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¹Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TT, UK²Department of Physiology, University College Cork, Cork, Ireland

	Abstract

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The present study investigated the effect of acute hypothermia on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) by generating baroreflex logistic function curves, using bolus doses of phenylephrine and sodium nitroprusside, in anaesthetized male Wistar rats at a core temperature (T_b) of 37°C, during acute severe hypothermia at T_b= 25°C and on rewarming to 37°C. Comparisons were made between rats without (euthermic, n= 6) and with (acclimated, n= 7) prior exposure to lower ambient temperatures and shorter photoperiod, simulating adaptation to winter conditions. In both groups of rats, acute hypothermia to T_b= 25°C shifted the baroreflex-RSNA curve slightly leftwards and downwards with decreases in the setpoint pressure and maximal gain, whereas it markedly impaired the baroreflex-HR curve characterized by decreases in response range by 90% (P < 0.001), minimum response by 10% (P < 0.05) and maximum gain by 95% (P < 0.001), from that at T_b= 37°C. All parameters were restored to precooling levels on rewarming. Electrical stimulation of cardiac vagal efferents induced a voltage-related bradycardia, the magnitude of which was partially reduced during acute hypothermia, and there was a significant prolongation of the electrocardiogram intervals indicating a delay in cardiac conduction. Mild suppression of baroreflex control of RSNA could contribute to hypothermic hypotension and may primarily reflect an effect of T_b on central drive. The marked attenuation of the baroreflex control of HR during hypothermia was likely to be due to an impairment of both the central and peripheral components of the reflex arc. Baroreflex control of RSNA and HR was similar between both groups of rats, which implied that the control was non-adaptive on chronic cold exposure.

(Received 10 December 2003; accepted after revision 18 February 2004; first published online 20 February 2004)
Corresponding author S. Egginton: Department of Physiology, The Medical School, University of Birmingham, Vincent Drive, Birmingham B15 2TT, UK. Email: s.egginton{at}bham.ac.uk

	Introduction

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Acute hypothermia can occur following exposure to a cold environment as a consequence of accidental exposure during recreational activity, and may be induced intentionally during surgery to protect the heart from ischaemic damage by decreasing the metabolic rate. In some animals (ground squirrels, hamsters) it is quite normal for core temperature to reach values only a few degrees above ambient levels during aestivation or hibernation. In anaesthetized rats acute hypothermia to a core temperature (T_b) of 25°C decreases mean arterial blood pressure (MABP) by 15% but heart rate (HR) by 50% (Broman et al. 1998) with a decrease in cardiac output (Granberg, 1991) and cardiac-related bursts of renal sympathetic nerve activity (RSNA) (Sabharwal et al. 2002). Such changes would normally be counteracted by the arterial baroreflex, which functions as a short-term negative feedback regulator of MABP, so that hypothermic hypotension would result in baroreflex- mediated tachycardia and sympathoexcitation. The lack of a compensatory reflex change suggests that the baroreceptor reflex regulation has been impaired by hypothermia.

There is evidence that thermal stimuli can modify the baroreceptor reflex, and that the interaction of the thermal and baroreceptor reflexes may occur centrally in the hypothalamus or the medulla (Heistad et al. 1973). However, the way in which thermal stimuli affect the baroreflex is unclear. Studies have suggested that baroreflex sensitivity may be enhanced (Zheng et al. 1996), attenuated (Angell James, 1971; Papanek et al. 1991), or unchanged (Kaul et al. 1973) by moderate hypothermia to a T_b of 25–30°C, depending on the species and experimental conditions. Hypothermia has also been reported to decrease aortic depressor nerve activity in rabbits (Angell James, 1971), increase cardiac muscarinic receptor affinity to agonists in rats (Phan et al. 1980), and modify sinus node electrophysiological activity in the isolated dog atrium (Kobayashi et al. 1985). These results suggest that hypothermia may modulate the baroreflex function at multiple sites within the reflex arc. However, the extent to which acute hypothermia influences baroreflex control of HR and RSNA has not been studied previously in anaesthetized rats. Furthermore, no information exists about whether this control is adaptive following chronic cold exposure.

Previous studies resulted in a confusing picture about the baroreflex response to lowered core temperatures, in part because they were limited by estimating slope of the response by linear regression analyses. In contrast, we have calculated the whole baroreceptor reflex curve, thereby providing a more complete description of the phenomenon. Additionally, we have examined the influence of chronic cold exposure on sensitivity of the baroreceptor reflex in order to determine the potential for functional adaptation. We hypothesized that the baroreflex stimulus–response curves for RSNA and HR would be different in euthermic and acclimated rats, and that changes in baroreflex-HR control were in part due to peripheral rather central influences.

