J. Physiol. (1976), 257, pp. 767-777 With 5 text-ftgurea Printed in Great Britain
767
THERMOREGULATION IN RABBITS DURING FEVER
BY W. I. CRANSTON, G. W. DUFF, R. F. HELLON AND D. MITCHELL* From Department of Medicine, St Thoms's Hospital Medical School, London, SEl 7EH and National Institute for Medical Research, London NW7 1AA
(Received 7 October 1975) SUMMARY
1. We have studied the effect of fever on the efficacy of the thermoregulatory control system in conscious rabbits. 2. The control system was challenged by a series of systemic thermal loads produced by the intravenous infusion of hot or cold isotonic solutions. The time integral of the consequent upward or downward displacement of brain temperature was used as an index of the response of the control system. Steady-state fever was induced by intravenous infusion of plasma containing leucocyte pyrogen. 3. With cold loads there was a linear relation between load and response. The regression coefficients were not significantly changed by fever in any of the six rabbits. With hot loads given to afebrile rabbits the regression of response on load was generally not statistically significant, but the responses were not demonstrably greater in the febrile state. 4. We were not able to demonstrate impairment in the capacity of the febrile animal to compensate for systemic thermal loads. INTRODUCTION
Whether fever affects the integrity of thermoregulatory mechanisms remains controversial. The controversy appears to depend largely upon a dichotomy in the nature of the experiments (Mitchell, Snellen & Atkins, 1970). On the one hand, experiments involving heat or cold stress applied systemically to animals have indicated that the sensitivity of the thermoregulatory system is unimpaired during fever (Haan & Albers, 1960; Cooper, Cranston & Snell, 1964; Macpherson, 1969; Cabanac & Massonnet, 1974; Al-Hachim & Frens, 1975). In the other type of experiment the thermosensitivity of hypothalamic neurones has been measured. Such * Present address: Department of Physiology, University of the Witwatersrand Medical School, Johannesburg, South Africa.
768 W. I. CRANSTON AND OTHERS experiments have all shown that pyrogens modify the response of these neurones to local temperature change (Wit & Wang, 1968; Cabanac, Stolwijk & Hardy, 1968; Eisenman, 1969; Schoener & Wang, 1975). If the thermosensitivity of the neurones contributes significantly to thermoregulation during fever, then the results of the two types of experiment are in conflict. A third general type of experiment has been reported, in which local hypothalamic temperature has been changed, and some effector responses measured. The results of these experiments do not clarify the issue. Some have led to the conclusion that the efficiency of the system is unimpaired by fever (Andersen, Hammel & Hardy, 1961; Sharp & Hammel, 1972) but others have led to the opposite conclusion (Grant & Adler, 1967; Eisenman, 1974). Mitchell et al. (1970) expressed reservations about some of the experiments involving systemic heat or cold stress; in particular, steady-state conditions were not always used and the thermal stress was not always measured adequately. In an attempt to surmount these limitations we employed hot and cold loads of measured magnitude during steady states before and after the induction of fever. We used the integrated deviation of core temperature as an indication of the efficiency of thermoregulatory mechanisms: the larger the integrated deviation of core temperature for a given thermal load the less efficient the control system. METHODS
Observations were made on twelve conscious rabbits, of Chinchilla or New Zealand White breeds; their weights ranged from 2-0 to 3-5 kg. At least a week before the experiment, a guide plate (Monnier & Gangloff, 1961) was affixed to the skull under general anaesthesia with alphaxolone and alphadolone (Althesin, Glaxo). The plate allowed introduction of a small thermistor above the pre-optic area, for measurement of brain temperature. All experiments were performed in a temperature controlled room at 22 + 1° C. The rabbits were restrained in conventional stocks. Rectal temperature was measured with a thermistor (Yellow Springs Model 403) introduced at least 80 mm. Brain temperature was measured with a miniature thermistor (STC model U23US) mounted, on the end of a fine needle (0-5 mm o.d.) and positioned using the head plate. The resistance of each of the thermistors was measured by a Wheatstone bridge circuit and recorded on a potentiometric recorder (Leeds and Northrup Speedomax W) at 18 s intervals. Thermal loads were applied by infusion of warm or cold compound sodium lactate injection (B.P. containing: Na+, 131; K+, 5; Ca2+, 2; Cl-, 111; lactate, 29 m mol 1.-i) into the marginal vein of an ear. The method employed was almost identical to that described by Cranston & Rosendorff (1967). In our experiments, cold loads were given by infusing fluids at room temperature and warm loads by passing the infusate through a copper coil immersed in a thermostatically controlled bath. The bath temperature was adjusted so that the temperature of the infused fluid was between 44 and 460 C. The mean temperature of each infusion was
THERMOREGULATION DURING FEVER
769
estimated by performing a duplicate infusion into a calorimeter (Cranston & Rosendorff, 1967). Infused volumes ranged between 20 and 50 ml. and the order of administration was randomized. The total fluid load administered to each animal in the course of one experiment was approximately 400 ml. Thermal loads were calculated as previously described by Cranston & Rosendorff (1967), namely by multiplying the infusion volume by the difference between the temperature of the infusion and of the animal's brain or rectum; the loads were expressed as kJ kg-'.
