Intrinsic autoregulation of cardiac output in rainbow trout (Oncorhynchus mykiss) at different heart rates
Department of Zoology, University of Göteborg, Box 463, S-405 30 Göteborg, Sweden
* Author for correspondence at present address: Department of Biology, IFM, Linköpings Universitet, SE-58183 Linköping, Sweden (e-mail: jordi{at}ifm.liu.se)
Accepted 3 October 2003
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Summary |
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Key words: venous pressure, heart rate, Frank-Starling mechanism, zatebradine, rainbow trout, fish, Oncorhynchus mykiss
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Introduction |
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Heart perfusion studies, however, operate at rates well above the in
vivo range of resting heart rate
(Altimiras and Larsen, 2000)
because the inhibitory cholinergic influence on the heart is abolished. At the
lower in vivo heart rates, the loading conditions of the heart will
differ due to a longer filling time
(Farrell and Jones, 1992
),
which in turn is expected to increase venous pressure.
This prediction has not been demonstrated experimentally due to technical
limitations in recording pressure in the sinus venosus
(PSV) in teleosts. Such a study, however, could shed light
on two separate aspects of the cardiac physiology of teleosts. First, it could
explain the shift from vis-à-fronte atrial filling to
vis-à-tergo filling
(Farrell, 1991). The unique
vis-àfronte filling mechanism observed in some fish species
relies on the generation of negative intrapericardiac pressures associated
with the elastic recoil of the ventricle during diastole. This mechanism
differs from vis-à-tergo filling, which depends on the
build-up of pressure in the central veins. Second, it could explain the role
of venous pressure in modulating cardiac output through the interdependence
between heart rate and stroke volume.
The aim of the study was to validate a new technique for measuring PSV, and to study PSV in trout at varying heart rates. Since truly resting heart rates were unattainable, a novel pharmacological approach to manipulate heart rate using the bradycardic agent zatebradine was developed and validated.
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Materials and methods |
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Series I. Effects of zatebradine on cardiovascular
parameters in vivo
Eight fish (580±53 g body mass) were used. The fish were
anaesthetized in a solution of MS-222 (100 mg l-1; Sigma) buffered
with sodium bicarbonate (200 mg l-1) until breathing movements
ceased, and placed ventral side up on an operating sling. The gills were
continuously irrigated with aerated water containing a diluted solution of
MS-222 (75 mg l-1). A polyethylene cannula (PE50, Clay Adams;
Becton Dickinson, Sparks, MD, USA) filled with heparinized (100 i.u.
ml-1) 0.9% NaCl was implanted into the dorsal aorta to measure
blood pressure (PDA) as previously described
(Hughes et al., 1983). The
cannula was secured with a suture on the back of the animal. In order to
measure cardiac output
(=ventral
aortic blood flow), the ventral aorta was exposed through an incision on the
left side of the isthmus. A cuff-type pulsed Doppler flow probe (Iowa Doppler
Products, Iowa City, IA, USA) with an internal diameter of 2.2-3.0 mm was
placed around the ventral aorta and the lead from the probe was secured with
two skin sutures.
After surgery, the animals were transferred to a holding chamber and allowed to recover for 24-36 h. The flow probes were connected to a Directional Pulsed Doppler Flowmeter (Model 545-4C, Iowa University, USA). The dorsal aortic cannula was connected to a Statham P23 (Hato Rey, Puerto Rico) pressure transducer connected to a bridge amplifier channel of a recorder (Grass Instruments, Quincy, MA, USA). The pressure transducer was calibrated against a static water column. Heart rate (fH) was obtained from the phasic blood flow signal using a Grass tachograph (model 7P44D).
The flow, pressure and tachograph signals were fed into a computer and stored to disk at 10 Hz using a custom-made program (Labview v.5.1, National Instruments, Austin, TX, USA).
The experimental protocol consisted of a control period followed by serial injections of zatebradine hydrochloride (Boehringer Ingelheim, Skärholmen, Sweden), corresponding to cumulative doses of zatebradine of 0.5, 1, 2, 4, 6 and 8 mg kg-1. A 30 min recording period was taken post-injection. The main decrease in heart rate occurred within the first 10 min post-injection.