	Methods

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Outbred male Wistar rats were used in accordance with UK legislation (Animals (Scientific Procedures) Act 1986), and had access to water and food ad libitum. Two groups of animals were used. Euthermic rats (n= 6) were sampled when 12 weeks old, with a final body mass of 300 ± 10 g, having been held on a 12 h: 12 h light: dark (L:D) photoperiod with an ambient temperature of 21 ± 1°C. Initial body mass of the cold exposed rats (n= 7) was 45 ± 5 g (4 weeks old) when they were transferred to an environmental chamber with light-tight ventilation ports and programmable low energy light source. The initial ambient temperature of 21°C was gradually reduced to 5 ± 1°C, accompanied by a gradual reduction in photoperiod of 1.5 h weekly from 8: 16 L:D to 1 h a day by the fourth week, maintained at this level for a further 4 weeks. Animals were removed at the end of this period, when they were 12 weeks old with body mass of 299 ± 14 g and taken into experimentation. This protocol avoids pathological changes associated with cold shock observed in earlier studies, and mimics the natural progression into winter conditions (Deveci et al. 2001).

Surgical preparation

Gaseous anaesthesia, 4% halothane in oxygen, was replaced with I.V. administration of a mixture of -chloralose and urethane (165 mg chloralose and 2.5 g urethane in 10 ml saline; Sigma-Aldrich, UK), approximately 0.7 ml given over 35 min, followed by 0.05 ml every 30 min, in normal physiological saline (150 mmol NaCl) at a rate of 3 ml h^–1 via the right femoral vein (representing a dose of 3.5 mg kg^–1 chloralose and 525 mg kg^–1 urethane). The animals were tracheotomised to aid spontaneous ventilation. Mean arterial blood pressure (MABP) and heart rate (HR) were measured via the right femoral artery catheter interfaced to a pressure transducer (MLT105, Precision, UK), with cannula patency maintained using heparinized saline (20 U heparin (ml saline)^–1). Blood pressure signals were amplified (PowerLab, ADInstruments, UK) and together with heart rate signals were recorded on a microcomputer (iMac DV) running a data acquisition program, Chart 4 (ADInstruments) at a sampling rate of 1000 s^–1. Animals were placed on a thermostatted plate connected to a temperature control unit to regulate the core temperature (T_b).

Electrocardiogram recording

To record electrocardiogram (ECG), an incision was made slightly lateral to midline of the overlying skin of the chest wall. The superficial pectoralis muscles were dissected so that the rib cage could be seen. ECG leads (ADInstruments, UK) were placed through the intercostal externi at position V4 and through the skin of the right forepaw. ECG signals were amplified using a Bioamplifier (ADInstruments) and recorded using the data acquisition programs Chart 4 and Scope v3.6/s (ADInstruments). The sampling rate was set at 2560 Hz with a sweep time of 200 ms. The ECG recorded showed a well defined P, QRS complex and T waves, and was used to calculate PR, QRS, ST, QT and RR intervals at different T_b values (Beinfield & Lehr, 1968; Gussak et al. 2000).

Measurement of responses to electrical stimulation of vagal efferent nerves

The right vagus was separated from the adjoining nerves and connective tissue in the cervical region and then carefully sectioned. The peripheral cut end of the right vagus nerve was placed on bipolar electrodes, sealed using silicone gel 932A and 932B (Wacker, Germany) and stimulated electrically, at the appropriate T_b, at 3 Hz, with 0.1 ms pulses of 10 s duration and varying voltage, delivered from a stimulator (Grass, model S8800, USA) through an isolation unit (Grass, SIU5B) and responses recorded using Chart software (ADInstruments).

Recordings of RSNA

Following cannulation of the bladder, the left kidney was exposed through a flank incision. A renal nerve bundle was identified, running on or beside the left renal artery, isolated and placed on bipolar electrodes and sealed using silicone gel 932A and 932B (Wacker). RSNA was recorded using an isolated preamplifier (Neurolog, UK) and fed through filters set at 0.1 Hz and 1 kHz to remove low and high frequency interference, passed via a Humbug (Quest Scientific, UK) to remove 50–60 Hz noise, and made audible with an audio speaker. The background noise for RSNA recordings was determined when nerve activity was minimized by increasing arterial blood pressure with phenylephrine (10 µg) at normal core temperature (T_b= 37°C) and subtracted from all records to yield actual RSNA responses. The amplified neurogram was rectified, integrated and expressed in millivolts (mV) over 1 s intervals (sampled at a rate of 1000 s^–1) and saved on the computer for offline analysis. To quantify RSNA response, percentage changes were calculated by taking the mean of the values during normal T_b as 100% RSNA.

Experimental protocol

Basal levels of variables were determined at 37°C (normal core temperature), moderate hypothermia at 31°C, severe hypothermia at 25°C, and on rewarming to 31° and 37°C by means of stepwise changes in the temperature of the thermostatted plate. The core temperature was changed, gradually, at a rate of 3°C every 15 min, and rats were maintained for 30 min at the chosen T_b values for data collection. Therefore, it took 1–1.5 h to reduce the core temperature to 25°C and a further 1–1.5 h to rewarm to 37°C.