A
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380
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B
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'
38-0 _
~~~~~~~Anaesthetized
/
37-8 0
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15
20
Fig. 1. Time course of change in brain temperature following the rapid intravenous infusion of 40 ml isotonic solution at 460 C. The moment of infusion is indicated by the arrows. A, from a conscious rabbit; B, for comparison, from the same rabbit anaesthetized with pentobarbitone sodium. The shaded part of (A) indicates the area used as a measure of the response to the hot load. The integrated deviations of brain and rectal temperatures ('C min) were determined by measurement of the area enclosed by the deflexion of measured temperature induced by the load, and an interpolated straight base line (Cranston & Rosendorff, 1967). This area will be called the 'response' of brain (or rectal) temperature. An example of this measurement is shown in Fig. 1 which also shows the temperature change following the same load in the same animal anaesthetized with pentobarbitone. In the conscious, but not the anaesthetized state, brain temperature returned to its previous level within 10 min. In the conscious animal the return of temperature to the initial level was determined by thermoregulatory mechanisms which were inactive during anaesthesia. In these experiments the whole area was measured since control mechanisms were already acting at the time of peak temperature displacement: in the same rabbit receiving the same hot load the peak temperature displacement during anaesthesia was twice as great as that when conscious (Fig. 1). The relation between the size of thermal load and magnitude of response is an index of the efficiency of the thermoregulatory
W. I. CRANSTON AND OTHERS
770
system: the greater the response to a particular thermal load the les efficient the system. In each experiment we measured the responses to five hot or six cold loads in the afebrile state. Fever was then induced by a priming injection into an ear vein of 3 0 ml plasma containing leucocyte pyrogen, followed by a sustaining infusion of the same material at a rate of 0-02 ml min-. After a stable febrile state had been attained, a further series of five hot or six cold loads was infused. For each set of hot or cold infusions, the relation between the loads and the responses was determined by least squares linear regression. Six animals received hot loads and six others cold. It was not possible to carry out hot and cold infusions in the same animals because of the difficulty of preserving suitable intravenous injection sites. Leucocyte pyrogen was prepared by incubating citrated rabbit whole blood with Proteus endotoxin (E pyrogen, Organon Laboratories) at a concentration of 3 ng ml-' blood for 18 h at 370 C. The plasma was separated by centrifuging at 2000 g for 30 min and stored at 40 C until used. I I 25 2-5
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Fig. 2. Examples from two rabbits of the relations between brain temperature response and cold load (A) or hot load (B). Open circles, values obtained before fever; filled circles, values obtained during fever. RESULTS
The average brain temperatures of each animal while afebrile and while febrile are shown in Tables 1 and 2. The average rise in brain temperature caused by the pyrogen was 1.20 C. Fig. 2 shows the responses of brain temperature to cold loads in the febrile and afebrile states in one animal, and similar responses to hot loads in another animal. It is clear that the brain temperature response
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773 THERMOREGULATION DURING FEVER is related to the thermal load, and the relation is not obviously different in the febrile and afebrile states. Tables 1 and 2 also show the correlation and regression coefficients for every experiment. Correlation coefficients, though usually high, frequently fail to attain the generally accepted levels of significance, especially in the case of hot loads, where only ten infusions were given to each animal. 0
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Fig. 3. Combined brain temperature responses to cold loads from six rabbits. Open circles and continuous regression line, before fever; filled circles and interrupted regression line, during fever. Regression details in Table 1.
