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Thermoregulatory responses of laying hens under cyclic environmental temperature to intraventricular calcium and sodium
Camp. Biochem. Physiol.
Vol. 89A, No. 3, pp. 415419, 1988
0300-9629/88 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain
RESPONSES OF LAYING THERMOREGULATORY HENS UNDER CYCLIC ENVIRONMENTAL TEMPERATURE TO INTRAVENTRICULAR CALCIUM AND SODIUM A. A. MAKI,* M. M. BECK,* E. W. GLEAVES* and J. A. DESHAZER~
*Animal Science Department and tAgricultura1 Engineering Department, University of Nebraska, Lincoln, NB 68583-0908, USA (Received 22 June 1987) Abstract-l.
Hens received ICV injections of Ca*+ (1.98 g/100 ml) or Na+ (7.25 g/100 ml) at 28°C and, following acclimation, at 37 or 2O”C,respectively. 2. At 28°C (thermoneutrality), rectal temperature rose (P -Z0.05) following Na+ and fell (P < 0.05) following Ca*+, similar to mammals and broiler chickens.
3. At 37°C Ca*+-induced hypothermia did not occur; nor did the Na+-associated hyperthermia at 20°C. 4. Acclimation to a high or low temperature may produce an endogenous shift in CSF ion levels that make additional ion administration ineffective in affecting body temperature.
MATERIALS AND METHODS
INTRODUCXION
Early this century, physiologists discovered that systemic injections of calcium and other cations cause an abrupt change in the body temperature of an animal (Freund, 1911, as cited by Meyers, 1982). By the 193Os, experimental technology had advanced to the extent that Hasama (1930) was able to examine the effect of cations injected directly into the hypothalamus. He found that excess sodium, potassium and barium produced hyperthermia in the unanesthetized cat, whereas excess calcium caused an intense reduction in body temperature. Many years later, and upon finding that calcium blocks the conscious cat’s sodium-induced temperature increase, Feldberg et al. (1970) speculated that the temperature’s set point could have its origin in a balance between these cations. Within a few years, it was discovered that there are apparently no differences between the mammalian species studied in the hyperand hypothermic effects evoked by Na+ and Ca*+ ions, respectively (Feldberg and Saxena, 1970; Myers et al., 1971; Myers and Brophy, 1972; Myers and Buckman, 1972; Greenleaf et al., 1974; Saxena, 1976). Denbow and Edens (1980), working with broilers under thermoneutral (26°C) conditions, found that rectal temperature increased or decreased in response to Na+ or Ca*+ introduced into the cerebrospinal fluid. Although the broiler response appears to be similar to that of mammals, no studies have been conducted with the laying hen, a species with very large dietary calcium requirements. In addition, recent studies at the University of Nebraska (Xin, 1985; Nasser, 1986) with the laying hen prompted further investigations into the ionic/cellular controls of thermoregulation under cyclic temperatures. The objective of this study was to characterize the temperature response to cations of the laying hen (Callus domesticus) under thermoneutral conditions, and near both limits of the thermoneutral zone.
Twenty-four White Leghorn laying hens (Gahs domesticus), aged 44 weeks, supplied by the Poultry Research Complex at the University of Nebraska, were caged individually- in environmentally controlled chambers- The daily temperature cycle, 20-37”C, was controlled by a sine-wave function such that the maximum occurred during photophase (12:00-3:00 p.m.). The temperature decreased gradually after this time to reach a minimum at 12:00-3:00 a.m. During each experimental period, all laying hens were housed in cyclic environmental chambers for at least 3 days prior to cation administration. Sixteen hr of light and 8 hr of darkness were maintained, with lights on from 5:OOa.m.-9%) p.m. Feed and water were available ud libiturn. Surgical preparation An 8-mm needle of a single 24gauge intravenous catheter placement unit was implanted stereotaxically into the third ventricle of each hen under chloropent anesthesia (Napentobarbital-chloralhydrate; Fort Dodge, IA). Stereotaxic coordinates (A6.5; V4.5; LO.OO),as suggested by Karten and Hodos (1967), were used. Three self-tapoina screws were fixed into the skull around the cannula, and an-acrylic dental cement (Coe tray plastic, Coe Labs, Chicago, IL) was used to secure the cannula which was covered by a removable plastic tip. Alcohol was applied to the incision to prevent bacterial infection and at least 2 weeks were allowed for recovery. Cannula placement was verified by cation injection under normal ambient temperature (28°C) (Denbow and Edens, 1980) and by visual examination of brains at the end of the experiment.
