Rad. and Environm. Biophys. 14, 251--256 (1977)
Radiation and Environmental Biophysics © Springer-Vedag 1977
Dose Rate and Oxygen Consumption Rate in Mice Confined in a Small Holder During Exposure to 2450 MHz Microwave Radiation Henry S. Ho and William P. Edwards U.S. Department of Health, Education, and Welfare, Pubfic Health Service, Food and Drug Administration, Bureau of Radiological Health, 5600 Fishers Lane, Rockville, Maryland 20857, USA
Summary. The average dose rate and oxygen consumption rate of an individual mouse in a small holder during exposure to 2450 MHz CW microwave radiation are determined. The environmental conditions are 24 ° C temperature, 55% relative humidity, and 78 ml/min airflow. A forward power of 1.7 W resulted in the average dose rates of 31.0 mW/g, and 23.6 mW/g respectively, for the animals irradiated in the small, and the large holders. The results support the hypothesis that previously observed reduction in microwave energy absorption during irradiation is due to the orientation and positioning of the animal's body with respect to the microwave field. Relatively higher rate of oxygen consumption of the tightly confined sham-irradiated animals in comparison to that of the animals in the large holder is observed. Although a decrease in oxygen consumption rate is observed during exposure for the microwave irradiated animals in the small holder, the magnitude of this decrease is not more than that of the animals irradiated in the large holder. Thus the lack of reduction in the absorption of microwave energy is not compensated by a correspondingly large decrease in oxygen consumption, resulting in a larger heat load and perhaps larger stress to animals confined in the small holder.
Introduction Previous investigations (Monahan and Ho, 1975; Monahan and Ho, 1976; Ho and Edwards, 1976) indicate decreased dose rate and oxygen consumption rate of mice during exposure to sufficiently intense 2450 MHz CW microwave radiation. It was hypothesized that the animals orient or position themselves to reduce their microwave energy absorption. This assumption is supported by measurements which indicated orientational dependence of microwave energy absorption of anesthetized animals. However, physical observation of active animals during irradiation was not possible in the enclosed waveguide apparatus, so that other explanations for this reduction, such as change in dielectric properties of the animal due to wetting during the irradiation may still be possible. In the present investigation, three-month old
252
H.S. Ho and W. P. Edwards
CFI male mice were singly irradiated with 2450 MHz CW microwave radiation in a small holder so that the animal could not effectively change its position or orientation. The dual purposes of this investigation are to determine whether the reduction in microwave energy absorption still occurs, and to examine the possible additional changes in oxygen consumption rate as a result of this tight confinement.
Methods and Materials
The microwave exposure system is an environmentally-controlled waveguide apparatus which was previously reported (Ho et al., 1973). The average absorbed dose rate (mW/g) was determined according to the procedure previously reported (Ho and Edwards, 1970; Ho et al., 1973; Christman et al., 1974). A block diagram of the irradiation apparatus is shown in Figure 1. Within the animal chamber, depending on the experimental protocol, the animal is either housed in a large holder 9.5 by 9.5 by 4.0 cm or small holder 3.3 by 10.0 by 2.2 cm, each made of Plexiglas (2.0 mm thick er = 2.6, tan d~ = 0.0057) and placed without support in the WR 430 waveguide. Air is pulled in from an opening at one end of the holder. The air exhausted from the holder is sampled by a temperature-controlled paramagnetic oxygen analyzer which measures the percentage of oxygen in the sample. The oxygen consumption rate is determined by the measured percentage of oxygen in the airflow sample and measured rate of airflow. The approximate specific metabolic rate (SMR) in
BLOCKDIAGRAMOF WAVEGUIDEIRRADIATIONAPPARATUS
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LOAD[_..J L.._JLOAD Fig. I. Block diagram of the environmentally-controlled2450 MHz waveguide irradiation apparatus
253
Oxygen Consumption Rate in Mice in Small Holder
mW/g of the animal can be calculated from the oxygen consumption rate and the weight of the animal by using the approximation that 4.8 calories is produced for every milliliter of oxygen consumed. Detailed description of this system has been previously reported (Ho and Edwards, 1976). Three-month-old CFI male mice were divided into four groups. A group containing 48 animals were sham-irradiated in the large holder. The remaining three groups contain 16 animals each. One group was sham-irradiated in the small holder. The remaining two groups were microwave-irradiated with a forward power of 1.7 W with one group in the large and the other in the small holders. The animals were irradiated singly in a controlled environment of 24 ° C temperature, 55% relative humidity and 78 ml/min airflow rate. The sham group were treated exactly as the microwave-irradiated groups except with 0 W forward power. The treatment was divided into three time periods of 30 min each. During the first (pre-irradiation) period, the rate of oxygen consumption of the animal was determined at 5-min intervals by means of the paramagnetic oxygen analyzer. During the second (irradiation) period, the microwave source was turned on and the oxygen consumption rate and the average absorbed dose rate were determined at 5-min intervals. The microwave power was turned off at the end of the irradiation period, and the animal was left undisturbed in the apparatus for 30 rain during the third (post-irradiation) period while the oxygen consumption rate was determined at 5-min intervals.
