In situ cardiac performance of Pacific bluefin tuna hearts in response to acute temperature change
Tuna Research and Conservation Center, Hopkins Marine Station, Department of Biological Sciences, Stanford University, Oceanview Boulevard, Pacific Grove, CA 93950, USA
* Author for correspondence (e-mail: bblock{at}stanford.edu)
Accepted 3 December 2003
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Summary |
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Key words: Pacific bluefin tuna, Thunnus orientalis, in situ heart preparation, temperature, citrate synthase
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Introduction |
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Current knowledge of physiological performance in tunas is primarily based
on research conducted on juvenile yellowfin and skipjack tunas, as both
species have been routinely maintained in land-based facilities
(Brill and Bushnell, 2001;
Farwell, 2001
). Standard
metabolic rates measured in juvenile yellowfin tuna are at least twofold
higher than those of active teleosts, including scombrids such as bonito
Sarda chiliensis and other species such as sockeye salmon
Oncorhynchus nerka and yellowtail Seriola quinqueradiata
(Brett and Glass, 1973
;
Yamamoto et al., 1981
;
Dewar and Graham, 1994a
;
Korsmeyer and Dewar, 2001
).
Albacore tuna placed in shipboard respirometers shortly after capture have
exhibited similarly high metabolic rates
(Graham and Laurs, 1982
).
The elevated metabolic rates of tunas are supported by a number of
distinctive morphological, biochemical and physiological specializations that
are unique to the tribe Thunnini. Large gill surface areas and thin
gill epithelia enhance oxygen uptake from the environment
(Muir and Hughes, 1969), while
well vascularized tissues
(Mathieu-Costello et al.,
1996
) with high levels of myoglobin
(Giovane et al., 1980
;
Marcinek, 2000
) and aerobic
enzymes (Dickson, 1995
;
Freund, 1999
) enhance oxygen
extraction by the tissues (Bushnell and
Brill, 1992
). High metabolic rates also demand high cardiac
outputs, which tunas achieve through high maximal heart rates and high stroke
volumes (Brill and Bushnell,
2001
).
To date, cardiovascular performance has primarily been studied in small
yellowfin, skipjack and albacore tunas. In yellowfin and skipjack tunas,
cardiac outputs of up to 84132 ml kg1
min1 are among the highest recorded in teleosts
(Bushnell and Brill, 1992;
Farrell et al., 1992
;
Brill and Bushnell, 2001
;
Blank et al., 2002
;
Mercier et al., 2002
). No
comparable studies have been conducted on closely related ectothermic taxa
such as bonito. Heart rates in spinally blocked yellowfin tuna at 25°C
range from 90 to 130 beats min1
(Bushnell and Brill, 1992
),
although heart rates in unstressed free-swimming yellowfin tuna are lower,
ranging from 67 to 100 beats min1
(Bushnell and Brill, 1991
;
Korsmeyer et al., 1997
). These
lower rates have been shown to result from tonic cholinergic input
(Keen et al., 1995
). Maximal
heart rates of 119 and 180 beats min1 recorded at 25°C
in yellowfin and skipjack tuna, respectively
(Keen et al., 1995
), exceed
those of most other teleosts. Stroke volumes of 1.11.3 ml
kg1 in yellowfin and skipjack tunas are similar to maximal
values in rainbow trout and two- to threefold higher than routine values in
teleosts including rainbow trout and yellowtail
(Yamamoto et al., 1981
;
Bushnell and Brill, 1992
;
Farrell and Jones, 1992
). It
is unclear whether values recorded in spinally blocked tunas represent routine
or maximal cardiac performance. Shipboard experiments on freshly caught
albacore tunas have yielded heart rates similar to those of the tropical tunas
while operating at 1519°C, but produced threefold lower cardiac
outputs due to lower stroke volumes (Lai
et al., 1987
; White et al.,
1988
).
The response of the tuna heart to temperature change has been studied in
free-swimming yellowfin, in situ preparations, and myocardial strip
preparations (Korsmeyer et al.,
1997; Freund,
1999
; Shiels et al.,
1999
; Blank et al.,
2002
). Measurements on instrumented yellowfin tuna swimming in a
flume indicate a trade-off between reduced heart rate and increased stroke
volume with falling temperatures, resulting in a decrease of cardiac output
(Korsmeyer et al., 1997
).
