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First published online March 2, 2006
Journal of Experimental Biology 209, 1024-1034 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02082
Temperature dependence of cardiac performance in the lobster Homarus americanus
1 Department of Neuroscience, University of Virginia, PO 801392,
Charlottesville, VA 22908, USA
2 Division of Biostatistics and Epidemiology, University of Virginia, PO
801392, Charlottesville, VA 22908, USA
* Author for correspondence (e-mail: mkw3k{at}virginia.edu)
Accepted 9 November 2005
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Summary |
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Key words: temperature, heart, lobster, Homarus americanus
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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).
Like other poikilotherms, lobsters are unable to regulate their own body
temperature. However, they apparently detect changes in water temperature with
great resolution, as they exhibit altered cardiac activity when temperature
shifts by less than 0.5°C (Jury and
Watson, 2000). Significant changes in environmental temperature
pose an obvious physiological challenge to the lobster: how can its organs
sustain appropriate performance, as each element of the system (including the
ion channels, cell membranes, neural circuits and contractile machinery) is
likely to depend on temperature in a different way? In the wild, lobsters
undergo seasonal migrations (Campbell,
1986; Campbell,
1989; Campbell and Stasko,
1986; Cooper and Uzmann,
1971; Ellis, 2001; Ennis,
1984; Estrella and Morrissey,
1997; Watson et al.,
1999). These migrations are thought to enable the lobsters to
reach or remain at water temperatures favorable for growth, development and
molting, all of which become compromised below 10°C (reviewed by
Ennis, 1995;
Waddy et al., 1995).
However, more recent data provide direct evidence that some lobster
populations do not escape the extremes of winter. Temperature probes affixed
to Gulf of Maine lobsters record extremely cold (0–6°C) water
temperatures throughout six months of winter and early spring (Cowan, 2004).
Under laboratory conditions, lobsters exposed to thermal gradients show a
preference for significantly warmer water temperatures, in the range
13–20°C, and avoid water at higher temperatures
(Crossin et al., 1998;
Reynolds and Casterlin, 1979).
In the wild, relatively warm temperatures are most likely to be found in
shallow waters in summer months. Water temperature in shallow water is
strongly affected by winds and tides. Temperature swings of greater than
12°C during the daily tides have been recorded at the level of the lobster
traps (Fig. 1)
(Manning, in press). Thus,
lobsters face thermal shifts not only throughout the seasons but on shorter
time scales, potentially even as they move in and out of thermoclines on the
ocean floor.
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In this study we explore how the cardiac system of the lobster responds to
temperature changes in the physiological range. Because crustacean hearts are
regulated by physiological inputs (neural, neurohormonal and chemosensory) as
well as environmental influences (oxygen levels and salinity)
(McMahon, 1999), it is also of
interest to know whether temperature affects the response of the heart to
other modulatory factors. We address this question by testing whether
modulation of the cardiac performance by serotonin is temperature dependent.
Serotonin is a neurohormone localized in and released from the pericardial
organs in the lobster (Cooke,
1966; Pulver and Marder,
2002). It has been shown previously that serotonin modulates
lobster hearts both in vivo and in vitro
(Battelle and Kravitz, 1978;
Guirguis and Wilkens, 1995;
Wilkens and Kuramoto, 1998;
Wilkens et al., 1996). Whether
these modulatory effects might depend on temperature is unknown.
Here we characterize the effects of temperature on cardiac performance in the lobster hearts by measuring the strength and frequency of the heartbeat and the cardiac output of the heart in vitro. For comparison, we examine the temperature dependence of heart rates in intact animals. In addition, we test whether the modulation of in vitro cardiac activity by serotonin depends on temperature. The results demonstrate that multiple parameters of cardiac performance depend on temperature, and that these temperature dependencies interact to produce a relatively constant relationship between cardiac output and temperature. In addition, serotonin modulates the strength and frequency of the lobster heartbeat in a manner that is independent of temperature.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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).
