MBF and
O2.
Alternatively, the rapid MbO2 resaturation and MBF recovery
kinetics reflect a two-step process. The recovering MBF at the start of eupnea
supplies O2 to the cell and resaturates Mb. This rate of
O2 resaturation (13 µmol l–1
s–1) far exceeds the eupneic
O2 of 3 µmol
l–1 s–1. In essence, the O2
supply alone does not control
O2 during the
apnea to eupnea transition, and MBF has an overcapacity in delivering
O2 supply above the cellular need, as observed in the functional
magnetic resonance imaging of brain cell activation
(Hyder et al., 2001).
Mb as an O2 buffer
In muscle, the initiation of contraction increases the energy demand, which
momentarily outstrips the vascular supply of O2. Such a transient
mismatch leans on Mb to buffer the O2 deficit. Indeed, Mb appears
to serve that role at the start of muscle contraction; it desaturates to a
steady-state within 30 s (Chung et
al., 2005).
During the 8–12 min apnea to eupnea cycle, seal muscle does not contract or move. So, the muscle energy demand has not increased to drive Mb desaturation. Instead, vascular O2 delivery has decreased. MBF declines to 50% of its final value within 2 min and 90% of its final value in approximately 4 min. The correlation of Mb desaturation kinetics with MBF indicates that the transient mismatch of O2 supply and demand during the eupnea to apnea transition triggers the release of O2 from the Mb. As observed at the initiation of muscle contraction, Mb serves as an O2 buffer during a transient mismatch of cellular supply and demand for O2.
The equipoise PO2 analysis, based on translational diffusion of Mb, indicates that Mb has a predominant role to facilitate O2 transport in the cell under both eupnea and apnea. This would suggest that Mb plays a key role in O2 buffering only at the beginning of apnea.
Implication of O2 gradient on apnea O2
The PO2 reflected in the Mb saturation data
and previously measured apneic venous PO2 data
allow assessment of the blood-to-muscle O2 gradient during sleep
apnea (Stockard et al., 2007).
During early apnea, Mb saturation, as reflected in the deoxy-Mb signal,
declines to 80%. Assuming a Mb P50 of 2 mmHg at 35°C,
80% saturation would correspond to a tissue PO2
of approximately 6 mmHg. A change from 90% to 80% Mb saturation represents a
PO2 decline from 18 to 6 mmHg inside the cell
(Araki et al., 1983;
Schenkman et al., 1997).
Venous PO2, on average, declines from 59 mmHg
at the start of apnea to 52 mmHg by 3 min into apnea
(Stockard et al., 2007). Using
venous PO2 as an estimate of the end capillary
mean PO2, the blood-to-muscle O2
gradient will remain about the same at the start of apnea (41 mmHg) and at 3
min into apnea (46 mmHg).
However, as apnea progresses, the venous PO2
continues to decline, reaching a value of 21 mmHg by 9 min into apnea
(Stockard et al., 2007). This
decline in venous PO2 is reflected by the
appearance of the deoxy-Hb signal in the 1H spectra. However,
intracellular PO2 remains at 6 mmHg as apnea
progresses. Consequently, the blood-to-muscle O2 gradient decreases
by a factor of 2.7 from 41 mmHg at the start of apnea to 15 mmHg at 9 min into
apnea.
Fick's diffusion law indicates that as the O2 gradient narrows,
O2 must
decrease. The narrowing of the O2 gradient implies that
O2 falls during
apnea. Otherwise, PO2 should continue to fall
to maintain the O2 gradient that continues a proper O2
flux to support a constant
O2. A similar
conclusion emerges from the analysis of MBF, arterial–venous difference
in O2 and
O2:
 | (5) |
As apnea progresses, the arterial–venous O2 difference
collapses (Stockard et al.,
2007); however, MBF also decreases. Consequently,
O2 must also
decrease. As no muscle movement occurs during apnea, the reduced
O2 implies that
the energy demand, presumably with ATP-dependent ion transporters, must
downregulate.