In situ heart preparations exhibit a similar trade-off between heart
rate and stroke volume, indicating that these responses are intrinsic to the
heart (Blank et al., 2002
).
Heart rates of yellowfin tuna in situ dropped from 109 beats
min1 at 25°C to 19.8 beats min1 at
10°C, while experimental maximal stroke volumes rose from 0.91 at 25°C
to 1.33 ml kg1 at 10°C
(Blank et al., 2002
). This
results in a drop in cardiac output from 98 ml kg1
min1 at 25°C to 28 ml kg1
min1 at 10°C. Experiments on isolated atrial and
ventricular strips from yellowfin tuna produced similar results, indicating
that falling temperatures increase contractile force but reduce sustainable
contraction frequencies (Freund,
1999
; Shiels et al.,
1999
).
In contrast to data on metabolic rate and cardiovascular function in
yellowfin, skipjack and albacore tunas, no measurements of heart performance
or metabolic rate have been made in any species of bluefin tuna either in
vivo or in situ. This is in large part due to the challenges of
maintaining captive bluefin tuna in land-based facilities. Several studies
have investigated cardiac morphology and metabolic biochemistry of the
Atlantic and Pacific bluefin tuna, Thunnus thunnus and Thunnus
orientalis (Basile et al.,
1976; Maresca et al.,
1976
; Balestrieri et al.,
1978
; Tota, 1978
;
Gemelli et al., 1980
;
Greco et al., 1982
;
Marcinek et al., 2001b
). These
studies have shown that Atlantic bluefin tuna hearts are proportionally larger
than those of skipjack and yellowfin tunas
(Poupa et al., 1981
), exhibit
a thick layer of compact myocardium (Tota,
1978
) and have extraordinarily high levels of myoglobin
(Giovane et al., 1980
;
Marcinek, 2000
). Experiments
on the spongy and compact layers of Atlantic bluefin ventricle indicate that
the spongy layer has higher mitochondrial enzyme activities
(Basile et al., 1976
;
Greco et al., 1982
), more
pronounced temperature dependence of oxidative enzymes
(Maresca et al., 1976
), and
greater capacity to metabolize lactate
(Gemelli et al., 1980
),
apparently reflecting the fact that spongy tissue is perfused by lumenal
venous blood and compact tissue receives arterial blood from the coronary
artery (Tota, 1978
).
Recently, the thermal physiology of free-swimming bluefin tunas, yellowfin
tuna and big-eye tuna has been examined by the deployment of electronic tags
on wild fish (Kitagawa et al.,
2000; Block et al.,
2001
; Gunn and Block,
2001
; Schaefer and Fuller,
2002
; Musyl et al.,
2003
). Archival tags indicate that all bluefin species experience
a wide range of environmental temperatures, with a maximal range of
2.831°C recorded thus far in Atlantic bluefin. To date, there is
little data from large Pacific bluefin tuna, but young Pacific bluefin tuna
similar in size to the animals studied in this paper have been acoustically
and archivally tracked in the eastern Pacific and experience temperatures from
1.8°C to 22°C (Marcinek et al.,
2001a
; Kitagawa et al.,
2002
; B. A. Block et al., unpublished data). Archival tagging of
juvenile Pacific bluefin tuna in the western Pacific indicates that these fish
primarily encounter temperatures of 1222°C but dive into waters as
cold as 5°C (Kitagawa et al.,
2000
,
2001
,
2002
). In the eastern Pacific,
where juveniles of both yellowfin and bluefin tuna have been tracked, acoustic
tracks indicate that yellowfin tuna primarily occupy waters above 17°C.
Periodic dives below this ambient temperature to 11°C lasted for only a
few minutes and the tuna quickly returned to the surface, suggesting a
physiological limitation (Block et al.,
1997
).