The steady state levels of RSNA, MABP, HR and ECG were measured by averaging over 3 min and then baroreflex control of RSNA and HR was assessed during pharmacological manipulation of MABP. The increases and decreases in MABP were achieved using bolus intravenous doses of 10 µg phenylephrine hydrochloride (Sigma, UK) given over 30 s, and 10 µg of sodium nitroprusside (Sigma) in a random order at each T_b. The corresponding responses in HR and RSNA to a change in MABP were recorded and the data fitted to sigmoid logistic function curves (Kent et al. 1972). ECG traces and the responses in HR with vagal stimulation were recorded at the appropriate T_b. At the end of each experiment, the animals were killed with an overdose of sodium pentabarbitone (Rhone-Merieux, Ireland).

Data analysis

To facilitate comparison of euthermic and acclimated rats the thermal extremes were used to elicit maximal baroreflex responses. A logistic sigmoid function equation, as described by Kent et al. (1972), was used to generate the baroreflex curves at T_b= 37°C, T_b= 25°C, and rewarming to 37°C (T_b,rew= 37°C)

(1)

where A₁ is the response range for HR or RSNA (difference between maximum and minimum response); A₂ is the gain coefficient (equivalent to the slope); A₃ is the midpoint pressure at the midrange of the curve (centring point); and A₄ is the minimum response for HR or RSNA. In each animal, MABP and RSNA or HR data were fitted to the logistic function to generate parameters A₁, A₂, A₃ and A₄ for each T_b using graphics software (DeltaGraph, Red Rock Software Inc., USA) (Miki et al. 2003).

Several other parameters were also calculated, including maximum response for HR and RSNA (i.e. the upper plateau of the curve). Saturation pressure for MABP (P_sat) and threshold pressure for MABP (P_thr) represent the MABP at which HR or RSNA was within 5% of its maximal or minimal response, respectively. Maximal gain is the gain value located at the centring point of the reflex (determined by the response range and gain coefficient). Operating range is the range of MABP over which HR or RSNA responded. Setpoint pressure (P_sp) is defined at the basal level prior to any pharmacological intervention and a point where the assumption is made that there are minimum regulatory influences, or where the input of the system equals the output. The variables were calculated according to the following equations (Saigusa et al. 1996; Miki et al. 2003):

(2)

Mean values of RSNA, HR and MABP for every 2 mmHg bin of MABP were used for curve fitting. Baroreflex curves were generated for each individual rat and then averaged over animals at each T_b. The mean A₁, A₂, A₃ and A₄ of each group at each T_b were then used to generate baroreflex-HR and baroreflex-RSNA curves for the euthermic and acclimated rats at T_b= 37°C, T_b= 25°C and rewarming to 37°C.

Statistical analysis

All data represent the average value calculated from individual rats and are expressed as means ±S.E.M. Statistical evaluation was performed using factorial and repeated measures analysis of variance (ANOVA), with Fisher's PLSD to estimate the post hoc significance (StatView SAS Institute, Cary, NC). Statistical significance was accepted at P < 0.05.

	Results

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Haemodynamic responses

There was a progressive decrease in HR and MABP in both euthermic and acclimated rats with cooling. The falls in HR at 31°C were greater (367 ± 4 and 373 ± 5 b.p.m. in euthermic and acclimated groups, respectively, both P < 0.01) than MABP and RSNA with a similar pattern observed upon rewarming (Table 1). Table 1 shows that there was 30% reduction in HR at 25°C versus basal levels taken at 37°C (P < 0.001) while there was 10% reduction of MABP at 25°C versus 37°C (P < 0.01) in euthermic rats. Basal levels of HR and MABP at 37°C for the acclimated rats were not significantly different from those of euthermic animals (Table 1). Reduction of T_b to 25°C in the acclimated rats resulted in 25% reduction in HR (P < 0.001) and 10% reduction in MABP (P < 0.01) versus 37°C. At T_b= 25°C, the magnitude of bradycardia in euthermic rats was more pronounced than in acclimated rats (P < 0.05). In both groups MABP and HR returned to precooling levels on rewarming. Cold acclimation did not produce any significant differences in the pattern of response between the two groups at any temperature (Table 1). RSNA was normalized to 100% at T_b= 37°C. There was a 5% decrease (n.s. versus 37°C) in RSNA in euthermic rats but a 25% reduction of RSNA (P < 0.05) in cold-acclimated rats at T_b= 25°C, both of which were restored on rewarming.

Figure 1 depicts the responses to change in MABP with phenylephrine (PE) or sodium nitroprusside (SNP) to generate baroreflex curves for RSNA and HR at different body temperatures. Increases in MABP caused an attenuation of RSNA and a bradycardia, whereas decreases in MABP reflexly caused a tachycardia and sympathoexcitation. At T_b= 25°C, increases or decreases in blood pressure, with PE and SNP, respectively, failed to have any corresponding effect on HR, unlike that seen at T_b= 37°C and T_b,rew= 37°C. By contrast the profile of the RSNA curves appeared much less altered at the lower temperature.