By contrast, during complete muscle ischemia in head-immersed Pekin ducks
(Anas platyrhynchos), the ATP turnover rate in pectoral muscle is
maintained at pre-submersion levels through depletion of the Mb oxygen store,
PCr breakdown and increased glycolysis (lactate accumulation)
(Stephenson and Jones, 1992).
This difference in muscle metabolic response between sleep apnea in the
elephant seal and head immersion in the Pekin duck may, of course, be
secondary to the physical (restraint/struggling) and physiological differences
between a spontaneous apnea of an elephant seal and the forced head immersion
of a duck. It is notable that paralysis of the muscle in the head-immersed
duck resulted in no detectable PCr breakdown or change in intracellular pH
during immersion. Presumably, the O2 store of Mb was adequate for
energy demand under these conditions. Complete depletion of the O2
reservoir of Mb over the duration of the immersion would have resulted in an
ATP turnover rate that was only 10% of that of a non-paralyzed muscle of a
restrained duck at rest.
It should be noted that declines in muscle
O2 in relation
to low MBF have also been observed in other species
(Duran and Renkin, 1974;
Gutierrez et al., 1988;
Mizuno et al., 2003). Thus,
the predicted decline in muscle
O2 during sleep
apnea in the elephant seal does not appear to represent a unique mechanism of
metabolic depression exclusive to diving animals. Although bradycardia and
decreased tissue blood flow during a breath hold will reduce metabolic rate in
muscle (above) and other organs
(Scholander, 1940;
Scholander et al., 1942), all
data of elephant seals during sleep apnea indicate that most metabolic
processes in other tissues continue
(Ponganis et al., 2006).
Indeed, the apneic
O2 and cardiac
output mirror resting-state values. During eupnea, the increased
O2 and cardiac
output reflect the increased respiratory costs and increased O2
uptake/consumption associated with increased blood flow to organs/muscle.
Rather than considering sleep apnea as a hypometabolic state, it is more
appropriate to consider the eupneic period as an elevated metabolic state.
This is the same conclusion reached by Castellini and Zenteno-Savin in an
allometric analysis of seal heart rates
(Castellini and Zenteno-Savin,
1997).
In addition, the present study's experimental data and the previously
estimated blood O2 contribution to metabolic rate during sleep
apnea (Stockard et al., 2007)
do not provide any evidence of whole-body hypometabolism. Using the blood
O2 depletion during sleep apnea in these seals in a prior study and
the net muscle O2 depletion (about 250 ml O2) determined
in the present study (20% decline in Mb saturation at a concentration of 45 g
kg–1 muscle tissue with muscle approximately 30% of a 70 kg
body mass), the analysis yields a combined blood and muscle O2
store depletion rate during a typical 7 min apnea of 4.7 ml O2
kg–1 body mass min–1
(Stockard et al., 2007). As
the lung O2 store in seals is only about 5% of the total
O2 store and neither lactate nor PCr shows any change, this
combined blood and muscle O2 depletion rate should closely
approximate the actual metabolic rate during the breath-hold period. However,
this value is still 26% greater than the Kleiber-predicted resting metabolic
rate for a 70 kg mammal at rest (Kleiber,
1961; Kleiber and Rogers,
1961).
PCr kinetics and O2
Throughout eupnea and apnea, the PCr and ATP levels remain constant,
despite the dynamic changes in intracellular
PO2 and
O2. Certainly,
the unchanging PCr/ATP ratio indicates that the tissue does not suffer from
any ischemia as MBF decreases to 30% of the eupneic level and vascular
PO2 drops from about 60 to 21 mmHg. During
apnea, metabolism can still rely on oxidative phosphorylation. The resultant
CO2 from oxidative metabolism can alter respiratory drive as well
as sleep structure (Milsom et al.
1996; Skinner and Milsom,
2004; Stephenson,
2005). Indeed, marine and terrestrial mammals appear to have
similar hypercarbic chemosensitivity but do not have the same thresholds as a
result of contrasting blood buffering capacity.