Briefly, a small section of the dorsal thoracic carapace was removed with the
heart attached and surrounding tissues were dissected away. The alary
ligaments and muscles were left intact, providing stretch to the
mechanoreceptors (Cooke, 2002).
The preparation was pinned in a recording chamber ventral side up and
submerged in 60 mls of lobster saline (462 mmol l–1 NaCl, 16
mmol l–1 KCl, 26 mmol l–1 CaCl2,
8 mmol l–1 MgCl2, 11 mmol l–1
glucose, 10 mmol l–1 Hepes, pH 7.4). The interior of the
recording chamber was encircled by several loops of Tygon tubing through which
refrigerated coolant (antifreeze:water in a 50:50 mix) was pumped by a
refrigerated circulator (Isotemp model 1016P, Fisher Scientific, Pittsburgh,
PA, USA) to control bath temperature in the range 2–25°C.
The organization of the lobster heart has been reviewed elsewhere
(McMahon, 1995). Briefly,
hemolymph enters the single chamber of the lobster heart through paired ostia
and is pumped out through seven arteries. Isolated hearts continue to beat
because of rhythmic neural output from the cardiac ganglion, located on the
dorsal inner wall of the heart. Following isolation of the heart, the sternal
artery was cannulated and perfused with temperature-controlled lobster saline
at 3.5 ml min–1 to maintain stretch. In some experiments the
antennal arteries were tied off with 6.0 surgical silk and attached to a
tension transducer (Grass Instruments, West Warwick, RI, USA; model FT-03) and
amplifier (CyberAmp model 320; Axon Instruments, Union City, CA, USA) to
record contractions of the heart during each heartbeat. Contractions were
measured as force (g). In some experiments cardiac output out via the
sternal artery was measured ultrasonically (in ml min–1)
using an inline flow transducer (Model T206, Transonic Systems, Ithaca, NY,
USA). In these experiments the cannula was led back into the water at zero
afterload and saline was perfused through the heart through one of the ostia.
Temperature in the bath was continually monitored. All physiological signals
were recorded on a video cassette recorder (VCR) and digitized by an analog to
digital converter using pClamp software (Digidata1200 A-D converter and pClamp
software from Axon Instruments-Molecular Devices, Union City, CA, USA).
In each experiment, the parameters of cardiac function were measured as the temperature was changed from 2 to 22°C at a rate of 0.9°C min–1. In some experiments, the temperature was returned to 2°C and serotonin (1 µmol l–1) was added to the saline perfusing the heart through the sternal artery cannula. After waiting for the serotonin effects to reach steady state (defined as a potentiation of contraction amplitude that varied by no more than 5% over a period of 5 min), temperature was again warmed from 2 to 22°C in the presence of serotonin.
In vivo experiments
To record cardiac activity in vivo two holes were drilled on the
midline of the dorsal carapace for chronic implantation of electrodes. The
holes were located 1 cm and 4 cm rostral to the posterior edge of the
carapace. Analysis of the location of the holes by later dissection showed
that they were in positions rostral and caudal to the perimeter of the
underlying heart. Two wires insulated to within 0.5 cm of their tips were
inserted into the holes, fixed in position with cryanoacrylate glue and duct
tape, and connected to an impedance converter (UFI model 2991). The DC and AC
outputs of the impedance converter were connected to a VCR as well as an
analog to digital converter to record data with pClamp software. Animals were
implanted at least 48 h before data were collected.
To record changes in cardiac activity as a function of the environmental temperature the animals were placed in a wire mesh cylinder surrounded by coils of tubing through which coolant was pumped by a refrigerated circulator. The cylinder was then submerged in an insulated chamber (23 cmx18 cmx16 cm) filled with 3.5 l of seawater (Crystal Sea Salt, Marine Enterprises International, Baltimore, MD, USA) until the dorsal carapace was 2–3 cm below the surface of the water. The temperature of the seawater bath was regulated by adjusting the refrigerated circulator. The bath temperature in the chamber was continually monitored by a temperature probe fixed in position over the dorsal carapace between the two recording electrodes. In all experiments the animals were acclimated to 2°C for at least 30 min before data were collected, during which time heart rate was monitored. During each experiment the temperature of the seawater was warmed from 2 to 22°C over a period of approximately 40–50 min.
