-Adrenergic regulation of systemic peripheral resistance and blood flow distribution in the turtle Trachemys scripta during anoxic submergence at 5°C and 21°C
1 Department of Biological Sciences, Simon Fraser University, Burnaby, BC,
Canada, V5A 1S6
2 Department of Zoophysiology, Aarhus University, Building 131, 8000 Aarhus
C, Denmark
* Author for correspondence (e-mail: jastecyk{at}sfu.ca)
Accepted 6 October 2003
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Anoxia was associated with an increased Rsys and
functional -adrenergic vasoactivity at both acclimation temperatures.
However, while anoxia at 21°C was associated with a high systemic
-adrenergic tone, the progressive increase of Rsys
at 5°C was not mediated by
-adrenergic control. A redistribution of
blood flow away from ancillary vascular beds towards more vital circulations
occurred with anoxia at both acclimation temperatures.
%
sys and absolute blood
flow were reduced to the digestive and urogenital tissues (approximately 2- to
15-fold), while %
sys and
absolute blood flows to the heart and brain were maintained at normoxic
levels. The importance of liver and muscle glycogen stores in fueling
anaerobic metabolism were indicated by increases in
%
sys to the muscle at
21°C (1.3-fold) and liver at 5°C (1.7-fold). As well, the crucial
importance of the turtle shell as a buffer reserve during anoxic submergence
was indicated by 40-50% of
sys being directed towards
the shell during anoxia at both 5°C and 21°C.
-Adrenergic
stimulation and blockade during anoxia caused few changes in
%
sys and absolute tissue
blood flow. However, there was evidence of
-adrenergic vasoactivity
contributing to blood flow regulation to the liver and shell during anoxic
submergence at 5°C.
Key words: red-eared slider, Trachemys scripta, anoxia, temperature, cardiovascular, systemic resistance, cardiac output, blood pressure, -adrenergic control, microsphere, blood flow distribution
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The cardiovascular system continues operating during anoxia to transport
metabolites between tissues, but because of the body's low metabolic demand
and direct effects of reduced oxygen on the myocardium, both heart rate
(fH) and systemic blood flow
(sys) are greatly depressed
(Herbert and Jackson, 1985b
;
Hicks and Wang, 1998
;
Hicks and Farrell, 2000a
).
While systemic blood pressure (Psys) also decreases during
anoxia, the reduction in
sys is considerably larger,
indicating a substantial (three- to fivefold) increase in systemic peripheral
resistance (Rsys)
(Hicks and Farrell, 2000a
).
The basis of this augmented Rsys, which maintains the
systemic circulation in a state of hypotension during anoxia, remains
unidentified (Hicks and Farrell,
2000b
).
In most vertebrates, Rsys is predominantly controlled
by -adrenergic innervation of the resistance vessels, and
-adrenergic mediated peripheral vasoconstriction occurs during hypoxia
in many species (Lillo, 1979
;
Fritsche and Nilsson, 1989
;
Axelsson and Fritsche, 1991
; J.
A. W. Stecyk and A. P. Farrell, manuscript submitted for publication).
Likewise, systemic
-adrenergic tone mediates the increased peripheral
vasoconstriction that occurs during diving in mammals and birds
(Butler and Jones, 1971
;
Butler, 1982
;
Lacombe and Jones, 1991
;
Signore and Jones, 1995
). In
freshwater turtles, adrenergic fibres innervate the systemic circulation
(Berger and Burnstock, 1979
),
and
-adrenergic stimulation increases Rsys in
normoxic animals (Comeau and Hicks,
1994
; Hicks and Farrell,
2000b
; Overgaard et al.,
2002
). Anoxic exposure of turtles is associated with high
concentrations of circulating catecholamines and it is possible that they
increase Rsys through arteriolar
-adrenergic
stimulation (Wasser and Jackson,
1991
; Keiver and Hochachka,
1991
; Keiver et al.,
1992
). However,
-adrenergic regulation of the
cardiovascular system seems depressed during anoxia, as the stimulatory
effects of adrenaline on fH and Psys
are blunted during anoxia at both cold and warm temperatures, and this is
correlated with a reduced density of ß-adrenergic receptors on the heart
(Hicks and Wang, 1998
;
Hicks and Farrell, 2000b
). The
-adrenergic control of vasomotor tone during anoxia has not been
directly investigated in turtles. Consequently, it remains unknown whether the
high levels of circulating catecholamines saturate the
-adrenergic
receptors, so that exogenous application does not affect
Rsys, or whether
-adrenergic control of vasomotor
tone is suppressed during anoxia. Thus, our first objective was to test the
hypothesis that increased
-adrenergic tone accounts for the augmented
Rsys during anoxia in the turtle Trachemys
scripta. We examined the
-adrenergic regulation of
sys,
Psys and Rsys with injections of
-adrenergic agonists and antagonists in normoxic and anoxic turtles
acclimated to 5°C or 21°C.
The increase in Rsys with hypoxia in vertebrate species
is usually accompanied by a redistribution of systemic blood flow that
reflects differences in metabolic needs among tissues, with critical systems,
such as the brain and the heart, receiving a high priority to prevent damage
from anoxia. For example, a high priority to cerebral blood flow has been
observed in fish that are tolerant to oxygen shortage during both anoxia and
severe hypoxia (Nilsson et al.,
1994; Yoshikawa et al.,
1995
; Söderström et
al., 1999
). Similarly, during hypoxia and underwater diving,
endotherms redistribute blood flow towards cerebral, myocardial and adrenal
vascular beds, while blood flow to visceral organs is reduced by a selective
vasoconstriction which, in many cases, is mediated by
-adrenergic
control (Johansen, 1964
;
Elsner et al., 1966
;
Chalmers et al., 1967
;
Krasney, 1971
;
Butler and Jones, 1971
;
Jones et al., 1979
;
Zapol et al., 1979
).
Consistent with these blood flow patterns, blood flow to various visceral
organs is reduced during short-term anoxia in anaesthetized turtles, while
brain blood flow is largely maintained (Davies,
1989
,
1991
;
Bickler, 1992
; Hylland et al.,
1994
,
1996
).
The redistribution of blood flow during short-term anoxia in anaesthetized
turtles (Davies, 1989)
indicates that the different vascular beds respond differently to hypoxia.
However, the anaesthetized turtle does not exhibit the otherwise
well-documented depression of cardiac activity during anoxia, which may
reflect the complex effects of anaesthetics on the heart and local blood flow
regulation (e.g. Smith and Wollman,
1972
; Marcus et al.,
1976
). Therefore, it is uncertain whether these findings can be
applied to unanaesthetized animals and to a prolonged period of anoxia that
can occur naturally. Thus, our second objective was to identify which critical
tissues receive a high priority of blood flow during prolonged anoxia when
cardiovascular function is depressed. Relative systemic blood flow
distribution and absolute tissue blood flows were determined during normoxia
and anoxia by the injection of coloured microspheres, while simultaneously
measuring absolute blood flows in the major arteries. Microspheres were also
injected following
-adrenergic stimulation and blockade during anoxia
to determine
-adrenergic control of blood flow redistribution.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Surgical procedures
Turtles were intubated with soft rubber tubing for artificial ventilation
with isoflurane (4% in room air prepared by a Halothane vaporizer;
Dräger, Lubeck, Germany) at a rate of 8-15 breaths min-1 and a
tidal volume of 10-20 ml kg-1 using a Harvard Apparatus Ventilator
(HI 665, Harvard Apparatus Inc., Holliston, MA, USA). Once a surgical plain of
anaesthesia was achieved, as determined by the lack of a pedal withdrawal
reflex, the isoflurane level was reduced to either 0.5% or 1% and maintained
at this level throughout the operation, which lasted approximately 40 min.
