Control of heart rate during thermoregulation in the heliothermic lizard Pogona barbata: importance of cholinergic and adrenergic mechanisms
1 School of Biological Sciences A08, The University of Sydney, NSW 2006, Australia and
2 Department of Zoology and Entomology, The University of Queensland, Brisbane Qld 4072, Australia
*Author for correspondence (e-mail: fseebach{at}bio.usyd.edu.au)
Accepted 2 October 2001
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Summary |
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Key words: thermoregulation, heart rate, neural control, cholinergic, adrenergic, reptile, lizard, Pogona barbata.
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Introduction |
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Several species of reptiles, including lizards, crocodilians and turtles, are known to increase their heart rate during basking, resulting in increased heat transfer between the warm animal surface and the cool core (Bartholomew and Tucker, 1963; Grigg and Seebacher, 1999
; Smith, 1976
; Voigt, 1975
). Conversely, when entering a cooling environment at high Tb, heart rate decreases so that heat transfer between the warm core and the cool surface decreases (Seebacher, 2000
). This pattern, where heart rate during heating is significantly faster than during cooling, is termed heart rate hysteresis, and it allows a reptile to stay warm for longer during the day by raising the body temperature faster during basking in the morning and reducing the rate of cooling in the evening (Seebacher, 2000
). Despite the functional significance of these changes in heart rate, the physiological mechanisms that effect changes in heart rate remain obscure.
The sympathetic (adrenergic) and the parasympathetic (cholinergic) nervous systems are principally responsible for short-term (on the scale of seconds or minutes) cardiovascular control in vertebrates (Akselrod et al., 1981). The cholinergic, vagal branch of the autonomic nervous system uses acetylcholine as a transmitter substance to depress heart rate by acting on heart muscarinic receptors. In contrast, spinal autonomic fibres effect an increase in heart rate, which is mediated by adrenaline acting on heart ß-adrenergic receptors (Axelsson et al., 1987
; Morris and Nilsson, 1994
). The autonomic fibres controlling the heart are continuously active, thereby creating a nervous tone which increases the efficacy of the heart rate response (Altimiras et al., 1997
; Hoffman and Romero, 2000
). There is evidence that changes in the cholinergic tone on the heart are the principle mechanism controlling heart rate during exercise in fish (Axelsson et al., 1987
; Altimiras et al., 1997
). In contrast, heart rate variability in a lizard (Gallotia galloti) appeared to be mediated primarily by ß-adrenergic receptor mechanisms (DeVera et al., 2000
). Moreover, there are species-specific differences in the relative importance of cholinergic and adrenergic autonomic control of the heart. For example, the heart of the toad Bufo paracnemis at rest was under the influence of both cholinergic and adrenergic tone (Hoffman and Romero, 2000
), whereas heart rate in resting Bufo marinus was regulated by cholinergic fibres alone, but tachycardia during exercise in this species was effected by adrenergic fibres (Wahlqvist and Campbell, 1988
).
Given the predominant role played by the autonomic nervous system in controlling heart rate of ectotherms, we postulated that autonomic neural mechanisms were responsible for heart rate regulation during body heating and cooling in the bearded dragon Pogona barbata, a heliothermic lizard. More specifically, we tested the hypothesis that changes in cholinergic and ß-adrenergic tone on the heart are responsible for the heart rate hysteresis observed during heating and cooling in reptiles. Identification of the mechanism controlling heart rate during heating and cooling is of importance, because it will help us to understand how reptiles regulate their body temperature physiologically, and indicate on which physiological systems selection pressures may have acted to produce the thermoregulatory strategies seen in vertebrates today.
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Materials and methods |
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For the duration of the experiments, lizards were kept in plastic containers (30x37x28 cm), which were large enough for the animals to sit comfortably on the bottom without, however, allowing extensive movement. Electrocardiograms (ECGs) were measured with a high gain AC amplifier (BioAmp, AD Instruments, Powerlab frontend) that was connected to a 4-channel PowerLab (AD Instruments). The signals were sampled at 30 Hz by Chart software run on a Toshiba Laptop computer, which also calculated heart rates. Electrodes consisted of insulated surgical stainless steel wire placed under the skin (after administration of Lignocaine as a local anaesthetic), one immediately ventral to the heart and a second at the base of the tail. The insulation was stripped off at the active ends of the electrodes, leaving approximately 1 cm of wire bared. Tb was measured with K-type thermocouples inserted 34 cm into the cloaca and also connected to the PowerLab.
During the experiment, lizards were heated from about 22.5°C to 32.5°C with an infrared heat lamp suspended above the plastic containers. The heat lamp was positioned at such a distance that the lizard surface received 600700 kW m2, which is similar to solar irradiation during basking on a summer morning (F. S., unpublished data; radiation intensity was measured with a Sol Data pyranometer connected to a datalogger). Once Tb reached 32°C the heat lamp was turned off and lizards were allowed to cool to within 1°C of their initial Tb. Heart rate and Tb were monitored continuously during the heating and cooling trials.
To control for potential effects of light, rather than heat, on heart rate, experimental trials were run with a cold, optical fibre light (Euromex) covered with red plastic foil instead of the infrared heat lamp.
Pharmacological protocol and treatments
The effect of the autonomic nervous system on heart rate during heating and cooling was investigated by chemically blocking the ß-adrenergic and muscarinic receptors. Atropine sulfate (1.5 mg kg body mass) was used as a muscarinic receptor (cholinergic) antagonist, and ß-adrenergic receptors were blocked with the antagonist sotalol hydrochloride (3.0 mg kg1 body mass). Atropine and sotalol were dissolved in 0.9 % saline and injected intraperitoneally. Saline solution was injected for control treatments.