475
Cation solutions Injection solutions of NaCl (7.25g/lOOml) and CaCl, (1.98g/lOOml) were prepared to contain 1.24M Na+ and 134.7 mM Ca*+ ions, respectively. These concentrations are equivalent to 8 and 45 times the respective concentration of Na+ (155 mM) and Ca*+ (3 mM), respectively, reported in
A. A. MAKIet al.
476
42-
I -20
1
I -10
I
, 10
I
, 20
,
I 56
I
I 70
,
I
, 20
I 110
TIME(min) Fig. 1. Body temperature responses to ICV injection of 10~1 Ca2+ or Na+ under euthermic (28°C) environment.
A
-----water
44
--cd’ 43 E +
42
41 1 f
I,, -30
, -10
,
, 10
,
, 30
,
* 50
,
, 70
,
, 90
.,
, 110
TIME(min) 8 -----water 44-
-NNd
43-
T,,,,,,,,,,,,,,,, -30
-10
10
30
50
70
90
110
tlME(mln) Fig. 2. (A) Body temperature response to ICV injection of 10~1 Ca2+ or ddH,O under hyperthermic (37°C) environment. (B) Body temperature response to ICV injection of 10 pl Na+ or ddH,O under 20°C ambient temperature.
ICV Ca2+ and Na+ in laying hens chicken cerebrospinal fluid (CSF) (Hillman, as cited by Denbow and Edens, 1980). Ten PI of solution were introduced through the cannula by a Hamilton syringe, Ca2+ at 37”C, Na+ at 20°C. Pyrogen-free double-distilled deionized water (ddH,O) was injected as a control solution. Physiological measurements
Thermistors (Yellow Spring Instruments model 401), inserted rectally and taped to the tail feathers, and elastic bellows fastened around the body, were used to record rectal temperature and respiration rate on a dynograph recorder (Beckman type 9858). Foot temperature was recorded as an indicator of peripheral temperature by means of thermocouples placed under a scale on the lateral right shank (Hillman, personal communication) and connected to a potentiometer (Hewlett strip chart recorder model 7100B). Rectal temperature, respiration rate, and peripheral temperature were recorded for a duration of 1 min at IO-min intervals for a total of 180min. The recording was begun 30min prior to, and continued 120min after, injection of Na+, Ca2+, or water. Data were analyscd by regression techniques (GLM) and analysis of variance (ANOVA). Differences imply statistical significance at P < 0.05, unless otherwise stated.
411 RESULTS
Body temperature (Ta) At an ambient temperature (To) of 28”C, a fall in TR was induced by the ICV injection of 10~1 of Caz+. Conversely, a sharp rise in TRwas produced by the introduction of Na+ into the same area (Fig. 1). These results are in agreement with the findings of Denbo and Edens (1980) in the broiler. When the ions were injected during high and low portions of the temperature cycle, the same responses were not noted. Elevating the ambient temperature to 37°C prior to Ca2+ injection blocked the calciuminduced fall in TR seen at 28°C. No significant differences were observed in TR after the injection of Ca2+ under hyperthermic conditions (Fig. 2A) until 2 hr post-injection, at which time TR began to rise sharply. Likewise, lowering the ambient temperature to 20°C prevented the hyperthermic response observed when Na+ was injected under euthermic conditions. No significant differences in TR prior to or after the injection of Na+ at 20°C were observed (Fig. 2B). Body temperature of hens injected with
A
---water
35
-CCd+
30 4
TIME(mln) 0
----water -Ne+
TIMEtmin) Fig. 3. (A) Respiration rate response to ICV injection of 10~1 Car+ or ddH,O under hypertherrnic environment (37°C). (B) Respiration rate response to XV injection of 10 ~1 Na+ or ddH,O under 20°C ambient temperature.