To verify whether the temperature within the thoracic cavity of a living animal approximated the temperature of the surrounding water we repeated the temperature protocol while measuring the temperature within the pericardial sinus. The temperature of the pericardial sinus becomes isothermal with the external temperature within the first 5–10 min of the 30 min acclimation period at 2°C. Fig. 2 shows that the temperature within the pericardial sinus rises as external water temperature warms. In three experiments of this type the difference between internal and external temperature was 1.31±0.42°C (mean ± s.d.) over the temperature range from 2 to 22°C, confirming that the internal body temperature in the region of the heart closely approximates external water temperature.
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).
Stroke volume was calculated by dividing cardiac output by the heart rate.
Statistical methods
Repeated measures models (Crowder and
Hand, 1990) were used for the analyses of experiments involving
multiple measurements on the same lobster as well as to investigate specific
hypotheses concerning comparisons between groups or to specific temperatures.
F tests were used to compare the values for cardiac parameters at
different temperatures. Analyses were done using SAS software (The SAS
Institute, Cary, NC, USA) version 9.1 module `PROC MIXED'.
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Temperature affects the amplitude of the heartbeat as well as the rates of contraction and relaxation
To describe these phenomena quantitatively, data from nine isolated hearts
were pooled. Contraction amplitude decreases significantly as the temperature
warms (Fig. 4). At temperatures
of 16°C and higher the mean amplitude of the heartbeat is less than 35% of
its initial value at 2°C. The temperature dependence of the shape of the
heartbeat is described in Fig.
5. Contractions develop and relax relatively slowly at the coldest
temperatures but become faster as temperature warms
(Fig. 5A). This phenomenon is
illustrated quantitatively by the phase plots
(Fig. 5B,E) describing the
relation of the rate of change in force to the contractile force recorded
during the heartbeat. These plots show pronounced differences in shape as
temperature changes because at warm temperatures the amplitude of the
heartbeat is comparatively small while the rate of change in force is
comparatively fast for the contraction and the relaxation phase of the
heartbeat. From 2 to 10°C the rates of contraction and relaxation of each
heartbeat more than double, before slowing as temperature becomes warmer
(Fig. 5C). To examine the
temperature dependence of contraction and relaxation without the confounding
effect of temperature on heartbeat strength, data from
Fig. 5C were replotted after
normalizing for contraction amplitude (Fig.
5F). Normalization changes the shape of these plots without
changing the conclusion of the analysis. Calculation of acute Q10
values for changes in rates of contraction and relaxation demonstrate these
parameters are strongly temperature dependent in the 2–4°C range
(acute Q10 values >20 and >7 for raw and normalized data,
respectively; Fig. 5D,G). At
warmer temperatures rates of contraction and relaxation vary from weakly
temperature dependent to temperature independent.
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Temperature affects heart rate both in vitro and in vivo
As shown in the raw data of Fig.
3, the frequency of the heartbeat tends to increase as a function
of temperature. Responses of ten individual isolated hearts to temperature
change are shown in Fig. 6A.
Heart rates increased in all hearts between 2 and 10°C, but there was
considerable variability at warmer temperatures with some of the isolated
hearts slowing or failing. Fig.
