〔543〕
Sensory
Irritation of the Upper Respiratory Tract by Sulfur Dioxide Ichiro
Department
Wakisaka
of Public Health, Faculty of Medicine, Kagoshima
University,
Kagoshima
INTRODUCTION
Studies on the mechanism of respiratory response to airborne chemical irritants are interesting especially regarding the problem of evaluating the irritant potential of air pollution. It has been shown that stimulation of the trigeminal nerve endings by chemical irritants is followed by a decrease in respiratory rate which is proportional to the inhaled concentration1,2). There are previous reports indicating that sulfur dioxide, a common chemical irritant in the atmospheric pollutants, can stimulate the trigeminal nerve endings in the nasal mucosa as well
as receptors in the tracheobronchial tree1,3-5).Alarie et al2) have shown
in mice that
stimulation of the cholinergic nerve endings of the trigeminus by sulfur dioxide was followed by a decrease in respiratory rate and that this reaction was accompanied with desensitization within a few minutes of continuous exposure. This pattern of respiratory response has been explained based on the reducing properties of sulfite to disulfide bonds in a receptor protein. In the present paper, some additional experiments were made to investigate the mode of sensory irritation and desensitization by sulfur dioxide. MATERIALS
AND
METHODS
The dd strain mice weighing about 25 gram were used in this experiment. Four mice were exposed simultaneously to each concentration of SO2. Each mouse was put into a small sealed plastic tube forming a body plethysmograph which was attached to a plastic exposure chamber with a volume of approximately 2 liters. The respiratory rate was obtained by recording pressure changes in the tube. The sulfur dioxide for exposure was obtained from a gas cylinder containing approximately 1000 ppm of SO2 in nitrogen. This mixture of gases was metered into the exposure chamber and diluted with the incoming room air to obtain different exposure concentrations. In all instances, samples taken of the exposure atmosphere were analyzed for SO2 by the West and Gaeke's method6). The respiratory rate was calculated for each 15-second interval and the percent decrease in respiratory rate was obtained by the comparison with the respiratory rate determined for each animal prior to exposure to SO2. The experimental conditions are as follows: 1. Seven groups, each of four mice, were exposed to SO2 at the concentration of 0 (room air), 23, 38, 75, 128, 250 or 500 ppm for 10 minutes. The respiratory rate was calculated for each 15-second interval during the first 5 minutes of exposure and every minute thereafter until the end of exposure. 2. Four groups, each of four mice, were exposed to xylocain aerosol of approximately 0.1 mg/liter air for 5 minutes immediately followed by exposure to SO2 at the concentration of 36, 75, 130 or 307 ppm for 10 minutes. For xylocain aerosol generation, a Dautrebande generator7) was used with 4% xylocain solution. 15 seconds.
The respiratory rate was calculated every
〔544〕JAPANESE
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3. Two groups, each of four mice, were exposed repeatedly to SO2 at the concentration of 89 ppm or 278 ppm for 2 minutes with 2 minutes of fresh room air for recovery between successive exposures. The respiratory rate was calculated every 15 seconds. 4. Six groups, each of four mice, were exposed to 23 ppm SO2 followed by 43, 108 or 308 ppm SO2, to 43 ppm SO2 followed by 114 or 316 ppm SO2, and to 108 ppm SO2 followed by 316 ppm SO2, respectively. In each 10 minutes' exposure, the respiratory rate was calculated every 15 seconds during the first 5 minutes of exposure and every minute thereafter. RESULTS
The results obtained when mice were exposed to various concentrations of SO2 for 10 minutes are shown in Figs. 1 and 2. Each data point represents the mean obtained with four mice exposed simultaneously and each line was obtained by plotting the moving average of each three consecutive data points. As shown in the figures, a rapid decrease in respiratory rate was observed at the beginning of exposure and the maximum response was obtained for each exposure. When the percent decrease in respiratory rate for the maximum response was plotted against the logarithm of the corresponding exposure consentration, a linear dose-response curve was obtained as shown in Fig. 3-A. The time needed for the respiratory rate to reach a maximum decrease was dependent