Series II. Twitch force and rate of force development in ventricular
strips
Six fish were used (450-550 g in body mass). Fish were killed by a sharp
blow to the head and the heart was rapidly excised from the animal and placed
on a chilled Petri dish, where three longitudinal strips under 1 mm width were
obtained from the ventral ridge of the pyramidal heart.
The strips were mounted for measurements of twitch force and rate of contraction using force transducers (Grass, FT-103) connected to a 4-channel bridge amplifier unit (Somedic, SenseLab 4CHAMP, Hörby, Sweden). The signals were stored digitally at 20 Hz for further analysis using a custom-made program (LabView 5.1, National Instruments). The Ringer solution was identical to the one used in the heart perfusion experiments and was also bubbled with 80 kPaO2/balance N2.
The strips were stretched to 90% of maximal contraction, paced at 0.2 Hz throughout the experiment and allowed to stabilize for 1 h. Following a 20 min control recording, the Ringer solution was quickly changed and the first concentration of zatebradine was tested. The three strips were run simultaneously, one being used as control and the other two as treatments. The concentrations of zatebradine tested were 0.5, 1, 2, 5 and 10 mg l-1.
Series III. Effects of zatebradine on venous pressure
Ten fish, mass 300-500 g, were used in the experiment. The animals were
instrumented with a dorsal aortic catheter for measurement of heart rate and
drug injection as described for Series I. Following this procedure, a catheter
was non-occlusively inserted in the left ductus of Cuvier (LDC) and forwarded
to the sinus venosus. The surgical procedure was as follows. The operculum and
the gills were retracted to expose the gill-free Vth branchial arch
and a 1 cm incision was made parallel to this arch (see
Fig. 1 for a graphical view).
The incision was initiated on top of the cleithrum bone and was followed
towards the ventral edge of the bone. The LDC was exposed at this location. It
was noticed that the motor nerve to the left pectoral fin runs on top of the
LDC and this was used as an anatomical landmark. A loose pocket of the LDC
wall was gently pulled and held with 3-0 suture thread
(Fig. 1C). A loose thread was
placed downstream and a small cut was made between the threads. A PE-50
catheter (with a bubble 35 mm from the tip) was inserted in the hole and
advanced to the sinus venosus (15-20 mm) before secured tightly in place. The
cannula was secured twice on the skin, once adjacent to the bubble on top of
the cleithrum bone and once close to the dorsal fin. The position of the
catheter tip was verified postmortem.
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This new procedure is similar to those previously used to measure the
pressure in the ducts of Cuvier (Olson et
al., 1997; Minerick et al.,
2003
), but allows the measurement of pressures in the sinus
venosus.
Fish were allowed to recover for 24 h before the effects of three doses of zatebradine (1, 2 and 4 mg kg-1) on heart rate and venous pressure were recorded.
Series IV. Heart rate-stroke volume relationship in the perfused
heart
Nine fish were used (505±24 g body mass, 469±31 mg wet
ventricular mass). Fish were killed by a sharp blow to the head and
transferred to an operating sling. Heparin (1 ml kg-1) was injected
via the caudal vessels to prevent clotting of the blood during
surgery. The heart was perfused in situ following previous protocols
with a few variations (Farrell et al.,
1986). Briefly, the abdominal cavity was opened, a double-bore
cannulae (as shown in Franklin and
Axelsson, 1994
) was inserted in the sinus venosus through a
hepatic vein and the perfusion of Ringer started from a reservoir. Other
hepatic veins, if present, were ligated to prevent leakage. The gill arches
were cut and Ringer flowed freely through the heart with each heart beat. The
isthmus was cut, exposing the ventral aorta and the afferent branchial
arteries, and a double bore cannula was inserted through the ventral aorta
into the bulbus arteriosus. The entire fish was then transferred to a constant
temperature stainless steel trough (15°C) filled with 0.9% NaCl. The
cannula to the atrium was connected to two water-jacketed reservoirs with
Ringer solution via a constant pressure device. The ventral aortic
cannula was connected to an output pressure head. The composition of the
perfusion solution was (in mmol l-1): 104.1 NaCl; 3.1 KCl; 0.9
MgSO4; 2.5 CaCl2. The buffer system was a Hepes-Tris
mixture (5 mmol l-1 Hepes adjusted to pH 7.8 with Tris to a final
concentration in the perfusate of 5 mmol l-1). The following
metabolic substrates were added: glucose, glutamate, fumarate and pyruvate (5
mmol l-1 each). The perfusate was gassed with 80 kPa
O2/balance N2 using a Gas Mixing Flowmeter (model
GF-3MP, Cameron Instruments, Port Aransas, TX, USA).