A. A. MAKI et al.
478
20II)-” 131 14
i -30
,
I
-10
,
t to
I,,
20
,
30
,
,,,I
TO
30
1,
t10
TIME(min)
Fig. 4. Peripheral (foot) temperature response to ICV injection of 10 pl CaZ+,Na+, or H,O at 37 or 20°C environmental temperatures.
ddHr0 rose slightly at 37°C and decreased slightly at 20°C. Respiration rate (RR) At 37”C, RR was significantly depressed by ICV Cazf but elevated in hens receiving ddH,O (Fig. 3A). In contrast, at ZO”C, Na+ produced a significant increase in RR at 20-30 min post-injection (Fig. 3B). By 40-50 min post-injection, RR decreased, and by 70-l 20 min, was significantly lower than pre-injection levels. Respiration rate of hens receiving ddHr0 at 20°C did not change.
Foottemperature (Tr) Intra~rebroventricular injection of Ca*+ or ddH,O at 37°C had no significant effects on Tr (Fig. 4). Moreover, there were no significant differences in T,following ICV Na+ or ddHr0, although it appeared to decrease in both groups over time (Fig. 4). DISCUSSION The results indicate that the laying hen responds to ICV Na+ and Ca*+ at 28°C by regulatory mechanisms similar to those of mammals (Myers, 1974) and broilers (Denbow and Edens, 1980). Under thermal challenge, however, the action of added Ca2+ on the cells of the hypoth~amus appears to be prevented. According to Myers (1982), this prevention occurs only when the animal is introduced into a high T, prior to injection of the cation, as was the case in the present study. It is hypothesized, therefore, in this study, that a thermal stressor may have triggered an endogenous ion shift in the hypothalamus to prevent the effect of added exogenous cation. If thermal loading on the set-point had already caused the threshold for endogenous Ca2+ to be exceeded, because hens were placed in 37°C prior to administration of Ca2+, addition of exogenous Ca2+ would not further affect TR.
Whether the unexpected drop in RR observed after Ca2+ injection is a direct or an indirect result of the cation, is unclear. Although several hypotheses might be advanced in explanation, it is possible that the RR depression may have been that normally seen during heat stress; even though hens in this study were not severely stressed, the added Ca2+ could have exacerbated the effect. Sodium, which under 28°C produced a significant increase in TR,had no similar effect at 20°C. The very slight increase observed in TR and RR after Na+ injection may have been caused by Na+-administration near the lower end of the thermoneutral zone of the hen; thus its normally hyperthermic effect was incomplete. Alternatively, an endogenous increase in Na+ may have, together with the added Na+, caused a slight increase in temperature above that which would have been observed in response to endogenous Na+ alone at 20°C. presumably, at yet lower Tu (i.e. well below thermoneutrality), an endogenous Na+ shift, similar to the Ca2+ shift at 37”C, would occur, making additional exogenous Na+ ineffective. In conclusion, Na+ and Ca2+ ions in the hypothalamus appear to function in the regulatory mechanisms of body temperature of the laying hen as in the broiler and many mammals. The ions may act initially to prevent changes in set-point temperature and, as a consequence, to the respiration center in the hy~thalamus. However, if an extreme heat challenge surpasses the upper threshold of the regulatory range, the endogenous cation concentration may respond to establish a new set-point. Likewise, temperatures below thermoneutral, although not measured explicitly in this study, appear to elicit regulatory shifts of Na+ in an attempt to prevent excessive drops in body temperature. Questions raised by this study include baseline levels of cations in avian CSF, as well as endogenous shifts in those levels and their ratios, in response to various environmental temperatures. Studies are underway to address these questions.