on the concentration of SO2, i.e., the higher the concentration of SO2, the faster the induction of maximum respiratory depression. Following the maximum decrease, the respiratory depression remained relatively constant for a short period of time and then gradually returned toward preexposure level while exposure continued. The recovery of respiratory rate proceeded slowly over a longer period of time and appeared to approach a different asymptote for each level of exposure concentration. In fact, the return of respiratory rate toward preexposure level was not complete at the end of 10 minutes' exposure for all but the lowest concentration tested. However, the slope of the recovery curve for each exposure appeared to be almost the same independently of the concentration of SO2 and indicates tration. The results obtained when
Fig. 1
that
the recovery
mice were exposed
○ ○ ○
0ppm 38ppm 128ppm
●
500ppm
rate
to xylocain
aerosol
by the exposure
concen-
for 5 minutes
immedi-
○ ○ ●
(control)
Time-response curves during exposure to sulfur dioxide (0, 38, 128 and 500 ppm)
is not affected
Fig. 2
23ppm 75ppm 250ppm
Time-response curves during exposure to sulfur dioxide (23, 75 and 250ppm)
VOL.
30, NO.
5, DECEMBER
1975〔545〕
○-36 ◇-75 □-130 ●-307
Fig.
4
Time-response
curves
to sulfur dioxide hetized mice Filled circles: normal mice, Open circles; xylocain anesthetized mice Curves fitted by the method of least squares, Y=37.716 log X-26.714 (A) for nornal mice, Y=33.490 log X-25.488 (B) for xylocain anesthetized
Fig.
3
Dose-response sulfur dioxide anesthetized
Fig.
curves with
during normal
with
during
ppm ppm ppm ppm
exposure
xylocain
anest-
mice.
exposure to and xylocain
mice
5
Time-response sulfur dioxide
curves obtained with a duration
of
for
room
air
recovery
○
Room
□ ●
Exposure Exposure
air
for
recovery
to SO2 to SO2
of 89 ppm of 278 ppm
with repeated exposures to of 2 minutes with 2 minutes
between
successive
exposures
ately followed by exposures to various concentrations of SO2 for 10 minutes are shown in Fig. 4. The data prints for exposure to xylocain aerosol were omitted because mice exposed for 5 minutes to xylocain aerosol alone did not elicit any change in respiratory rate. With exposure to SO2, the pattern of respiratory response was exactly similar to that for normal mice not previously exposed to xylocain aerosol; i.e., a rapid decrease in respiratory rate was followed by a gradual return toward control level. A does-response curve obtained with these local anesthetized mice is presented in Fig. 3-B and comparable to that obtained with normal mice. Fig. 5 shows the results obtained when mice were exposed repeatedly to SO2 at the concentration of 89 or 278 ppm for 2 minutes with a recovery period of 2 minutes between successive exposures. The maximum response for each exposure showed a gradual decrease with the repitition of this process and the plots of each maximum response for each concentra-
〔546〕JAPANESE
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tion of SO2 against the number of repeat were fitted by parallel lines indicating an independence of exposure concentration on the decreasing rate of response. In fact, after the third exposure to 89 ppm SO2 and after the fifth one to 278 ppm SO2, no responses were observed. Fig. 6 presents the results obtained when two 10 minutes' exposures were made in such a way that an exposure to a given concentration of SO2 was immediately followed by another higher concentration of SO2. Each data point represents the percent decrease in respiratory rate for the maximum response at each concentration of SO2. The maximum Open circles represent the maximum percent decrease in respiratory rate for the first exposure to a given response to each subsequent exposure was concentration of SO2 and filled circles represent those for the subsequent exposure to another higher concentcalculated from the maximum decrease during ration of SO2. Each line connects the two maximum percent decreaseduring exposures to two concentrations the subsequent exposure and from the rate of SO2 in the course of one experiment. The dashed line gives a dose-response curve for normal during the last 15 seconds of the preceding mice. one. For the first exposure to a given conFig. 6 Maximum percent decrease in respiratory centration of SO2, the maximum response was rate obtained with exposures to given comparable in magnitude to the values derived concentrations of SO2 immediately folfrom the dose-response curve presented in lowed by another higher concentrations of SO2 Fig. 3-A. The mice previously exposed to a given concentration of SO2 still reacted to another higher concentration of SO2, but the magnitude of the response to this exposure was far less than the values derived from the dose-response curve for normal mice. Moreover, the maximum response to the subsequent exposure seems to be less for high than for low level of the preceding exposure concentration. DISCUSSION