A tonic adrenergic stimulation (5 nmol l-1 adrenaline
bitartrate; Sigma) was maintained by pumping a 2.5 µmol l-1
adrenaline-Ringer solution through a side port of the inflow tubing to the
heart using a peristaltic pump (Minipuls 3, Gilson, Villiers le Bel, France).
It has been established that tonic adrenergic stimulation is essential for the
long-term viability of perfused hearts
(Graham and Farrell, 1989).
The flow rate of the pump was automatically adjusted to follow changes in
cardiac output so that the dilution rate of the adrenaline solution was
constant (1:500). The same system was used to deliver a constant zatebradine
concentration (1 mg l-1) to the heart.
Cardiac output was measured using an in-line flow probe (Transonic 2N, Ithaca, NY, USA; 2 mm internal diameter) coupled to a transit-time flowmeter (Transonic, T106). Preload and afterload pressures were measured using disposable pressure transducers (DPT6100, Peter von Berg Medizintechnik, Kirchseeon/Eglharting, Germany) connected to a 4-channel bridge amplifier unit (SenseLab 4CHAMP). Pressures were calibrated against static water columns and the calibration checked every 30 min. The signals were recorded on a Grass recorder (model 7WU), which also logged instantaneous heart rate using a tacograph unit (Grass, 7P44). In parallel, the data was digitally stored using a custom made program (LabView 5.1, National Instruments). Preload and afterload pressures, cardiac output and heart rate were averaged for 5 s and stored.
The experimental protocol consisted of a 30 min stabilization period, followed by a recording period, during which zatebradine-free perfusate (Control group, N=4) or perfusate with 1 mg l-1 zatebradine (ZAT group, N=5) was supplied to the heart. The zatebradine dose was chosen after preliminary experiments. Larger doses (2, 3 and 4 mg l-1) elicited faster changes in fH but were unsuitable for prolonged exposure because the heart became arrhythmic before reaching a heart rate of 30 beats min-1, which was the lowest target fH intended in the experiment.
During the stabilization period, afterload was set to 5 kPa and cardiac output was adjusted to 30 ml min-1 kg-1 via changes in preload pressure. In the ZAT group, preload pressure was allowed to change as heart rate decreased following administration of zatebradine. The perfused hearts in the Control group were also subjected to the zatebradine after the 2 h control trial.
Calculations and statistics
In Series I, stroke volume (VS) was calculated as the
ratio between and
fH. Systemic vascular resistance
(Rsys) was calculated as PDA divided
by
, assuming that venous pressure was
zero and did not change during the experimental protocol. It was also assumed
that no significant changes in blood viscosity took place during the
experiment.
, VH
and Rsys are presented as percent changes from the control
value.
The dose-effect relationship of zatebradine (ZAT) on fH resembles that of a Michaelis-Menten enzymatic reaction if the effects are plotted as the decrease in fH from control values. Thus, a Levenberg-Marquardt non-linear fitting procedure was used to adjust the fH versus [ZAT] data for each animal to the Michaelis-Menten equation to obtain the concentration of zatebradine that induces half maximal effects on fH, referred here as [ZAT]0.5.
Control values of the different cardiovascular parameters were subsequently compared against those values at [ZAT]0.5 for normalization purposes. If the [ZAT]0.5 deviated more than 10% of the closest nominal [ZAT] used in the study, the values for [ZAT]0.5 were linearly interpolated between the two values closest to [ZAT]0.5. [ZAT]0.5 was in the range 1.5-3 mg l-1. Since the window around Km in the Michaelis-Menten equation is highly linear, the error induced by interpolation is minimized.