View larger version (27K): [in this window] [in a new window]   Figure 1.  Original recordings from an individual rat of arterial blood pressure (ABP), heart rate (HR) and renal sympathetic nerve activity (RSNA) during pharmacological manipulation of ABP with phenylephrine (PE) and sodium nitroprusside (SNP) at different core temperatures A bolus intravenous dosage of 10 µg PE and 10 µg SNP was given to generate the stimulus–response curves for RSNA and HR.

Hypothermia modestly shifted the baroreflex curve for RSNA to the left and downwards in both the euthermic (Fig. 2A) and acclimated rats (Fig. 2B). This was characterized by slight decreases in both X- and Y-axis parameters, namely the response range (A₁), gain coefficient (A₂), midpoint pressure (A₃), minimum response (A₄), threshold pressure (P_thr), saturation pressure (P_sat) and a slight increase in operating range (Tables 2 and 3). Maximum response decreased significantly in euthermic rats by 12% (P < 0.05) and in acclimated rats by 26% (P < 0.05) at T_b= 25°C versus 37°C (Table 3). The maximal gain, which is dependent on the response and operating ranges, decreased by 13% (P < 0.05) and 28% (P < 0.05) on cooling to 25°C in euthermic and acclimated rats, respectively (Table 3). The pattern of response was similar in both groups of animals at all T_b values. In the acclimated rats, A₄ was higher, by 18% (P < 0.05), while A₁, maximal response and maximal gain were lower, by 30%, 16% and 29%, respectively (all P < 0.05), compared to the euthermic rats (Tables 2 and 3). On rewarming, the baroreflex gain curve parameters for RSNA were almost identical with those obtained before cooling, with all variables returning towards basal levels in both groups (Tables 2 and 3).

View larger version (14K): [in this window] [in a new window]   Figure 2.  Baroreflex-RSNA curves in euthermic rats (A) and acclimated rats (B) at different core temperatures Curves reflect averaged data from euthermic (n= 6) and acclimated rats (n= 7). Continuous lines indicate responses at 37°C, dotted lines responses at 25°C and dashed lines responses on rewarming to 37°C.

The baroreflex curve for HR in euthermic (Fig. 3A) and acclimated (Fig. 3B) animals was markedly attenuated at T_b= 25°C. Acute hypothermia markedly decreased A₁ by 93% (P < 0.001), A₄ by 15% (P < 0.05) and maximum response by 32% (P < 0.01) compared to 37°C in both groups (Tables 2 and 3) whereas A₃, P_thr, P_sat and operating range did not change significantly on cooling relative to normal core T_b of 37°C (Tables 2 and 3). Maximal gain of baroreflex control of HR reduced significantly in euthermic rats (by 96%versus37°C, P < 0.001) and acclimated rats (by 94%versus37°C, P < 0.001) at T_b= 25°C, and it was 45% higher (P < 0.05) in acclimated rats compared to euthermic rats at T_b= 25°C. In acclimated rats, A₄ was lower, by 5% (P < 0.05), while A₁, maximal response and maximal gain were higher, by 25% (P < 0.05), 2% and 15% (P < 0.05), respectively, compared to the euthermic rats. On rewarming the baroreflex curve for HR was almost identical with that obtained before cooling, with all variables returning towards basal levels, in both groups (Tables 2 and 3).

View larger version (14K): [in this window] [in a new window]   Figure 3.  Baroreflex-HR curves in euthermic (A) and acclimated (B) rats at different core temperatures Curves reflect averaged data from euthermic (n= 6) and acclimated rats (n= 7). Continuous lines indicate responses at 37°C, dotted lines responses at 25°C and dashed lines responses on rewarming to 37°C.

Acute hypothermia (T_b= 25°C) increased the length of each cardiac cycle prolonging the RR interval by 40% (P < 0.001) and all the constituent intervals, PR (by 35%, P < 0.001), QRS (by 25%, P < 0.001), ST (by 35%, P < 0.001) and QT (by 25%, P < 0.001) versus 37°C in both groups (Fig. 4 and Table 4), all of which returned to precooling levels when the animals were rewarmed. Cold acclimation did not produce any significant differences in ECG waveform between the two groups at any temperature (Table 4).

View larger version (19K): [in this window] [in a new window]   Figure 4.  Original ECG traces taken from an euthermic rat at different core temperatures

Figure 5 illustrates the effect of stimulating the peripheral cut end of the right vagus on HR at different core T_b. Electrical stimulation of cardiac vagal efferents evoked voltage-dependent recruitment of nerve fibres to decrease HR at T_b= 37°C and T_b,rew= 37°C with the magnitude of bradycardia proportional to voltage in euthermic and acclimated rats (Figs 6A and B). At T_b= 37°C and T_b,rew= 37°C, 2 V stimulation decreased HR by 95 b.p.m. versus zero voltage (P < 0.01) whereas with 10 V stimulation HR reduced by 180 b.p.m. versus zero voltage (P < 0.001) in both groups of rats. However, at 25°C in both normothermic and acclimated rats, with 2 V there was a slight reduction in HR by 9 b.p.m. versus zero voltage (n.s.) which was significantly much lower (80 b.p.m. versus same voltage, P < 0.001) compared to responses obtained either before or after cooling. At 25°C, a higher voltage was required to evoke a bradycardia when compared to pre or post hypothermic responses, 10 V decreased HR by 110 b.p.m. versus zero voltage (P < 0.001) in both groups of rats (Figs 6A and B).