In the case of cold loads, the differences between the regression coefficients were tested by analysis of variance; the F values are shown in Table 1. In no case was there any significant difference between regression coefficients for the febrile and afebrile states.
W. I. CRANSTON AND OTHERS Figs. 3 and 4 show the pooled results for all animals with cold and hot infusions respectively. The coefficients calculated from the combined results are also shown in Tables 1 and 2. For cold loads the slopes of the regression equations are not significantly different in the febrile and 774
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Fig. 4. Combined brain temperature responses to hot loads from six rabbits. Open circles, before fever; filled circles, during fever. Continuous line shows regression for afebrile rabbits (details in Table 2); interrupted lines indicate 95 % prediction limits.
afebrile states. For hot loads in febrile rabbits, the correlation coefficient is very low (r = +0X071, P > 0.1). This makes it difficult to interpret the comparison of the regression coefficients. Fig. 4 shows the individual points for the hot loads in febrile and afebrile rabbits, and the regression line and 95 % confidence limits for afebrile rabbits. It is manifest that the responses of febrile rabbits are not greater than those of the afebrile rabbits. Of thirty responses to hot loads in febrile rabbits, twenty-seven fall within the 95 % confidence limits established in experiments upon afebrile rabbits. All the preceding results refer to brain temperature responses. Rectal temperatures were measured in all animals, and the same calculation yielded results similar to those obtained from brain temperature measurements. In general, changes of rectal temperature were smaller than those of brain temperature. Fig. 5 shows the relation between responses of brain and rectal temperatures in all animals. The relation appears to be
775 THERMOREGULATION DURING FEVER linear and is independent of the nature of the thermal load and of the presence offever. The correlation coefficient is highly significant (r = + 0881, P < 0.001). Nothing further would be added by analyzing the rectal temperature responses in detail.
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Fig. 5. Relation between brain temperature response and rectal temperature response to the same cold and hot loads for twelve rabbits before fever (open circles) and during fever filledd circles). Continuous line shows the regression fitted to all data points (y = 049Ix-OeI6; r = +0*881; S.E. Of estimate = 0.42). Interrupted lines indicate 95%°/ prediction limits. DISCUSSION
The slope of a regression line relating the displacement of body temperature to the size of a thermal load delivered intravenously is a measure of the efficiency of the temperature control system of an intact animal; the steeper the slope the less efficient the control. We measured deviations of both brain and rectal temperatures in rabbits subjected to graded hot and cold loads. The rectal temperature deviation bore a linear relation to the brain temperature deviation, so only brain temperature deviations were analysed in detail. In afebrile rabbits the slope of the linear regression of brain temperature deviation on thermal loads was about 2.50 C mmn kg kJ'l for both hot and cold loads. In the case of cold loads it was possible to demonstrate that the slope was unaltered during fever; in the case of hot loads the temperature displacements in the febrile state were certainly no greater than those in the afebrile
776 W. I. CRANSTON AND OTHERS state. Thus the ability of the control system to minimize displacements
of body temperature was not reduced during fever. The greater variability of the results obtained from experiments with hot loads arises partly from an unavoidable limitation of the size of the loads. To prevent discomfort in the conscious animals, we limited the temperatures of the infusions to 460 C, i.e. 670 C above the prevailing deep body temperature. In contrast the temperatures of the cold infusions were at least 150 C below body temperature. The problem arising from this limitation of the hot loads was exacerbated in the febrile state by the greater variability of body temperature (see s.E. of mean of brain temperatures in Tables 1 and 2) and by the fever.