In Series II, twitch force and maximal rate of contraction were determined
as previously described (Hove-Madsen and
Gesser, 1989).
In Series IV, power was calculated as:
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All data are presented as means ± S.E.M.
Wilcoxon's signed-ranks test for paired samples (two-tailed) were used to
evaluate the statistical significance of control and [ZAT]0.5
cardiovascular variables in Series I and III and the cardiac variables at
different heart rates in Series IV. A Kruskal-Wallis non-parametric paired
test was used to compare the effects of zatebradine in Series II. Asterisks in
the figures indicate significant changes between paired samples. In the case
of repeated tests, a modified Bonferroni procedure was used to reduce the risk
of discarding a true null hypothesis
(Holm, 1979).
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Results |
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The effects of zatebradine were highly significant. At [ZAT]0.5,
fH decreased by 50%, PDA decreased by
14% and by 15%, while
VS increased by 66% and Rsys remained
unchanged (Figs 2 and
3).
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Series II. Twitch force and rate of force development in ventricular
strips
The application of zatebradine to ventricular strips did not show any
significant changes on twitch force or on the maximum rate of force
development at any of the doses employed
(Table 1).
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Series III. Venous pressure
The stepwise decrease in heart rate induced with zatebradine was coupled to
a progressive increase in venous pressure. At a control fH
of 58.3±3.5 beats min-1 (N=10), venous pressure was
subambient (-0.06±0.04 kPa) and this increased significantly to
-0.02±0.04 kPa and 0.07±0.05 kPa after injection of zatebradine
(2 mg kg-1 and 4 mg kg-1, respectively)
(Fig. 4).
|
Series IV. Heart rate-stroke volume relationship in the perfused
heart
The control experiments revealed little deterioration of
fH and Pin in the perfused heart after
2 h (Fig. 5).
Pin increased from -0.023±0.013 kPa (N=4)
to 0.003±0.020 kPa and fH decreased from
69±4 beats min-1 to 59±3 beats min-1.
decreased from 2.49±0.17 mW
g-1 28.1±1.2 ml min-1 kg-1
25.6±1.3 ml min-1 kg-1, VS
increased from 0.41±0.03 ml kg-1 to 0.44±0.03 ml
kg-1 and power output decreased from 2.52±0.18 mW
g-1 to 2.27±0.17 mW g-1 (not shown). In
comparison, zatebradine-treated preparations (1 mg kg-1,
N=7) showed a significant change in preload and heart rate after 90
min perfusion. The decrease in fH was almost linear down
to 40 beats min-1 and slowly leveled thereafter at 28.3±2.5
beats min-1 (Fig.
5).
|
A small but significant change in
and power output of the heart occurred at a heart rate of 30 beats
min-1 in comparison to 60 beats min-1 (control)
(Fig. 6).
decreased from 29.4±0.2 ml
min-1 kg-1 (N=7) to 26.8±0.5 ml
min-1 kg-1 and power output decreased from
2.74±0.21 mW g-1 to at fH=30 beats
min-1 to and fH=60 beats min-1,
respectively. At the same time, Pin increased
significantly from 0.000±0.007 kPa (N=7) to 0.053±0.013
kPa and VS increased from 0.49±0.00 ml
kg-1 to 0.88±0.02 ml kg-1
(Fig. 6).
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Discussion |
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Zatebradine is a suitable bradycardic agent
Zatebradine, a bradycardic agent, was tested as a means of manipulating
heart rate within its physiological range
(Altimiras and Larsen, 2000).
Zatebradine-induced bradycardia occurs as a result of the inhibition of the
hyperpolarization-activated current in pacemaker cells, as shown in different
mammalian species (Kobinger and Lillie,
1984
; Schipke et al.,
1991
).