ICV Ca2* and Na+ in laying hens Acknowledgements-The authors wish to thank Drs J. Kinder and G. Tharp for critical review of the manuscript, and K. Hannah for manuscript preparation. Research was funded by USDA Northeast Regional Research Funds, Project NE-127. REFERENCES Denbow D. M. and Edens F. W. (1980) Effects of intraventricular injections of sodium and calcium on body temperature in the chicken. Am. J. Physiol. 239, R62-R65. Feldberg W. and Saxena P. N. (1970) Mechanism of action of pyrogens. J. Physiol. 2x1, 245-261. Feldberg W., Myers R. D. and Veale W. L. (1970) Perfusion from cerebral ventricle to cisterna magna in the unanesthetized cat. Effect of calcium on body temperature, J. Physiol. 207, 403416. Greenleaf J. E., Kozlowski S., Nazara K., Kaciuba-Usciko H. and Brezezinska Z. (1974) Temperature responses to infusion of electrolytes huring exercise. In Temperature Re~tion and Druz Action (Edited bv Lomax P.. Schonbairn E. and Jacob J.), pp..352-360:Karger, Basel. Hasama B. (1930) Pharmakologische and Physiologische Studien uber die Schweissexentren. Arch. exp. Puthol. Pharmakol. 153, 291-308. Karten H. and Hodos W. (1967) Siereotuxic Atlas of the Brain of the Pigeon. Johns Hopkins Press, Baltimore.
479
Myers R. D. (1974) Ionic concepts of the set-point for body temperature. In Recent Studies of Hypothalamic Function (Edited by Lederis K. and Cooper K. E.), pp. 371-390. Karger, Base]. Myers R. D. (1982) The role of ions in ~e~o~~ation and fever. In Handbook of Exwrimental Pharmacotozv, 1. Vol. 60, 151-186. Myers R. D. and Brophy P. D. (1972) Temperature changes in the rat produced by altering the sodium-calcium ratio in the cerebral ventricles. Neurophnrmacotogy 11, 351-361. Myers R. D. and Buckman 3. E. (1972) Deep hypothermia induced in the golden hamster altering cerebral calcium levels. Am. J. Physiot. 223, 1313-1318. Myers R.D., Veale W.L. and Yaksh T. L. (1971) Changes in body temperature of the unanesthetized monkey produced by sodium and calcium ions perfused through the cerebral ventricles. J. Physiot. 217, 381-392. Nasser A. Y. (1986) Performance of old and young laying hens as affe&d by two cyclic temperature p&e&. %I,?% Thesis, University of Nebraska. Lincoln. NE. Saxena P. N. (1976) Sodium and c&ium ions in the control of temperature set-point in the pigeon. Br. J. Pharm. 56, 187-192. Xin H. (1985) Poultry energetics as influenced by atmospheric ammonia and temperature. M.S. Thesis, University of Nebraska, Lincoln, NE.
Vol. 89A, No. 3, pp. 415419, 1988
0300-9629/88 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain
RESPONSES OF LAYING THERMOREGULATORY HENS UNDER CYCLIC ENVIRONMENTAL TEMPERATURE TO INTRAVENTRICULAR CALCIUM AND SODIUM A. A. MAKI,* M. M. BECK,* E. W. GLEAVES* and J. A. DESHAZER~
*Animal Science Department and tAgricultura1 Engineering Department, University of Nebraska, Lincoln, NB 68583-0908, USA (Received 22 June 1987) Abstract-l.
Hens received ICV injections of Ca*+ (1.98 g/100 ml) or Na+ (7.25 g/100 ml) at 28°C and, following acclimation, at 37 or 2O”C,respectively. 2. At 28°C (thermoneutrality), rectal temperature rose (P -Z0.05) following Na+ and fell (P < 0.05) following Ca*+, similar to mammals and broiler chickens.
3. At 37°C Ca*+-induced hypothermia did not occur; nor did the Na+-associated hyperthermia at 20°C. 4. Acclimation to a high or low temperature may produce an endogenous shift in CSF ion levels that make additional ion administration ineffective in affecting body temperature.