Nasal mucosa is abundantly supplied with fine unmyelinated nerve endings which are readily stimulated by airborne chemical irritants in the upper airways8,9). Sulfur dioxide is a gas soluble in water and retained in the upper airways in proportion depending on the inhaled concentration, which stimulates the afferent trigeminal nerve endings followed by inhibition of the phrenic nerve activity in mice1,10). Alarie et al2) have shown in mice that the intensity of sensory irritation by SO2 was directly related to the concentration of this agent and that the sensory irritation reaction for SO2 gradually disappeared while the exposure continued. The mechanism of the time-dependent change in the sensitivity of a sensory system for exposure to SO2 has been well discussed in the previous report, suggesting that the reducing properties of sulfite toward disulfide bonds in a receptor protein induce changes in membrane permeability. The results obtained in this paper also demonstrated that sensory irritation by SO2 was accompanied with a respiratory depression which reached a maximum and was followed by a gradual recovery toward normal control level while exposure continued. These results agree approximately with those obtained previously2) but are not in complete agreement. That is, it was observed in this experiment with dd strain mice that the onset of a return of respiratory rate toward preexposure level took a relatively long time to develop and that the rapidity of desensitization process was slow, so that complete return toward preexposure level
VOL.
30, NO.
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could not be reached within 10 minutes' exposure so long as the concentration of SO2 was not extremely low. The possible difference in the strain of the experimental animal is suggested to be one of the reasons for this disagreement. Immediately following 10 minutes' exposure to a given concentration of SO2, the mice still reacted to another higher concentration of SO2 in a fashion less sensitive than the control animal not previously exposed to SO2. In addition, it seems that there is a limit in the magnitude of desensitization which depends on the concentration of SO2. The return of respiratory rate toward preexposure level was also observed with repeated brief exposure to SO2 with brief room air for recovery between successive exposures and the rate of the return was independent of the concentration of SO2. The rate of desensitization may be dependent on time of exposure rather than on concentration of SO2. Therefore, if the exposure to SO2 were turned off at any time before the maximum desensitization, the magnitude of desensitization at that time may be constant independently of the concentration of SO2. Xylocain aerosol did not inhibit the respiratory response to SO2. This result parallels the observation by Amdur11) that the cough reflex elicited by SO2 is not inhibited by procain, and indicates that the receptors sensitive to SO2 are not influenced by local anesthesia. It has been shown that the mice exposed to SO2 while breathing through a tracheal cannula so that the gas came in contact only with the lung and not with the trachea failed to react similar to control mice breathing through the nose5). This indicates that the reflex respiratory depression by SO2 is induced by stimulation of the trigeminal nerve endings in the upper respiratory tract. Nadel et al12) studied the mechanism of bronchoconstriction produced by SO2 in cats, indicating that exposing only the upper airways produced an increase of pulmonary resistance which was blocked by atropine or by cooling of cervical vagosympathetic nerves. Similar results were reported by Widdicombe5) for cough reflex elicited by inhalation of SO2 or fine dust. He also found that after several inhalations of SO2, the animals became completely refractory to the gas but gave normal response to mechanical stimulation of the trachea. Frank et al13) have reported that in human subjects when SO2 was administered