6B compares the mean heart rates recorded as a function of
temperature from isolated hearts in vitro and from hearts in intact
animals in vivo. In both isolated hearts and intact animals,
increases in heart rate occurred gradually as temperature warmed with no
observed periods of bradycardia or tachycardia. Both sets of data show
significant increases in heart rate as temperature warms (see
Fig. 6 legend). Over the
temperature range from 2 to 22°C, the heart rates of isolated hearts
increased approximately fourfold in vitro (closed squares) whereas
heart rates measured in vivo increased approximately 2.5-fold (closed
circles). Isolated hearts tend to beat more slowly than hearts in intact
animals, these difference are statistically significant at 2°C and in the
temperature range from 12 to 22°C. In isolated hearts, the maximum heart
rate observed was 1.53 Hz (99 beats min–1) at a temperature
of 20°C. In intact animals the maximum heart rate observed was 1.85 Hz
(111 beats min–1) at 22°C, slightly faster than the
previously reported upper limit of 100 beats min–1 for
lobsters walking on treadmills (O'Grady et
al., 2001). Finally, to test whether heart rates in intact animals
might thermoadapt over long periods at thermal equilibrium, lobsters were
warmed from 2°C to a new steady state level of 12°C.
Fig. 6C shows that heart rate
increases as temperature warms from 2 to 12°C and remains constant for 45
min under steady state temperature conditions. Thus, we find no evidence that
thermoadaptation alters heart rate. This observation is consistent with the
fact that our data, recorded from lobsters under laboratory conditions, is in
good agreement with heart rates measured in lobsters in the wild
(Fig. 6B, open symbols).
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The strength and the frequency of the heartbeat are independent parameters
Because the spontaneously beating heart is driven by rhythmic output of the
cardiac ganglion, the relationship between the frequency of the heartbeat and
the strength of the heartbeat is of particular interest. The neurocardiac
synapses on the heart muscle gate calcium influx into heart muscle through
voltage-dependent channels, and these synapses facilitate strongly as a
function of frequency of motoneuron firing
(Anderson and Cooke, 1969;
Mahadevan et al., 2004).
Previously, Mahadevan et al. have observed a positive correlation between
frequency and strength of the lobster heart beat that they attributed to
enhanced synaptic depolarization at high burst frequencies
(Mahadevan et al., 2004).
However, in our experiments we observed that these parameters behave
differently in response to temperature change: the strength of contractions
decreases (Fig. 4) whereas the
frequency of contractions increases (Fig.
6; see also raw data in Fig.
3).
To examine the potential coupling between strength and frequency of the heartbeat we plotted the relation between these parameters for each of seven isolated hearts that were exposed to temperatures warming from 2 to 22°C (Fig. 8). None of these plots shows a positive relationship between the amplitude and the frequency of contraction. In general, the contraction amplitude of the heartbeat appears to decrease as frequency increases. Almost all of the plots showed regions where similar amplitudes were recorded at different frequencies, or temperature ranges where contraction amplitude changed while heartbeat frequency remained constant. Therefore, measured as a function of changing temperature, the parameters of heart rate and strength of the heartbeat do not positively correlate.
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Cardiac output is relatively constant as a function of temperature
The cardiac output depends both on the stroke volume and the heart rate. To
test the possibility that the temperature-dependent decrease in heartbeat
strength (Fig. 4) might be
offset by the concurrent temperature-dependent increase in heart rate
(Fig. 6) we measured cardiac
output directly. Fig. 9A shows
examples of cardiac flow recorded from the same isolated heart at two
different temperatures. Measurements of this type in four isolated hearts were
analyzed over the temperature range from 2 to 22°C. In agreement with
earlier results (Fig. 6B),
heart rate increases with temperature from 2°C up to 12°C to 14°C
(Fig. 9B). In contrast, stroke
volume is maximal at the coldest temperatures and decreases as temperature
warms (Fig. 9C). Interestingly,
cardiac output shows a temperature-dependent trend that is well described by a
quadratic equation with a maximum at approximately 10°C
(Fig. 9D). Between 2 and
20°C there are no statistically significant differences in cardiac output,
thus cardiac output appears relatively constant over this temperature range.
At 22°C cardiac output decreases significantly compared to the data at
2°C.
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Modulatory effects of serotonin are temperature independent
In agreement with earlier reports, serotonin increases both the strength
and the frequency of the heartbeat (Fig.