For placement of catheters and flow probes, the heart and central vascular
blood vessels were accessed by excision of a 3 cmx4 cm piece of the
plastron using a bone saw. An occlusive catheter (PE-50 containing saline with
100 i.u. ml-1 heparin) was advanced from the left thyroid artery
into the right subclavian artery originating from the right aortic arch. For
blood flow measurements, 1.0-1.5 cm sections of major systemic blood vessels
were freed from the surrounding connective tissue for placement of ultrasonic
blood flow probes (sizes 2-3 mm; Transonic Systems Inc., Ithaca, NY, USA).
Turtles exposed to anoxia were instrumented with flow probes around the left
aortic arch (LAo), the right aortic arch (RAo), and a single probe around both
the left subclavian and left carotid arteries (Lsubcar). The use of one flow
probe for monitoring blood flow through two vessels has previously been
validated in turtles (Wang and Hicks,
1996; see also Akagi et al.,
1987
). In addition, the left atrium of these animals was
cannulated for the injection of coloured microspheres into the systemic
circulation. A PE-90 catheter, flared at the end to prevent withdrawal, was
inserted through a 0.3 cm incision of the atrial wall into the lumen and
fastened to the atrial wall by surgical silk (4-0). The pericardium was
subsequently closed with two or three sutures (4-0 surgical silk). Control
normoxic turtles were instrumented with a single flow probe around the LAo,
and
sys was determined from
the equation
sys=2.8x
lao,
as previously verified (Comeau and Hicks,
1994
; Wang and Hicks,
1996
). Acoustic gel was infused between the blood vessels and flow
probes to enhance the signal and the excised piece of the plastron was
resealed in its original position using surgical tape and fast-drying epoxy
resin.
After completion of the operation, turtles were ventilated with room air until they resumed spontaneous ventilation. Turtles were then allowed to recover in individual water-filled aquaria (40 cmx30 cmx30 cm), covered with black plastic to minimize visual disturbance, for either 48 h at 21°C or 72 h at 5°C.
Experimental protocol
All experiments were performed on unrestrained turtles that were free to
move within the aquaria. Prior to any experimental manipulation, arterial
blood samples were obtained through the arterial cannula for the measurement
of hematocrit and arterial pH. Turtles studied during anoxia were denied air
access while the aquarium water was continuously bubbled with N2
(water PO2 <0.3 kPa) and were exposed to anoxia for
either 6 h at 21°C or 12 days at 5°C. Normoxic turtles had free access
to room air throughout the experimentation period.
Injections of -adrenergic agonists and antagonists were used to
examine the
-adrenergic regulation of Rsys. Anoxic
turtles were treated sequentially with the
-adrenergic agonist
phenylephrine (5 µg kg-1 and subsequently 50 µg
kg-1), the
-adrenergic antagonist phentolamine (3 mg
kg-1) and, finally, phenylephrine (50 µg kg-1). After
injections of phenylephrine, cardiovascular variables were allowed to return
to baseline values before continuing the protocol. Following phentolamine
injection, cardiovascular function was allowed to stabilize before subsequent
drug injections. Control normoxic turtles were only treated with phentolamine
(3 mg kg-1). All chemicals, purchased from Sigma-Aldrich, Denmark,
were dissolved in physiological turtle saline (in mmol l-1: NaCl,
105; KCl, 2.5; CaCl2, 1.3; MgSO4, 1; NaHCO3,
15; NaH2PO4, 1; pH 7.8) and injected as a single 0.5-1.0
ml bolus through the arterial cannula, which was subsequently flushed with
saline. The total volume injected never exceeded 2 ml and control saline
injections of 2 ml did not cause haemodynamic changes.
Coloured polystyrene microspheres (25 µm in diameter) (Dye Track, Triton Technologies, San Diego, CA, USA) were used to measure regional blood flow distribution. Microspheres, suspended in the manufacturer supplied saline, which contained 0.05% Tween 80 to prevent agglomeration and 0.01% Thimerosal to act as a bacteriostat, to a final concentration of 4.0x105 spheres ml-1 were injected into the anoxic exposed group of turtles in 1 ml portions through the left atrial cannula. This procedure was conducted during normoxia while the animals were not ventilating their lungs (tangerine microspheres), during anoxia (orange microspheres), during the maximal haemodynamic response to the first 50 µg kg-1 phenylephrine injection (lemon microspheres) and, finally, after subsequent injection of phentolamine (canary microspheres), when cardiovascular function stabilized. A minimum of 30 min was allowed between microsphere injection and any subsequent experimental manipulation. To minimize conglomeration of the microspheres in the heart, the microsphere solution was sonicated for 1 min and vortexed for 30 s immediately before injection. After microsphere delivery, which lasted approximately 1 min, the syringe and cannula were flushed twice (0.75 ml each time) with physiological turtle saline and rinsed four times with acidified ethanol (0.2% v:v HCl, 37% ethanol) and the liquid retained such that the number of spheres remaining in the syringe could be determined. A reference blood sample (0.3-0.5 ml) was taken from the arterial cannula 20 min after the normoxic and anoxic microsphere injections to assess whether the microspheres had indeed been trapped in the tissues.
At the completion of the protocol, turtles were euthanized with a vascular injection of pentobarbital (100 µg kg-1). All animals were then dissected to separate organs and tissues, including the integument, red and white muscle, bones, esophagus, stomach, intestines (including pancreas), spleen, ventricle, atria, brain, liver (including gallbladder), kidneys, gonads, fat, connective tissue (included major blood vessels, bladder and thyroid gland) and eyes, which were cut into 7-15 g pieces and placed in individual polypropylene conical centrifuge tubes. The shell was subsampled, using seven representative samples of 1.0-8.9 g (mean 3.8±0.2 g) and representing 6.9±0.4% of total shell mass. Three samples were taken from the plastron, one from each of the anterior, medial and posterior sections, and four from the carapace, one from each side of the shell (costal scutes) and two from the vertebral scutes, one anterior and one posterior. Tissue samples were stored at room temperature for up to 8 weeks, allowing for unaided tissue degradation to occur, before chemical digestion and microsphere recovery from the tissues.
Tissue digestion and microsphere recovery
Immediately prior to tissue digestion, 3000 control spheres (blue, 25 µm
diameter), suspended in saline containing 0.05% Tween 80 and 0.01% Thimerosal,
were added to each sample to evaluate the efficiency and quality of the
recovery process. Extraction of microspheres from reference blood samples and
soft tissues was as follows:
Microsphere recovery from bone and shell samples followed the same protocol as described above, but included two additional steps. After step 3, the pellet was resuspended by sonication in approximately 5x the original tissue volume of `bone digesting reagent' (0.12% EDTA, 5.38% HCl, 94% H2O). Tubes were then maintained at 60°C for at least 12 h with intermittent sonication, centrifuged for 5 min at 1500 g and aspirated to a safe level. The procedure was repeated if bone fragments remained following centrifugation. Once the bone or shell was completely dissolved, the remaining pellet was resuspended by sonication in 15% Triton-X solution and the protocol described above was followed (i.e. alkaline digestion if a large pellet remained, or two acidified ethanol washes if the pellet was not visible).