The suitability of zatebradine as a chronotropic agent also requires that
it has no inotropic effects and no effect on peripheral resistance. As shown
in Fig. 2, a dose of
zatebradine of 2.79±0.47 mg l-1 decreased heart rate by half
the maximum change, which compares well with similar experiments in mammals
(Franke et al., 1987;
Kalman et al., 1995
;
Ryu et al., 1996
;
Schipke et al., 1991
). Thus,
zatebradine is also a bradycardic agent in trout, indicating that the
hyperpolarization current (If) is also a component of the
pacemaker currents in nodal cells in this species.
The results from the Series II experiments indicate that zatebradine has no significant effect on the peripheral vasculature because systemic resistance is unchanged (Fig. 3). The significant decrease in dorsal aortic pressure is due to the concomitant decrease in cardiac output when heart rate decreases to half the maximum effect (Fig. 3).
Finally, no significant inotropic effects of zatebradine were found in
isolated ventricular strips (Table
1), as was the case for mammals
(Chen and Slinker, 1992).
Altogether these results indicate that zatebradine is an appropriate pharmacological tool to manipulate heart rate without other significant cardiovascular changes and as such, is well suited to study both in vivo and in vitro how heart rate affects intrinsic cardiac regulation.
Pressure in the sinus venosus and mechanisms of atrial filling
Pressure in the sinus venosus in control conditions was negative in seven
out of ten fish, with a mean value of -0.06±0.04 kPa
(Fig. 4). To our knowledge
these are the first pressure measurements in the sinus venosus of a teleost
fish and directly corroborate the predictions based on data from perfused
hearts (Farrell and Jones,
1992). Values for central venous pressure in trout obtained in
other studies indicate values slightly above ambient, averaging 0.37 kPa in
the Cuverian ducts (at an average heart rate of 62.8 beats min-1 at
12°C; Olson et al., 1997
)
or 0.19 kPa in the common cardinal vein (at an average heart rate of 32 beats
min-1 at 10°C; Kiceniuk and
Jones, 1977
). Collectively, these values portray a pressure
gradient from slightly above ambient in the central veins to slightly
subambient in the sinus venosus that could support the operation of the
vis-à-fronte mechanism for atrial filling
(Farrell and Jones, 1992
).
Vis-à-fronte filling is allegedly associated with resting
conditions and low stroke volumes in active teleosts because filling pressures
above ambient (vis-à-tergo filling) are required for normal
and elevated stroke volumes (Farrell and
Jones, 1992). In resting conditions, however, heart rates are
considerably lower. Recent studies in trout using surgery-free methods
indicate that true resting heart rates at 15°C are in the order of 30
beats min-1 (Altimiras and
Larsen, 2000
), well below the control fH value
obtained in this study (58.3±3.5 beats min-1). Such low
heart rates could not be attained in surgically instrumented animals, so
zatebradine was used instead. Lowering the heart rate lengthens the filling
time and PSV increases (0.07±0.05 kPa at
fH=24.6±3.7 beats min-1;
Fig. 4), which questions the
existence of negative pressures in the sinus venosus in resting conditions
and, at the same time, casts doubts on the in vivo relevance of
vis-à-fronte atrial filling. Minerick et al.
(2003
) have recently reached
the same conclusion by demonstrating a dynamic coupling between venous
pressure and cardiac output.
Role of heart rate in the regulation of cardiac output
The rise of PSV associated to the progressive decrease
in heart rate from the Series III experiments also provided a mechanistic
explanation of the autoregulation of cardiac output observed in vivo.
Thus, the small decrease in down to
87.5±4.6% when fH was halved with zatebradine would
be explained by a rise in PSV that would allow a
concomitant increase in VS to 165±13%
(Fig. 3), according to the
Frank-Starling relationship. Complete compensation was not attained, perhaps
because maximal stroke volume was reached
(Forster and Farrell,
1994
).
The mechanism of cardiac output autoregulation related to variations in heart rate and filling pressure (denominated time-dependent autoregulation in the rest of the Discussion) was further verified in perfused hearts with the simultaneous measurement of VS and Pin at varying heart rates. As shown in Fig. 6, the zatebradine-mediated drop in heart rate is coupled to a simultaneous increase in Pin and VS similar in magnitude to the in vivo values.