MATERIALS AND METHODS
INTRODUCXION
Early this century, physiologists discovered that systemic injections of calcium and other cations cause an abrupt change in the body temperature of an animal (Freund, 1911, as cited by Meyers, 1982). By the 193Os, experimental technology had advanced to the extent that Hasama (1930) was able to examine the effect of cations injected directly into the hypothalamus. He found that excess sodium, potassium and barium produced hyperthermia in the unanesthetized cat, whereas excess calcium caused an intense reduction in body temperature. Many years later, and upon finding that calcium blocks the conscious cat’s sodium-induced temperature increase, Feldberg et al. (1970) speculated that the temperature’s set point could have its origin in a balance between these cations. Within a few years, it was discovered that there are apparently no differences between the mammalian species studied in the hyperand hypothermic effects evoked by Na+ and Ca*+ ions, respectively (Feldberg and Saxena, 1970; Myers et al., 1971; Myers and Brophy, 1972; Myers and Buckman, 1972; Greenleaf et al., 1974; Saxena, 1976). Denbow and Edens (1980), working with broilers under thermoneutral (26°C) conditions, found that rectal temperature increased or decreased in response to Na+ or Ca*+ introduced into the cerebrospinal fluid. Although the broiler response appears to be similar to that of mammals, no studies have been conducted with the laying hen, a species with very large dietary calcium requirements. In addition, recent studies at the University of Nebraska (Xin, 1985; Nasser, 1986) with the laying hen prompted further investigations into the ionic/cellular controls of thermoregulation under cyclic temperatures. The objective of this study was to characterize the temperature response to cations of the laying hen (Callus domesticus) under thermoneutral conditions, and near both limits of the thermoneutral zone.
Twenty-four White Leghorn laying hens (Gahs domesticus), aged 44 weeks, supplied by the Poultry Research Complex at the University of Nebraska, were caged individually- in environmentally controlled chambers- The daily temperature cycle, 20-37”C, was controlled by a sine-wave function such that the maximum occurred during photophase (12:00-3:00 p.m.). The temperature decreased gradually after this time to reach a minimum at 12:00-3:00 a.m. During each experimental period, all laying hens were housed in cyclic environmental chambers for at least 3 days prior to cation administration. Sixteen hr of light and 8 hr of darkness were maintained, with lights on from 5:OOa.m.-9%) p.m. Feed and water were available ud libiturn. Surgical preparation An 8-mm needle of a single 24gauge intravenous catheter placement unit was implanted stereotaxically into the third ventricle of each hen under chloropent anesthesia (Napentobarbital-chloralhydrate; Fort Dodge, IA). Stereotaxic coordinates (A6.5; V4.5; LO.OO),as suggested by Karten and Hodos (1967), were used. Three self-tapoina screws were fixed into the skull around the cannula, and an-acrylic dental cement (Coe tray plastic, Coe Labs, Chicago, IL) was used to secure the cannula which was covered by a removable plastic tip. Alcohol was applied to the incision to prevent bacterial infection and at least 2 weeks were allowed for recovery. Cannula placement was verified by cation injection under normal ambient temperature (28°C) (Denbow and Edens, 1980) and by visual examination of brains at the end of the experiment.
475
Cation solutions Injection solutions of NaCl (7.25g/lOOml) and CaCl, (1.98g/lOOml) were prepared to contain 1.24M Na+ and 134.7 mM Ca*+ ions, respectively. These concentrations are equivalent to 8 and 45 times the respective concentration of Na+ (155 mM) and Ca*+ (3 mM), respectively, reported in
A. A. MAKIet al.
476
42-
I -20
1
I -10
I
, 10
I
, 20
,
I 56
I
I 70
,
I
, 20
I 110
TIME(min) Fig. 1. Body temperature responses to ICV injection of 10~1 Ca2+ or Na+ under euthermic (28°C) environment.
A
-----water
44
--cd’ 43 E +
42
41 1 f
I,, -30
, -10
,
, 10
,
, 30
,
* 50
,
, 70
,
, 90
.,
, 110
TIME(min) 8 -----water 44-
-NNd
43-
T,,,,,,,,,,,,,,,, -30
-10
10
30
50
70
90
110
tlME(mln) Fig. 2. (A) Body temperature response to ICV injection of 10~1 Ca2+ or ddH,O under hyperthermic (37°C) environment. (B) Body temperature response to ICV injection of 10 pl Na+ or ddH,O under 20°C ambient temperature.
ICV Ca2+ and Na+ in laying hens chicken cerebrospinal fluid (CSF) (Hillman, as cited by Denbow and Edens, 1980). Ten PI of solution were introduced through the cannula by a Hamilton syringe, Ca2+ at 37”C, Na+ at 20°C. Pyrogen-free double-distilled deionized water (ddH,O) was injected as a control solution. Physiological measurements
Thermistors (Yellow Spring Instruments model 401), inserted rectally and taped to the tail feathers, and elastic bellows fastened around the body, were used to record rectal temperature and respiration rate on a dynograph recorder (Beckman type 9858). Foot temperature was recorded as an indicator of peripheral temperature by means of thermocouples placed under a scale on the lateral right shank (Hillman, personal communication) and connected to a potentiometer (Hewlett strip chart recorder model 7100B). Rectal temperature, respiration rate, and peripheral temperature were recorded for a duration of 1 min at IO-min intervals for a total of 180min. The recording was begun 30min prior to, and continued 120min after, injection of Na+, Ca2+, or water. Data were analyscd by regression techniques (GLM) and analysis of variance (ANOVA). Differences imply statistical significance at P < 0.05, unless otherwise stated.