Measurement of microsphere distribution and terminology
Following tissue digestion, 250 µl of 2-(2-ethoxyethoxy)ethylacetate was
added to each tube and vortexed to extract the dye from the microspheres. The
dye was allowed to extract for 20 min and then centrifuged at 1500 g for 5
min. 200 µl of the supernatant was then transferred to a microplate and the
absorption of each sample was measured at five wavelengths with a microplate
spectrophotometer (Molecular Devices Corporation, Sunnyvale, California, USA)
referenced to 200 µl of 2-(2-ethoxyethoxy)ethylacetate. The five
wavelengths used were those of maximal absorbance for each microsphere colour
label, namely 390 nm (lemon), 440 nm (canary), 495 nm (orange), 525 nm
(tangerine) and 672 nm (blue). Samples with absorbance readings greater than
1.3 units were diluted and reanalyzed to ensure linearity between absorbance
and dye concentration. Correction for spectral overlap among the five colour
labels and subsequent determination of the number of each colour of
microsphere per sample was resolved through a matrix conversion computer
program (Triton Technologies Inc., San Diego, CA, USA).
The total number of each colour of microsphere recovered per animal was
determined as the sum of all microspheres recovered and those estimated to be
trapped in the non-digested portion of the shell (the number of microspheres
recovered from each of the representative shell samples did not differ
statistically, so the amount of microspheres in the non-digested portion of
the shell was assumed to be identical). Recovery of injected microspheres was
expressed relative to the number of microspheres injected (i.e.
4.0x105 minus the number of spheres remaining in the syringe
after injection). The fraction of systemic cardiac output
(%sys) directed to each
tissue was calculated as the quotient of the number of microspheres recovered
per tissue and total microspheres recovered from systemic tissues. Absolute
blood flows (µl min-1 g-1) to systemic tissues were
calculated by multiplying
%
sys for each tissue by
systemic cardiac output
(
sys), as measured by the
ultrasonic flow probes (see below).
Calculation of haematological and cardiovascular variables
Hematocrit was determined as the fractional erythrocyte volume in a
capillary tube following 3 min of centrifugation at 10 000 g.
Arterial pH was measured using a Radiometer pH electrode (PS-1 204,
Copenhagen, Denmark) maintained and calibrated at the acclimation temperature
of the experimental animal in a BMS Mk3 electrode set-up. Electrode output was
displayed on a Radiometer PHM 73 pH monitor. Systemic blood pressure
(Psys) was measured by attaching the arterial cannula to a
disposable pressure transducer (Baxter Edward, model PX600, Irvine, CA, USA)
calibrated daily against a static water column. The signal from the pressure
transducer was amplified using an in-house built preamplifier. Flow probes
were connected to two, dual-channel blood flow meters (T201, Transonic Systems
Inc., Ithaca, NY, USA). All signals were continuously recorded with a Biopac
MP100 computer assisted data acquisition system (Biopac Systems Inc., Goleta,
CA, USA) at 50 Hz and data recordings were analysed offline using AcqKnowledge
data analysis software (version 3.5.7; Biopac Systems Inc., Goleta, CA,
USA).
Systemic cardiac output
(sys) was calculated as
LAo+
RAo+2
Lsubcar
for the anoxic exposed turtles, whereas
sys was estimated as
2.8x
LAo for the
control normoxic turtles. Heart rate (fH) was derived from
the beat-to-beat interval of the Psys trace. Systemic
stroke volume (VSsys) was calculated as
sys/fH
and systemic resistance (Rsys) as
Psys/
sys
with the assumption that right atrial pressure is negligible. Systemic power
output (POsys) was calculated as
sysxPsys/Mv,
where Psys is measured in kPa and Mv
is ventricular mass (g).
Data analysis and statistics
Normoxic control values for all experimental groups were recorded for a
30-60 min period immediately before drug injections or anoxic exposure. At
21°C, haemodynamic variables were recorded continuously throughout the 6
hanoxic period; reported haemodynamic values from this time period were
averaged from 5 min periods at each hour of anoxic exposure. At 5°C,
haemodynamic variables were recorded for 30 min on days 3, 8 and 12 of the
anoxic exposure and continuously throughout the period of drug and microsphere
injections on day 12. At both temperatures, we report the maximal response
following injections of the -adrenergic agonist phenylephrine, which
normally occurred within 5 min and 15 min after injection at 21°C and
5°C, respectively, while new steady state values were attained 0.5 h and 1
h after injection of the
-adrenergic antagonist phentolamine at
21°C and 5°C, respectively.
Values presented for all haematological and cardiovascular variables,
%sys and absolute tissue
blood flows at each sample time are means ± S.E.M.
Within-group comparisons of cardiovascular variables were determined using a
one-way repeated measures (RM) analysis of variance (ANOVA), and comparisons
of cardiovascular variables between acclimation temperatures were performed
using a t-test. Similarly, comparisons of
%
sys and absolute tissue
blood flow between acclimation temperatures were performed using
t-tests. Changes in
%
sys between routine
normoxic and routine anoxic conditions, as well as following
-adrenergic stimulation and blockade, were determined using a two-way
RM-ANOVA on arcsine-transformed
%
sys data, with tissue
%
sys and condition as the
two factors. Changes in absolute blood flow to different tissues during
normoxic control, anoxic control, and following the
-adrenergic agonist
and antagonist drug injections, were assessed with a one-way RM-ANOVA unless
the data were not normally distributed, in which case a Friedman RM-ANOVA on
ranks was used. Where appropriate, multiple comparisons were performed using
Student-Newman-Keuls tests and in all instances significance was accepted when
P<0.05.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Secondly, injected microspheres must be adequately mixed with blood at the
injection site to provide a homogenous solution such that the same
concentration of microspheres prevails at all arterial sites. We injected the
microspheres into the left atrium, and while atrial and ventricular
contraction are likely to have ensured good mixing, the undivided ventricle of
turtles allows for some of the injected microspheres to be shunted into the
pulmonary circulations (White et al.,
1989). During normoxia, we therefore injected the microspheres
during breath-hold, when pulmonary blood flow and Left-to-Right shunting is
low (e.g. White et al., 1989
;
Hicks, 1994
;
Wang and Hicks, 1996
). During
anoxia, at least at warm temperatures, pulmonary blood flow is greatly reduced
due to hypoxic pulmonary vasoconstriction
(Hicks and Wang, 1998
;
Crossley et al., 1998
). These
precautions seemed effective as the relative number of spheres recovered from
the lungs was low (5.2±1.1%; N=44) and did not change with
either injection time or acclimation temperature.