twice in the course of one experiment, the response to the second exposure was less than that to the first one. The adaptation phenomenon in human subjects or in cats seems to be similar to the desensitization phenomenon in mice. However, no desensitization phenomenon has been observed in guinea pigs14). A rapid decrease in respiratory rate can be considered as a quick and convenient function to protect harmful dose of irritant chemicals from reaching the alveoli. It has been shown that if mortality is chosen as a criterion of the toxicity, mice were less resistant than guinea pigs at high concentration of SO2 whereas guinea pigs were less resistant than mice at low concentration15,16). The state of reduced sensitivity following exposure to SO2 will increase tolerance of animals to the gas wtihin a short period of time but it may be one of the factors in the production of toxic effects with a prolonged period of exposure. SUMMARY
The respiratory reaction produced by sulfur dioxide was studied with the dd strain mice. Sensory irritation by sulfur dioxide was accompanied with a rapid decrease in respiratory rate which reached a peak within a few minutes and was followed by a gradual recovery while exposure continued. The degree of maximum response was directly proportional to the exposure concentration. Xylocain aerosol did not change the respiratory response to sulfur dioxide, indicating that the receptor sensitive to sulfur dioxide can not be affected by local anesthesia. The mice desensitized by a given concentration of SO2 to its sensory irritation still reacted to another
〔548〕JAPANESE
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higher concentration of SO2 in a fashion less sensitive than the normal mice not previously exposed to SO2. The rate of desensitization was dependent on time rather than on concentration of SO2, but it seems that there is a certain limit in the magnitude of desensitization which depends on the concentration of SO2. REFERENCES
1) Alarie, Y.: Irritating Properties of Airborne Materials to the Upper Respiratory Tract, Arch. Environ. Health, 13, 443-449 (1966). 2) Alarie. Y., I. Wakisaka and S. Oka: Sensory Irritation by Sulfur Dioxide and Chlorobenzilidene Malononitrile, Environ. Physiol. Biochem., 3, 53-64 (1973). 3) Widdicombe, J. G.: Respiratory Reflex from the Trachea and Bronchi of the Cat, J. Physiol., 122, 55-70 (1954). 4) Nadel, J. A., Salem, H., Tamplin, B. and Tokiwa, Y.: Mechanism of Bronchoconstriction during Inhalation of Sulfer Dioxide, J. Apply. Physiol., 20, 164-167 (1965). 5) Widdicombe, J. G.: Receptor in the Trachea and Bronchi of the Cat, J. Physiol.,123, 71-104 (1954). 6) West, P. and Gaeke, G. C.: Fixation of Sulfur Dioxide as Disulfite-mercurate (II) and Subsequent Colorimetric Estimation, Anal. Chem., 28, 1816-1819 (1956). 7) Dautrebande, H. Beckmann and W. Walkanhorst : New Studies on Aerosols (III); Production of Solid, Smallsized Aerosols, Arch. Int. Pharmacodyn. CXVI, No. 1-2, 170-186 (1959). 8) Cauna, N., Hinderer, K. H, and Wentges, R. T.: Sensory receptor organs of the human nasal respiratory mucosa, Amer. J. Anat., 124, 187-193 (1969). 9) Ulrich, C. E., Haddock, M. P, and Alarie, A.: Airborne chemical irritants; role of the trigeminal nerve, Arch. Environ. Health, 24, 37-42 (1972). 10) Amdur, M. O.: The Physiological Response of Guinea Pigs to Atmospheric Pollutants, Int. J. Air Pollution, 1, 170-183 (1959). 11) Amdur, M. O.: Respiratory Absorption Data and SO2 Dose-Response Curves, Arch. Environ. Health, 12, 729-732 (1966). 12) Nadel, J. A., Salem, H., Tamplin, B., and Tokiwa, Y.: Mechanism of Bronchoconstriction, Arch. Environ. Health, 10, 175-178 (1965). 13) Frank, N. R., Amdur, M. O., and Whittenberger, J. L.: A Comparison of the Acute Effects of SO2 Administered Alone or in Combination with NaCl Particles on the Respiratory Mechanics of Healthy Adults, Int. J. Air Water Pollution, 8, 125-133 (1964). 14) Amdur, M. O. and Mead, J.: A method for studying the mechanical properties of the lungs of unanesthetized animals: Application to the study of respiratory irritants, Proceedings of 3rd National Air Pollution Symposium, Pasadena, California, p. 150-159 (1955). 15) Weedon, F. R.: Experimental Acute Gastric Ulcer Produced in Animals by Exposure to Sulfur Dioxide Gas, New York State J. Med., 42, 620-623 (1942). 16) Leong, K. J., MacFarland, H. A. and Sellers, E. A.: Acute SO2 Toxicity: Effects of Histamine Liberation, Arch. Environ. Health 3, 668-675 (1961).