Many previous experiments involving several species have led to the conclusion that the ability of intact animals to compensate for systemic thermal loads is apparently unaffected by fever (Al-Hachim & Frens, 1975; Cabanac & Massonnet, 1974; Cooper et al. 1964; Macpherson, 1969; Haan & Albers, 1960). Our experiments, in which well-defined hot and cold loads and steady-state fevers were employed, lead to the same conclusion. In contrast, all experiments in which the thermosensitivity of single hypothalamic neurones has been determined have indicated that this thermosensitivity is affected by fever (Wit & Wang, 1968; Cabanac et al. 1968; Eisenman, 1969; Schoener & Wang, 1975). It now seems unlikely that the conflict between the experiments on hypothalamic neuronal thermosensitivity and those employing thermal loads arises entirely from technical inadequacies. One explanation, which cannot be ruled out, is that hypothalamic thermosensitive neurones are not crucially concerned in the mechanisms whereby animals compensate for systemic thermal loads during fever. We thank Margaret Tester for help with the experiments, and G. J. S. Ross of the Department of Statistics, Rothamsted Experimental Station, for statistical advice.
REFERENCES AL-HACHIM, G. M. & FRENS, J. (1975). Analysis of shivering in non-peripheral cooling during pyrogen fever. Int. J. Bioclim. Biomet. 19, 53-55. ANDERSEN, H. T., HAMMEL, H. T. & HARDY, J. D. (1961). Modifications of the febrile response to pyrogen by hypothalamic heating and cooling in the unanaesthetized dog. Acta physiol. sand. 53, 247-254. CABANAC, M. & MASSONNET, B. (1974). Temperature regulation during fever: change of set point or change of gain? A tentative answer from a behavioural study in man. J. Physiol. 238, 561-568. CABANAC, M., STOLWIJK, J. A. J. & HARDY, J. D. (1968). Effect of temperature and pyrogens on single-unit activity in the rabbit's brain stem. J. apple. Physiol. 24, 645-652. COOPER, K. E., CRANSTON, W. I. & SNELL, E. S. (1964). Temperature regulation during fever in man. ClGn. Sci. 27, 345-356. CRANSTON, W. I. & ROSENDORFF, C. (1967). Central temperature regulation in the conscious rabbit after monoamine oxidase inhibition. J. Physiol. 193, 359- 373.
THERMOREGULATION DURING FEVER
777
EISENMAN, J. S. (1969). Pyrogen-induced changes in the thermosensitivity of septal and preoptic neurons. Am. J. Physiol. 216, 330-334. EISENMAN, J. S. (1974). Depression of preoptic thermosensitivity by bacterial pyrogen in rabbits. Am. J. Phy8eiol. 227, 1067-1073. GRANT, R. & ADLER, R. D. (1967). Responses to leucocytic pyrogen (LP) in hyperthermic and hypothalamus-heated rabbits: a challenge to 'reset' hypothesis of fever. Phypiologi8t, Wa8h. 10, 186. HAAN, J. & ALBERs, C. (1960). Uber die Auslosung Thermoregulatorischer Reaktionen beim Hund unter Pyrogen-Einwirkung. Pflugerm Arch. gee. Phy-iol. 271, 537-547. MACPHERSON, R. K. (1969). The effect of fever on temperature regulation in man. Clin. Sci. 18, 281-287. MITCHELL, D., SNETIT N, J. W. & ATKINs, A. R. (1970). Thermoregulation during fever: changes of set-point or change of gain. Pflugger Arch. gee. Phyuiol. 321, 292-302. MONNIER, M. & GANGLOFF, H. (1961). Atlas for Stereotaxic Brain Research on -the Conscious Rabbit. Amsterdam: Elsevier Publishing. SCHOLNER, E. P. & WANG, S. C. (1975). Leukocytic pyrogen and sodium acetylsalicylate on hypothalamic neurons in the cat. Am. J. Physiol. 229, 185-190. SHARP, F. R. & HAMMEL, H. T. (1972). Effect of fever on salivation response in the resting and exercising dog. Am. J. Physiol. 223, 77-82. SNEDECOR, G. W. & COCIRAN, W. G. (1967). Statictical Methods. Ames, Iowa: Iowa State University Press. WIT, A. & WANG, S. C. (1968). Temperature-sensitive neurons in the preopticI anterior hypothalamic region: actions of pyrogen and acetylsalicylate. Am. J. Physiol. 215, 1160-1169.