The coloured microsphere technique also relies on an efficient microsphere isolation and purification protocol such that measured absorbance intensities correlate directly with the number of microspheres. Indeed, analysis of reference microsphere samples in the present study revealed linear relationships (P<0.001 in all cases) between microsphere number and measured absorbance for each of the five colour labels (blue, y=0.0002x, r2=0.93; tangerine, y=0.0022x, r2=0.98; orange, y=0.0004x, r2=0.96; canary, y=0.0006x, r2=0.98; lemon, y=0.0003x, r2=0.87). Furthermore, there was a linear relationship (P<0.001 in all cases) between the number of microspheres determined through matrix conversion and the number of microspheres in each experimental sample for all five colour labels (blue, y=0.9712x, r2=0.93; tangerine, y=0.9873x, r2=0.98; orange, y=1.6673x, r2=0.96; canary, y=1.2674x, r2=0.98; lemon, y=1.3199x, r2=0.93). Further, tissue type did not influence the efficiency of microsphere recovery. The relative recovery of blue control spheres (73±2%; N=1215), used to assess microsphere loss during the extraction procedure, did not differ between tissue types. Relative recovery of injected microspheres differed among the different microsphere colour labels. Tangerine coloured spheres (injected at normoxic control) had a recovery of 29±5%, whereas the lemon coloured spheres (injected after phenylephrine during anoxia) had a recovery of 140±2%. 74±13% of the canary (injected after phentolamine during anoxia) and 103±10% of the orange (routine anoxic injection) microspheres were recovered.
The accuracy of the microsphere technique for determination of
%sys can be quantified in
the present study because
sys was measured
simultaneously with blood flow probes (Fig.
1). %
sys to
tissues perfused by the left subclavian (left foreleg integument, bone and
muscle) and carotid arteries (head and neck bones, integument, muscle,
esophagus, trachea, thyroid, brain and eyes) compared to the relative blood
flow in these vessels were linearly related, but the microsphere technique
underestimated %
sys by
approximately 30%.
|
Blood flow ratios among the major systemic blood vessels
Blood flows in all major systemic arteries, with the exception of the right
carotid and right subclavian arteries, were successfully measured in 9 of the
14 turtles exposed to anoxia and instrumented with three ultrasonic flow
probes. Assuming that these vessels receive the same flows as the left carotid
and left subclavian arteries, we estimate that
LAo,
RAo and
Lsubcar account for
36.3±0.01%, 35.9±0.01% and 13.9±0.01%, respectively
(N=9 in all cases), of
sys in turtles. This
proportional distribution was not affected by acclimation temperature, oxygen
availability (Table 1) or
injections of phenylephrine and phentolamine during anoxia (data not
shown).
|
Effect of acclimation temperature on normoxic haematological
variables and -adrenergic control of cardiovascular function
Normoxic hematocrit did not vary significantly between acclimation
temperatures, but arterial pH was significantly greater in 5°C normoxic
turtles than 21°C normoxic turtles
(Table 2). Cold-acclimation
resulted in large reductions in fH and
sys, while
Rsys increased significantly
(Table 3).
sys, fH
and POsys were 5-11x lower at 5°C than at
21°C, with respective Q10 values of 2.6, 2.8 and 4.7. In
contrast, Rsys was twofold greater at 5°C than at
21°C. As a result, Psys was reduced by only 46% at
5°C despite a 4.7-fold decrease in
sys.
|
|
-Adrenergic regulation of cardiovascular function differed between
acclimation temperatures in normoxic turtles
(Table 3). At 21°C,
injection of the
-adrenergic antagonist phentolamine did not affect any
of the measured haemodynamic variables, whereas injection at 5°C
significantly reduced Rsys, which was manifested as a
decrease in Psys and augmented
sys through an increase in
VSsys. Thus, systemic
-adrenergic tone
was inversely related with acclimation temperature in normoxic turtles.
Cardiovascular function during anoxia
sys,
fH and Psys were significantly reduced
with anoxia at both acclimation temperatures (Figs
2 and
3;
Table 4). At. 21°C,
2.1-fold and 2.6-fold reductions in fH and
sys, respectively, occurred
by 6 h of anoxia and the accompanying fall in Psys led to
an almost fourfold reduction in POsys. Correspondingly,
Rsys increased 2.3-fold by 6 h of anoxic exposure. At
5°C, the proportional changes in cardiovascular status during anoxia were
larger than those observed at 21°C. fH,
sys and
POsys were reduced by 1.3- to 3.4-fold by day 3 of anoxia,
and these initial reductions were then followed by a slower, gradual decline
such that by day 12 of anoxia, fH,
sys and
POsys were maximally reduced by 4.7-fold, 4.3-fold and
6.7-fold, respectively. Similar to the response at 21°C, there was a
corresponding increase in Rsys (2.9-fold by day 12 of
anoxic exposure) at 5°C. However, absolute Rsys was
2.3 times greater during 5°C anoxia than 21°C anoxia
(Table 4).
|
|
|
Systemic blood flow distribution during normoxia and anoxia
Blood flow distribution differed with acclimation temperature under
normoxic conditions (Fig. 4A,C;
Table 5), with cold turtles
having a significantly higher
%sys to the integument, but
a lower %
sys to the
intestines. Absolute tissue flows were greater to muscle (4.8-fold), bone
(2.6-fold), intestines (10.4-fold), liver (2.5-fold), gonads (2.4-fold) and
fat (2.5-fold) in warm turtles.
|
|
Systemic blood flow distribution was altered with anoxic submergence. After
6 h of anoxia at 21°C,
%sys decreased
significantly in the stomach (6.2-fold) and intestines (3.8-fold), and
increased significantly in the muscle (1.3-fold) and shell (1.7-fold)
(Fig. 4C,D; Table 5). Absolute tissue blood
flow decreased significantly to the intestines (14.4-fold), stomach
(11.8-fold), kidneys (10.7-fold) and muscle (1.9-fold)
(Table 5). After a 12-day
anoxic exposure at 5°C,
%
sys decreased
significantly in the kidneys (2.7-fold) and gonads (2.2-fold) and increased
significantly in the liver (1.7-fold) and shell (1.2-fold)
(Fig. 4A,B;
Table 5). Absolute blood flow
decreased to all systemic tissues during anoxic submergence at 5°C
(Table 5), with the largest
decreases occurring in the digestive and urogenital tissues and the smallest
decreases occurring in the brain, heart and liver
(Table 6).
|
-Adrenergic control of cardiovascular function and systemic
blood flow distribution during anoxia
Systemic -adrenergic tone in anoxic turtles differed with
acclimation temperature, but in contrast to normoxic turtles, systemic
-adrenergic tone increased with acclimation temperature. With the
exception of a significant, but minor increase in Psys
after a low dose of phenylephrine (5 µg kg-1), there were no
significant effects of
-adrenergic stimulation at 21°C, although
Rsys did tend to increase
(Table 4). In contrast to these
small effects of
-adrenergic stimulation, injection of the
-adrenergic antagonist phentolamine elicited a threefold decrease in
Rsys at 21°C and completely abolished the effects of
subsequent injection of phenylephrine. Thus, the large anoxia-induced increase
in Rsys at 21°C was a result of an elevated
-adrenergic tone.
In anoxic turtles at 5°C, there was a clear dose-dependent increase in
Psys and Rsys following injection of
phenylephrine, but phentolamine did not significantly affect routine anoxic
Rsys (Table
4). Thus, although -adrenergic receptors remained
functional, as attested by the eliminated effects of phenylephrine following
phentolamine (Table 4), the
increase in Rsys during anoxia was not a result of an
increased
-adrenergic tone. Consequently, the anoxia-induced
-adrenergic mediated systemic vasoconstriction was blunted at 5°C
despite being central to the systemic vascular tone during normoxia at this
temperature.
