Phy~iol.
252,
J. (1975), pp. 481-490 With 4 text -ft gure8 Printed in Great Britain
481
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS FOLLOWING EXPOSURE TO COLD AND WATER DEPRIVATION
By M. MASKREY AND S. C. NICOL From the Department of Physiology, University of Tasmania, G.P.O. Box 2520, Hobart, Tasmania, 7001, Australia
(Received 14 April 1975) SUMMARY
1. Rabbits were clipped and exposed in turn to four environmental conditions: control (0), cold exposure (CE), water deprivation (WD) and water deprivation and cold exposure together (WD/CE). 2. Following each type of treatment, the rabbits were exposed for 1 hr to an ambient temperature (T.) of 350 C. During this time, respiratory frequency (RF), rectal temperature (Tre), activity and oxygen consumption (JV02) were recorded. 3. It was found that under both cold exposure and water deprivation conditions, the mean respiratory frequency during the first 30 min of heat exposure was reduced when compared with controls. This was associated with a delay in the onset of thermal tachypnoea. Under conditions of water deprivation and cold exposure together, the mean respiratory frequency was further reduced and the length of the delay was increased. 4. Previous cold exposure led to an increase in the V102 measured at 350 C, whereas the jx02 after water deprivation and water deprivation and cold exposure together were not significantly different from the control. 5. Neither the initial Tire nor the change in the Tre during the course of the heat exposure were significantly different from the controls under any of the experimental conditions. 6. It is concluded that both water deprivation and previous cold exposure cause a- block to panting in the heat and that the blocking mechanisms involved are closely interrelated. It is also concluded that neither the metabolic rate of the animal nor its initial or final Tre are important factors in determining the degree to which thermal tachypnoea is inhibited.
482
M. MASKREY AND S. C. NICOL INTRODUCTION
When exposed to a high ambient temperature (T.), the rabbit uses evaporation from the upper respiratory tract in order to dissipate excess heat. Thermal tachypnoea is normally present in resting rabbits at Ta of 250 C and above (Gonzalez, Kluger & Hardy, 1971). Stimulation of peripheral cold receptors brought about by shearing followed by exposure to a low Ta has been shown to inhibit thermal tachypnoea during subsequent heat exposure. This was first reported for the shorn sheep (Bligh, 1963) and further reports of this phenomenon have arisen from investigations on the same species (Phillips & Raghavan, 1970; Maskrey, 1970, 1974; Slee, 1973). However, delays in the onset of thermal tachypnoea are also encountered in clipped rabbits which have been previously exposed to cold (Bligh & Allen, 1970). Under conditions of water deprivation, animals tend to reduce their evaporative water loss. This is especially noticeable in animals living under arid conditions (Maloiy, 1970; Taylor, 1970a, b, 1972; Shkolnik, Borut & Choshniak, 1972). The water-deprived rabbit shows similar tendencies to conserve water by reducing respiratory frequency (RF) and by failing to respond to a centrally applied warm stimulus with an increase in respiratory evaporative cooling (Turlejska-Stelmasiak, 1973, 1974). The purpose of the present study is to investigate the mechanisms involved in the inhibition of respiratory evaporative heat loss produced by previous cold exposure and by water deprivation. METHODS Nine rabbits, with body weights of 2.0-3-5 kg, were used in the study. These were of both sexes and comprised a variety of breeds. At the beginning of the investigation, the fur was removed from the trunk and hind quarters of all rabbits. The animals were then kept in this condition by regular clipping throughout the period in which the experiments were carried out. Between experiments the animals were individually housed in wire cages in a room where T. was kept constant at about 20° C. Rabbits were fed on dry bran pellets throughout. Each of the nine rabbits was subjected in turn to four different sets of conditions as follows: Control (C). Under control conditions, rabbits received water ad libitum up until the time of the experiment. Immediately before testing they were kept for 18 hr in a climatic chamber at a T. of 250 C. Cold-eXpo8ed (CE). Cold-exposed rabbits also received water ad libitum up until the time of the experiment. Immediately before testing they were exposed for 18 hr to a T. of 50 C. Water-deprived (WD). Under conditions of water deprivation rabbits were denied drinking water for 72 hr before testing and exposed to a T. of 25° C for 18 hr
immediately before testing.