The presence of a non-limiting oxidative phosphorylation addresses a
long-standing postulate that envisions PCr breakdown supplementing glycolysis
during a dive to compensate for the presence of hypoxemia
(Hochachka and McClelland,
1997). During sleep apnea, this does not occur because muscle
tissue hypoxemia does not exist. From a conventional vantage, blood flow and
intracellular PO2 fall correspondingly to
maintain an O2 gradient that sustains the
O2. Oxygen
delivery dynamically matches the oxygen demand. The release of O2
from the Mb then provides a temporary source of O2 as the gradient
adjusts. Once the gradient has adjusted, Hb can provide an adequate flux of
O2 for cellular respiration. With adequate delivery of
O2, no deficit in oxidative ATP generation exists. Creatine kinase
does not need to mobilize PCr to buffer any ATP loss. The cell can still rely
on aerobic metabolism. The PCr level should then not change.
But as the gradient narrows later on during apnea and blood flow declines
to 30% the eupneic level,
O2 must fall by
a factor of five, well below the value at the start of apnea. Yet no signs of
tissue hypoxia or ischemia, as measured by elevated lactate production,
appear. Either muscle
O2 can decrease
significantly without producing any tissue hypoxia or the efficiency of energy
utilization has improved dramatically or the current measurements do not
adequately detect the shift in metabolic regulation
(Whalen et al., 1974).
Recent studies have suggested that experimental measurements might not
accurately track the transient metabolic response. Standard measurements of
PCr with 31P NMR show no change in the PCr intensity ascribed to a
single twitch. Yet, NMR techniques tailored to capture metabolic transients
show a significant use of ATP and PCr per contraction cycle, far greater than
the conventional assessments reported in the literature
(Chung et al., 1998). Such
metabolic transients raise questions about the fluxes that maintain the
cellular bioenergetics and suggest a dynamic control of energy flux from
glycogen, glucose and respiration as proposed in the glycogen shunt model
(Shulman and Rothman, 2001).
This model does not restrict lactate as only a metabolic end product
reflecting hypoxemia or ischemia. Instead, studies have shown that lactate can
become oxidized to serve as a direct mitochondrial precursor for oxidative
phosphorylation (Brooks et al.,
1999). Conventional 31P NMR and metabolic measurements
would miss these dynamic fluxes.
Finally, if the
O2 has decreased
dramatically during apnea, then the returning O2 during the apnea
to eupnea transition will restore the O2 and the
O2 to the higher
eupneic level. The postulated
O2 recovery from
a lower rate still shows no corresponding change in PCr. Thus, PCr recovery
rate does not always reflect
O2 recovery or
oxidative capacity (Chung et al.,
2006; Foley and Meyer,
1993).
Indeed, the metabolic adaptation in seal muscle during a breath hold as
reflected in the eupnea to apnea transition indicates a complex relationship
between O2 and
PCr, which requires additional investigation.
Conclusion
In conclusion, NMR spectroscopy has provided new insights into Mb function
and blood-to-muscle O2 transfer during the breath holds of sleeping
elephant seals. Aerobic metabolism in muscle is maintained, despite the fall
in intracellular and vascular O2. PCr and ATP levels do not change.
As MBF declines during apnea, Mb buffers the initial loss of the O2
supply by releasing its O2 store. MbO2 desaturates
quickly to a steady-state level, which reflects an adjustment of the
O2 gradient from the vasculature to the cell. No switch from free
to Mb-facilitated O2 diffusion occurs, as under all physiological
conditions the equipoise analysis of Mb translation diffusion asserts a
predominant role for Mb as the intracellular O2 transporter.
Despite the continuing decline in the vascular
PO2 during sleep apnea, the intracellular
PO2 of muscle does not change significantly
after its initial fall. A constant intracellular
PO2, in association with (1) a declining
vascular PO2 and a narrowing O2
gradient, (2) constant PCr and ATP levels, and (3) a lack of lactate
accumulation, implies a downregulation of ATP demand. Such declines in muscle
metabolic rate in relation to decreased MBF have been observed in other
species. Even with such regulation of muscle metabolic rate, sleep apnea
should not be considered a hypometabolic state. The combined depletion of
blood and muscle O2 stores during the breath hold yields a
whole-body O2
greater than the Kleiber-predicted resting metabolic rate.