411 RESULTS
Body temperature (Ta) At an ambient temperature (To) of 28”C, a fall in TR was induced by the ICV injection of 10~1 of Caz+. Conversely, a sharp rise in TRwas produced by the introduction of Na+ into the same area (Fig. 1). These results are in agreement with the findings of Denbo and Edens (1980) in the broiler. When the ions were injected during high and low portions of the temperature cycle, the same responses were not noted. Elevating the ambient temperature to 37°C prior to Ca2+ injection blocked the calciuminduced fall in TR seen at 28°C. No significant differences were observed in TR after the injection of Ca2+ under hyperthermic conditions (Fig. 2A) until 2 hr post-injection, at which time TR began to rise sharply. Likewise, lowering the ambient temperature to 20°C prevented the hyperthermic response observed when Na+ was injected under euthermic conditions. No significant differences in TR prior to or after the injection of Na+ at 20°C were observed (Fig. 2B). Body temperature of hens injected with
A
---water
35
-CCd+
30 4
TIME(mln) 0
----water -Ne+
TIMEtmin) Fig. 3. (A) Respiration rate response to ICV injection of 10~1 Car+ or ddH,O under hypertherrnic environment (37°C). (B) Respiration rate response to XV injection of 10 ~1 Na+ or ddH,O under 20°C ambient temperature.
A. A. MAKI et al.
478
20II)-” 131 14
i -30
,
I
-10
,
t to
I,,
20
,
30
,
,,,I
TO
30
1,
t10
TIME(min)
Fig. 4. Peripheral (foot) temperature response to ICV injection of 10 pl CaZ+,Na+, or H,O at 37 or 20°C environmental temperatures.
ddHr0 rose slightly at 37°C and decreased slightly at 20°C. Respiration rate (RR) At 37”C, RR was significantly depressed by ICV Cazf but elevated in hens receiving ddH,O (Fig. 3A). In contrast, at ZO”C, Na+ produced a significant increase in RR at 20-30 min post-injection (Fig. 3B). By 40-50 min post-injection, RR decreased, and by 70-l 20 min, was significantly lower than pre-injection levels. Respiration rate of hens receiving ddHr0 at 20°C did not change.
Foottemperature (Tr) Intra~rebroventricular injection of Ca*+ or ddH,O at 37°C had no significant effects on Tr (Fig. 4). Moreover, there were no significant differences in T,following ICV Na+ or ddHr0, although it appeared to decrease in both groups over time (Fig. 4). DISCUSSION The results indicate that the laying hen responds to ICV Na+ and Ca*+ at 28°C by regulatory mechanisms similar to those of mammals (Myers, 1974) and broilers (Denbow and Edens, 1980). Under thermal challenge, however, the action of added Ca2+ on the cells of the hypoth~amus appears to be prevented. According to Myers (1982), this prevention occurs only when the animal is introduced into a high T, prior to injection of the cation, as was the case in the present study. It is hypothesized, therefore, in this study, that a thermal stressor may have triggered an endogenous ion shift in the hypothalamus to prevent the effect of added exogenous cation. If thermal loading on the set-point had already caused the threshold for endogenous Ca2+ to be exceeded, because hens were placed in 37°C prior to administration of Ca2+, addition of exogenous Ca2+ would not further affect TR.