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 483 Water-deprived/cold-exposed (WDICE). The rabbits were again denied drinking
water for 72 hr and, in addition, exposed for 18 hr to a T. of 50 C. During the last 2 hr of the 18 hr exposure period respiratory frequency was observed and counted with the aid of a stopwatch. These measurements were used to determine the pre-test respiratory frequency. A note was also made of the activity and posture of the animal. For each experiment the following procedure was adopted. Rabbits were removed from the climatic chamber in which they had been kept for the previous 18 hr, their rectal temperature (Tre) was measured using a clinical thermometer, and they were placed in a water-jacketed metabolic chamber in which the T. was maintained at 35 +10C with a relative humidity of between 60 and 70 %. The rabbits remained in the chamber for exactly 1 hr. During this period, the following measurements were made: Respiratory frequency. Respiratory frequency was measured by recording changes in the electrical resistance of a pneumograph belt made from a 15 cm length of rubber tubing filled with copper sulphate solution which was fastened around the chest of the animal. Electrical connexions from each end of the pneumograph were taken to the input of a Beckman 9892A skin resistance coupler. Rectal temperature. As well as measuring Tre before and after heat exposure, in five experiments Tre was continuously monitored by telemetry. The body of the transmitter was securely taped to the tail of the rabbit and the thermo-sensitive probe inserted 5 cm into the rectum. The output of the transmitter was received by a commercial FM tuner and the click rate measured by a Beckman 9857 cardiotachometer coupler. Tre and respiratory frequency were both recorded on a Beckman type R Dynograph recorder. Activity. Activity in the metabolic chamber was monitored using a tuned oscillator actograph (S. C. Nicol & S. B. Rofe, in preparation). Oxygen consumption. gI0 was recorded using an open circuit technique. Outlet air from the metabolic chamber was dried and passed through a Servomex OA.184 oxygen analyser used in the ratio mode with an oxygen range of 20-21 %. The difference between outlet oxygen concentration and atmospheric oxygen concentration was displayed on a Rikadenki B 341 potentiometric recorder, as were chamber temperature and activity. Oxygen consumption was calculated using the tables of Hill (1972). Following heat exposure, the rabbits were removed from the metabolic chamber and their Tre measured with a clinical thermometer. At the same time blood was collected from a marginal ear vein in haematocrit tubes for estimation of the packed cell volume (P.c.v.). In the analysis of the results the significance of differences between means was computed using Student's t test. RESULTS
The results are summarized in Table 1.
Respiratory frequency Mean pre-test respiratory frequency was significantly less under coldexposed, water-deprived, and water-deprived and cold-exposed together conditions than under control conditions (P < 0.001) (Fig. 1 A) though there were no significant differences between the pre-test respiratory frequency under the three experimental conditions.
484
M. MASKREY AND S. C. NICOL M I" 0 +l +l +lIU +I +I +I
10 co
aq
.0.. >
~~~~T- cq
> ~ i~
4~ oO ~
C;i
4ooc
V
~~~~o
Q
_
+l +1 +1 +1 +1 +1 +1
MO
q X
o~ U4U )°° C4 ~~
0 M c~ ~ ~ ~ ~ ~~c
t
4a 04
+1 +1 o
1°+l +l
00
04
~ ~ ~
~
~~
(D~~~~~~~~~~~~~U
04
~
~ ~ ~
~~~
Hao o00
E
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 485 When the animals were exposed to 350 C, the degree of respiratory evaporative heat loss occurring was estimated on the basis of two criteria, these being the mean respiratory frequency measured during the first 30 min in the heat (Fig. 1B) and the length of the delay before the onset of thermal tachypnoea (Fig. 1C). The controls showed an increase in respiratory frequency as soon as they entered the heat. Under the three experimental conditions, however, there was a considerable delay before panting was seen (see Fig. 2). B
A
C
100
D300
WD
C
EU
C
C
WD/CElE
L~~~~~~~~~~~~~~~~~C .0 ~~~~~~~.0 U~~~~~~~~~~~~E so
40
~~~~~~~~~0
-
20 L
C~~~~~~~~~~~0
L~~~~~~~~C
~~~~0
0 CE
WD/CE
CE
0
WD/CE
CE
WD/CE
Fig. 1. Respiratory responses of clipped rabbits to heat exposure (T., 350 C). In this and following figures, C stands for control rabbits; CE, cold-exposed; WD, water-deprived; and WD/CE, water-deprived and cold-exposed. The histograms represent mean values and the bars indicate S.E. of mean. A, respiratory frequency before heat exposure. B, mean respiratory frequency during the first 30 min of heat exposure. C, delay in the onset of thermal tachypnoea.