View larger version (23K): [in this window] [in a new window]   Figure 5.  Original recordings of effects of vagal stimulation on ECG of a rat at different core temperatures Stimulus parameters were 3 Hz, 210 , 0.1 ms pulses for 10 s duration with varying voltage.

View larger version (19K): [in this window] [in a new window]   Figure 6.  Responses on decreases in heart rate to stimulation of right vagal efferents with varying voltages in euthermic rats (A) and acclimated rats (B) at different core temperatures Data points represent means ±S.E.M. from euthermic (n= 6) and acclimated rats (n= 7). Continuous lines indicate responses at 37°C, dotted lines responses at 25°C and dashed lines responses on rewarming to 37°C. *P < 0.01, **P < 0.001versus zero voltage; #P < 0.05, ##P < 0.01versus same voltage at 37°C (ANOVA). Mean HR values at each Tb are mentioned in Table 1.

	Discussion

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During acute hypothermia in conscious animals, the autonomic nervous system elicits a range of thermoregulatory and cardiovascular responses which oppose any decrease in T_b and help to maintain MABP and HR (Hellon, 1983). While HR showed a graded response with fall in T_b, there is substantial compensation in MABP and RSNA during moderate hypothermia (between T_b= 37°C to 31°C) that is impaired on exposure to more severe hypothermia (between T_b= 31°C to 25°C). The magnitude of haemodynamic responses is determined by thermosensitive areas in the preoptic and anterior hypothalamic nuclei of hypothalamus (Morishima & Gale, 1972), and also by the arterial baroreceptor reflex (Kregel et al. 1994) that initiates compensatory cardiovascular adjustments during a thermal challenge (Massett et al. 2000). It may well be that these thermoregulatory and cardiovascular responses may occur even though the present study used anaesthetized rats. However, in the present study T_b was reduced below the normal thermoregulatory range, and since temperature in the brain is decreased it would be expected that conduction and synaptic transmission were also impaired. This is supported by the suppression in the baroreflex-RSNA function curves. In the case of baroreflex-HR curves it is clear that several different effector pathways influence the degree of impairment.

Acute hypothermia modestly shifted the baroreflex curve for RSNA, leftwards and downwards, which was accompanied by a decrease in setpoint pressure and decreased sensitivity of the baroreflex that was restored on rewarming. Small decreases in the maximum and minimum response of the baroreflex-RSNA curve at T_b= 25°C would imply less suppression of the inhibitory effects originating from the baroreceptors. The maximum response of RSNA has been considered to reflect the number and/or synchronicity of bursting population of the sympathetic motoneurones at the lowest baroreceptor activity (Saigusa et al. 1996; Head & Burke, 2000). Since it has been suggested that the tonic drive of the sympathetic nerves is initiated at the rostral ventrolateral medulla (RVLM) (Jordan, 1995), it is possible that hypothermia directly influences the RVLM neurones to decrease the sympathetic tone (level of RSNA) and therefore reset the setpoint to a lower pressure, contributing to modest hypotension associated with hypothermia. This resetting of the reflex where the gain is maximum ensures that the reflex operates effectively as a buffer to fluctuations in blood pressure at that T_b (Head, 1995).

The marked suppression of the baroreflex control of HR at T_b= 25°C was accompanied by attenuation in response range, and maximum and minimum response. This implied that the ability to elicit reflex bradycardia and tachycardia in response to changes in MABP was severely impaired during cooling, and hence the buffering capacity of the baroreceptor-HR reflex was diminished at T_b= 25°C. Marked attenuation of upper and lower plateaus contributing to suppression of the response range could be due to reduced sympathetic and vagal activity (Head, 1995). In addition, the significant decrease in maximal gain, in both groups of animals, reflects a reduction in the sensitivity of the baroreflex during hypothermia. The fact that for the kidney, at least, baroreflex control of sympathetic outflow was only modestly affected by lowering of body temperature, suggests that if a similar situation pertains with the cardiac sympathetic outflow, the control of vagal outflow was more susceptible to body temperature. However, it is known that changes in sympathetic nerve activity to other vascular beds may not necessarily covary with RSNA (Coote, 1988). Many workers, including ourselves, use RSNA as an index of overall sympathetic nerve responses but it does not rule out the possibility that baroreflex-sympathetic curves may be different in sympathetic outflows to other organs, since regional differences between cardiac and renal nerves have already been reported (Matsukawa et al. 1993). For example, during hypothermia, a loss of cardiac-related bursts accompanied by a decrease in RSNA was found in the present study, whereas Kenney et al. (1999) reported an increase in lumbar nerve activity despite a loss of cardiac-related bursts. Interestingly, the dissociation between baroreflex control of HR and peripheral sympathetic nerve activity has been observed in response to other physiologically relevant stimuli such as during exercise, angiotensin infusion, sleep, etc. (Korner, 1979; Abboud & Thames, 1983; Kumagai & Reid, 1994; Massett et al. 2000; Miki et al. 2003). Baroreflex control of HR can be modulated by the activation of thermoreceptors, cardiopulmonary baroreceptors, hypothalamic nuclei and ventilation of the animal (Abboud & Thames, 1983). It has been reported previously that the anterior hypothalamus participates in baroreflex regulation of HR by altering parasympathetic tone but without affecting cardiac nerve activity or RSNA (Miyajima & Buñag, 1985). Recently, other studies have shown similar findings during induced hypothermia, although they used linear regression techniques to estimate overall baroreflex function in terms of calculated slope (Xu et al. 2000; Tanaka et al. 2001).