亜硫酸 ガスに よる上部呼吸道 の刺激 鹿児島大学医学部公衆衛生学教室 脇 亜 硫 酸 ガ スに よ る呼 吸 反応 をddマ
阪
一
郎
ウス を用 い て研 究 した。 亜 硫 酸 ガ ス に よ る刺 激感 覚 は, 急 速 な 呼 吸数 減 少
を ひ き お こ し, 数分 後 には そ の極 に達 し, しか る後, ば くろ 中 に除 々に 回復 して い った。 最 大 反 応 の程 度 は, ば くろ 濃度 と直 接 関 係が あ る。 キ シ ロカ イ ンエ ロゾル は, 亜 硫酸 ガ スに 対 して の呼 吸 反 応 に変 化 を あ た え なか った 。 この こ とは, 亜 硫 酸 ガ スに 対 して の感 覚 受 容 器 は, 局 所 麻 酔 に よ って 影 響 を うけな い こ とを示 す 。 あ る濃 度 の 亜 硫 酸 ガス に よ って, そ の刺 激 に対 す る感 受 性 を失 っ た マ ウス は, 別 な よ り高 い濃 度 の 亜硫 酸 ガ スに 対 して 尚反 応 を示 した が, あ らか じめ亜 硫 酸 ガス へ の ば くろを うけて い な い正 常 マ ウ スよ りも感 受 性 が低 い。 感 受 性 喪失 の速 度 は, 亜 硫 酸 ガス の濃 度 よ りもむ しろ 時 間 に関 係 して い たが, 感 受性 喪 失 の 強 さに は, あ る限度 が あ り, それ は 亜 硫 酸 ガ ス の濃 度 に よ る ら しい。 (受付1975年10月2日)
Sensory
Irritation of the Upper Respiratory Tract by Sulfur Dioxide Ichiro
Department
Wakisaka
of Public Health, Faculty of Medicine, Kagoshima
University,
Kagoshima
INTRODUCTION
Studies on the mechanism of respiratory response to airborne chemical irritants are interesting especially regarding the problem of evaluating the irritant potential of air pollution. It has been shown that stimulation of the trigeminal nerve endings by chemical irritants is followed by a decrease in respiratory rate which is proportional to the inhaled concentration1,2). There are previous reports indicating that sulfur dioxide, a common chemical irritant in the atmospheric pollutants, can stimulate the trigeminal nerve endings in the nasal mucosa as well
as receptors in the tracheobronchial tree1,3-5).Alarie et al2) have shown
in mice that
stimulation of the cholinergic nerve endings of the trigeminus by sulfur dioxide was followed by a decrease in respiratory rate and that this reaction was accompanied with desensitization within a few minutes of continuous exposure. This pattern of respiratory response has been explained based on the reducing properties of sulfite to disulfide bonds in a receptor protein. In the present paper, some additional experiments were made to investigate the mode of sensory irritation and desensitization by sulfur dioxide. MATERIALS
AND
METHODS
The dd strain mice weighing about 25 gram were used in this experiment. Four mice were exposed simultaneously to each concentration of SO2. Each mouse was put into a small sealed plastic tube forming a body plethysmograph which was attached to a plastic exposure chamber with a volume of approximately 2 liters. The respiratory rate was obtained by recording pressure changes in the tube. The sulfur dioxide for exposure was obtained from a gas cylinder containing approximately 1000 ppm of SO2 in nitrogen. This mixture of gases was metered into the exposure chamber and diluted with the incoming room air to obtain different exposure concentrations. In all instances, samples taken of the exposure atmosphere were analyzed for SO2 by the West and Gaeke's method6). The respiratory rate was calculated for each 15-second interval and the percent decrease in respiratory rate was obtained by the comparison with the respiratory rate determined for each animal prior to exposure to SO2. The experimental conditions are as follows: 1. Seven groups, each of four mice, were exposed to SO2 at the concentration of 0 (room air), 23, 38, 75, 128, 250 or 500 ppm for 10 minutes. The respiratory rate was calculated for each 15-second interval during the first 5 minutes of exposure and every minute thereafter until the end of exposure. 2. Four groups, each of four mice, were exposed to xylocain aerosol of approximately 0.1 mg/liter air for 5 minutes immediately followed by exposure to SO2 at the concentration of 36, 75, 130 or 307 ppm for 10 minutes. For xylocain aerosol generation, a Dautrebande generator7) was used with 4% xylocain solution. 15 seconds.