Results
Figure 2 shows the mean and standard error of the specific metabolic rate (SMR) in mW/g of the sham-irradiated animals in the large holder at 5-min intervals. The initial 5-min interval represents a transitional period for the instrumentation and apparatus, and therefore the indication of lower oxygen consumption rate in this first 100
T = 24°C RH=55% Air Flow = 78 ml/min
80
60
40
0
10
20
30
40
50
60
70
80
90
Time IMinutes)
Fig. 2. Mean and standard error of the specific metabolic heat rate of 48 CF1 male mice singly sham irradiated in the small holder
254
H . S . H o a n d W . P. E d w a r d s
1 O0
T = 24°C RH = 55% Air Flow = 78 ml/min
80
60
E 40
0
10
20
30
40 50 60 Time (Minutes)
70
80
90
Fig. 3. M e a n s a n d s t a n d a r d e r r o r s o f the specific m e t a b o l i c r a t e o f 16 C F I m a l e m i c e singly s h a m i r r a d i a t e d in the small h o l d e r
100
T = 24°C RH = 55% Air F l o w = 7 8 ml/min
80
6C E
0
10
20
30
40
50
60
70
80
90
Time (Minutes)
Fig. 4. Memas and standard errors of the specific metabolic rate (lower bar) and average dose rate
(upper bar) of 16 CFI male mice singlyirradiated with a forward power of 1.7 W in the large holder
period should be ignored. There is a higher oxygen consumption rate in the first 30 min than in the second and third 30-rain periods. This difference may be due to the animal exploring a new environment initially and then settling down and reducing its activity subsequently. Figure 3 shows the mean and standard error for the specific metabolic rate (SMR) of the sham-irradiated animals in the small holder. The specific metabolic rate for the animals in the small holder is generally higher than that of the animals in the larger holder. This higher value for the small holder case is sustained throughout the experiment in contrast to the decreasing characteristics for the large holder case. Figure 4 shows the means and standard errors of the specific
Oxygen Consumption Rate in Mice in Small Holder
255
100 T = 24°C RH = 55% Air Flow = 78 ml/min
80
60
E 40
~
I
2C /
-z--]-~--~
0
10
! i
20
30
40 Time
50
60
70
80
90
(Minutes)
Fig. 5. Means and standard errors of the specific metabolic rate (lower bar) and average dose rate (upper bar) of 16 CFI male mice singly irradiated with a forward power of 1.7 W in the small holder
metabolic rate and the average dose rate of 16 animals irradiated with a forward power of 1.7 W in the large holder. During the irradiation, the microwave energy absorption decreases from 28 mW/g to 21 mW/g accompanied by a decrease of specific metabolic rate from 18 mW/g to 14 mW/g. The depressed specific metaboric rate recovers somewhat during the post-irradiation period. Figure 5 is similar to Figure 4 except that the group of 16 animals were irradiated in the small holder. The average dose rate, indicated by the upper portion of the bars, remains relatively constant at 31.0 mW/g. The specific metabolic rate, however, decreases from 18.7 mW/g to 16.3 mW/g during irradiation, in contrast to the mean value of 24.1 mW/g during the pre-irradiation period. There is also a trend towards the recovery of the depressed specific metabolic rate during the post-irradiation period.