Due to large individual variation there were no significant changes in
either relative or absolute tissue blood flows following -adrenergic
manipulation in anoxic turtles at 21°C
(Table 5). At 5°C,
phenylephrine injection significantly increased
%
sys in muscle and liver,
while %
sys to the shell
decreased. Similarly, absolute blood flow to the liver increased following
-adrenergic stimulation. However, only the increased liver
%
sys, which occurred with
-stimulation, was restored to routine anoxic levels with phentolamine
injection. Shell %
sys
increased significantly following
-adrenergic blockade, but did not
fully return to the pre-
-adrenergic stimulation value.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The reduction in sys
with reduced temperature during normoxia is consistent with other measurements
from ectothermic vertebrates, including freshwater turtles
(Farrell and Jones, 1992
;
Hicks and Farrell,
2000a
,b
;
Stecyk and Farrell, 2002
; J.
A. W. Stecyk and A. P. Farrell, manuscript submitted for publication), and
mirrors the reduction in metabolic rate
(Jackson and Schmidt-Nielsen,
1966
; Jackson,
1968
; Herbert and Jackson,
1985b
). Although Psys was also reduced with
decreased temperature, Rsys was greatly elevated at
5°C.
-Adrenergic regulation of systemic vasomotor tone seems
important in this response since
-adrenergic blockade with phentolamine
injection at 5°C reduced Rsys to the 21°C normoxic
level, but was without effect on Rsys at 21°C (Tables
3,
4). The increased
-adrenergic vasomotor tone in 5°C normoxic turtles supplements the
suppression of cholinergic inhibition of the heart at low temperature, which
probably offsets the negative effects of temperature on cardiac activity
(Hicks and Farrell, 2000b
),
and may represent an important mechanism for regulating blood flow
distribution between priority and less-essential tissues. In fact, differences
in absolute blood flow and
%
sys existed between warm-
and cold-acclimated turtles. Absolute blood flow was decreased to muscle,
bone, intestines, liver, gonads and fat in 5°C acclimated turtles relative
to 21°C acclimated turtles. Additionally,
%
sys was increased to the
integument at 5°C, which may reflect the increased reliance on cutaneous
gas exchange at this temperature (Herbert
and Jackson, 1985b
; Ultsch and
Jackson, 1982
). The decrease in
%
sys to the intestines at
5°C compared to 21°C may reflect the fact that these animals had
fasted during the 1.5-month acclimation period. However, verification of an
-adrenergic involvement in these phenomena is still needed.
Control of systemic peripheral resistance during anoxia
As previously reported, anoxia was accompanied by large depressions in
fH, sys
and Psys at both 5°C and 21°C (Figs
2,
3;
Table 4). These cardiovascular
changes closely resemble those previously described at warm and cold
temperatures (Herbert and Jackson,
1985b
; Hicks and Wang,
1998
; Hicks and Farrell,
2000a
), although our
sys value was higher than
that reported by Hicks and Farrell
(2000a
,b
)
due to an elevated VSsys. The marked increase
in Rsys accompanying anoxia at both temperatures is also
consistent with earlier studies (Hicks and
Wang, 1998
; Hicks and Farrell,
2000a
,b
),
but contrasts with the normal vasodilatory effects of oxygen lack, decreased
pH and increased levels of vasoactive metabolites that are present during
anoxia. Thus, the increased Rsys may be due to activation
of vascular
-adrenergic receptors by the elevated levels of circulating
catecholamines (Wasser and Jackson,
1991
; Keiver and Hochachka,
1991
; Keiver et al.,
1992
) and/or increased sympathetic nerve activity. Indeed,
-adrenergic peripheral vasoconstriction during oxygen limitation is
well documented in different vertebrates
(Butler and Jones, 1971
;
Lillo, 1979
;
Butler, 1982
;
Fritsche and Nilsson, 1989
;
Axelsson and Fritsche, 1991
;
Lacombe and Jones, 1991
;
Signore and Jones, 1995
; J. A.
W. Stecyk and A. P. Farrell, manuscript submitted for publication), and
Trachemys scripta certainly has
-adrenergic receptor mediated
control of Rsys (e.g.
Overgaard et al., 2002
).
However, autonomic regulation of cardiac activity during anoxia in turtles is
dependent upon acclimation temperature. Autonomic control is more pronounced
at warm acclimation temperatures, while the direct effects of oxygen lack and
acidosis seem to account for most of the decreased cardiac performance during
cold, anoxic submergence (Hicks and Wang,
1998
; Hicks and Farrell,
2000b
).
The present study reveals that the -adrenergic system remains
functional during anoxia at both warm and cold acclimation temperatures, but
that the
-adrenergic contribution to the increased
Rsys varies with temperature. Specifically, a large
-adrenergic tone accounting for the increased Rsys
during anoxia at 21°C was revealed by the lack of haemodynamic responses
following injection of phenylephrine and the large (threefold) reduction in
Rsys following
-adrenergic blockade with
phentolamine. In fact, phentolamine reduced Rsys to the
21°C normoxic level (Tables
3,
4). The small effect of
phenylephrine injection on cardiovascular function may possibly reflect the
high levels of circulating catecholamines (>56 nmol norepinephrine;
Wasser and Jackson, 1991
)
fully saturating the systemic
-adrenergic receptors during anoxia.
In contrast to the high -adrenergic tone on Rsys
during anoxia at 21°C, the large progressive increase in
Rsys accompanying anoxia at 5°C does not seem to be
mediated by
-adrenergic vasoactivity. While inhibition of the
-adrenergic receptors with phentolamine eliminated the effects of the
preceding
-adrenergic stimulation with phenylephrine,
-adrenergic blockade did not affect routine anoxic
Rsys at 5°C. This finding is peculiar because it
demonstrates that
-adrenergic vasoactivity remains operational during
anoxia, but that the systemic
-adrenergic tonus is low. Thus, the
increased concentration of plasma catecholamines present in cold, anoxic
turtles (approximately 25 nmol norepinephrine;
Wasser and Jackson, 1991
)
seemingly does not elicit
-adrenergic mediated systemic
vasoconstriction, perhaps because of increased receptor density, decreased
receptor affinity, or reduced signal transduction efficacy. Nevertheless, the
low
-adrenergic tone during anoxia at 5°C is consistent with an
overall blunting of the autonomic regulation of the cardiovascular system
during cold anoxic submergence, when only small cholinergic and
ß-adrenergic tones exist on the cardiovascular system
(Hicks and Farrell,
2000b
).
Given the low -adrenergic tone on the systemic circulation during
anoxia at 5°C, other regulatory mechanisms must be responsible for the
increased Rsys. Hicks and Farrell
(2000a
) suggested that the
hypotension associated with anoxia at 5°C could directly affect
Rsys if Psys failed to surpass the
critical closing pressure of certain vessels. However, our results from
normoxic turtles at 5°C argue against such a mechanism because injection
of phentolamine caused a very severe hypotension (<1.0 kPa) while
Rsys remained low
(Table 3). Nonetheless, vessel
diameters may be reduced at low blood flows and pressures, leading to a higher
resistance (Lipowsky et al.,
1978
). Similarly, blood vessel tension is increased with cold
temperature, and thus may also contribute to the increased
Rsys (Friedman et al.,
1968
; Dinnar,
1981
), which would be exacerbated by the increased blood viscosity
as temperature and flow decrease (Langille
and Crisp, 1980
). Finally, the low
-adrenergic vasomotor
tone during anoxia may represent increased non-adrenergic, non-cholinergic
regulation of Rsys.