The respiratory rate was calculated every
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3. Two groups, each of four mice, were exposed repeatedly to SO2 at the concentration of 89 ppm or 278 ppm for 2 minutes with 2 minutes of fresh room air for recovery between successive exposures. The respiratory rate was calculated every 15 seconds. 4. Six groups, each of four mice, were exposed to 23 ppm SO2 followed by 43, 108 or 308 ppm SO2, to 43 ppm SO2 followed by 114 or 316 ppm SO2, and to 108 ppm SO2 followed by 316 ppm SO2, respectively. In each 10 minutes' exposure, the respiratory rate was calculated every 15 seconds during the first 5 minutes of exposure and every minute thereafter. RESULTS
The results obtained when mice were exposed to various concentrations of SO2 for 10 minutes are shown in Figs. 1 and 2. Each data point represents the mean obtained with four mice exposed simultaneously and each line was obtained by plotting the moving average of each three consecutive data points. As shown in the figures, a rapid decrease in respiratory rate was observed at the beginning of exposure and the maximum response was obtained for each exposure. When the percent decrease in respiratory rate for the maximum response was plotted against the logarithm of the corresponding exposure consentration, a linear dose-response curve was obtained as shown in Fig. 3-A. The time needed for the respiratory rate to reach a maximum decrease was dependent on the concentration of SO2, i.e., the higher the concentration of SO2, the faster the induction of maximum respiratory depression. Following the maximum decrease, the respiratory depression remained relatively constant for a short period of time and then gradually returned toward preexposure level while exposure continued. The recovery of respiratory rate proceeded slowly over a longer period of time and appeared to approach a different asymptote for each level of exposure concentration. In fact, the return of respiratory rate toward preexposure level was not complete at the end of 10 minutes' exposure for all but the lowest concentration tested. However, the slope of the recovery curve for each exposure appeared to be almost the same independently of the concentration of SO2 and indicates tration. The results obtained when
Fig. 1
that
the recovery
mice were exposed
○ ○ ○
0ppm 38ppm 128ppm
●
500ppm
rate
to xylocain
aerosol
by the exposure
concen-
for 5 minutes
immedi-
○ ○ ●
(control)
Time-response curves during exposure to sulfur dioxide (0, 38, 128 and 500 ppm)
is not affected
Fig. 2
23ppm 75ppm 250ppm
Time-response curves during exposure to sulfur dioxide (23, 75 and 250ppm)
VOL.
30, NO.
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1975〔545〕
○-36 ◇-75 □-130 ●-307
Fig.