These findings during sleep apnea contrast with those during forced
submersion (severe bradycardia/vasoconstriction, complete circulatory
isolation of muscle and depletion of muscle O2, and conservation of
blood O2 for the heart and brain). During dives, we suspect that
the intensity and magnitude of these physiological and metabolic responses
will range between these two extremes, dependent on the nature of a given dive
and the associated reduction in heart rate. As Scholander emphasized,
O2 consumption and the rate of blood O2 depletion will
ultimately depend on the heart rate response
(Scholander, 1940). Similarly,
the role of the high Mb concentrations in seal muscle (facilitation of
O2 diffusion vs an O2 store) will also be
dependent on the intensity of the cardiovascular response during a dive.
Although our analyses predict that basal muscle
O2 during sleep
apnea decreases in relation to even moderate declines in MBF, muscle
O2 during dives
will probably be most dependent on the locomotory workload of muscle.
|
Acknowledgments |
|---|
|
References |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Andrews, R. D., Jones, D. R., Williams, J. D., Thorson, P. H., Oliver, G. W., Costa, D. P. and Le Boeuf, B. J. (1997). Heart rates of Northern elephant seals while diving at sea and resting on the beach. J. Exp. Biol. 200,2083 -2095.[Abstract]
Araki, R., Tamura, M. and Yamazaki, I. (1983).
The effect of intracellular oxygen concentration on lactate release, pyridine
nucleotide reduction, and respiration rate in the rat cardiac tissue.
Circ. Res. 53,448
-455.
Blei, M., Conley, K. E. and Kushmerick, M. J.
(1993). Separate measures of ATP utilization and recovery in
human skeletal muscle. J. Physiol.
465,203
-222.
Blix, A. S., Elsner, R. W. and Kjekhus, J. K. (1983). Cardiac output and its distribution through capillaries and A-V shunts in diving seals. Acta Physiol. Scand. 118,109 -116.[Medline]
Brooks, G. A., Dubouchaud, H., Brown, M., Sicurello, J. P. and
Butz, C. E. (1999). Role of mitochondrial lacate
dehydrogenase and lactate oxidation in the intracellular lactate shuttle.
Proc. Natl. Acad. Sci. USA
96,1129
-1134.
Castellini, M. A. and Zenteno-Savin, T. (1997). Heart rate scaling with body mass in pinnipeds. Mar. Mamm. Sci. 13,149 -155.[CrossRef]
Castellini, M. A., Costa, D. P. and Huntley, A. (1986). Hematocrit variation during sleep apnea in elephant seal pups. Am. J. Physiol. 251,R429 -R431.[Medline]
Castellini, M. A., Kooyman, G. L. and Ponganis, P. J.
(1992). Metabolic rates of freely diving Weddell seals:
correlations with oxygen stores, swim velocity and diving duration.
J. Exp. Biol. 165,181
-194.
Castellini, M. A., Milsom, W. K., Berger, R. J., Costa, D. P., Jones, D. R., Castellini, J. M., Rea, L. D., Bharma, S. and Harris, M. (1994). Patterns of respiration and heart rate during wakefulness and sleep in elephant seal pups. Am. J. Physiol. 266,R863 -R869.[Medline]
Chung, Y., Sharman, R., Carlsen, R., Unger, S. W., Larson, D. and Jue, T. (1998). Metabolic fluctuation during a muscle contraction cycle. Am. J. Physiol. 274,C846 -C852.[Medline]
Chung, Y., Molé, P. A., Sailasuta, N., Tran, T. K., Hurd, R. and Jue, T. (2005). Control of respiration and bioenergetics during muscle contraction. Am. J. Physiol. 288,C730 -C738.[CrossRef]
Chung, Y., Molé, P. A., Tran, T. K., Sailasuta, N., Masuda, K., Hurd, R. and Jue, T. (2006). Muscle bioenergetics during exercise recovery. Med. Sci. Sports. Med.38 , (11), S14.