Bluefin tunas conserve metabolic heat and face a unique physiological
challenge when diving below the thermocline or encountering cold surface
waters at high latitude. The heart rapidly equilibrates to cold ambient
temperatures (Brill, 1987;
Brill et al., 1994
), but
vascular countercurrent heat exchangers defend the temperature of the brain,
eyes, swimming muscles and viscera, maintaining warm tissues which demand high
cardiac outputs (Brill et al.,
1994
). This raises the possibility that cardiac performance in
cold ambient conditions limits the tuna's thermal niche and behavioral
performance (Brill et al.,
1998
,
1999
;
Block et al., 1997
;
Marcinek et al., 2001a
). What
has not been directly addressed in many studies is how and why this
physiological limitation occurs.
In this study, the in situ heart preparation of Farrell et al.
(1992) was used to measure
cardiac performance, including heart rate, stroke volume and cardiac output in
bluefin tuna across a range of temperatures likely to be encountered in the
wild (230°C). In situ preparations have been used
successfully to study heart function in a variety of fish species (Farrell et
al., 1983
,
1988
), including yellowfin and
skipjack tuna (Farrell et al.,
1992
). The use of the in situ technique allows comparison
of the data presented here with similar measurements of temperature
sensitivity in yellowfin tuna hearts in situ
(Blank et al., 2002
). The
in situ preparation was also applied to one albacore tuna Thunnus
alalunga that became available during the course of the experiments, and
data are reported here for comparison. Because aerobic capacity is an
important component of cardiac performance, citrate synthase activity, a
commonly used index of tissue aerobic capacity, was measured in homogenates of
Pacific bluefin tuna atrium and ventricle.
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Materials and methods |
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Fish handling and surgical procedures
Fish were captured in a nylon sling, transported out of the tank in an
envelope of sea water and euthanized by pithing. The spinal cord was ablated
by insertion of a 30 cm piece of 120 kg test monofilament to eliminate
post-mortem swimming motions. Surgical procedures were identical to
those described previously (Blank et al.,
2002). The peritoneal cavity was opened ventrally and the sinus
venosus was cannulated and perfused with oxygenated Ringer's solution
via a hepatic vein. A second cannula was inserted into the ventral
aorta to receive output from the heart. The coronary artery was cannulated and
perfused with oxygenated Ringer's successfully in seven of nine preparations.
The pericardium was kept intact. After surgery, the entire fish was
transferred to a 75 l insulated water bath filled with saline at 15°C. The
input cannula was connected to insulated perfusate reservoirs and recycling of
perfusate was initiated by moving the output tubing back to the perfusate
reservoirs, which set output pressure at approximately 6 kPa. At the
completion of each experiment, the atrium and ventricle were removed, cut
open, blotted dry on paper towels and weighed. Mean atrial and ventricular
masses of Pacific bluefin tuna were 3.85±0.66 g and 21.8±3.0 g
(0.057±0.009% and 0.322±0.025% of body mass), respectively.
Atrial and ventricular masses of one albacore tuna were 2.01 g and 11.27 g
(0.038% and 0.213% of body mass), respectively.
Cardiac performance tests
Once the fish was placed into the saline bath and the heart was
successfully recycling fluid to the reservoir, a set of tests was completed,
comprising measurements at standard conditions, maximum flow, maximum power,
and standard conditions again. Standard conditions were defined as input
pressure of 00.05 kPa and output pressure of approximately 6 kPa. To
determine the maximum flow that the heart could produce, input pressure was
elevated to 2.4 kPa and cardiac output was allowed to stabilize (maximal
flow conditions). Following a brief recovery period, input pressure was again
elevated to
2.4 kPa and output pressure was simultaneously increased to
10 kPa and then elevated in additional 1 kPa steps until power no longer
increased (maximal power conditions). Standard conditions were intended to
approximate in vivo conditions for a fish in a relaxed state, while
conditions of maximal flow and pressure were intended to evoke the maximal
performance of the heart.
Temperature experiments
With input and output pressures at standard conditions, the temperatures of
the bath saline and perfusate were simultaneously adjusted over a period of
35 min and the preparation was allowed to equilibrate for 35 min
prior to measurements at the new temperature. Control tests at 15°C were
completed between measurements at each test temperature. In cases where
cardiac output at standard conditions declined by more than 10% from the
initial control test, data at the test temperature were normalized by the
ratio of initial values to control values bracketing the test temperature.