10). At 2°C the effects of serotonin on seven isolated hearts
reached steady state within 12.2±8.5 min (mean ± s.d.; range
2.3–21 min). In these experiments serotonin increased contraction
amplitude of the heart by an average of 128% (range 13–497%) and
increased the frequency of the heartbeat by an average of 64% (range
18–171%). Fig. 11 shows
the effects of serotonin on contraction amplitude and frequency over the
entire temperature range, expressed as a ratio between the value measured in
the presence of serotonin as compared to the control value at that
temperature. In some experiments performed in summer months serotonin was
particularly potent in increasing contraction amplitude at the warm
temperatures of 16–18°C (raw data not shown). However, results
averaged from all seven experiments showed no statistically significant
temperature dependence of the effect of serotonin on contraction amplitude
(Fig. 11A). Serotonin's effect
in increasing the heart rate was also temperature independent, serotonin
increased the frequency of the heartbeat approximately two fold at all
temperatures (Fig. 11B). The
shapes of the phase plots for heartbeats in the presence and absence of
serotonin are similar (Fig.
11C), indicating that the effects of serotonin on contraction and
relaxation rates are proportional to its effect in increasing contraction
amplitude.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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A major finding in this study is that heart rates increase with temperature
both in vitro and in vivo. These results are in agreement
with several previous reports on Homarus
(Hokkanen and DeMont, 1997;
McMahon, 1999;
Schreiber et al., 2000;
Schreiber et al., 1998) as
well as other species of lobster (Nakamura
et al., 1994; Zainal et al.,
1992). We observe faster heart rates in intact lobsters compared
to isolated hearts, especially in the range 12–20°C. Similar
observations have been made previously, both in lobsters
(Schreiber et al., 1998) and
crabs (Wilkens, 1994), and attributed to the overall cardioexcitatory effects
of the cardioregulatory nerves and endogenous hormones in intact animals. It
is also possible that the different heart rates observed in isolated and
intact hearts reflect differences in filling pressure, vascular resistance and
stretch of the heart muscle.
Our demonstration that the acute Q10 values for heart rate are
similar in isolated heart and in intact animals (except at the extreme of
2°C) suggests that the temperature dependence of the lobster heart rate
arises mainly from the intrinsic properties of the cardiac ganglion, rather
than from activity in the cardioregulatory nerves or from hormonal signals
arising from other regions of the CNS. Interestingly, Jury and Watson
(Jury and Watson, 2000)
reached a different conclusion. Based on their observations that severing the
cardioregulatory nerves in intact animals abolished the response of the heart
to changes in temperature, they suggested that the cardioregulatory nerves
mediate temperature dependence of the heart rate
(Jury and Watson, 2000). It is
not entirely clear how to reconcile their results with ours. However, we note
that their experiments were performed in a relatively narrow temperature range
(starting at 15°C and warming or cooling by 1.5°C) and are in
agreement with our data showing a small but significant difference in
temperature dependence of intact and isolated hearts at 14°C (see
Fig. 7). Nevertheless, over the
relatively large temperature range of 4–22°C our observation that
the temperature dependence of heart rate is similar in vitro and
in vivo supports the hypothesis that temperature dependence of the
heartbeat arises mainly from temperature effects on the cardiac ganglion
neurons. A minor contribution by cardioregulatory nerves and/or hormones to
the temperature dependence of the heart rate is suggested by the difference in
the slopes of the plots describing the temperature dependence of the heart
rate in vivo and in vitro, as shown in
Fig. 6B.
McMahon has emphasized the importance of not relying solely on the heart
rate as an indicator of cardiac performance
(McMahon, 1999;
McMahon, 2001) and our
observations strongly support this view. Whereas the lobster heart rate
increases linearly as a function of temperature over most of the range from 2
to 22°C two other parameters of cardiac performance, the strength of the
heartbeat and the stroke volume, both decrease. Similar observations have been
reported for crab (Cancer magister) hearts
(DeWachter and McMahon, 1996;
DeWachter and Wilkens, 1996).