The mean respiratory frequency during the first 30 min of heat exposure was much reduced in the treated as compared with the control conditions (P < 0 001). What is more, in the cold-exposed plus water-deprived condition the respiratory frequency was significantly reduced as compared with either cold-exposed or water-deprived alone (P < 0-01). The mean delay before the onset of thermal tachypnoea was significantly greater for the water-deprived plus cold-exposed condition than when rabbits were either water-deprived or cold-exposed alone (P < 0-001). When the cold-exposed and water-deprived conditions were
~M. MASKREY AND S. C. NICOL there was found to be no significant difference between the
486
486
compared mean delays.
Oxygen consumption V2measured at 350 C was significantly increased over the control level following cold exposure (P < 0.05) but not significantly different under either of the other experimental conditions. These results are illustrated in Fig. 3. 600-
EU
40 U
30
0 C
300
10
0
10
20
30 Time (min)
40
50
60
Fig. 2. Respiratory frequency of a clipped rabbit at 350 C after different pre-exposure conditions. 0, control (C); EL, cold-exposed (CE); U, waterdeprived (WD); 0, water-deprived and cold-exposed (WDICE).
* ~~~~Rectal temperature The mean initial Tre did not differ significantly between conditions. All rabbits showed an increase in Tre during the 60 min at 350 C. The greatest increase was under the water-deprived conditions while the least was under cold-exposed plus water-deprived conditions. Although these were significantly different from one another (P < 0.01) neither of them was significantly different from either the control or the coldexposed condition.
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 487 In the five experiments in which Tre was monitored throughout the 60 min at 350 C it was observed that Tre did not increase uniformly throughout the period of heat exposure (see Fig. 4). Instead it appeared that the rate of rise of Tre increased sharply at the time at which thermal tachypnoea was initiated.
06
T
0
CL b0
E C
0
UA 0 0
C
CE
WD WD/CE
Fig. 3. Oxygen consumption of clipped rabbits during exposure to an ambient temperature of 350 C.
Activity and posture When exposed to 250 C for 18 hr, the rabbits remained alert and active. Rabbits exposed to 50 C for 18 hr adopted a huddled posture and shivered
visibly. When placed in the metabolic chamber at 350 C, the control-conditioned rabbits spread themselves upon the floor of the chamber almost immediately. When they had been exposed to cold or to water deprivation or to both before heat exposure, the rabbits adopted this spread posture only after they had begun to pant. Apart from a bout of activity within the first 5 min after the animal had been introduced into the metabolic chamber, the rabbits were most active in the few minutes immediately before the onset of thermal tachypnoea. For the rest of the time they remained quiet.
M. MASKREY AND S. C. NICOL
488
Packed cell volume The mean P.v.c. from the rabbits when subjected to the water-deprived or cold-exposed plus water-deprived conditions was significantly higher than the control (P < 0 01). This confirms that the rabbits when denied drinking water had in fact become dehydrated. The mean P.c.v. of the rabbits after cold exposure alone, however, was not significantly different from the control. -
400
410
-
(Uc
1
300
40.5S L
U C
v00
400 E
L
100 0
0
10
20
139.5 30 Time (min)
40
50
60
39
Fig. 4. Respiratory frequency (U) and rectal temperature (0) in a clipped rabbit during heat exposure (T. = 350 C) after previous water deprivation and cold exposure. DISCUSSION
The inhibition of thermal tachypnoea encountered in a heat-exposed animal which has previously been clipped and exposed to cold has been attributed to the stimulation of peripheral cold sensors (Bligh, 1963). This view is supported by the observation that the length of the delay before the onset of panting can be correlated with the mean skin temperature of the animal before entering the heat (Maskrey, 1974). The inhibition of thermal tachypnoea in water-deprived rabbits on the other hand has been attributed to cellular dehydration (Turlejska-Stelmasiak, 1974). Presumably the control of thermal tachypnoea involves the medullary respiratory centre, the neuronal activity of which determines the frequency or the depth of respiration. Neither the metabolic rate of the rabbit nor its original core temperature appear to be major factors in determining the degree of inhibition of thermal tachypnoea. Following cold exposure oxygen consumption at