Time-dependent autoregulation enhances the repertoire of intrinsic cardiac
regulation to include the effects of varying filling time. Strictly speaking,
the mechanism is dependent on the extrinsic neurohumoral modulation of heart
rate, but its compensatory role is exerted through changes in stroke volume by
riding on the Starling curve of the heart. The impact of time-dependent
autoregulation, however, is limited by the interaction with other mechanisms
regulating venous pressure and stroke volume. Adrenaline, for instance, is
known to increase venous pressure (Zhang
et al., 1998).
At high heart rates, the low venous pressures expected from short filling
times are likely to be counteracted by the positive inotropic effect related
to adrenergic activation. This prediction needs to be confirmed
experimentally, but it is already known that energetically demanding
conditions such as exercise require elevated cardiac outputs achieved by
increasing heart rate and stroke volume simultaneously, and this is
incompatible with subambient or low venous pressures
(Farrell et al., 1996). In the
absence of adrenergic estimulation, a reciprocal relationship between
fH and VS has already been shown in
paced trout hearts in vitro
(Farrell et al., 1989
).
Besides the possibility that short filling times limit stroke volume, a
reduced force of contraction resulting from the negative staircase effect was
also speculated (Farrell and Jones,
1992
).
At low heart rates, a longer filling time promotes an increase in venous
pressure that results in atrial distension. In turn, atrial stretch triggers
the release of atrial natriuretic factor (ANF)
(Cousins and Farrell, 1996),
which might break down the reciprocal
fH-VS relationship because ANF is a
potent vasodilator that lowers venous pressure
(Olson et al., 1997
).
Such a cardioprotective role of ANF
(Farrell and Olson, 2000) is
not incompatible with the time-dependent autoregulatory mechanism proposed in
this study, which is targeted at a more physiological range of sinus venosus
pressures. ANF release in freshwater- and seawater-acclimated perfused trout
hearts is evident at filling pressures above 0.1 kPa
(Cousins et al., 1997
), while
the results presented indicate the operation of the time-dependent
autoregulatory mechanism from subambient to pressures slightly below 0.1
kPa.
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Acknowledgments |
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References |
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---|
Altimiras, J. and Larsen, E. (2000). Non-invasive recording of heart rate and ventilation rate in rainbow trout during rest and swimming. Fish go wireless! J. Fish. Biol. 57,197 -209.[CrossRef]
Chen, Z. and Slinker, B. K. (1992). The sinus node inhibitor UL-FS 40 lacks significant inotropic effect. J. Cardiovasc. Pharmacol. 19,264 -271.[Medline]
Cousins, K. L. and Farrell, A. P. (1996). Stretch-induced release of atrial natriuretic factor (ANF) from the heart of rainbow trout (O. mykiss). Can. J. Zool. 74,380 -387.
Cousins, K. L., Farrell, A. P., Sweeting, R. M., Vesely, D. L.
and Keen, J. E. (1997). Release of atrial natriuretic factor
prohormonepeptides 1-30, 31-67 and 99-126 from freshwater- and
seawater-acclimated perfused trout (Oncorhynchus mykiss) hearts.
J. Exp. Biol. 200,1351
-1362.
Farrell, A. P. (1991). From hagfish to tuna: A perspective on cardiac function in fish. Physiol. Zool. 64,1137 -1164.
Farrell, A. P., Gamperl, A. K., Hicks, J. M. T., Shiels, H. A.
and Jain, K. E. (1996). Maximum cardiac performance of
rainbow trout (Oncorhynchus mykiss) at temperatures approaching their
upper lethal limit. J. Exp. Biol.
199,663
-672.
Farrell, A. P. and Jones, D. R. (1992). The heart. In Fish Physiology, vol.XIIB (ed. W. S. Hoar, D. J. Randall and A. P. Farrell), pp. 1-87. New York: Academic Press.
Farrell, A. P., McLeod, K. and Driedzic, W. R. (1982). The effects of preload, after load, and epinephrine on cardiac performance in the sea raven, Hemitripterus americanus. Can. J. Zool. 60,3165 -3171.