Moreover, cardiac output in the lobster displays a pattern of temperature
dependence that probably results from the opposing temperature-dependent
effects on heart rate and stroke volume. Cardiac output was maximal at
10°C, but became compromised as the temperature warmed above 20°C,
with a significant decrease in cardiac output in hearts that continued to beat
and an increase in the number of hearts failing. It is possible that cardiac
performance in intact animals might be protected somewhat from thermal stress
by endogenous neural and hormonal signals, since nearly all the hearts in
intact animals (20 out of 21) continued to beat at 22°C. Although we did
not make measurements at temperatures warmer than 22°C, it is likely that
cardiac performance will degrade as animals approach their lethal temperatures
of 25 to 30°C (McLeese,
1956).
It is important to note that the rate of temperature change in our experiments (approximately 1°C min–1) is considerably faster than temperature changes occurring in the wild during a tide or a season. (For example, the fastest rate of temperature change during the course of tide is approximately 3°C h–1 in the record shown in Fig. 1.) In the ocean, it is possible that the animals might be able to acclimate to warmer temperatures if temperature changes sufficiently slowly, thereby improving cardiac performance at warm temperatures. However, it is also likely that animals moving through thermoclines on the ocean floor will experience relatively rapid and extreme changes in temperature. It is interesting to note that our measurements of heart rate in lobsters exposed to acute changes in temperature in the lab are in good agreement with those obtained from ocean animals experiencing seasonal temperature changes over the course of a year (see Fig. 6B). These results suggest that our relatively fast thermal challenges in the laboratory stimulate cardiac responses very similar to those in lobsters experiencing seasonal thermal shifts in their natural habitat. Future experiments will be directed toward examining whether the temperature dependence of cardiac performance differs for animals acclimated to different seasonal temperatures or animals exposed to temperature change at different rates.
Finally, the temperature dependence of cardiac performance in the lobster
differs in several respects from that reported for crab C. magister
(DeWachter and Wilkens, 1996),
a Pacific coast species that inhabits a similar thermal range. In isolated
crab hearts the temperature dependence of the heart rate is only one third
that measured in intact animals and cardiac output is maximal at 2°C and
decreases as a function of temperature until heart failure occurs at 20°C.
In contrast, isolated lobster hearts appear to be more robust in the face of
warming temperature. Most are viable up to 22°C, show
temperature-dependent increases in heart rate that are similar to those
observed in hearts in intact animals, and exhibit an increase in cardiac
output as temperature warms above 2°C that reaches a maximum at
10°C.
Frequency and amplitude of the heartbeat
A previous study of isolated lobster hearts concluded that the strength of
the heartbeat and the frequency of the heartbeat are coupled
(Mahadevan et al., 2004),
because these parameters show a positive correlation in spontaneously beating
hearts and in hearts paced by external stimuli. This interpretation seems
reasonable, given that the same study showed that the magnitude of synaptic
depolarization at neurocardiac synapses is linearly related to burst
frequency. However, the experiments were performed at constant temperature
(fixed between 10 and 14°C), thereby constraining the range of possible
physiological responses. Our results, collected over a broad temperature
range, show no evidence for coupling between the strength and frequency of the
heartbeat. We favor the viewpoint that strength and frequency of the heartbeat
are independent parameters and independently regulated. At constant
temperatures heartbeat frequency will be a critical determinant of contraction
amplitude, but changes in temperature will affect both parameters
independently, and alter any relationship between them. The `uncoupling' of
heartbeat frequency and amplitude in the lobster has been observed previously
in response to hypoxia (McMahon,
1999) and during adaptation to stimulation by nitrous oxide
(Mahadevan et al., 2004).