Discussion
The results of this investigation indicate that when an active mouse cannot effectively change its orientation and position, the microwave energy absorption remain practically constant. This finding supports the previously reported (Monahan and Ho, 1975) assumption that animals orient or position themselves to reduce their microwave energy absorption since other possible explanations such as the change of dielectric properties of the animal due to wetting during the irradiation are eliminated by the present findings. Comparison between the oxygen consumption rate of the animals in the small and the large holders indicates a continually nigher oxygen consumption rate, and perhaps higher level of muscular activity due to struggling for the tightly confined animals. The reduction of oxygen consumption rate of the animals in the small holder during microwave irradiation indicates that the animals in a limited way
256
H.S. Ho and W. P. Edwards
compensate for the increased heat load b y reducing its own heat production. However, this reduction o f oxygen consumption is not more than that of the animals irradiated in the large holder. Hence the lack o f ability to reduce microwave absorption is not compensated b y an additional reduction in oxygen consumption rate. The additional heat load must be dissipated b y some other means such as a d d e d heat exchange between the animal b o d y and the environment.
References Christman, C. L., Ho, H. S., Yarrow, S.: A microwave dosimetry system for measured sampled integral-dose rate. IEEE Trans. on Microwave Theory and Techniques MTT-22(12), 1267-1272 (1974) Ho, H. S., Ginns, E. I., Christman, C. L.: Environmentally-controlled waveguide irradiation facility. IEEE Trans. on Microwave Theory and Techniques MTT-21(12), 837--840 (1973) Ho, H. S., Edwards, W. P.: Dose rate related effects on the oxygen consumption of mice during and after microwave irradiation. Special Supplement to Radio Science (in press) (1977) Monahan, J. C., Ho, H. S.: Microwave-induced avoidance behavior in the mouse. Biological Effects of Electromagnetic Waves, Selected Papers of the USNC/URSI Annual Meeting, Boulder Colorado, October 20-23, 1975, Vol. I, pp. 274-282, DHEW Publication No. (FDA) 77-8010 (1976) Monahan, J. C., Ho, H. S.: Temperature dependence of microwave avoidance. Special Supplement to Radio Science (in press) (1977) Received March 14, 1977
Radiation and Environmental Biophysics © Springer-Vedag 1977
Dose Rate and Oxygen Consumption Rate in Mice Confined in a Small Holder During Exposure to 2450 MHz Microwave Radiation Henry S. Ho and William P. Edwards U.S. Department of Health, Education, and Welfare, Pubfic Health Service, Food and Drug Administration, Bureau of Radiological Health, 5600 Fishers Lane, Rockville, Maryland 20857, USA
Summary. The average dose rate and oxygen consumption rate of an individual mouse in a small holder during exposure to 2450 MHz CW microwave radiation are determined. The environmental conditions are 24 ° C temperature, 55% relative humidity, and 78 ml/min airflow. A forward power of 1.7 W resulted in the average dose rates of 31.0 mW/g, and 23.6 mW/g respectively, for the animals irradiated in the small, and the large holders. The results support the hypothesis that previously observed reduction in microwave energy absorption during irradiation is due to the orientation and positioning of the animal's body with respect to the microwave field. Relatively higher rate of oxygen consumption of the tightly confined sham-irradiated animals in comparison to that of the animals in the large holder is observed. Although a decrease in oxygen consumption rate is observed during exposure for the microwave irradiated animals in the small holder, the magnitude of this decrease is not more than that of the animals irradiated in the large holder. Thus the lack of reduction in the absorption of microwave energy is not compensated by a correspondingly large decrease in oxygen consumption, resulting in a larger heat load and perhaps larger stress to animals confined in the small holder.
Introduction Previous investigations (Monahan and Ho, 1975; Monahan and Ho, 1976; Ho and Edwards, 1976) indicate decreased dose rate and oxygen consumption rate of mice during exposure to sufficiently intense 2450 MHz CW microwave radiation. It was hypothesized that the animals orient or position themselves to reduce their microwave energy absorption. This assumption is supported by measurements which indicated orientational dependence of microwave energy absorption of anesthetized animals. However, physical observation of active animals during irradiation was not possible in the enclosed waveguide apparatus, so that other explanations for this reduction, such as change in dielectric properties of the animal due to wetting during the irradiation may still be possible. In the present investigation, three-month old
252
H.S. Ho and W. P. Edwards
CFI male mice were singly irradiated with 2450 MHz CW microwave radiation in a small holder so that the animal could not effectively change its position or orientation. The dual purposes of this investigation are to determine whether the reduction in microwave energy absorption still occurs, and to examine the possible additional changes in oxygen consumption rate as a result of this tight confinement.