Experiments were completed within a period of 90150 min following
surgery.
Solutions
Ringer's solution consisted of (in mmol l1) 185.7 NaCl,
1.1 MgCl2, 7.0 KCl, 3.22 CaCl2, 10 sodium pyruvate and
10 Hepes. The pH was adjusted to 7.8 at 20°C by addition of NaOH.
Epinephrine was maintained in the Ringer's perfusate at 1 nmol
l1. The perfusate was bubbled with 100% oxygen throughout
the experiments. Saline for the 75 l bath was made up as a 1:3 mixture (v/v)
of seawater and tapwater (or ice as needed).
Instrumentation, calibrations and analysis
Flows were measured with a Transonic 6-N in-line flow probe connected to a
Transonic T-106 flow meter (Transonic Systems Inc., Ithaca, NY, USA). Both
input and output pressures were measured with pressure transducers from
ADInstruments (Sydney, Australia). Flow and pressure signals were read by a
Powerlab 8s hooked to a Macintosh G4 computer running Chart 4.0 software
(ADInstruments). Flow signals were calibrated by weighing the saline output
over a measured time. Pressure signals were calibrated with a water manometer.
Mean flow, pressures, power and heart rate were calculated from 5 or 6 beats
at each temperature and experimental condition using the Powerlab program.
Power output is expressed as mW g1 heart (ventricle+atrium)
mass. Results at different temperatures within the standard or maximal
condition were compared by single-factor analysis of variance (ANOVA) and
StudentNewmanKeuls post-hoc tests. Differences between
standard and maximal conditions at individual temperatures were assessed by
paired t-tests. Linear regressions of performance at different
temperatures were compared among species by ANOVA. Significance was assessed
at P0.05. Data are presented as means ±
S.E.M.
Citrate synthase (CS) assays
Atria and ventricles were removed from wild Pacific bluefin tuna at sea or
within 5 days of arrival at the TRCC and freeze-clamped with copper tongs
cooled in liquid nitrogen. Crude homogenates were prepared and CS activity was
determined by the reduction of 5,5'-dithiobis(2-nitrobenzoic acid)
(DTNB) and the resulting change in absorbance at 412 nm using a Perkin-Elmer
Lamda 6 spectrophotometer (Norwalk, CT, USA). The reaction mixture contained
0.4 mmol l1 acetyl CoA, 0.25 mmol l1 DTNB,
0.5 mmol l1 oxaloacetate and 50 mmol l1
imidazole, pH 7.83 (Hansen and Sidell,
1983). Reactions were conducted at 25°C.
Archival tagging
Atlantic bluefin tuna were archival tagged as reported in Block et al.
(2001). Implantable archival
records in Fig. 5 are from
Wildlife Computers MK7 (Redmond, WA, USA) archival tags placed in bluefin tuna
of 208 cm and 219 cm curved fork length in January 1999. Data were recorded at
2 min intervals for ambient external temperature, internal temperature and
pressure records, and obtained when the tuna were recaptured and the data
downloaded from recovered tags.
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Results |
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Cardiac output and power output were unaffected by temperature when the heart was perfused under standard conditions (Fig. 2C,D, open circles). Although heart rate decreased with decreasing temperature, there was a compensatory rise in stroke volume, which increased significantly from 0.18±0.019 ml kg1 at 25°C to 0.86±0.012 ml kg1 at 2°C (P<0.001, Fig. 2B). Cardiac outputs were 20.6±2.6 ml kg1 min1 and power outputs were 0.67±0.11 mW g1 at 10°C. Stroke volumes and cardiac outputs at 30°C are not reported due to failure of the preparations to return to control conditions at 15°C.