There was no difference in the pattern of response of baroreflex-RSNA and baroreflex-HR curves between euthermic and acclimated rats at each chosen T_b. However, the difference between the two groups of animals lay primarily in the levels of response range, maximum response, minimum response and maximal gain. These small differences could be due to altered patterning of RSNA (Sabharwal et al. 2002), altered basal levels of HR and RSNA during hypothermia, increased sympathetic innervation of brown adipose tissue (Himms-Hagen, 1990), enhanced metabolic activity (Jansky, 1979) and elevated catecholamine levels (Young et al. 1982) such that the acclimated rats prefer cooler ambient temperatures when placed on thermal gradients (Owen et al. 1991). Despite exposure to severe hypothermic conditions and a near total suppression of baroreflex-HR curves it was interesting that there was a complete recovery on rewarming, suggestive of transient suppression of cardiac neural control and a rapid restoration of function. Even though anaesthesia is known to have a depressive effect on the autonomic control of HR (Watkins & Maxiner, 1991) and sympathetic output (Suzuki et al. 1993), it is likely that during induced hypothermia, as in surgery, the magnitude of response may be different compared to the present study, but nonetheless the pattern would be similar in terms of baroreflex control of HR and RSNA.

Another objective of the study was to determine if some of the changes in baroreflex control of HR could be attributed to changes taking place in the heart during acute hypothermia. Changes in the ECG traces during acute hypothermia are reflective of a direct effect of T_b as well as autonomic influence on the heart. Prolongation of the cardiac cycle showed no evidence of a differential thermal sensitivity in the component intervals. The delayed conductance during hypothermia is attributed to the inhibition of the Na⁺–K⁺ pump (Schneider & Gillis, 1966), while a delay in atrioventricular conduction causes lengthening of the PR interval (Bashour et al. 1989). Widening of QRS complex was due to the reduced rate of depolarization (Bjornstad et al. 1991). Furthermore, hypothermia delays repolarization and inactivates voltage-sensitive inward Ca²⁺ to prolong the ST interval (Gussak et al. 1995) and QT intervals (Bjornstad et al. 1991), respectively. That the relative changes were similar between euthermic and acclimated rats shows that the effect of T_b on rodent electrocardiogram is non-adaptive.

To further evaluate the impact of hypothermia on the peripheral component we electrically stimulated the peripheral nerve endings using spatial summation to recruit more nerve fibres dependent on voltage (Eccles, 1957). At 37°C and rewarming to 37°C, electrical stimulation of right vagal efferents evoked a bradycardia, with falls in HR being proportional to stimulation parameters. Interestingly, at T_b= 25°C lower stimulation parameters failed to evoke a similar response, whereas previously it had been thought that acute hypothermia augments vagal tone in both humans and animals. This could be due to either an impaired release of neurotransmitter from nerve endings or more efferent fibres needing to be recruited in order to evoke a similar response, indicative of peripheral influences on suppression of baroreflex control of HR.

Moreover, it has been observed that there is a retardation of energy-dependent metabolic processes that affects the heart and central nervous system before other organs (Reuler, 1978), a direct effect of cooling on the sinus nodes of atrium which slows the heart (Schneider & Gillis, 1966), absence of HR responses to pharmacological blood pressure changes below 34°C in anaesthetized rabbits (Xu et al. 2000), occurrence of marked bradycardia even in bilaterally vagotomised animals (Cookson & DiPalma, 1955) and increased cardiac cholinergic receptor affinity to agonists in rats (Phan et al. 1980). These further support our hypothesis that changes taking place in the heart, due to hypothermia, can have an impact on baroreflex control of HR. It is worthy of note that in the present study, the lowering of body temperature had similar actions in depressing vagally mediated responses in the heart of both the euthermic and cold acclimated rats.