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 489 350 C is raised, while following water deprivation it is slightly reduced. However, the degree of inhibition of thermal tachypnoea under the two conditions is similar. On the other hand, following both water deprivation and cold exposure, oxygen consumption was similar to that seen after water deprivation alone, though the degree to which panting was inhibited was quite different under the two sets of conditions. It is interesting to note that water deprivation completely blocked the metabolic response to cold exposure. This may have resulted from the voluntary decrease in food intake commonly associated with water deprivation. Bligh (1963) proposed that the delay before the onset of thermal tachypnoea in shorn sheep was due to a persistent central block between the temperature receptors and the respiratory centre. He suggested that the blocking agent accumulates during the period in which the animal is exposed to cold, then the intensity of the agent decays exponentially from the time at which the animal enters the heat. A similar blocking mechanism could be visualized in water-deprived animals. The simplest hypothesis is to propose that the same blocking agent accumulates at a single site whether the initial stimulus was peripheral cold or cellular dehydration. Even if the blocking agents are different in the two cases or act at separate sites, their actions must be closely interrelated. This becomes clear when the delays before the onset of thermal tachypnoea, or the mean respiratory frequency during-the first 30 min in the heat, under the three experimental conditions are compared. If water deprivation and cold exposure were acting through wholly independent mechanisms, then subjecting the rabbit to a combination of both water deprivation and cold exposure would result in a degree of inhibition corresponding to the more potent factor. Instead, summation of the block occurs so that water deprivation and cold exposure together result in a far greater inhibition than either condition alone. Although peripheral cold stimulation ceases as soon as heat exposure begins, cellular dehydration continues to be present during the heat exposure. Therefore it is insufficient to postulate that the inhibition of panting gradually disappears because the blocking agent slowly declines when the initiating stimulus is removed. Instead, the stimulation of warm sensors must also play a part in removal of the blocking agent, otherwise water-deprived rabbits would never pant in the heat. Even under control conditions in which the rabbits panted throughout the duration of the exposure to heat, Tre increased. This is in agreement with the findings of Gonzalez et al. (1971). The absence of a significant difference between the change in Tre in the control and experimental conditions also supports the suggestion by these authors that panting in the rabbit is rather inefficient. In fact, in the five experiments in which
490 ~M. MASKRE Y AND S. C. NICOL Tre was monitored throughout heat exposure, the rate of increase in Tre was greater after the onset of panting than before. The sudden change from the non-panting to the panting animal is accompanied by changes in activity and posture. This may have important thermoregulatory consequences (McEwen & Heath, 1973). Also it may indicate that the conscious sensation of peripheral warm stimulation first becomes evident to the animal at this time. The thermoregulatory and metabolic changes which occur at the onset of thermal tachypnoea require further investigation before positive statements concerning the primary stimulus for the initiation of panting can be made. 490
We would like to thank Mr S. B. Rofe for designing and constructing the tem-
perature telemetry transmitter. REFERENCES