Methods and Materials
The microwave exposure system is an environmentally-controlled waveguide apparatus which was previously reported (Ho et al., 1973). The average absorbed dose rate (mW/g) was determined according to the procedure previously reported (Ho and Edwards, 1970; Ho et al., 1973; Christman et al., 1974). A block diagram of the irradiation apparatus is shown in Figure 1. Within the animal chamber, depending on the experimental protocol, the animal is either housed in a large holder 9.5 by 9.5 by 4.0 cm or small holder 3.3 by 10.0 by 2.2 cm, each made of Plexiglas (2.0 mm thick er = 2.6, tan d~ = 0.0057) and placed without support in the WR 430 waveguide. Air is pulled in from an opening at one end of the holder. The air exhausted from the holder is sampled by a temperature-controlled paramagnetic oxygen analyzer which measures the percentage of oxygen in the sample. The oxygen consumption rate is determined by the measured percentage of oxygen in the airflow sample and measured rate of airflow. The approximate specific metabolic rate (SMR) in
BLOCKDIAGRAMOF WAVEGUIDEIRRADIATIONAPPARATUS
TELEPHONE LINE TOCOMPUTERo
i
r"---'--1
[ J [TERMINAL[ [ HAZALTINE CRT] [ MODEM~---~ INM~ICEI . . ~ 1 TERMINALWITH~ TAPECASSETTE
[
I
~
POWER METER 4 PORT VARIABLE
VOLTAGE INTEGRATOR DATAACQUISITION INTERFACE
VOLTAGE INTEGRATOR
POWER METER == I Pr
I
POWER METER
TPt
OIRECTIONAL[ AN,MAL I DIRECTIONALH COUPLER CHAMBER COUPLER No.1 No.2
LOADI
LOAD[_..J L.._JLOAD Fig. I. Block diagram of the environmentally-controlled2450 MHz waveguide irradiation apparatus
253
Oxygen Consumption Rate in Mice in Small Holder
mW/g of the animal can be calculated from the oxygen consumption rate and the weight of the animal by using the approximation that 4.8 calories is produced for every milliliter of oxygen consumed. Detailed description of this system has been previously reported (Ho and Edwards, 1976). Three-month-old CFI male mice were divided into four groups. A group containing 48 animals were sham-irradiated in the large holder. The remaining three groups contain 16 animals each. One group was sham-irradiated in the small holder. The remaining two groups were microwave-irradiated with a forward power of 1.7 W with one group in the large and the other in the small holders. The animals were irradiated singly in a controlled environment of 24 ° C temperature, 55% relative humidity and 78 ml/min airflow rate. The sham group were treated exactly as the microwave-irradiated groups except with 0 W forward power. The treatment was divided into three time periods of 30 min each. During the first (pre-irradiation) period, the rate of oxygen consumption of the animal was determined at 5-min intervals by means of the paramagnetic oxygen analyzer. During the second (irradiation) period, the microwave source was turned on and the oxygen consumption rate and the average absorbed dose rate were determined at 5-min intervals. The microwave power was turned off at the end of the irradiation period, and the animal was left undisturbed in the apparatus for 30 rain during the third (post-irradiation) period while the oxygen consumption rate was determined at 5-min intervals.