767
THERMOREGULATION IN RABBITS DURING FEVER
BY W. I. CRANSTON, G. W. DUFF, R. F. HELLON AND D. MITCHELL* From Department of Medicine, St Thoms's Hospital Medical School, London, SEl 7EH and National Institute for Medical Research, London NW7 1AA
(Received 7 October 1975) SUMMARY
1. We have studied the effect of fever on the efficacy of the thermoregulatory control system in conscious rabbits. 2. The control system was challenged by a series of systemic thermal loads produced by the intravenous infusion of hot or cold isotonic solutions. The time integral of the consequent upward or downward displacement of brain temperature was used as an index of the response of the control system. Steady-state fever was induced by intravenous infusion of plasma containing leucocyte pyrogen. 3. With cold loads there was a linear relation between load and response. The regression coefficients were not significantly changed by fever in any of the six rabbits. With hot loads given to afebrile rabbits the regression of response on load was generally not statistically significant, but the responses were not demonstrably greater in the febrile state. 4. We were not able to demonstrate impairment in the capacity of the febrile animal to compensate for systemic thermal loads. INTRODUCTION
Whether fever affects the integrity of thermoregulatory mechanisms remains controversial. The controversy appears to depend largely upon a dichotomy in the nature of the experiments (Mitchell, Snellen & Atkins, 1970). On the one hand, experiments involving heat or cold stress applied systemically to animals have indicated that the sensitivity of the thermoregulatory system is unimpaired during fever (Haan & Albers, 1960; Cooper, Cranston & Snell, 1964; Macpherson, 1969; Cabanac & Massonnet, 1974; Al-Hachim & Frens, 1975). In the other type of experiment the thermosensitivity of hypothalamic neurones has been measured. Such * Present address: Department of Physiology, University of the Witwatersrand Medical School, Johannesburg, South Africa.
768 W. I. CRANSTON AND OTHERS experiments have all shown that pyrogens modify the response of these neurones to local temperature change (Wit & Wang, 1968; Cabanac, Stolwijk & Hardy, 1968; Eisenman, 1969; Schoener & Wang, 1975). If the thermosensitivity of the neurones contributes significantly to thermoregulation during fever, then the results of the two types of experiment are in conflict. A third general type of experiment has been reported, in which local hypothalamic temperature has been changed, and some effector responses measured. The results of these experiments do not clarify the issue. Some have led to the conclusion that the efficiency of the system is unimpaired by fever (Andersen, Hammel & Hardy, 1961; Sharp & Hammel, 1972) but others have led to the opposite conclusion (Grant & Adler, 1967; Eisenman, 1974). Mitchell et al. (1970) expressed reservations about some of the experiments involving systemic heat or cold stress; in particular, steady-state conditions were not always used and the thermal stress was not always measured adequately. In an attempt to surmount these limitations we employed hot and cold loads of measured magnitude during steady states before and after the induction of fever. We used the integrated deviation of core temperature as an indication of the efficiency of thermoregulatory mechanisms: the larger the integrated deviation of core temperature for a given thermal load the less efficient the control system. METHODS
Observations were made on twelve conscious rabbits, of Chinchilla or New Zealand White breeds; their weights ranged from 2-0 to 3-5 kg. At least a week before the experiment, a guide plate (Monnier & Gangloff, 1961) was affixed to the skull under general anaesthesia with alphaxolone and alphadolone (Althesin, Glaxo). The plate allowed introduction of a small thermistor above the pre-optic area, for measurement of brain temperature. All experiments were performed in a temperature controlled room at 22 + 1° C. The rabbits were restrained in conventional stocks. Rectal temperature was measured with a thermistor (Yellow Springs Model 403) introduced at least 80 mm. Brain temperature was measured with a miniature thermistor (STC model U23US) mounted, on the end of a fine needle (0-5 mm o.d.) and positioned using the head plate. The resistance of each of the thermistors was measured by a Wheatstone bridge circuit and recorded on a potentiometric recorder (Leeds and Northrup Speedomax W) at 18 s intervals. Thermal loads were applied by infusion of warm or cold compound sodium lactate injection (B.P. containing: Na+, 131; K+, 5; Ca2+, 2; Cl-, 111; lactate, 29 m mol 1.-i) into the marginal vein of an ear. The method employed was almost identical to that described by Cranston & Rosendorff (1967). In our experiments, cold loads were given by infusing fluids at room temperature and warm loads by passing the infusate through a copper coil immersed in a thermostatically controlled bath. The bath temperature was adjusted so that the temperature of the infused fluid was between 44 and 460 C. The mean temperature of each infusion was