BLIGH, J. (1963). Inhibition of thermal polypnea in the closely shorn sheep. J. Physiol. 168, 764-781. BLIGH, J. & ALLEN, T. E. (1970). A comparitive consideration of the modes of evaporative heat loss from mammals. In Phyaiological and Behavioral Temperature Regulation, ed. HARDY, J. D., GAGGE, A. P. & STOLWIJK, J. A. J., pp. 97-107. Springfield: Thomas. GONZALE, R. R., KLUGER, M. J. & HARDY, J. D. (1971). Partitional calorimetry of the New Zealand White rabbit at temperatures 5-35O C. J. apple. Phy8iol. 31, 728-734. HILLa, R. W. (1972). Determination of oxygen consumption by use of the paramagnetic oxygen analyzer. J. appl. Phy8iol. 33, 261-263. McEwTEN, G. N. & HEATH, J. E. (1973). Resting metabolism and thermoregulation in the unrestrained rabbit. J. a~ppl. Physiol. 35, 884-886. MALoiy, G. M. 0. (1970). Water economy in the Somali donkey. Am. J. Phy8iol. 219, 1522-1527. MASKREY, M. (1970). A comparison of the effects of close shearing and of injections of noradrenaline into a lateral cerebral ventricle in the Welsh Mountain sheep. J. Phy~iol. 210, 102-103 P. MAsKREY, M. (1974). Delay in the onset of thermal tachypnoea in shorn sheep exposed to 420 C in winter and summer. Aust. J. biol. Sci. 27, 259-266. PHILLIPS, G. D. & RAGHAVAN, G. V. (1970). Responses of unshorn and shorn sheep to thermal stress. J. Phy8iOl. 208, 317-328. SHKOLNIK, A., BoRuTr, A. & CHOSHNAXu, J.- (1972). Water economy of the Beduin Goat. Symp. zool. Soc. Lond. 31, 229-242. SuEE, J. (1973). Cold-induced inhibition of thermal panting in shorn sheep. 1. Effect of intensity of cold exposure. Anim. Prod. 16, 271-283. TAYLOR, C. R. (1970a). Strategies of temperature regulation: effect on evaporation in East African ungulates. Am. J. Phy8iol. 219, 1131-1135. TAYLOR, C. R. (1970b). Dehydration and heat: effects on temperature regulation of East African ungulates. Am. J. Phy8iol. 219, 1136-1139. TAYLOR, C. R. (1972). The desert gazelle: a paradox resolved. Symp. zool. Soc. Lond. 31, 215-227. TuRLi~sKA-STELMASTIC, E. (1973). Thermoregulatory responses to hypothalamic heating in dehydrated rabbits. Experientia, 29, 51. TuRLisKA-STELMASIAx, E. (1974). The influence of dehydration on heat dissipation mechanisms in the rabbit. J. Phyeiol., Pari8 68, 5-15.
252,
J. (1975), pp. 481-490 With 4 text -ft gure8 Printed in Great Britain
481
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS FOLLOWING EXPOSURE TO COLD AND WATER DEPRIVATION
By M. MASKREY AND S. C. NICOL From the Department of Physiology, University of Tasmania, G.P.O. Box 2520, Hobart, Tasmania, 7001, Australia
(Received 14 April 1975) SUMMARY
1. Rabbits were clipped and exposed in turn to four environmental conditions: control (0), cold exposure (CE), water deprivation (WD) and water deprivation and cold exposure together (WD/CE). 2. Following each type of treatment, the rabbits were exposed for 1 hr to an ambient temperature (T.) of 350 C. During this time, respiratory frequency (RF), rectal temperature (Tre), activity and oxygen consumption (JV02) were recorded. 3. It was found that under both cold exposure and water deprivation conditions, the mean respiratory frequency during the first 30 min of heat exposure was reduced when compared with controls. This was associated with a delay in the onset of thermal tachypnoea. Under conditions of water deprivation and cold exposure together, the mean respiratory frequency was further reduced and the length of the delay was increased. 4. Previous cold exposure led to an increase in the V102 measured at 350 C, whereas the jx02 after water deprivation and water deprivation and cold exposure together were not significantly different from the control. 5. Neither the initial Tire nor the change in the Tre during the course of the heat exposure were significantly different from the controls under any of the experimental conditions. 6. It is concluded that both water deprivation and previous cold exposure cause a- block to panting in the heat and that the blocking mechanisms involved are closely interrelated. It is also concluded that neither the metabolic rate of the animal nor its initial or final Tre are important factors in determining the degree to which thermal tachypnoea is inhibited.