Although we have not investigated the physiological mechanisms underlying
temperature dependence in this system, our results suggest several possible
mechanisms. As heart rate is determined by the rhythmic neural output of the
cardiac ganglion, it is possible that temperature regulates the frequency and
patterns of bursting of the Homarus cardiac ganglion motoneurons and
pacemaker cells. Temperature-dependent decreases in the strength of the
heartbeat might arise either from the properties of the Homarus
cardiac muscle fibers, or of the cardiac ganglion motoneuron synapses that
innervate them. For example, temperature may affect both the size of the
neurocardiac synaptic potentials and the extent to which they facilitate, as
shown in the myocardium of another lobster species Panulirus
japonicus (Kuramoto,
1994). In addition, it is also probable that the calcium dynamics
within the cardiac muscle fibers might be temperature dependent, thereby
regulating the availability of calcium in the cytoplasm and the rate at which
it activates the contractile machinery. Further experiments will be required
to investigate these possibilities.
Serotonergic modulation of heart rate and contraction amplitude is temperature independent
In agreement with earlier reports, we find that serotonin increases both
the rate and the strength of the heartbeat. The onset of serotonin effects on
the heart at 2°C is comparatively slow, reaching steady state in
approximately 12 min, on average. By comparison, other studies performed at
12–13°C have demonstrated that potentiation of lobster heart rate by
serotonin reaches steady state within 5 min
(Guirguis and Wilkens, 1995;
Kuramoto et al., 1995). In our
experiments modulation of the heart by serotonin was effective over the whole
temperature range tested. Although there was some variability between
preparations, we did not observe a statistically significant temperature
dependence of the modulatory effects overall. In contrast, serotonin elicits
several temperature-dependent effects on a lobster skeletal muscle
neuromuscular system. Serotonin increases muscle tonus most strongly at the
coldest temperatures (2°C), increases nerve-evoked contractions most
strongly at warm temperatures (16–18°C) and enlarges the temperature
range over which the inhibitory motoneuron is effective in relaxing the muscle
(M.K.W., unpublished observations).
There are likely to be multiple sites of action for serotonin in the
lobster cardiac system. Serotonin increases the heartbeat of intact animals
even after the cardioregulatory nerves are severed
(Guirguis and Wilkens, 1995),
suggesting that serotonin acts directly on the cardiac ganglion. This was
subsequently verified by Berlind's observation that serotonin increases and
prolongs the bursting output of the cardiac ganglion in vitro
(Berlind, 1998). Such effects
would be consistent with serotonin increasing heartbeat frequency and
contraction amplitude. In Homarus serotonin also decreases output
through the sternal arterial valve, thereby redirecting arterial flow
(Wilkens et al., 1996).
However, it is also probable that serotonin acts on the neurocardiac synapses
as well as the muscle fibers to increase contractility. Cooke reported
preliminary evidence that serotonin potentiates synaptic transmission at
synapses on the lobster heart (Cooke,
1966). Serotonin potentiates both synaptic transmission and muscle
contractility in lobster dactyl opener neuromuscular systems
(Kravitz et al., 1980) as well
as other invertebrate neuromuscular systems
(Worden, 1998).
Finally, it is important to note that the temperature can be a confounding
variable in physiology experiments, because multiple physiological processes
underlie cellular function and each can depend differently on temperature. For
example, a recent study of excitation–contraction coupling in lobster
cardiac muscle at constant temperature concluded with the prediction that an
increase in heart rate will enhance calcium loading of the sarcoplasmic
reticulum, thereby accelerating relaxation of the heartbeat, increasing
diastolic filling and contraction amplitude
(Shinozaki et al., 2002).
Further, these authors predicted that heartbeat frequency-dependent increases
in contraction amplitude will cause stroke volume and cardiac output to
increase. However, using an experimental protocol that changes temperature we
observe different relationships between the parameters underlying cardiac
performance. As the lobster heart rate increases both contraction amplitude
and stroke volume decrease, while cardiac output itself remains relatively
constant with a maximum at 10°C and a significant decrease at temperatures
warmer than 20°C. Changes in temperature can therefore alter the
relationships between parameters of cardiac function to produce physiological
effects that cannot be predicted from observations at a constant fixed
temperature.
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Acknowledgments |
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