This study has shown that during acute hypothermia there is a decrease in maximal gain and setpoint pressure of baroreflex control of RSNA slightly shifting the curve leftwards and downwards. This shift could contribute to modest hypotension during hypothermia and may primarily reflect an effect of T_b on central drive. Acute hypothermia severely impairs baroreflex control of HR with marked reductions in maximal gain and response range. The baroreflex curves for RSNA and HR on rewarming were no different from before cooling, indicative of a transient suppression of cardiovascular controls during acute hypothermia. Acclimation does not alter the pattern of response, implying baroreflex control the mechanism is non-adaptive following chronic cold exposure. The effect of stimulation of cardiac vagal efferents on HR was partially reduced and there was a significant lengthening of each interval in ECG at 25°C. This increases the conduction time and thereby contributes to the prolongation of R–R interval. Therefore, impairment of both central and peripheral components of the baroreflex contributes towards marked attenuation of the baroreflex-HR curve.

	References

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Abboud FM & Thames MD (1983). Interaction of cardiovascular reflexes in circulatory control. Handbook of Physiology, section 2, The Cardiovascular System, vol. III, Peripheral Circulation and Organ Blood Flow, ed. Shepherd JT & Abboud FM, part 2, chap. 19, pp. 675–753. American Physiological Society, Bethesda, MD, USA.

Angell James JE (1971). The responses to aortic arch and right subclavian baroreceptors to changes of non-pulsatile pressure and their modification by hypothermia. J Physiol 14, 201–223.

Bashour TT, Gualberto A & Ryan C (1989). Atrioventricular block in accidental hypothermia – a case report. Angiology 40, 63–66.[Medline]

Beinfield WH & Lehr D (1968). QRS-T variations in the rat electrocardiogram. Am J Physiol 214, 197–204.[Free Full Text]

Bjornstad H, Tande PM & Refsum H (1991). Cardiac electrophysiology during hypothermia. Implications for medical treatment. Arctic Med Res 50 (Suppl. 6), 71–75.[Medline]

Broman M, Källskog Ö, Kopp UC & Wolgast M (1998). Influence of the sympathetic nervous system on renal function during hypothermia. Acta Physiol Scand 163, 241–249.[CrossRef][Medline]

Cookson BA & DiPalma JR (1955). Severe bradycardia of profound hypothermia in the dog. Am J Physiol 182, 447–453.[Free Full Text]

Coote JH (1988). The organisation of cardiovascular neurons in the spinal cord. Rev Physiol Biochem Pharmacol 110, 147–285.[Medline]

Deveci D, Stone PC & Egginton S (2001). Differential effect of cold acclimation on blood composition in rats and hamsters. J Comp Physiol [B] 171, 135–143.[CrossRef][Medline]

Eccles JC (1957). The Physiology of Nerve Cells, pp. 30–96. The Johns Hopkins University Press, Baltimore, MD, USA.

Granberg PO (1991). Human physiology under cold exposure. Arctic Med Res 50 (Suppl. 6), 23–27.[Medline]

Gussak I, Bjerregaard P, Egan TM & Chaitman BR (1995). ECG phenomenon called the J wave. History, pathophysiology, and clinical significance. J Electrocardiol 28, 49–58.[CrossRef][Medline]

Gussak I, Chaitman BR, Kopecky SL & Nerbonne JM (2000). Rapid ventricular repolarization in rodents: electrocardiographic manifestations, molecular mechanisms, and clinical insights. J Electrocardiol 33, 159–170.[Medline]

Head GA (1995). Baroreflexes and cardiovascular regulation in hypertension. J Cardiovasc Pharmacol 26 (Suppl. 2), S7–S16.[Medline]

Head GA & Burke SL (2000). Comparison of renal sympathetic baroreflex effects of rilmenidine and alpha-methylnoradrenaline in the ventrolateral medulla of the rabbit. J Hypertens 18, 1263–1276.[CrossRef][Medline]

Heistad DD, Abboud FM, Mark AL & Schmid PG (1973). Interaction of thermal and baroreceptor reflexes in man. J Appl Physiol 35, 581–586.[Free Full Text]

Hellon R (1983). Thermoreceptors. Handbook of Physiology, section 2, The Cardiovascular System, vol. III, Peripheral Circulation and Organ Blood Flow, ed. Shepherd JT & Abboud FM, part 2, chap. 18, pp. 659–673. American Physiological Society, Bethesda, MD, USA.

Himms-Hagen J (1990). Brown adipose tissue thermogenesis: Role in thermoregulation, energy regulation and obesity. In Thermoregulation: Physiology and Biochemistry, ed. Schonbaum E & Lomax P. Pergamon Press, Toronto.

Jansky L (1979). Heat production. In Body Temperature, Regulation, Drug Effects, and Therapeutic Implications, ed. Lomax P & Schonbaum E, pp. 89–117. Marcel Dekker Inc, New York.

Jordan D (1995). CNS integration of cardiovascular regulation. In Cardiovascular Regulation, ed. Jordan D & Marshall J, pp. 1–14. Portland Press, London.

Kaul SU, Armstrong DJ & Millar RA (1973). Preganglionic sympathetic activity and baroreceptor responses during hypothermia. Br J Anaesth 54, 433–439.