482
M. MASKREY AND S. C. NICOL INTRODUCTION
When exposed to a high ambient temperature (T.), the rabbit uses evaporation from the upper respiratory tract in order to dissipate excess heat. Thermal tachypnoea is normally present in resting rabbits at Ta of 250 C and above (Gonzalez, Kluger & Hardy, 1971). Stimulation of peripheral cold receptors brought about by shearing followed by exposure to a low Ta has been shown to inhibit thermal tachypnoea during subsequent heat exposure. This was first reported for the shorn sheep (Bligh, 1963) and further reports of this phenomenon have arisen from investigations on the same species (Phillips & Raghavan, 1970; Maskrey, 1970, 1974; Slee, 1973). However, delays in the onset of thermal tachypnoea are also encountered in clipped rabbits which have been previously exposed to cold (Bligh & Allen, 1970). Under conditions of water deprivation, animals tend to reduce their evaporative water loss. This is especially noticeable in animals living under arid conditions (Maloiy, 1970; Taylor, 1970a, b, 1972; Shkolnik, Borut & Choshniak, 1972). The water-deprived rabbit shows similar tendencies to conserve water by reducing respiratory frequency (RF) and by failing to respond to a centrally applied warm stimulus with an increase in respiratory evaporative cooling (Turlejska-Stelmasiak, 1973, 1974). The purpose of the present study is to investigate the mechanisms involved in the inhibition of respiratory evaporative heat loss produced by previous cold exposure and by water deprivation. METHODS Nine rabbits, with body weights of 2.0-3-5 kg, were used in the study. These were of both sexes and comprised a variety of breeds. At the beginning of the investigation, the fur was removed from the trunk and hind quarters of all rabbits. The animals were then kept in this condition by regular clipping throughout the period in which the experiments were carried out. Between experiments the animals were individually housed in wire cages in a room where T. was kept constant at about 20° C. Rabbits were fed on dry bran pellets throughout. Each of the nine rabbits was subjected in turn to four different sets of conditions as follows: Control (C). Under control conditions, rabbits received water ad libitum up until the time of the experiment. Immediately before testing they were kept for 18 hr in a climatic chamber at a T. of 250 C. Cold-eXpo8ed (CE). Cold-exposed rabbits also received water ad libitum up until the time of the experiment. Immediately before testing they were exposed for 18 hr to a T. of 50 C. Water-deprived (WD). Under conditions of water deprivation rabbits were denied drinking water for 72 hr before testing and exposed to a T. of 25° C for 18 hr
immediately before testing.
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 483 Water-deprived/cold-exposed (WDICE). The rabbits were again denied drinking
water for 72 hr and, in addition, exposed for 18 hr to a T. of 50 C. During the last 2 hr of the 18 hr exposure period respiratory frequency was observed and counted with the aid of a stopwatch. These measurements were used to determine the pre-test respiratory frequency. A note was also made of the activity and posture of the animal. For each experiment the following procedure was adopted. Rabbits were removed from the climatic chamber in which they had been kept for the previous 18 hr, their rectal temperature (Tre) was measured using a clinical thermometer, and they were placed in a water-jacketed metabolic chamber in which the T. was maintained at 35 +10C with a relative humidity of between 60 and 70 %. The rabbits remained in the chamber for exactly 1 hr. During this period, the following measurements were made: Respiratory frequency. Respiratory frequency was measured by recording changes in the electrical resistance of a pneumograph belt made from a 15 cm length of rubber tubing filled with copper sulphate solution which was fastened around the chest of the animal. Electrical connexions from each end of the pneumograph were taken to the input of a Beckman 9892A skin resistance coupler. Rectal temperature. As well as measuring Tre before and after heat exposure, in five experiments Tre was continuously monitored by telemetry. The body of the transmitter was securely taped to the tail of the rabbit and the thermo-sensitive probe inserted 5 cm into the rectum. The output of the transmitter was received by a commercial FM tuner and the click rate measured by a Beckman 9857 cardiotachometer coupler. Tre and respiratory frequency were both recorded on a Beckman type R Dynograph recorder. Activity. Activity in the metabolic chamber was monitored using a tuned oscillator actograph (S. C. Nicol & S. B. Rofe, in preparation). Oxygen consumption. gI0 was recorded using an open circuit technique. Outlet air from the metabolic chamber was dried and passed through a Servomex OA.184 oxygen analyser used in the ratio mode with an oxygen range of 20-21 %. The difference between outlet oxygen concentration and atmospheric oxygen concentration was displayed on a Rikadenki B 341 potentiometric recorder, as were chamber temperature and activity. Oxygen consumption was calculated using the tables of Hill (1972). Following heat exposure, the rabbits were removed from the metabolic chamber and their Tre measured with a clinical thermometer. At the same time blood was collected from a marginal ear vein in haematocrit tubes for estimation of the packed cell volume (P.c.v.). In the analysis of the results the significance of differences between means was computed using Student's t test. RESULTS
The results are summarized in Table 1.
Respiratory frequency Mean pre-test respiratory frequency was significantly less under coldexposed, water-deprived, and water-deprived and cold-exposed together conditions than under control conditions (P < 0.001) (Fig. 1 A) though there were no significant differences between the pre-test respiratory frequency under the three experimental conditions.