In contrast to standard conditions, maximal cardiac output and power output increased directly with temperature due to increases in heart rate combined with constant stroke volume. Stroke volumes were greater at maximal flow conditions relative to standard conditions at all temperatures (P<0.05). This difference was more pronounced at warmer temperatures, where maximal stroke volume at 25°C showed a nearly sixfold increase over standard conditions. However, maximal stroke volume showed no significant effects of temperature, rising only slightly from 1.06±0.07 ml kg1 at 25°C to 1.32±0.10 ml kg1 at 2°C (Fig. 2B, filled circles). Maximal cardiac output showed a strong temperature effect, increasing from 18.1±1.7 ml kg1 min1 at 2°C to 106±5.1 ml kg1 min1 at 25°C (Q10=2.0) (P<0.001, Fig. 2C, filled circles). Maximal power outputs also increased significantly at higher temperatures, rising from 0.43±0.05 mW g1 at 2°C to 4.38±0.68 mW g1 at 25°C (Q10=2.75) (P<0.001, Fig. 2D, filled circles).
In Fig. 3, cardiac
performance measured with the in situ perfused heart technique is
compared in three species of tuna. The maximal cardiac performance of a single
albacore tuna tested in situ from 10°C to 20°C is shown in
comparison with data from bluefin tuna and yellowfin tuna tested with an
identical preparation (Blank et al.,
2002). All three species show a strong temperature-dependence of
heart rate with an apparent difference in the lower limit. Heart rate of the
single albacore tuna decreased from 88 beats min1 at
20°C to 38 beats min1 at 10°C
(Fig. 3A). For all three
species, stroke volume is less sensitive to temperature than heart rate. In
albacore tuna, stroke volume rose from 0.93 ml kg1 at
20°C to 1.43 ml kg1 at 10°C
(Fig. 3B). Cardiac output fell
from 82 ml kg1 min1 at 20°C to 54 ml
kg1 min1 at 10°C
(Fig. 3C). Power output was
greater in the albacore tuna than in bluefin tuna when normalized to the
smaller heart mass of the albacore (Fig.
3D).
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To examine the aerobic capacity of the Pacific bluefin heart, CS activity was measured in atria and ventricles at 25°C. CS activities in atria and ventricles were 77±3.1 and 95±3.1 U g1 wet mass, respectively.
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Discussion |
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Temperature effects on in situ cardiac performance
Pacific bluefin tuna heart rates ranged from 156±9.4 beats
min1 at 30°C to 13±0.9 beats
min1 at 2°C (Table
1; Fig. 2A). Heart
rates of albacore tuna (Fig.
3A) were similar to those of bluefin tuna from 10°C to
20°C and slightly lower than those previously recorded in anesthetized
albacore tuna (Lai et al.,
1987; White et al.,
1988
). While the absolute values of Pacific bluefin tuna heart
rate were within the range of heart rates previously reported in yellowfin
tuna at 25°C, Pacific bluefin heart rates were significantly higher than
those of yellowfin tuna at temperatures from 15°C to 2°C. The most
important distinction among the species is that the Pacific bluefin tuna heart
rates showed a significantly lower temperature dependence than those of
yellowfin across the tested temperatures (P<0.0001,
Fig. 3A). The Q10
for Pacific bluefin heart rate was 2.1 between 25°C and 10°C compared
to 3.1 for yellowfin across the same temperatures
(Blank et al., 2002
). Maximal
stroke volumes of 1.11.3 ml kg1 measured in Pacific
bluefin tuna were not significantly affected by temperature change between
25° and 2°C and were similar to those of yellowfin tuna. Albacore tuna
stroke volumes exceeded those previously recorded in albacore tuna more than
threefold, resulting in over twofold higher maximal cardiac outputs
(Lai et al., 1987
;
White et al., 1988
). The
albacore heart yielded similar stroke volume and cardiac output to that of
bluefin tuna despite a smaller relative heart mass (0.21% relative ventricular
mass). This resulted in greater myocardial power output when normalized to
heart mass. The maximal cardiac outputs of 106.2±5.1 ml
kg1 min1 in Pacific bluefin tuna at
25°C were within the range previously recorded in yellowfin and skipjack
tuna (Bushnell and Brill,
1992
; Farrell et al.,
1992
; Jones et al.,
1993
; Blank et al.,
2002
). Importantly, maximal cardiac outputs of Pacific bluefin
tuna were significantly less temperature sensitive than those of yellowfin
tuna across the measured temperature range (P<0.01), resulting in
significantly higher cardiac outputs at low temperatures
(Fig. 3C)
(Blank et al., 2002
).