Duran, W. N. and Renkin, E. M. (1974). Oxygen
consumption and blood flow in resting mammalian skeletal muscle.
Am. J. Physiol. 226,173
-177.
Elsner, R. W., Franklin, D. L. and VanCitters, R. L. (1964). Cardiac output during diving in an unrestrained sea lion. Nature 202,809 -810.[CrossRef][Medline]
Foley, J. M. and Meyer, R. A. (1993). Energy cost of twitch and tetanic contractions of rat muscle estimated in situ by gated 31P NMR. NMR Biomed. 6, 32-38.[Medline]
Glover, G. H. and Schneider, E. (1991). Three-point Dixon technique for true water/fat decomposition with Bo inhomogeneity correction. Magn. Reson. Med. 18,371 -383.[Medline]
Grinnell, S. W., Irving, L. and Scholander, P. F. (1942). Experiments on the relation between blood flow and heart rate in the living seal. J. Cell. Comp. Physiol. 19,341 -350.[CrossRef]
Gutierrez, G., Pohil, R. J. and Strong, R.
(1988). Effect of flow on O2 consumption during
progressive hypoxemia. J. Appl. Physiol.
65,601
-607.
Guyton, G. P., Stanek, K. S., Schneider, R. C., Hochachka, P.
W., Hurford, W. E., Zapol, D. G., Liggins, G. C. and Zapol, W. M.
(1995). Myoglobin saturation in free-diving Weddell seals.
J. Appl. Physiol. 79,1148
-1155.
Hochachka, P. W. and McClelland, G. B. (1997). Cellular metabolic homeostasis during large-scale change in ATP turnover rates in muscles. J. Exp. Biol. 200,381 -386.[Abstract]
Hyder, F., Kida, I., Behar, K. L., Kennan, R. P., Maciejewski, P. K. and Rothman, D. L. (2001). Quantitative functional imaging of the brain: towards mapping neuronal activity by BOLD fMRI. NMR Biomed. 14,413 -431.[CrossRef][Medline]
Irving, L. and Hart, J. S. (1957). The metabolism and insulation of seals as bare-skinned mammals in cold water. Can. J. Zool. 35,498 -511.
Jue, T. (2004). Bioenergetics implication of metabolic fluctuation during muscle contraction. In Metabolomics by in vivo NMR (ed. R. G. Shulman and D. L. Rothman), pp.104 -117. Chichester: John Wiley.
Kleiber, M. (1961). Fire of Life. New York: John Wiley & Sons.
Kleiber, M. and Rogers, T. (1961). Energy metabolism. Annu. Rev. Physiol. 23, 5-36.[Medline]
Kreutzer, U. and Jue, T. (1995). Critical intracellular oxygen in the myocardium as determined with the 1H NMR signal of myoglobin. Am. J. Physiol. 268,H1675 -H1681.[Medline]
Kreutzer, U., Wang, D. S. and Jue, T. (1992).
Observing the 1H NMR signal of the myoglobin Val-E11 in myocardium:
an index of cellular oxygenation. Proc. Natl. Acad. Sci.
USA 89,4731
-4733.
Kreutzer, U., Chung, Y., Butler, D. and Jue, T. (1993). 1H-NMR characterization of the human myocardium myoglobin and erythrocyte hemoglobin signals. Biochim. Biophys. Acta 1161,33 -37.[CrossRef][Medline]
Kreutzer, U., Mekhamer, Y., Tran, T. K. and Jue, T. (1998). Role of oxygen in limiting respiration in the in situ myocardium. J. Mol. Cell Cardiol. 30,2651 -2655.[CrossRef][Medline]
Le Boeuf, B. J., Crocker, D. E., Grayson, J., Gedamke, J., Webb, P. M., Blackwell, S. B. and Costa, D. P. (2000). Respiration and heart rate at the surface between dives in northern elephant seals. J. Exp. Biol. 203,3265 -3274.[Abstract]
Lin, P. C., Kreutzer, U. and Jue, T. (2007a). Anisotropy and temperature dependence of myoglobin translational diffusion in myocardium: implication on oxygen transport and cellular architecture. Biophys. J. 92,2608 -2620.[CrossRef][Medline]
Lin, P. C., Kreutzer, U. and Jue, T. (2007b).