484
M. MASKREY AND S. C. NICOL M I" 0 +l +l +lIU +I +I +I
10 co
aq
.0.. >
~~~~T- cq
> ~ i~
4~ oO ~
C;i
4ooc
V
~~~~o
Q
_
+l +1 +1 +1 +1 +1 +1
MO
q X
o~ U4U )°° C4 ~~
0 M c~ ~ ~ ~ ~ ~~c
t
4a 04
+1 +1 o
1°+l +l
00
04
~ ~ ~
~
~~
(D~~~~~~~~~~~~~U
04
~
~ ~ ~
~~~
Hao o00
E
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 485 When the animals were exposed to 350 C, the degree of respiratory evaporative heat loss occurring was estimated on the basis of two criteria, these being the mean respiratory frequency measured during the first 30 min in the heat (Fig. 1B) and the length of the delay before the onset of thermal tachypnoea (Fig. 1C). The controls showed an increase in respiratory frequency as soon as they entered the heat. Under the three experimental conditions, however, there was a considerable delay before panting was seen (see Fig. 2). B
A
C
100
D300
WD
C
EU
C
C
WD/CElE
L~~~~~~~~~~~~~~~~~C .0 ~~~~~~~.0 U~~~~~~~~~~~~E so
40
~~~~~~~~~0
-
20 L
C~~~~~~~~~~~0
L~~~~~~~~C
~~~~0
0 CE
WD/CE
CE
0
WD/CE
CE
WD/CE
Fig. 1. Respiratory responses of clipped rabbits to heat exposure (T., 350 C). In this and following figures, C stands for control rabbits; CE, cold-exposed; WD, water-deprived; and WD/CE, water-deprived and cold-exposed. The histograms represent mean values and the bars indicate S.E. of mean. A, respiratory frequency before heat exposure. B, mean respiratory frequency during the first 30 min of heat exposure. C, delay in the onset of thermal tachypnoea.
The mean respiratory frequency during the first 30 min of heat exposure was much reduced in the treated as compared with the control conditions (P < 0 001). What is more, in the cold-exposed plus water-deprived condition the respiratory frequency was significantly reduced as compared with either cold-exposed or water-deprived alone (P < 0-01). The mean delay before the onset of thermal tachypnoea was significantly greater for the water-deprived plus cold-exposed condition than when rabbits were either water-deprived or cold-exposed alone (P < 0-001). When the cold-exposed and water-deprived conditions were
~M. MASKREY AND S. C. NICOL there was found to be no significant difference between the
486
486
compared mean delays.
Oxygen consumption V2measured at 350 C was significantly increased over the control level following cold exposure (P < 0.05) but not significantly different under either of the other experimental conditions. These results are illustrated in Fig. 3. 600-
EU
40 U
30
0 C
300
10
0
10
20
30 Time (min)
40
50
60
Fig. 2. Respiratory frequency of a clipped rabbit at 350 C after different pre-exposure conditions. 0, control (C); EL, cold-exposed (CE); U, waterdeprived (WD); 0, water-deprived and cold-exposed (WDICE).
* ~~~~Rectal temperature The mean initial Tre did not differ significantly between conditions. All rabbits showed an increase in Tre during the 60 min at 350 C. The greatest increase was under the water-deprived conditions while the least was under cold-exposed plus water-deprived conditions. Although these were significantly different from one another (P < 0.01) neither of them was significantly different from either the control or the coldexposed condition.
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 487 In the five experiments in which Tre was monitored throughout the 60 min at 350 C it was observed that Tre did not increase uniformly throughout the period of heat exposure (see Fig. 4). Instead it appeared that the rate of rise of Tre increased sharply at the time at which thermal tachypnoea was initiated.
06
T
0
CL b0
E C
0
UA 0 0
C
CE
WD WD/CE
Fig. 3. Oxygen consumption of clipped rabbits during exposure to an ambient temperature of 350 C.
Activity and posture When exposed to 250 C for 18 hr, the rabbits remained alert and active. Rabbits exposed to 50 C for 18 hr adopted a huddled posture and shivered
visibly. When placed in the metabolic chamber at 350 C, the control-conditioned rabbits spread themselves upon the floor of the chamber almost immediately. When they had been exposed to cold or to water deprivation or to both before heat exposure, the rabbits adopted this spread posture only after they had begun to pant. Apart from a bout of activity within the first 5 min after the animal had been introduced into the metabolic chamber, the rabbits were most active in the few minutes immediately before the onset of thermal tachypnoea. For the rest of the time they remained quiet.