THERMOREGULATION DURING FEVER
769
estimated by performing a duplicate infusion into a calorimeter (Cranston & Rosendorff, 1967). Infused volumes ranged between 20 and 50 ml. and the order of administration was randomized. The total fluid load administered to each animal in the course of one experiment was approximately 400 ml. Thermal loads were calculated as previously described by Cranston & Rosendorff (1967), namely by multiplying the infusion volume by the difference between the temperature of the infusion and of the animal's brain or rectum; the loads were expressed as kJ kg-'.
A
|.1
382
e
Unanaesthetized
380
E38-2
B
4.'
'
38-0 _
~~~~~~~Anaesthetized
/
37-8 0
5
10 Time (min)
15
20
Fig. 1. Time course of change in brain temperature following the rapid intravenous infusion of 40 ml isotonic solution at 460 C. The moment of infusion is indicated by the arrows. A, from a conscious rabbit; B, for comparison, from the same rabbit anaesthetized with pentobarbitone sodium. The shaded part of (A) indicates the area used as a measure of the response to the hot load. The integrated deviations of brain and rectal temperatures ('C min) were determined by measurement of the area enclosed by the deflexion of measured temperature induced by the load, and an interpolated straight base line (Cranston & Rosendorff, 1967). This area will be called the 'response' of brain (or rectal) temperature. An example of this measurement is shown in Fig. 1 which also shows the temperature change following the same load in the same animal anaesthetized with pentobarbitone. In the conscious, but not the anaesthetized state, brain temperature returned to its previous level within 10 min. In the conscious animal the return of temperature to the initial level was determined by thermoregulatory mechanisms which were inactive during anaesthesia. In these experiments the whole area was measured since control mechanisms were already acting at the time of peak temperature displacement: in the same rabbit receiving the same hot load the peak temperature displacement during anaesthesia was twice as great as that when conscious (Fig. 1). The relation between the size of thermal load and magnitude of response is an index of the efficiency of the thermoregulatory
W. I. CRANSTON AND OTHERS
770
system: the greater the response to a particular thermal load the les efficient the system. In each experiment we measured the responses to five hot or six cold loads in the afebrile state. Fever was then induced by a priming injection into an ear vein of 3 0 ml plasma containing leucocyte pyrogen, followed by a sustaining infusion of the same material at a rate of 0-02 ml min-. After a stable febrile state had been attained, a further series of five hot or six cold loads was infused. For each set of hot or cold infusions, the relation between the loads and the responses was determined by least squares linear regression. Six animals received hot loads and six others cold. It was not possible to carry out hot and cold infusions in the same animals because of the difficulty of preserving suitable intravenous injection sites. Leucocyte pyrogen was prepared by incubating citrated rabbit whole blood with Proteus endotoxin (E pyrogen, Organon Laboratories) at a concentration of 3 ng ml-' blood for 18 h at 370 C. The plasma was separated by centrifuging at 2000 g for 30 min and stored at 40 C until used. I I 25 2-5
0
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A
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C~~~~
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W
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15
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02
s~
f
04
0
06
Fig. 2. Examples from two rabbits of the relations between brain temperature response and cold load (A) or hot load (B). Open circles, values obtained before fever; filled circles, values obtained during fever. RESULTS
The average brain temperatures of each animal while afebrile and while febrile are shown in Tables 1 and 2. The average rise in brain temperature caused by the pyrogen was 1.20 C. Fig. 2 shows the responses of brain temperature to cold loads in the febrile and afebrile states in one animal, and similar responses to hot loads in another animal. It is clear that the brain temperature response
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773 THERMOREGULATION DURING FEVER is related to the thermal load, and the relation is not obviously different in the febrile and afebrile states. Tables 1 and 2 also show the correlation and regression coefficients for every experiment. Correlation coefficients, though usually high, frequently fail to attain the generally accepted levels of significance, especially in the case of hot loads, where only ten infusions were given to each animal. 0
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Fig. 3. Combined brain temperature responses to cold loads from six rabbits. Open circles and continuous regression line, before fever; filled circles and interrupted regression line, during fever. Regression details in Table 1.