Myoglobin translational diffusion in myocardium and its implication on
intracellular oxygen transport. J. Physiol.
578,595
-603.
Masuda, K., Truscott, K., Lin, P. C., Kreutzer, U., Chung, Y., Sriram, R. and Jue, T. (2008). Determination of myoglobin concentration in blood-perfused tissue. Eur. J. Appl. Physiol. 104,41 -48.[CrossRef][Medline]
Milsom, W., Castellini, M., Harris, M., Castellini, J., Jones, D., Berger, R., Bahrma, S., Rea, L. and Costa, D. (1996). Effects of hypoxia and hypercapnia on patterns of sleep-associated apnea in elephant seal pups. Am. J. Physiol. 271,R1017 -R1024.[Medline]
Mizuno, M., Kimura, Y., Iwakawa, T., Oda, K., Ishii, K.,
Ishiwata, K., Nakamura, Y. and Muraoka, I. (2003). Regional
differences in blood flow and oxygen consumption in resting muscle and their
relationship during recovery from exhaustive exercise. J. Appl.
Physiol. 95,2204
-2210.
Morris, G. A. and Freeman, R. (1978). Selective excitation in Fourier transform nuclear magnetic resonance. J. Magn. Reson. 29,433 -462.
Nicolay, K., Braun, K. P., Graaf, R. A., Dijkhuizen, R. M. and Kruiskamp, M. J. (2001). Diffusion NMR spectroscopy. NMR Biomed. 14,94 -111.[CrossRef][Medline]
Papadopoulos, S., Endeward, V., Revesz-Walker, B., Jurgens, K.
D. and Gros, G. (2001). Radial and longitudinal diffusion of
myoglobin in single living heart and skeletal muscle cells. Proc.
Natl. Acad. Sci. USA 98,5904
-5909.
Ponganis, P. J., Kooyman, G. L. and Castellini, M. A. (1993a). Determinants of the aerobic dive limit of Weddell seals: analysis of diving metabolic rates, postdive end tidal pO2`s, and blood and muscle oxygen stores. Physiol. Zool. 66,732 -749.
Ponganis, P. J., Kooyman, G. L., Castellini, M. A., Ponganis, E. P. and Ponganis, K. V. (1993b). Muscle temperature and swim velocity profiles during diving in a Weddell seal, Leptonychotes weddellii. J. Exp. Biol. 183,341 -346.[Abstract]
Ponganis, P. J., Kreutzer, U., Sailasuta, N., Knower, T., Hurd,
R. and Jue, T. (2002). Detection of myoglobin desaturation in
Mirounga angustirostris during apnea. Am. J. Physiol.
Regul. Integr. Comp. Physiol. 282,R267
-R272.
Ponganis, P. J., Stockard, T. K., Levenson, D. H., Berg, L. and Baranov, E. A. (2006). Cardiac output and muscle blood flow during rest-associated apneas of elephant seals. Comp. Biochem. Physiol. 144,105 -111.
Price, W. S. (1997). Pulsed-field gradient nuclear magnetic resonance as a tool for studying translational diffusion.1: basic theory. Concepts Magn. Reson. 9, 299-336.[CrossRef]
Reynafarje, B. (1963). Simplified method for the determination of myoglobin. J. Lab. Clin. Med. 61,138 -145.[Medline]
Saltin, B., Gagge, A. P. and Stolwijk, J. A. J.
(1968). Muscle temperature during submaximal exercise in man.
J. Appl. Physiol. 25,679
-688.