Results
Figure 2 shows the mean and standard error of the specific metabolic rate (SMR) in mW/g of the sham-irradiated animals in the large holder at 5-min intervals. The initial 5-min interval represents a transitional period for the instrumentation and apparatus, and therefore the indication of lower oxygen consumption rate in this first 100
T = 24°C RH=55% Air Flow = 78 ml/min
80
60
40
0
10
20
30
40
50
60
70
80
90
Time IMinutes)
Fig. 2. Mean and standard error of the specific metabolic heat rate of 48 CF1 male mice singly sham irradiated in the small holder
254
H . S . H o a n d W . P. E d w a r d s
1 O0
T = 24°C RH = 55% Air Flow = 78 ml/min
80
60
E 40
0
10
20
30
40 50 60 Time (Minutes)
70
80
90
Fig. 3. M e a n s a n d s t a n d a r d e r r o r s o f the specific m e t a b o l i c r a t e o f 16 C F I m a l e m i c e singly s h a m i r r a d i a t e d in the small h o l d e r
100
T = 24°C RH = 55% Air F l o w = 7 8 ml/min
80
6C E
0
10
20
30
40
50
60
70
80
90
Time (Minutes)
Fig. 4. Memas and standard errors of the specific metabolic rate (lower bar) and average dose rate
(upper bar) of 16 CFI male mice singlyirradiated with a forward power of 1.7 W in the large holder
period should be ignored. There is a higher oxygen consumption rate in the first 30 min than in the second and third 30-rain periods. This difference may be due to the animal exploring a new environment initially and then settling down and reducing its activity subsequently. Figure 3 shows the mean and standard error for the specific metabolic rate (SMR) of the sham-irradiated animals in the small holder. The specific metabolic rate for the animals in the small holder is generally higher than that of the animals in the larger holder. This higher value for the small holder case is sustained throughout the experiment in contrast to the decreasing characteristics for the large holder case. Figure 4 shows the means and standard errors of the specific
Oxygen Consumption Rate in Mice in Small Holder
255
100 T = 24°C RH = 55% Air Flow = 78 ml/min
80
60
E 40
~
I
2C /
-z--]-~--~
0
10
! i
20
30
40 Time
50
60
70
80
90
(Minutes)
Fig. 5. Means and standard errors of the specific metabolic rate (lower bar) and average dose rate (upper bar) of 16 CFI male mice singly irradiated with a forward power of 1.7 W in the small holder
metabolic rate and the average dose rate of 16 animals irradiated with a forward power of 1.7 W in the large holder. During the irradiation, the microwave energy absorption decreases from 28 mW/g to 21 mW/g accompanied by a decrease of specific metabolic rate from 18 mW/g to 14 mW/g. The depressed specific metaboric rate recovers somewhat during the post-irradiation period. Figure 5 is similar to Figure 4 except that the group of 16 animals were irradiated in the small holder. The average dose rate, indicated by the upper portion of the bars, remains relatively constant at 31.0 mW/g. The specific metabolic rate, however, decreases from 18.7 mW/g to 16.3 mW/g during irradiation, in contrast to the mean value of 24.1 mW/g during the pre-irradiation period. There is also a trend towards the recovery of the depressed specific metabolic rate during the post-irradiation period.
Discussion
The results of this investigation indicate that when an active mouse cannot effectively change its orientation and position, the microwave energy absorption remain practically constant. This finding supports the previously reported (Monahan and Ho, 1975) assumption that animals orient or position themselves to reduce their microwave energy absorption since other possible explanations such as the change of dielectric properties of the animal due to wetting during the irradiation are eliminated by the present findings. Comparison between the oxygen consumption rate of the animals in the small and the large holders indicates a continually nigher oxygen consumption rate, and perhaps higher level of muscular activity due to struggling for the tightly confined animals. The reduction of oxygen consumption rate of the animals in the small holder during microwave irradiation indicates that the animals in a limited way
256
H.S. Ho and W. P. Edwards
compensate for the increased heat load b y reducing its own heat production. However, this reduction o f oxygen consumption is not more than that of the animals irradiated in the large holder. Hence the lack o f ability to reduce microwave absorption is not compensated b y an additional reduction in oxygen consumption rate. The additional heat load must be dissipated b y some other means such as a d d e d heat exchange between the animal b o d y and the environment.
References Christman, C. L., Ho, H. S., Yarrow, S.: A microwave dosimetry system for measured sampled integral-dose rate. IEEE Trans. on Microwave Theory and Techniques MTT-22(12), 1267-1272 (1974) Ho, H. S., Ginns, E. I., Christman, C. L.: Environmentally-controlled waveguide irradiation facility. IEEE Trans. on Microwave Theory and Techniques MTT-21(12), 837--840 (1973) Ho, H. S., Edwards, W. P.: Dose rate related effects on the oxygen consumption of mice during and after microwave irradiation. Special Supplement to Radio Science (in press) (1977) Monahan, J. C., Ho, H. S.: Microwave-induced avoidance behavior in the mouse. Biological Effects of Electromagnetic Waves, Selected Papers of the USNC/URSI Annual Meeting, Boulder Colorado, October 20-23, 1975, Vol. I, pp. 274-282, DHEW Publication No. (FDA) 77-8010 (1976) Monahan, J. C., Ho, H. S.: Temperature dependence of microwave avoidance. Special Supplement to Radio Science (in press) (1977) Received March 14, 1977