Changes in systemic blood flow distribution with anoxic exposure
A redistribution of blood flow towards oxygen-sensitive tissues such as the
brain and heart is critical to survival and is a commonly used survival
strategy among vertebrates when exposed to hypoxia
(Johansen, 1964;
Elsner et al., 1966
;
Chalmers et al., 1967
;
Krasney, 1971
;
Butler and Jones, 1971
;
Jones et al., 1979
;
Zapol et al., 1979
;
Boutilier et al., 1986
; Davies,
1989
,
1991
;
Bickler, 1992
; Hylland et al.,
1994
,
1996
;
Nilsson et al., 1994
;
Yoshikawa et al., 1995
;
Söderström et al.,
1999
). Here, we provide a quantitative description of systemic
blood flow distribution during the large depression in cardiac status
occurring with anoxic submergence in the anoxia-tolerant freshwater turtle. We
clearly show that perfusion is sacrificed in subsidiary tissues, while the
cerebral and myocardial circulations, as well as other critical organs,
receive a priority of blood flow. Specifically, anoxia led to substantial
depressions in %
sys and
absolute tissue flow to digestive and urogenital organs at both temperatures,
while %
sys was increased or
maintained to the shell (5°C and 21°C), muscle (21°C) and liver
(5°C) (Fig. 4,
Table 5). These findings are in
agreement with the redistribution of blood flow away from the renal and
splanchnic circulatory beds observed after 30 min of N2 ventilation
in anaesthetized turtles (Davies,
1989
) and are consistent with the greatly reduced renal function
in conscious anoxic turtles (Warburton and
Jackson, 1991
; Jackson et al.,
1996
). Furthermore, the small number of microspheres directed
towards the lungs during anoxia at 5°C implies the presence of a similar
pulmonary vasoconstriction and reduced Left-to-Right shunt with anoxic
exposure at 5°C to that exhibited during anoxic submergence at warm
temperatures (Hicks and Wang,
1998
; Crossley et al.,
1998
).
The importance of tissues containing large glycogen stores, specifically
the liver and skeletal muscle, in fostering anoxic survival is highlighted in
the present study. During anoxia, turtles must meet their energy demands
through anaerobic metabolism, with glucose as the primary substrate. Glucose
is derived from catecholamine-mediated breakdown of hepatic and skeletal
muscle glycogen stores (Daw et al.,
1967; Penny, 1974
;
Keiver and Hochachka, 1991
;
Wasser and Jackson, 1991
;
Keiver et al., 1992
), thus,
maintained blood flow to the liver and muscle during anoxia may facilitate
glucose export to other organs. Indeed, 6 h of anoxia at 21°C resulted in
an increased %
sys to muscle
and the maintenance of liver absolute blood flow at control normoxic levels.
Similarly, %
sys to the
liver was increased during anoxia at 5°C, and the reduction in absolute
flows to the liver and muscle were minimal compared with the overall reduction
in
sys (4.3-fold,
Table 4) and the reductions in
absolute blood flow to the bulk of systemic tissues
(Table 6).
Anaerobic energy metabolism potentially threatens anoxic survival because
of the accumulation of lactate and the ensuing acidosis
(Herbert and Jackson, 1985b).
However, the shell of the turtle acts as a powerful buffer reserve and
diminishes the acidosis and accumulation of lactate in body fluids (reviewed
by Jackson, 2000
,
2002
). The increased demand of
blood flow to the shell observed in the present study is consistent with the
increased demand on the shell as a buffer reserve during anoxia. In fact,
%
sys directed to the shell
increased significantly during anoxia at both temperatures, such that after
cardiac depression, 40-50% of
sys was directed towards
the shell. Furthermore, the homogenous distribution of microspheres in the
shell is consistent with the uniform lactate accumulation of the entire shell
during anoxia (Jackson et al.,
1996
; Jackson,
1997
).
Turtles in the present study did not display the increased
%sys or absolute blood flow
to the brain or myocardial circulations documented during short-term hypoxic
exposure in other vertebrates (Johansen,
1964
; Elsner et al.,
1966
; Chalmers et al.,
1967
; Krasney,
1971
; Butler and Jones,
1971
; Jones et al.,
1979
; Zapol et al.,
1979
; Nilsson et al.,
1994
; Yoshikawa et al.,
1995
; Söderström et
al., 1999
), including anaesthetized freshwater turtles (Davies,
1989
,
1991
;
Bickler, 1992
; Hylland et al.,
1994
,
1996
). Nevertheless, the
importance of the brain and heart for anoxic survival and their corresponding
demand for blood flow is clearly signified in the present study. At 21°C,
brain and myocardial %
sys
and absolute blood flow were maintained at control normoxic levels following
the 6 h exposure period despite a 2.6-fold decrease in
sys
(Fig. 2, Tables
4,
5). Likewise, 5°C cerebral
and myocardial %
sys were
maintained at control normoxic levels after 12 days of anoxia, while absolute
blood flows were reduced less than the overall reduction in
sys (4.3-fold;
Table 4), as well as the
reductions in absolute blood flows to the bulk of the systemic tissues
(Table 6). These differences in
cerebral and myocardial blood supply during anoxia may simply reflect the long
duration of anoxia in our study, which resulted in a complete transition from
aerobic to anaerobic metabolism. Typically, normal cellular functions are
maintained at the onset of anoxia and organ ATP levels preserved through
activation of glycolysis. Consequently, increased tissue blood flow may be
required for increased glucose delivery and waste removal. However, once
biochemical reorganization has occurred, and glycolytic inhibition (reviewed
by Storey, 1996
) and metabolic
depression are established, an increase in blood flow is no longer required.
In fact, at 20°C, brain blood flow of anaesthetized anoxic turtles returns
to normoxic levels within 1-2 h of anoxia (Hylland et al.,
1994
,
1996
).
-Adrenergic control of systemic blood flow distribution during
anoxia
It is well established that -adrenergic control mediates peripheral
vasoconstriction and subsequent redistribution of blood flow among tissues
during hypoxia or diving in many groups of vertebrates
(Butler and Jones, 1971
;
Butler, 1982
;
Lacombe and Jones, 1991
;
Signore and Jones, 1995
). In
the present study, the use of microspheres was unable to resolve many major
changes in blood flow distribution between tissues following injection of
-adrenergic agonists and antagonists, and this, to some extent, may
reflect a limitation of the methodology. However, given that
-adrenergic stimulation did not increase Rsys
during anoxia at 21°C, and given that Rsys is not
-adrenergically mediated during anoxia at 5°C, major changes in
blood flow distribution may not be expected after
-adrenergic
manipulation. Nevertheless, there seems to be an
-adrenergic mediated
dilation of the liver and constriction in the shell during anoxia at 5°C
(Table 5). Conversely, the
general lack of changes at 21°C may simply reflect a global response of
all tissues to
-adrenergic manipulation, with the resistances in all
tissue beds changing simultaneously such that no overall redistribution
occurs. Differentiation between the two possibilities is outside the scope of
the present study and thus caution must be exercised in interpreting the
observed changes in %
sys
and absolute blood flow as a reflection of tissue-specific
-adrenergic
regulation.