In Pacific bluefin tuna as in yellowfin tuna, there is a substantial
decrease in cardiac performance at low temperature due to decreases in heart
rate. This cold-induced bradycardia is accompanied by a low cardiac scope, or
ability to increase cardiac output from standard to maximal conditions through
changes in stroke volume at lower temperatures
(Fig. 4). The calculated value
for scope indicates the cardiac response to increased filling pressure (i.e.
the Starling effect). It is possible that cardiac performance at maximal
conditions would be further increased by adrenergic stimulation, although only
small effects of epinephrine were seen in yellowfin tuna hearts in
situ (Blank et al., 2002).
In addition, perfusion with Ringer's solution rather than blood may impose an
oxygen limitation on the heart, particularly at higher temperatures, where
workload increases while oxygen supply decreases. At maximal conditions,
improved lumenal perfusion accompanying increases in cardiac output may help
to offset such oxygen limitation. This might explain the ability of bluefin
hearts to function at 30°C at our maximal flow conditions but not at
standard conditions. Whether our maximal in situ conditions succeed
in reproducing maximal in vivo cardiac output is unclear due to the
absence of in vivo measurements of cardiac output in bluefin tuna. In
our previous study (Blank et al.,
2002
), in situ values for maximal cardiac output in
yellowfin tuna matched values recorded in spinally blocked fish. However,
maximal cardiac outputs of both yellowfin and bluefin tuna are similar to
those measured in recent in situ studies in triploid brown trout
(Mercier et al., 2002
),
despite the difference in metabolic rates between trout and tunas
(Altimiras et al., 2002
). This
discrepancy suggests that tuna hearts may be capable of greater cardiac
outputs than those revealed by in situ preparations.
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Cellular factors influencing cardiac temperature sensitivity
The most notable feature of this study is that the Pacific bluefin tuna
heart continues to function at cold ambient temperatures (2°C). The
cellular adaptations that allow a bluefin tuna heart to function in the cold
remain largely unknown. High aerobic capacity may be an important contributor
to maintenance of ATP supplies in hearts working at very cold temperatures,
although cold-acclimation has little effect on aerobic capacity in the striped
bass heart (Rodnick and Sidell,
1997). Citrate synthase activities of 77±3.1 and
95±3.1 U g1 wet mass at 25°C in atrium and
ventricle of Pacific bluefin tuna indicate that aerobic capacity is elevated
relative to those of teleosts such as trout and striped bass
(Rodnick and Sidell, 1997
;
Clark and Rodnick, 1998
).
However, these values are similar to aerobic capacities measured in
endothermic and ectothermic scombrids, including yellowfin tuna, Eastern
pacific bonito and Pacific mackerel
(Freund, 1999
), suggesting
that differences in thermal sensitivity of tuna hearts are not related to the
aerobic capacity of the myocardium. Several lines of evidence point to
specializations in excitationcontraction (EC) coupling proteins
as a critical factor in achieving high heart rates and cold tolerance of tuna
hearts (Shiels et al., 2002a
;
Landeira-Fernandez et al.,
2004
). New measurements of L-type calcium channel
function in cardiac myocytes of Pacific bluefin tuna indicate that peak
Ca2+ current amplitudes and kinetics in the atrium are enhanced
relative to those of many other teleosts
(Shiels et al., 2002a
).
L-type calcium channel function is enhanced in atrium of bluefin
tuna relative to Pacific mackerel, but the contribution of this channel to
EC coupling in other tuna species remains unexplored.
Biochemical studies of ventricles of several tuna species indicate that
increased performance and cold-tolerance in Pacific bluefin tuna hearts may
depend on increased levels of sarcoplasmic reticulum (SR) proteins, in
particular the SR Ca2+ ATPase
(Landeira-Fernandez et al.,
2004) and SR calcium release channel (J. M. Morrissette and B. A.