Farrell, A. P., McLeod, K. R. and Chancey, B. (1986). Intrinsic mechanical properties of the perfused rainbow trout heart and the effects of catecholamines and extracellular calcium under control and acidotic conditions. J. Exp. Biol. 125,319 -345.[Abstract]
Farrell, A. P. and Olson, K. R. (2000). Cardiac natriuretic peptides: a physiological lineage of cardioprotective hormones? Physiol. Biochem. Zool. 73, 1-11.[CrossRef][Medline]
Farrell, A. P., Small, S. and Graham, M. S. (1989). Effect of heart rate and hypoxia on the performance of a perfused trout heart. Can. J. Zool. 67,274 -280.
Forster, M. E. and Farrell, A. P. (1994). The volumes of the chambers of the trout heart. Comp. Biochem. Physiol. 109A,127 -132.[CrossRef]
Franke, H., Su, C. A. P. F., Schumaker, K. and Seiberling, M. (1987). Clinical pharmacology of two specific bradycardic agents. Eur. Heart. J. 8(Suppl. L), 91-98.
Franklin, C. E. and Axelsson, M. (1994). The
intrinsic properties of an in situ perfused crocodile heart.
J. Exp. Biol. 186,269
-288.
Graham, M. S. and Farrell, A. P. (1989). The effect of temperature acclimation and adrenaline on the performance of a perfused trout heart. Physiol. Zool. 62, 38-61.
Holm, S. (1979). A simple sequentially rejective multiple test procedure. Scand. J. Stat. 6, 65-70.
Hove-Madsen, L. and Gesser, H. (1989). Force frequency relation in the myocardium of rainbow trout. Effects of K+ and adrenaline. J. Comp. Physiol. B 159, 61-70.[Medline]
Hughes, G. M., Albers, C., Muster, D. and Götz, K. H. (1983). Respiration of the carp, Cyprinus carpio L., at 10° and 20°C and the effects of hypoxia. J. Fish. Biol. 22,613 -628.
Kalman, J. M., Tonkin, A. M. and Power, J. M. (1995). Specific effects of zatebradine on sinus node function: suppression of automaticity, prolongation of sinoatrial conduction and pacemaker shift in the denervated canine heart. J. Pharmacol. Exp. Ther. 272,85 -93.[Abstract]
Kiceniuk, J. W. and Jones, D. R. (1977). The oxygen transport system in trout (Salmo gairdneri) during sustained exercise. J. Exp. Biol. 69,247 -260.
Kobinger, W. and Lillie, C. (1984). Cardiovascular characterization of UL FS 49, 1,3,4,5-tetrahydro-7,8-dimethoxy-3-[3-[[2-(3,4-dimethoxyphenyl) ethyl]methylimino]propyl]-2H-3-benzazepin-2-on hydrochloride, a new `specific bradycardic agent'. Eur. J. Pharmacol. 104, 9-18.[CrossRef][Medline]
Minerick, A. R., Chang, H.-C., Hoagland, T. M. and Olson, K. R. (2003). Dynamic synchronization analysis of venous pressure-driven cardiac output in rainbow trout. Am. J. Physiol. Regul. Integr. Comp. Physiol. 285,889 -896.
Olson, K. R., Conklin, D. J., Farrell, A. P., Keen, J. E., Takei, Y., Weaver, L., Smith, M. P. and Zhang, Y. (1997). Effect of natriuretic peptides and nitroprusside on venous function in trout. Am. J. Physiol. 273,R527 -R539.[Medline]
Ryu, K.-H., Tanaka, N. and Ross, J., Jr (1996). Effects of a sinus node inhibitor on the normal and failing rabbit heart. Basic. Res. Cardiol. 91,131 -139.[Medline]
Schipke, J. D., Harasawa, Y., Sugiura, S., Alexander, J., Jr and Burkhoff, D. (1991). Effect of a bradycardic agent on the isolated blood-perfused canine heart. Cardiovasc. Drugs. Ther. 5,481 -488.[Medline]
Zhang, Y., Weaver, L., Jr, Ibeawuchi, A. and Olson, K. R.
(1998). Catecholaminergic regulation of venous function in the
rainbow trout. Am. J. Physiol. Regul. Integr. Comp.
Physiol. 274,R1195
-R1202.
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