Concluding remarks
In summary, our study reveals that -adrenergic regulation of
Rsys in the freshwater turtle during anoxic submergence is
temperature-dependent. The increased Rsys during anoxia at
21°C can largely be ascribed to an increased
-adrenergic tone,
whereas an
-adrenergic tone does not seem to contribute to the marked
increase in Rsys accompanying anoxia at 5°C. The large
-adrenergic tone on Rsys during anoxia at 21°C
is consistent with the importance of autonomic regulation of the
cardiovascular system during anoxia at warm temperatures, and the blunting of
this response with cold anoxic exposure is consistent with the suppression of
autonomic control during cold anoxic submergence. However, while the intrinsic
effects of anoxia and acidosis are predominantly responsible for the
depression in cardiac activity during anoxic submergence at 5°C, the
primary determinants of the increased Rsys and regulated
hypotension remain to be identified.
The overall redistribution of systemic blood flow and changes in absolute
blood flows to specific tissues during anoxia are consistent with tissue
metabolism and/or their respective importance for survival during anoxia.
Following 6 h of anoxia at 21°C,
%sys and absolute blood
flow were reduced to the digestive and urogenital tissues while
%
and absolute blood flows to the
cerebral and myocardial circulations were maintained at control normoxic
levels. Following 12 days of anoxia at 5°C,
%
sys was reduced to the
urogenital tissues, but maintained at control normoxic levels in the brain and
heart. This indicates that the digestive and urogenital tissues are of reduced
importance, whereas the myocardial and cerebral circulations remain a
priority. Similarly, the increased importance of liver and muscle glycogen
stores in fueling anaerobic metabolism during anoxia was indicated by the
increased %
sys to the
muscle (21°C) and liver (5°C) and minimally reduced absolute blood
flow to the liver at 5°C. Finally, the crucial and increased importance of
the turtle shell as a buffer reserve during anoxic submergence (Jackson,
2000
,
2002
) was highlighted by the
increased %
sys directed
towards the shell with anoxia at both 5°C and 21°C.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Akagi, K., Endo, C., Saito, J., Onodera, M., Kozu, M., Tanigawara, S., Okamura, K., Yajima, A. and Sato, A. (1987). Ultrasonic transit-time measurement of blood flow in the animal chronic preparation model. Jpn. J. Med. Ultrasonics 14,104 -110.
Axelsson, M. and Fritsche, R. (1991). Effects of exercise, hypoxia and feeding on the gastrointestinal blood flow in the Atlantic cod Gadus morhua. J. Exp. Biol. 158,181 -198.[Abstract]
Berger, P. J., and Burnstock, G. (1979). Autonomic nervous system. In Biology of the Reptilia vol 10B (ed. C. Gans and W. Dawson), pp.1 -39. New York: Academic Press.
Bickler, P. E. (1992). Effects of temperature and anoxia on regional cerebral blood flow in turtles. Am. J. Physiol. 262,R538 -R541.[Medline]
Boutilier, R. G., Glass, M. L. and Heisler, N. (1986). The relative distribution of pulmocutaneous blood flow in Rana catesbeiana: effects of pulmonary or cutaneous hypoxia. J. Exp. Biol. 126,33 -39.[Abstract]
Butler, P. J. (1982). Respiratory and cardiovascular control during diving in birds and mammals. J. Exp. Biol. 100,195 -221.[Abstract]
Butler, P. J. and Jones, D. R. (1971). The effect of variations in heart rate and regional distribution of blood flow on the normal pressor response to diving in ducks. J. Physiol. 214,457 -459.[Medline]
Chalmers, J. P., Korner, P. I. and White, S. W. (1967). Local and reflex factors affecting the distribution of the peripheral blood flow during arterial hypoxia in the rabbit. J. Physiol. 192,537 -548.[Medline]
Comeau, S. G. and Hicks, J. W. (1994). Regulation of central vascular blood flow in the turtle. Am. J. Physiol. 267,R569 -R578.[Medline]
Crossley, D., Altimiras, J. and Wang, T.
(1998). Hypoxia elicits an increase in pulmonary vascular
resistance in anaesthetized turtles (Trachemys scripta).
J. Exp. Biol. 201,3367
-3375.
Davies, D. G. (1989). Distribution of systemic blood flow during anoxia in the turtle, Chrysemys scripta. Resp. Physiol. 78,383 -390.[CrossRef][Medline]
Davies, D. G. (1991). Chemical regulation of cerebral blood flow in turtles. Am. J. Physiol. 260,R382 -384.[Medline]
Daw, J. C., Wenger, D. P. and Berne, R. M. (1967). Relationship between cardiac glycogen and tolerance to anoxia in the western painted turtle, Chrysemys picta bellii. Comp. Biochem. Physiol. 22, 69-73.[CrossRef][Medline]
Dinnar, U. (1981). Cardiovascular Fluid Dynamics, pp. 1-252. Boca Raton: CRC Press, Inc.
Elsner, R., Franklin, D. L., Van Citters, R. L. and Kenney, D. W. (1966). Cardiovascular defense against asphyxia. Science 153,941 -949.[Medline]
Farrell, A. P. and Jones, D. R. (1992). The Heart. In Fish Physiology, vol.XII , part A (ed. W. S. Hoar, D. J. Randall and A. P. Farrell), pp. 1-88. San Diego: Academic Press, Inc.
Friedman, S. M., Nakashima, M. and Friedman, C. L. (1968). Effects of cooling and rewarming on Na, K, and tension changes in rat tail artery. Can. J. Physiol. Pharmacol. 46,25 -34.[Medline]
Fritsche, R. and Nilsson, S. (1989). Cardiovascular responses to hypoxia in the Atlantic cod, Gadus morhua. Exp. Biol. 48,153 -160.[Medline]
Herbert, C. V. and Jackson, D. C. (1985a). Temperature effects on the responses to prolonged submergence in the turtle Chrysemys picta bellii. I. Blood acid-base and ionic changes during and following anoxic submergence. Physiol. Zool. 58,665 -669.
Herbert, C. V. and Jackson, D. C. (1985b). Temperature effects on the responses to prolonged submergence in the turtle Chrysemys picta bellii. II. Metabolic rate, blood acid-base and ionic changes and cardiovascular function in aerated and anoxic water. Physiol. Zool. 58,670 -681.
Hicks, J. M. T. and Farrell, A. P. (2000a). The
cardiovascular responses of the red-eared slider (Trachemys scripta)
acclimated to either 22 or 5°C. I. Effects of anoxia exposure on in
vivo cardiac performance. J. Exp. Biol.
203,3765
-3774.
Hicks, J. M. T. and Farrell, A. P. (2000b). The
cardiovascular responses of the red-eared slider (Trachemys scripta)
acclimated to either 22 or 5°C. II. Effects of anoxia on adrenergic and
cholinergic control. J. Exp. Biol.
203,3775
-3784.
Hicks, J. W. (1994). Adrenergic and cholinergic regulation of intracardiac shunting. Physiol. Zool. 67,1325 -1346.