4
Time-response
curves
to sulfur dioxide hetized mice Filled circles: normal mice, Open circles; xylocain anesthetized mice Curves fitted by the method of least squares, Y=37.716 log X-26.714 (A) for nornal mice, Y=33.490 log X-25.488 (B) for xylocain anesthetized
Fig.
3
Dose-response sulfur dioxide anesthetized
Fig.
curves with
during normal
with
during
ppm ppm ppm ppm
exposure
xylocain
anest-
mice.
exposure to and xylocain
mice
5
Time-response sulfur dioxide
curves obtained with a duration
of
for
room
air
recovery
○
Room
□ ●
Exposure Exposure
air
for
recovery
to SO2 to SO2
of 89 ppm of 278 ppm
with repeated exposures to of 2 minutes with 2 minutes
between
successive
exposures
ately followed by exposures to various concentrations of SO2 for 10 minutes are shown in Fig. 4. The data prints for exposure to xylocain aerosol were omitted because mice exposed for 5 minutes to xylocain aerosol alone did not elicit any change in respiratory rate. With exposure to SO2, the pattern of respiratory response was exactly similar to that for normal mice not previously exposed to xylocain aerosol; i.e., a rapid decrease in respiratory rate was followed by a gradual return toward control level. A does-response curve obtained with these local anesthetized mice is presented in Fig. 3-B and comparable to that obtained with normal mice. Fig. 5 shows the results obtained when mice were exposed repeatedly to SO2 at the concentration of 89 or 278 ppm for 2 minutes with a recovery period of 2 minutes between successive exposures. The maximum response for each exposure showed a gradual decrease with the repitition of this process and the plots of each maximum response for each concentra-
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tion of SO2 against the number of repeat were fitted by parallel lines indicating an independence of exposure concentration on the decreasing rate of response. In fact, after the third exposure to 89 ppm SO2 and after the fifth one to 278 ppm SO2, no responses were observed. Fig. 6 presents the results obtained when two 10 minutes' exposures were made in such a way that an exposure to a given concentration of SO2 was immediately followed by another higher concentration of SO2. Each data point represents the percent decrease in respiratory rate for the maximum response at each concentration of SO2. The maximum Open circles represent the maximum percent decrease in respiratory rate for the first exposure to a given response to each subsequent exposure was concentration of SO2 and filled circles represent those for the subsequent exposure to another higher concentcalculated from the maximum decrease during ration of SO2. Each line connects the two maximum percent decreaseduring exposures to two concentrations the subsequent exposure and from the rate of SO2 in the course of one experiment. The dashed line gives a dose-response curve for normal during the last 15 seconds of the preceding mice. one. For the first exposure to a given conFig. 6 Maximum percent decrease in respiratory centration of SO2, the maximum response was rate obtained with exposures to given comparable in magnitude to the values derived concentrations of SO2 immediately folfrom the dose-response curve presented in lowed by another higher concentrations of SO2 Fig. 3-A. The mice previously exposed to a given concentration of SO2 still reacted to another higher concentration of SO2, but the magnitude of the response to this exposure was far less than the values derived from the dose-response curve for normal mice. Moreover, the maximum response to the subsequent exposure seems to be less for high than for low level of the preceding exposure concentration. DISCUSSION
Nasal mucosa is abundantly supplied with fine unmyelinated nerve endings which are readily stimulated by airborne chemical irritants in the upper airways8,9). Sulfur dioxide is a gas soluble in water and retained in the upper airways in proportion depending on the inhaled concentration, which stimulates the afferent trigeminal nerve endings followed by inhibition of the phrenic nerve activity in mice1,10). Alarie et al2) have shown in mice that the intensity of sensory irritation by SO2 was directly related to the concentration of this agent and that the sensory irritation reaction for SO2 gradually disappeared while the exposure continued. The mechanism of the time-dependent change in the sensitivity of a sensory system for exposure to SO2 has been well discussed in the previous report, suggesting that the reducing properties of sulfite toward disulfide bonds in a receptor protein induce changes in membrane permeability. The results obtained in this paper also demonstrated that sensory irritation by SO2 was accompanied with a respiratory depression which reached a maximum and was followed by a gradual recovery toward normal control level while exposure continued. These results agree approximately with those obtained previously2) but are not in complete agreement. That is, it was observed in this experiment with dd strain mice that the onset of a return of respiratory rate toward preexposure level took a relatively long time to develop and that the rapidity of desensitization process was slow, so that complete return toward preexposure level
VOL.