Whether the unexpected drop in RR observed after Ca2+ injection is a direct or an indirect result of the cation, is unclear. Although several hypotheses might be advanced in explanation, it is possible that the RR depression may have been that normally seen during heat stress; even though hens in this study were not severely stressed, the added Ca2+ could have exacerbated the effect. Sodium, which under 28°C produced a significant increase in TR,had no similar effect at 20°C. The very slight increase observed in TR and RR after Na+ injection may have been caused by Na+-administration near the lower end of the thermoneutral zone of the hen; thus its normally hyperthermic effect was incomplete. Alternatively, an endogenous increase in Na+ may have, together with the added Na+, caused a slight increase in temperature above that which would have been observed in response to endogenous Na+ alone at 20°C. presumably, at yet lower Tu (i.e. well below thermoneutrality), an endogenous Na+ shift, similar to the Ca2+ shift at 37”C, would occur, making additional exogenous Na+ ineffective. In conclusion, Na+ and Ca2+ ions in the hypothalamus appear to function in the regulatory mechanisms of body temperature of the laying hen as in the broiler and many mammals. The ions may act initially to prevent changes in set-point temperature and, as a consequence, to the respiration center in the hy~thalamus. However, if an extreme heat challenge surpasses the upper threshold of the regulatory range, the endogenous cation concentration may respond to establish a new set-point. Likewise, temperatures below thermoneutral, although not measured explicitly in this study, appear to elicit regulatory shifts of Na+ in an attempt to prevent excessive drops in body temperature. Questions raised by this study include baseline levels of cations in avian CSF, as well as endogenous shifts in those levels and their ratios, in response to various environmental temperatures. Studies are underway to address these questions.
ICV Ca2* and Na+ in laying hens Acknowledgements-The authors wish to thank Drs J. Kinder and G. Tharp for critical review of the manuscript, and K. Hannah for manuscript preparation. Research was funded by USDA Northeast Regional Research Funds, Project NE-127. REFERENCES Denbow D. M. and Edens F. W. (1980) Effects of intraventricular injections of sodium and calcium on body temperature in the chicken. Am. J. Physiol. 239, R62-R65. Feldberg W. and Saxena P. N. (1970) Mechanism of action of pyrogens. J. Physiol. 2x1, 245-261. Feldberg W., Myers R. D. and Veale W. L. (1970) Perfusion from cerebral ventricle to cisterna magna in the unanesthetized cat. Effect of calcium on body temperature, J. Physiol. 207, 403416. Greenleaf J. E., Kozlowski S., Nazara K., Kaciuba-Usciko H. and Brezezinska Z. (1974) Temperature responses to infusion of electrolytes huring exercise. In Temperature Re~tion and Druz Action (Edited bv Lomax P.. Schonbairn E. and Jacob J.), pp..352-360:Karger, Basel. Hasama B. (1930) Pharmakologische and Physiologische Studien uber die Schweissexentren. Arch. exp. Puthol. Pharmakol. 153, 291-308. Karten H. and Hodos W. (1967) Siereotuxic Atlas of the Brain of the Pigeon. Johns Hopkins Press, Baltimore.
479
Myers R. D. (1974) Ionic concepts of the set-point for body temperature. In Recent Studies of Hypothalamic Function (Edited by Lederis K. and Cooper K. E.), pp. 371-390. Karger, Base]. Myers R. D. (1982) The role of ions in ~e~o~~ation and fever. In Handbook of Exwrimental Pharmacotozv, 1. Vol. 60, 151-186. Myers R. D. and Brophy P. D. (1972) Temperature changes in the rat produced by altering the sodium-calcium ratio in the cerebral ventricles. Neurophnrmacotogy 11, 351-361. Myers R. D. and Buckman 3. E. (1972) Deep hypothermia induced in the golden hamster altering cerebral calcium levels. Am. J. Physiot. 223, 1313-1318. Myers R.D., Veale W.L. and Yaksh T. L. (1971) Changes in body temperature of the unanesthetized monkey produced by sodium and calcium ions perfused through the cerebral ventricles. J. Physiot. 217, 381-392. Nasser A. Y. (1986) Performance of old and young laying hens as affe&d by two cyclic temperature p&e&. %I,?% Thesis, University of Nebraska. Lincoln. NE. Saxena P. N. (1976) Sodium and c&ium ions in the control of temperature set-point in the pigeon. Br. J. Pharm. 56, 187-192. Xin H. (1985) Poultry energetics as influenced by atmospheric ammonia and temperature. M.S. Thesis, University of Nebraska, Lincoln, NE.