Hicks, J. W. and Wang, T. (1998). Cardiovascular regulation during anoxia in the turtle: An in vivo study. Physiol. Zool. 71, 1-14.[Medline]
Hylland, P., Nilsson, G. and Lutz, P. (1994). Time course of anoxia-induced increase in cerebral blood flow rate in turtles: Evidence for a role of adenosine. J. Cereb. Blood Flow Metab. 14,877 -881.[Medline]
Hylland, P., Nilsson, G. and Lutz, P. (1996). Role of nitric oxide in the elevation of cerebral blood flow induced by acetylcholine and anoxia in the turtle. J. Cereb. Blood Flow Metab. 16,290 -295.[Medline]
Jackson, D. C. (1968). Metabolic depression and
oxygen depletion in the diving turtle. J. Appl.
Physiol. 24,503
-509.
Jackson, D. C. (1997). Lactate accumulation in
the shell of the turtle Chrysemys picta bellii during anoxia at
3°C and 10°C. J. Exp. Biol.
200,2295
-2300.
Jackson, D. C. (2000). Living without oxygen: lessons from the freshwater turtle. Comp. Biochem. Physiol. 125A,299 -315.
Jackson, D. C. (2002). Hibernating without
oxygen: physiological adaptations of the painted turtle. J.
Physiol. 543,731
-737.
Jackson, D. C. and Schmidt-Nielsen, K. (1966). Heat production during diving in the fresh water turtle, Pseudemys scripta. J. Cellular Physiol. 67,225 -231.[Medline]
Jackson, D. C. and Ultsch, G. R. (1982). Long-term submergence at 3°C of the turtle Chrysemys picta bellii, in normoxic and severely hypoxic water: II. Extracellular ionic responses to extreme lactic acidosis. J. Exp. Biol. 96, 29-43.
Jackson, D. C., Toney, V. I. and Okamoto, S. (1996). Lactate distribution and metabolism during and after anoxia in the turtle Chrysemys picta bellii. Am. J. Physiol. 271,R409 -R416.[Medline]
Johansen, K. (1964). Regional distribution of circulating blood during submersion asphyxia in the duck. Am. J. Physiol. 205,1167 -1171.
Johlin, J. M. and Moreland, F. B. (1933). Studies of the blood picture of the turtle after complete anoxia. J. Biol. Chem. 103,107 -114.
Jones, D. R., Bryan, R. M., West, N. H., Lord, N. H. and Clark, B. (1979). Regional distribution of blood flow during diving in the duck (Anas platyrhynchos). Can. J. Zool. 57,995 -1002.
Keiver, K. M. and Hochachka, P. W. (1991). Catecholamine stimulation of hepatic glycogenolysis during anoxia in the turtle Chrysemys picta. Am. J. Physiol. 261,R1241 -R1345.
Keiver, K. M., Weinberg, J. and Hochachka, P. W. (1992). The effect of anoxic submergence and recovery on circulating levels of catecholamines and corticosterone in the turtle, Chrysemys picta. Gen. Comp. Endocrinol. 85,308 -315.[Medline]
Krasney, J. A. (1971). Regional circulatory
responses to arterial hypoxia in the anaesthetized dog. Am. J.
Physiol. 220,699
-704.
Lacombe, A. M. A. and Jones, D. R. (1991). Neural and humoral effects on hindlimb vascular resistance of ducks during forced submergence. Am. J. Physiol. 261,R1579 -R1586.[Medline]
Langille, B. L. and Crisp, B. (1980). Temperature dependence of blood viscosity in frogs and turtles: effect on heat exchange with environment. Am. J. Physiol. 239,R248 -253.[Medline]
Lillo, R. S. (1979). Autonomic cardiovascular control during submergence and emergence in bullfrogs. Am. J. Physiol. 237,R210 -R216.[Medline]
Lipowsky, H. H., Kovalcheck, S. and Zwefach, B. W. (1978). The distribution of blood rheological parameters in the microvasculature of cat mesentery. Circ. Res. 43,738 -749.[Abstract]
Lutz, P. L. and Storey, K. B. (1997). Adaptations to variations in oxygen tension by vertebrates and invertebrates. In Handbook of Comparative Physiology, Section 13, Volume II (ed. William H. Dantzler), pp.1472 -1522. New York: Oxford University Press.
Marcus, M. L., Heistad, D. D., Ehrhardt, J. C. and Abboud, F. M. (1976). Total and regional cerebral blood flow measurement with 7-10, 15-, 25-, and 50-µm microspheres. J. App. Phyiol. 40,501 -507.
Nilsson, G. E., Hylland, P. and Löfman, C. O. (1994). Anoxia and adenosine induce increased cerebral blood flow rate in crucian carp. Am. J. Physiol. 267,R590 -R595.[Medline]
Overgaard, J., Stecyk, J. A. W., Farrell, A. P. and Wang, T.
(2002). Adrenergic control of the cardiovascular system in the
turtle (Trachemys scripta). J. Exp. Biol.
205,3335
-3345.
Penny, D. G. (1974). Effects of prolonged diving anoxia on the turtle, Pseudemys scripta elegans. Comp. Biochem. Physiol. 47A,933 -941.[CrossRef]
Shelton, G. and Burggren, W. (1976). Cardiovascular dynamics of the chelonia during apnea and lung ventilation. J. Exp. Biol. 64,323 -343.[Abstract]
Signore, P. E. and Jones, D. R. (1995). Effect of pharmacological blockade on cardiovascular responses to voluntary and forced diving in muskrats. J. Exp. Biol. 198,2307 -2315.[Medline]
Smith, A. L. and Wollman, H. (1972). Cerebral blood flow and metabolism: Effects of anesthetic drugs and techniques. Anesthesiology 36,378 -400.[Medline]
Söderström, V., Renshaw, G. M. and Nilsson, G. E.
(1999). Brain blood flow and blood pressure during hypoxia in the
epaulette shark Hemiscyllium ocellatum, a hypoxia tolerant
elasmobranch. J. Exp. Biol.
202,829
-835.
Stecyk, J. A. W. and Farrell, A. P. (2002).
Cardiorespiratory responses of the common carp (Cyprinus carpio) to
severe hypoxia at three acclimation temperatures, J. Exp.
Biol. 205,759
-768.
Storey, K. B. (1996). Metabolic adaptations supporting anoxia tolerance in reptiles: Recent advances. Comp. Biochem. Physiol. 113B,23 -35.
Ultsch, G. R. and Jackson, D. C. (1982). Long-term submergence at 3°C of the turtle Chrysemys picta bellii, in normoxic and severely hypoxic water: I. Survival, gas exchange and acid-base status. J. Exp. Biol. 96, 11-28.
Wang, T. and Hicks, J. W. (1996).
Cardiorespiratory synchrony in turtles. J. Exp. Biol.
199,1791
-1800.
Warburton, S. J. and Jackson, D. C. (1991). Turtle (Chrysemis picta bellii) shell mineral content is altered by exposure to prolonged anoxia. Physiol. Zool. 68,783 -798.
Wasser, J. S. and Jackson, D. C. (1991). Effects of anoxia and graded acidosis on the levels of circulating catecholamines in turtles. Resp. Physiol. 84,363 -377.[CrossRef][Medline]
White, F. N., Hicks, J. W. and Ishimatsu, A. (1989). Relationship between respiratory state and intracardiac shunts in turtles. Am. J. Physiol. 256,R240 -R247.[Medline]
Yoshikawa H., Ishida, Y., Kawata, K., Kawai, F. and Kanamori, M. (1995). Electroencephalograms and cerebral blood flow in carp, Cyprinus carpio, subjected to acute hypoxia. J. Fish Biol. 46,114 -122.
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.