Schenkman, K. A., Marble, D. R., Burns, D. H. and Feigl, E.
O. (1997). Myoglobin oxygen dissociation by multiwavelength
spectroscopy. J. Appl. Physiol.
82, 86-92.
Scholander, P. F. (1940). Experimental investigations on the respiratory function in diving mammals and birds. Hvalradets. Skr. 22,1 -131.
Scholander, P. F., Irving, L. and Grinnell, S. W.
(1942). Aerobic and anaerobic changes in seal muscles during
diving. J. Biol. Chem.
142,431
-440.
Sessler, D. I. (2000). Perioperative heat balance. Anesthesiology 92,578 -596.[Medline]
Shulman, R. G. and Rothman, D. L. (2001). The
`glycogen shunt' in exercising muscle: a role for glycogen in muscle
energetics and fatigue. Proc. Natl. Acad. Sci. USA
98,457
-461.
Skinner, L. A. and Milsom, W. K. (2004). Respiratory chemosensitivity during wake and sleep in harbour seal pups (Phoca vitulina richardsii). Physiol. Biochem. Zool. 77,847 -863.[CrossRef][Medline]
Stejskal, E. O. and Tanner, J. E. (1965). Spin diffusion measurements: spin echoes in the presence of a time-dependent field gradient. J. Chem. Phys. 42,288 -292.[CrossRef]
Stephenson, R. (2005). Physiological control of
diving behaviour in the Weddell seal Leptonychotes weddellii: a model
based on cardiorespiratory control theory. J. Exp.
Biol. 208,1971
-1991.
Stephenson, R. and Jones, D. R. (1992). Metabolic responses to forced dives in Pekin duck measured by indirect calorimetry and 31P-MRS. Am. J. Physiol. 263,R1309 -R1317.[Medline]
Stockard, T. K., Levenson, D. H., Berg, L., Fransiolo, J. R.,
Baranov, E. A. and Ponganis, P. J. (2007). Blood oxygen
depletion during rest-associated apneas of northern elephant seals
(Mirounga angustirostris). J. Exp. Biol.
210,2607
-2617.
Torrey, H. C. (1956). Bloch equations with diffusion terms. Phys. Rev. 104,563 -565.[CrossRef]
Tran, T. K., Sailasuta, N., Kreutzer, U., Hurd, R., Chung, Y., Mole, P., Kuno, S. and Jue, T. (1999). Comparative analysis of NMR and NIRS measurements of intracellular PO2 in human skeletal muscle. Am. J. Physiol. 276,R1682 -R1690.[Medline]
Whalen, W. J., Nair, P., Buerk, D. and Thuning, C. A.
(1974). Tissue PO2 in normal and
denervated cat skeletal muscle. Am. J. Physiol.
227,1221
-1225.
Zapol, W. M., Liggins, G. C., Schneider, R. C., Qvist, J.,
Snider, M. T., Creasy, R. K. and Hochachka, P. W. (1979).
Regional blood flow during simulated diving in the conscious Weddell seal.
J. Appl. Physiol. 47,968
-973.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
Related articles in JEB:
This article has been cited by other articles:
|
|
 |
|
  K. Phillips HOW SEALS MANAGE THEIR INTERNAL SCUBA TANK J. Exp. Biol., October 15, 2008; 211(20): ii - ii. [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
and β signals of ATP, respectively.
describes the linear rise of free O2 flux with
PO2. Krogh's diffusion coefficient,
K0, is the proportionality constant and the slope. The straight
line shows the rate of change at a K0 value of
2.52x10–5 ml O2 cm–1
min–1 atm–1 at 23°C. The broken line
graphs the equation
,
the Mb-facilitated O2 diffusion as a function of
PO2, P50=1.5 mmHg (1
mmHg=0.133 kPa), CMb=3.8 mmol l–1 and
DMb=4.5x10–7 cm2
s–1. The equipoise PO2 67 mmHg
corresponds to the intersection of the two curves. Below 67 mmHg, the Mb
contribution dominates.