Block, unpublished data). SR contributes to cardiac function by providing an
intracellular calcium store, thereby reducing diffusion distances and
accelerating rates of force development and relaxation
(Bers, 2002
). Increased SR
contribution to EC coupling is commonly implicated in myocardial cold
tolerance in studies of cold-acclimation in fish
(Keen et al., 1994
;
Aho and Vornanen, 1998
) and
hibernation in mammals (Liu et al.,
1997
). While the SR was long thought to play a minimal role in
teleost hearts (Farrell and Jones,
1992
), recent work has begun to clarify its importance in a
variety of fish species. Application of ryanodine to block SR function in
myocardial strips has been shown to reduce force in a variety of teleost
cardiac tissues (Hove-Madsen,
1992
; Møller-nielsen
and Gesser, 1992
; Keen et al.,
1994
; Aho and Vornanen,
1998
; Shiels et al.,
1998
). In addition, patch-clamp techniques and in vitro
Ca2+ uptake assays have demonstrated SR Ca2+ uptake in
trout myocardium (Aho and Vornanen,
1998
; Hove-Madsen et al.,
1998
). Application of ryanodine to block SR function in myocardial
strips has particularly pronounced effects on force development of yellowfin
tuna heart (Shiels et al.,
1999
). New measurements of Ca2+ uptake and
Ca2+ ATPase activity in SR vesicles of scombrid hearts indicate
that Pacific bluefin ventricle has high SR Ca2+ ATPase activities
that exceed those of yellowfin and albacore tuna
(Landeira-Fernandez et al.,
2004
). While many other factors regulate heart rate in
situ as well as in wild fish, high ventricular SR Ca2+ ATPase
activity may be a critical factor permitting bluefin tuna to sustain heart
rate in extraordinarily cold waters. As indicated by Landeira-Fernandez et al.
(2004
), the enhancement of SR
Ca2+ ATPase protein expression in all tunas in comparison to their
sister taxa may be a key specialization underlying increased cardiac
performance in Thunnus.
Ecological implications of cardiac thermal sensitivity
Our results indicate that improved cardiac performance in the cold at the
cellular and organismal levels may contribute to the ability of the bluefin
tuna to penetrate into a cooler thermal niche. Bluefin tunas occupy temperate
and sub-polar latitudes where ambient water temperatures at the surface and at
depth are cool. The ability of the bluefin tuna heart to maintain contractile
performance at extremely low temperatures is critical to this ecological niche
expansion. Yellowfin tuna studied with acoustic tags at the northern (coolest)
portion of their range primarily occupied waters with surface temperatures
greater than 17°C and occasionally dived to depths where ambient
temperatures were 11°C (Block et al.,
1997). Most dives below these temperatures lasted less than a
minute or two and yellowfin tissue temperatures were unlikely to equilibrate
to such low ambient temperatures. The physiological limitations that prevent
yellowfin tuna from remaining below the thermocline to forage extensively have
remained unknown. Most studies of yellowfin tuna, including tracking studies
in warmer waters, show similar `bounce' diving
(Brill et al., 1999
) where
time in cool deep waters in short. Electronic tagging of juvenile Pacific
bluefin tuna in the eastern Pacific demonstrated that this species makes
regular migrations to cold temperate latitudes where surface waters are
significantly colder (1114°C). During these prolonged periods of
feeding at high latitudes the juvenile Pacific bluefin tuna dive frequently,
encountering waters as cool as 1.87°C for short durations
(Block et al., 2001
; B. A.
Block, unpublished data). This differs from the western Pacific, where
electronically tagged juvenile bluefin tuna primarily occupy warmer
temperatures (Kitagawa et al.,
2002
).
Electronic tagging data from Atlantic and Pacific bluefin tunas indicate
that larger bluefin maintain significantly larger temperature differentials
between mean body temperature and ambient water temperature while foraging in
cold seas (Block et al., 2001).
This most likely results from increased thermal inertia, the increased mass of
aerobic muscle and viscera contributing to whole body heat production, and the
extensive retia mirabilia. Thus far, the only internal peritoneal
temperature data from bluefin tuna exceeding 150 kg have come from the
Atlantic Ocean. Mature Atlantic bluefin tuna (200400 kg) tracked with
archival tags spend up to 7 months of the year north of 50°N latitude,
feeding in waters with surface temperatures as cool as 79°C (Block
et al., 2001
,
2003
). Such long-term residency
in temperate and sub-polar seas by Atlantic bluefin tuna poses a challenge for
the heart that only mature bluefin tuna appear to have solved. Some, but not
all, mature Atlantic bluefin tuna (150 kg or larger) display a repetitive
diving pattern where peritoneal temperature remains constant, despite
repetitive dives into cool waters (Fig.