In the case of cold loads, the differences between the regression coefficients were tested by analysis of variance; the F values are shown in Table 1. In no case was there any significant difference between regression coefficients for the febrile and afebrile states.
W. I. CRANSTON AND OTHERS Figs. 3 and 4 show the pooled results for all animals with cold and hot infusions respectively. The coefficients calculated from the combined results are also shown in Tables 1 and 2. For cold loads the slopes of the regression equations are not significantly different in the febrile and 774
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afebrile states. For hot loads in febrile rabbits, the correlation coefficient is very low (r = +0X071, P > 0.1). This makes it difficult to interpret the comparison of the regression coefficients. Fig. 4 shows the individual points for the hot loads in febrile and afebrile rabbits, and the regression line and 95 % confidence limits for afebrile rabbits. It is manifest that the responses of febrile rabbits are not greater than those of the afebrile rabbits. Of thirty responses to hot loads in febrile rabbits, twenty-seven fall within the 95 % confidence limits established in experiments upon afebrile rabbits. All the preceding results refer to brain temperature responses. Rectal temperatures were measured in all animals, and the same calculation yielded results similar to those obtained from brain temperature measurements. In general, changes of rectal temperature were smaller than those of brain temperature. Fig. 5 shows the relation between responses of brain and rectal temperatures in all animals. The relation appears to be
775 THERMOREGULATION DURING FEVER linear and is independent of the nature of the thermal load and of the presence offever. The correlation coefficient is highly significant (r = + 0881, P < 0.001). Nothing further would be added by analyzing the rectal temperature responses in detail.
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The slope of a regression line relating the displacement of body temperature to the size of a thermal load delivered intravenously is a measure of the efficiency of the temperature control system of an intact animal; the steeper the slope the less efficient the control. We measured deviations of both brain and rectal temperatures in rabbits subjected to graded hot and cold loads. The rectal temperature deviation bore a linear relation to the brain temperature deviation, so only brain temperature deviations were analysed in detail. In afebrile rabbits the slope of the linear regression of brain temperature deviation on thermal loads was about 2.50 C mmn kg kJ'l for both hot and cold loads. In the case of cold loads it was possible to demonstrate that the slope was unaltered during fever; in the case of hot loads the temperature displacements in the febrile state were certainly no greater than those in the afebrile
776 W. I. CRANSTON AND OTHERS state. Thus the ability of the control system to minimize displacements
of body temperature was not reduced during fever. The greater variability of the results obtained from experiments with hot loads arises partly from an unavoidable limitation of the size of the loads. To prevent discomfort in the conscious animals, we limited the temperatures of the infusions to 460 C, i.e. 670 C above the prevailing deep body temperature. In contrast the temperatures of the cold infusions were at least 150 C below body temperature. The problem arising from this limitation of the hot loads was exacerbated in the febrile state by the greater variability of body temperature (see s.E. of mean of brain temperatures in Tables 1 and 2) and by the fever.