M. MASKREY AND S. C. NICOL
488
Packed cell volume The mean P.v.c. from the rabbits when subjected to the water-deprived or cold-exposed plus water-deprived conditions was significantly higher than the control (P < 0 01). This confirms that the rabbits when denied drinking water had in fact become dehydrated. The mean P.c.v. of the rabbits after cold exposure alone, however, was not significantly different from the control. -
400
410
-
(Uc
1
300
40.5S L
U C
v00
400 E
L
100 0
0
10
20
139.5 30 Time (min)
40
50
60
39
Fig. 4. Respiratory frequency (U) and rectal temperature (0) in a clipped rabbit during heat exposure (T. = 350 C) after previous water deprivation and cold exposure. DISCUSSION
The inhibition of thermal tachypnoea encountered in a heat-exposed animal which has previously been clipped and exposed to cold has been attributed to the stimulation of peripheral cold sensors (Bligh, 1963). This view is supported by the observation that the length of the delay before the onset of panting can be correlated with the mean skin temperature of the animal before entering the heat (Maskrey, 1974). The inhibition of thermal tachypnoea in water-deprived rabbits on the other hand has been attributed to cellular dehydration (Turlejska-Stelmasiak, 1974). Presumably the control of thermal tachypnoea involves the medullary respiratory centre, the neuronal activity of which determines the frequency or the depth of respiration. Neither the metabolic rate of the rabbit nor its original core temperature appear to be major factors in determining the degree of inhibition of thermal tachypnoea. Following cold exposure oxygen consumption at
INHIBITION OF THERMAL TACHYPNOEA IN RABBITS 489 350 C is raised, while following water deprivation it is slightly reduced. However, the degree of inhibition of thermal tachypnoea under the two conditions is similar. On the other hand, following both water deprivation and cold exposure, oxygen consumption was similar to that seen after water deprivation alone, though the degree to which panting was inhibited was quite different under the two sets of conditions. It is interesting to note that water deprivation completely blocked the metabolic response to cold exposure. This may have resulted from the voluntary decrease in food intake commonly associated with water deprivation. Bligh (1963) proposed that the delay before the onset of thermal tachypnoea in shorn sheep was due to a persistent central block between the temperature receptors and the respiratory centre. He suggested that the blocking agent accumulates during the period in which the animal is exposed to cold, then the intensity of the agent decays exponentially from the time at which the animal enters the heat. A similar blocking mechanism could be visualized in water-deprived animals. The simplest hypothesis is to propose that the same blocking agent accumulates at a single site whether the initial stimulus was peripheral cold or cellular dehydration. Even if the blocking agents are different in the two cases or act at separate sites, their actions must be closely interrelated. This becomes clear when the delays before the onset of thermal tachypnoea, or the mean respiratory frequency during-the first 30 min in the heat, under the three experimental conditions are compared. If water deprivation and cold exposure were acting through wholly independent mechanisms, then subjecting the rabbit to a combination of both water deprivation and cold exposure would result in a degree of inhibition corresponding to the more potent factor. Instead, summation of the block occurs so that water deprivation and cold exposure together result in a far greater inhibition than either condition alone. Although peripheral cold stimulation ceases as soon as heat exposure begins, cellular dehydration continues to be present during the heat exposure. Therefore it is insufficient to postulate that the inhibition of panting gradually disappears because the blocking agent slowly declines when the initiating stimulus is removed. Instead, the stimulation of warm sensors must also play a part in removal of the blocking agent, otherwise water-deprived rabbits would never pant in the heat. Even under control conditions in which the rabbits panted throughout the duration of the exposure to heat, Tre increased. This is in agreement with the findings of Gonzalez et al. (1971). The absence of a significant difference between the change in Tre in the control and experimental conditions also supports the suggestion by these authors that panting in the rabbit is rather inefficient. In fact, in the five experiments in which
490 ~M. MASKRE Y AND S. C. NICOL Tre was monitored throughout heat exposure, the rate of increase in Tre was greater after the onset of panting than before. The sudden change from the non-panting to the panting animal is accompanied by changes in activity and posture. This may have important thermoregulatory consequences (McEwen & Heath, 1973). Also it may indicate that the conscious sensation of peripheral warm stimulation first becomes evident to the animal at this time. The thermoregulatory and metabolic changes which occur at the onset of thermal tachypnoea require further investigation before positive statements concerning the primary stimulus for the initiation of panting can be made. 490
We would like to thank Mr S. B. Rofe for designing and constructing the tem-
perature telemetry transmitter. REFERENCES
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