5). This diving pattern suggests a potential physiological
limitation that may be induced by cooling of the heart, as was observed
previously in electronic tracking records of big-eye tuna (Brill et al.,
1998
.,
1999
). Tunas foraging in cold
seas face the challenge of maintaining oxygen delivery to warm aerobic tissues
while heart temperature declines. Although metabolic rate has not been
measured in any species of the bluefin tuna group, it is likely that bluefin
tuna have metabolic rates similar to, or higher than, tropical tunas of
similar size. The high level of heat production and heat conservation in adult
bluefin (Carey and Teal, 1969
;
Carey et al., 1984
) is likely
to maintain high rates of oxygen consumption in the slow-oxidative muscle and
visceral organs and high tissue metabolic rates even in the face of cold
ambient temperatures. It has been suggested that high myoglobin levels in tuna
slow-twitch axial muscle may serve as an oxygen store during brief periods of
cold-induced bradycardia in bluefin tuna, in a manner analogous to the oxygen
reserve of marine mammals (Marcinek,
2000
). While the adaptations for enhanced performance lead to
impressive cold tolerance in giant Atlantic bluefin, thermal limits are still
apparent. Atlantic bluefin tuna encountering cold conditions (ambient waters
<4°C) may be unable to elevate cardiac output sufficiently to support
strenuous feeding activities at depth for extended periods and may be forced
to return continuously to the surface where warmer ambient waters contribute
to higher heart rates, higher cardiac outputs, and reoxygenation of the tissue
and myoglobin stores. This `bounce' diving behavior is less apparent in giant
bluefin tuna of large size encountering similar conditions in sub-polar and
polar seas, suggesting that they have somehow overcome this limitation.
While cold tolerance is clearly important for allowing bluefin tuna to
exploit prey resources at higher latitudes and in deep waters, the ability to
maintain function in warm waters is also important. All tunas of the genus
Thunnus breed in warm waters ranging from 23°C to 31°C
(Schaefer, 2001;
Block et al., 2001
; S. L. Teo
and B. A. Block, unpublished data). These warm waters may constitute another
major physiological challenge for giant adult bluefin. In adult bluefin tunas,
peritoneal temperatures as high as 33°C have been recorded
(Block et al., 2001
; B. A.
Block, unpublished data), suggesting that extraordinarily high tissue
oxygenation demands occur during breeding in warm waters with relatively low
oxygen content. Giant bluefin tunas breeding in warm waters may approach the
limits of their cardiac capacity. As heart rate reaches an upper thermal
maximum, the tendency for stroke volume to fall with increasing temperatures
imposes a limit to increases in cardiac output in fish encountering acute high
temperatures (Farrell et al.,
1996
). Falling stroke volumes at higher temperatures result from a
combination of factors, including the negative forcefrequency curve of
most teleost ventricular tissues in combination with the Starling
(lengthtension) effect (Shiels et
al., 2002b
). The low temperature-dependence of maximal stroke
volume in Pacific bluefin tuna hearts suggests that they may have the ability
to maintain contractile force at higher frequencies associated with higher
temperatures. As a result, oxygen delivery can be maintained while the fish
are on the warm spawning grounds. Although heart rate continues to increase at
temperatures up to 30°C, there may be an upper limit associated with
calcium cycling enzyme kinetics in the bluefin myocyte that result in a
ceiling above which performance declines. Ultimately, this may limit cardiac
performance and oxygen delivery. Evidence for such an upper limit may be seen
in the high mortality associated with capture (a sympathetic stress) of giant
bluefin on scientific longlines in the Gulf of Mexico (A. M. Boustany and B.
A. Block, unpublished data). Improving techniques to allow measurement of
cardiac performance at high temperatures should elucidate the critical upper
limit of cardiac performance.
Maintenance of high cardiac outputs at high as well as low temperatures are likely to play a crucial role in increasing the thermal niche of bluefin tunas. Understanding how the bluefin tuna heart performs over this wide temperature range should provide important information about vertebrate cardiac performance.
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