The experiment consisted of four treatments: Control 1, heating and cooling after injection with saline solution; Control 2, cold red light after injection of saline solution; Treatment 1, heating and cooling after blocking the cholinergic nervous system by injecting atropine; Treatment 2, heating and cooling after administration of atropine and sotalol injected simultaneously. The pharmacological protocol follows details described by Altimiras et al. (1997). Lizards were injected 2 h before conducting heating and cooling trials to minimize the effect of handling stress. Blockade of the cholinergic and adrenergic systems was established from stabilization of heart rate, which typically occurred 3060 min after injection of the antagonists. During each treatment, heart rate and Tb were monitored at least 5 min before the heat/cold lamp was switched on, and experimental equipment could be operated without disturbing the animals. As a rule, lizards sat quietly in the plastic container, but some of the animals moved occasionally, and this was clearly discernible on the ECG trace by the presence of electromyograms from skeletal muscular activity. These data were omitted from the analysis.
Analysis
Changes in heart rate were analysed statistically by a three-way analysis of variance (ANOVA) with heating/cooling, treatment (control, atropine, atropine and sotalol) and lizard (16) as factors. To overcome possible dependence of sequential measurements, heart rate measurements were randomized, and a random sub-sample of 15 measurements per level of each factor were used in the analysis. Cholinergic and ß-adrenergic tones were calculated as follows (Altimiras et al., 1997):
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where HRcontrol is heart rate during control treatment, HRchol is heart rate during cholinergic blockade (atropine treatment), and HRcomplete is heart rate during complete autonomic blockade (atropine + sotalol).
Changes in ß-adrenergic and cholinergic tone during heating and cooling were analysed by model 1 linear regression analysis with tone as the dependent variable and Tb as the independent variable. Data were presented in chronological order, but rather than plotting time on the x-axis, tone was plotted against Tb so that the problem of slightly different body masses and, therefore, different heating and cooling times of the study animals, was overcome.
Rates of heating and cooling were expressed as the transient rate of change in the internal temperature of the lizards. Body temperature was expressed as the dimensionless temperature =(TbTe)/(TiTe), where Te is the operative temperature during the heating or cooling trial, and Ti is the initial body temperature of a lizard at the beginning of each heating or cooling episode (Seebacher, 2000
). Rates of heating with the different treatments were compared by regressing ln(
) over time for the heating trial of each lizard, and comparing the slopes of the regression by a one-way analysis of variance with treatment as factor.
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Results |
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Rates of heating were significantly different between the different treatments (F2,15=4.55, P<0.03; Fig. 7) indicating that the autonomic nervous system may play a role in thermoregulation. Lizards heated faster with a cholinergic blockade than under control conditions, and rates of heating decreased when the autonomic nervous system was totally blocked (Fig. 7).
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Discussion |
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Considering mean values from all lizards, it would appear that heart rate of P. barbata is regulated primarily by variation of the ß-adrenergic tone on the heart. This is in contrast to fish, where heart rate during exercise is regulated primarily by variation in the cholinergic tone on the heart (Axelsson, 1988; Altimiras et al., 1997
; Axelsson et al., 2001
). Moreover, antarctic fish were found to increase the cholinergic tone on their heart when heated, counteracting a temperature-induced increase in heart rate (Q10 effect), so that heart rate was thermally independent (Franklin et al., 2000
). Care has to be taken, however, in drawing the conclusion that heart rate in P. barbata is primarily controlled by ß-adrenergic receptors, because we found significant differences between individual lizards.
All lizards responded with a sudden drop in heart rate as the heat lamp was switched off, and this response appears to be a cholinergically mediated reflex as it disappears with the administration of atropine. The extremely rapid response to the removal of the heat source could indicate that thermal sensors in the skin may instigate a cholinergic response via the action of prostaglandins, for example (Robleto and Herman, 1988). The existence of peripheral control mechanisms is also indicated by the fact that the increase in wash-out rates of radioactive Xe may precede an increase in heart rate after application of heat to the skin surface of the marine iguana (Morgareidge and White, 1972
).
In exercising fish, cholinergic tone decreased by from 38 % to 15 % and adrenergic tone increased from 21 % to 28 % compared to resting values (Axelsson et al., 1987). Lizards in this study showed much greater variability in tone on the heart in response to heating and cooling but, again, there were pronounced differences between individuals. The ß-adrenergic tone decreased by over 60 %, and the cholinergic tone by over 40 % between heating and cooling, so it must be concluded that there is a pronounced response to heating and cooling. It seems contradictory, however, that both ß-adrenergic and cholinergic tones changed in the same direction and were lower during cooling than during heating. Akin to the fish example quoted above, we expected that as ß-adrenergic tone increased, cholinergic tone would decrease, and vice versa, so that the total neural effect on heart rate is compounded. This was not the case, and it appears that the cholinergic and ß-adrenergic branches work against each other. This relationship is also seen in the individual short-term responses to the sudden removal of the heat source where, if there is a change, cholinergic and ß-adrenergic tone tend to change in the same direction.
Does an autonomic neural mechanism of thermoregulation exist in P. barbata? The cholinergic and ß-adrenergic control systems certainly have an impact on heart rate, which would influence rates of internal heat transfer (Seebacher, 2000). However, in P. barbata these neural pathways are not responsible for the major cardiovascular mechanism in thermoregulation, i.e. the heart rate hysteresis pattern. It appears, therefore, that there are other regulatory mechanisms controlling heart rate during heating and cooling and these may be more important in thermoregulation.
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References |
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