Kenney MJ, Claassen DE, Fels RJ & Saindon CS (1999). Cold stress alters characteristics of sympathetic nerve discharge bursts. J Appl Physiol 87, 732–742.[Abstract/Free Full Text]

Kent BB, Drane JW, Blumenstein B & Manning JW (1972). A mathematical model to assess changes in the baroreceptor reflex. Cardiology 57, 295–310.[Medline]

Kobayashi M, Godin D & Nadeau R (1985). Sinus node responses to perfusion pressure changes, ischaemia and hypothermia in the isolated blood-perfused dog atrium. Cardiovasc Res 19, 20–26.[Medline]

Korner PI (1979). Central nervous control of autonomic cardiovascular function. Handbook of Physiology, section 2. The Cardiovascular System, vol. I, The Heart, ed. Page E, Fozzard HA & Solaro RJ, pp. 691–740. American Physiological Society, Bethesda, MD, USA.

Kregel KC, Stauss H & Unger T (1994). Modulation of autonomic nervous system adjustments to heat stress by central ANG II receptor antagonism. Am J Physiol 266, R1985–R1991.[Medline]

Kumagai K & Reid IA (1994). Angiotensin II exerts differential actions on renal nerve activity and heart rate. Hypertension 24, 451–456.[Abstract/Free Full Text]

Massett MP, Lewis SJ, Stauss HM & Kregel KC (2000). Vascular reactivity and baroreflex function during hyperthermia in conscious rats. Am J Physiol Regul Integr Comp Physiol 279, R1282–R1289.[Abstract/Free Full Text]

Matsukawa K, Ninomiya I & Nishiura N (1993). Effects of anesthesia on cardiac and renal sympathetic nerve activities and plasma catecholamines. Am J Physiol 265, R792–R797.[Medline]

Miki K, Yoshimoto M & Tanimizu M (2003). Acute shifts of baroreflex control of renal sympathetic nerve activity induced by treadmill exercise in rats. J Physiol 548, 313–322.[Abstract/Free Full Text]

Miyajima E & Buñag RD (1985). Anterior hypothalamic lesions impair reflex bradycardia selectively in rats. Am J Physiol 248, H937–H944.[Medline]

Morishima MS & Gale CC (1972). Relationship of blood pressure and heart rate to body temperature in baboons. Am J Physiol 223, 387–395.[Free Full Text]

Owen TL, Spencer RL & Duckles SP (1991). Effect of age on cold acclimation in rats: metabolic and behavioral responses. Am J Physiol 260, R284–R289.[Medline]

Papanek PE, Wood CE & Fregly MJ (1991). Role of the sympathetic nervous system in cold-induced hypertension in rats. J Appl Physiol 71, 300–306.[Abstract/Free Full Text]

Phan NT, Wei JW & Sulakhe PV (1980). Temperature dependent alterations in the cardiac muscarinic receptor conformation. Eur J Pharmacol 67, 497–498.[CrossRef][Medline]

Reuler JB (1978). Hypothermia: pathophysiology, clinical settings, and management. Ann Intern Med 89, 519–527.[Medline]

Sabharwal R, Johns EJ & Egginton S (2002). Effects of hypothermia on renal function in normothermic and cold-acclimated anaesthetised rats. J Physiol P, 88P.

Saigusa T, Iriki M & Arita J (1996). Brain angiotensin II tonically modulates sympathetic baroreflex in rabbit ventrolateral medulla. Am J Physiol 271, H1015–H1021.[Medline]

Schneider FH & Gillis CN (1966). Hypothermic potentiation of chronotropic response of isolated atria to sympathetic nerve stimulation. Am J Physiol 211, 890–896.[Free Full Text]

Suzuki S, Ando S, Imaizumi T & Takeshita A (1993). Effects of anethesia on sympathetic nerve rhythm: power spectral analysis. J Auton Nerv Syst 43, 51–58.[Medline]

Tanaka M, Nagasaki G & Nishikawa T (2001). Moderate hypothermia depresses arterial baroreflex control of heart rate during, and delays its recovery after, general anesthesia in humans. Anesthesiology 95, 51–55.[CrossRef][Medline]

Watkins L & Maxiner W (1991). The effect of pentobarbital anesthesia on the autonomic nervous system control of heart rate during baroreceptor activation. J Auton Nerv Syst 36, 107–114.[CrossRef][Medline]

Xu H, Aibiki M, Seki K, Ogura S, Yokono S & Ogli K (2000). Effects of induced hypothermia on renal sympathetic nerve activity and baroreceptor reflex in urethane-anesthetized rabbits. Crit Care Med 28, 3854–3860.[CrossRef][Medline]

Young JB, Saville E, Rothwell NJ, Stock MJ & Landsberg L (1982). Effect of diet and cold exposure on norepinephrine turnover in brown adipose tissue of the rat. J Clin Invest 69, 1061–1071.[Medline]

Zheng F, Kidd C & Bowser-Riley F (1996). Effects of moderate hypothermia on baroreflex and pulmonary chemoreflex heart rate response in decerebrate ferrets. Exp Physiol 81, 409–420.[Abstract]

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