Many previous experiments involving several species have led to the conclusion that the ability of intact animals to compensate for systemic thermal loads is apparently unaffected by fever (Al-Hachim & Frens, 1975; Cabanac & Massonnet, 1974; Cooper et al. 1964; Macpherson, 1969; Haan & Albers, 1960). Our experiments, in which well-defined hot and cold loads and steady-state fevers were employed, lead to the same conclusion. In contrast, all experiments in which the thermosensitivity of single hypothalamic neurones has been determined have indicated that this thermosensitivity is affected by fever (Wit & Wang, 1968; Cabanac et al. 1968; Eisenman, 1969; Schoener & Wang, 1975). It now seems unlikely that the conflict between the experiments on hypothalamic neuronal thermosensitivity and those employing thermal loads arises entirely from technical inadequacies. One explanation, which cannot be ruled out, is that hypothalamic thermosensitive neurones are not crucially concerned in the mechanisms whereby animals compensate for systemic thermal loads during fever. We thank Margaret Tester for help with the experiments, and G. J. S. Ross of the Department of Statistics, Rothamsted Experimental Station, for statistical advice.
REFERENCES AL-HACHIM, G. M. & FRENS, J. (1975). Analysis of shivering in non-peripheral cooling during pyrogen fever. Int. J. Bioclim. Biomet. 19, 53-55. ANDERSEN, H. T., HAMMEL, H. T. & HARDY, J. D. (1961). Modifications of the febrile response to pyrogen by hypothalamic heating and cooling in the unanaesthetized dog. Acta physiol. sand. 53, 247-254. CABANAC, M. & MASSONNET, B. (1974). Temperature regulation during fever: change of set point or change of gain? A tentative answer from a behavioural study in man. J. Physiol. 238, 561-568. CABANAC, M., STOLWIJK, J. A. J. & HARDY, J. D. (1968). Effect of temperature and pyrogens on single-unit activity in the rabbit's brain stem. J. apple. Physiol. 24, 645-652. COOPER, K. E., CRANSTON, W. I. & SNELL, E. S. (1964). Temperature regulation during fever in man. ClGn. Sci. 27, 345-356. CRANSTON, W. I. & ROSENDORFF, C. (1967). Central temperature regulation in the conscious rabbit after monoamine oxidase inhibition. J. Physiol. 193, 359- 373.
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777
EISENMAN, J. S. (1969). Pyrogen-induced changes in the thermosensitivity of septal and preoptic neurons. Am. J. Physiol. 216, 330-334. EISENMAN, J. S. (1974). Depression of preoptic thermosensitivity by bacterial pyrogen in rabbits. Am. J. Phy8eiol. 227, 1067-1073. GRANT, R. & ADLER, R. D. (1967). Responses to leucocytic pyrogen (LP) in hyperthermic and hypothalamus-heated rabbits: a challenge to 'reset' hypothesis of fever. Phypiologi8t, Wa8h. 10, 186. HAAN, J. & ALBERs, C. (1960). Uber die Auslosung Thermoregulatorischer Reaktionen beim Hund unter Pyrogen-Einwirkung. Pflugerm Arch. gee. Phy-iol. 271, 537-547. MACPHERSON, R. K. (1969). The effect of fever on temperature regulation in man. Clin. Sci. 18, 281-287. MITCHELL, D., SNETIT N, J. W. & ATKINs, A. R. (1970). Thermoregulation during fever: changes of set-point or change of gain. Pflugger Arch. gee. Phyuiol. 321, 292-302. MONNIER, M. & GANGLOFF, H. (1961). Atlas for Stereotaxic Brain Research on -the Conscious Rabbit. Amsterdam: Elsevier Publishing. SCHOLNER, E. P. & WANG, S. C. (1975). Leukocytic pyrogen and sodium acetylsalicylate on hypothalamic neurons in the cat. Am. J. Physiol. 229, 185-190. SHARP, F. R. & HAMMEL, H. T. (1972). Effect of fever on salivation response in the resting and exercising dog. Am. J. Physiol. 223, 77-82. SNEDECOR, G. W. & COCIRAN, W. G. (1967). Statictical Methods. Ames, Iowa: Iowa State University Press. WIT, A. & WANG, S. C. (1968). Temperature-sensitive neurons in the preopticI anterior hypothalamic region: actions of pyrogen and acetylsalicylate. Am. J. Physiol. 215, 1160-1169.