30, NO.
5, DECEMBER
1975〔547〕
could not be reached within 10 minutes' exposure so long as the concentration of SO2 was not extremely low. The possible difference in the strain of the experimental animal is suggested to be one of the reasons for this disagreement. Immediately following 10 minutes' exposure to a given concentration of SO2, the mice still reacted to another higher concentration of SO2 in a fashion less sensitive than the control animal not previously exposed to SO2. In addition, it seems that there is a limit in the magnitude of desensitization which depends on the concentration of SO2. The return of respiratory rate toward preexposure level was also observed with repeated brief exposure to SO2 with brief room air for recovery between successive exposures and the rate of the return was independent of the concentration of SO2. The rate of desensitization may be dependent on time of exposure rather than on concentration of SO2. Therefore, if the exposure to SO2 were turned off at any time before the maximum desensitization, the magnitude of desensitization at that time may be constant independently of the concentration of SO2. Xylocain aerosol did not inhibit the respiratory response to SO2. This result parallels the observation by Amdur11) that the cough reflex elicited by SO2 is not inhibited by procain, and indicates that the receptors sensitive to SO2 are not influenced by local anesthesia. It has been shown that the mice exposed to SO2 while breathing through a tracheal cannula so that the gas came in contact only with the lung and not with the trachea failed to react similar to control mice breathing through the nose5). This indicates that the reflex respiratory depression by SO2 is induced by stimulation of the trigeminal nerve endings in the upper respiratory tract. Nadel et al12) studied the mechanism of bronchoconstriction produced by SO2 in cats, indicating that exposing only the upper airways produced an increase of pulmonary resistance which was blocked by atropine or by cooling of cervical vagosympathetic nerves. Similar results were reported by Widdicombe5) for cough reflex elicited by inhalation of SO2 or fine dust. He also found that after several inhalations of SO2, the animals became completely refractory to the gas but gave normal response to mechanical stimulation of the trachea. Frank et al13) have reported that in human subjects when SO2 was administered twice in the course of one experiment, the response to the second exposure was less than that to the first one. The adaptation phenomenon in human subjects or in cats seems to be similar to the desensitization phenomenon in mice. However, no desensitization phenomenon has been observed in guinea pigs14). A rapid decrease in respiratory rate can be considered as a quick and convenient function to protect harmful dose of irritant chemicals from reaching the alveoli. It has been shown that if mortality is chosen as a criterion of the toxicity, mice were less resistant than guinea pigs at high concentration of SO2 whereas guinea pigs were less resistant than mice at low concentration15,16). The state of reduced sensitivity following exposure to SO2 will increase tolerance of animals to the gas wtihin a short period of time but it may be one of the factors in the production of toxic effects with a prolonged period of exposure. SUMMARY
The respiratory reaction produced by sulfur dioxide was studied with the dd strain mice. Sensory irritation by sulfur dioxide was accompanied with a rapid decrease in respiratory rate which reached a peak within a few minutes and was followed by a gradual recovery while exposure continued. The degree of maximum response was directly proportional to the exposure concentration. Xylocain aerosol did not change the respiratory response to sulfur dioxide, indicating that the receptor sensitive to sulfur dioxide can not be affected by local anesthesia. The mice desensitized by a given concentration of SO2 to its sensory irritation still reacted to another
〔548〕JAPANESE
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higher concentration of SO2 in a fashion less sensitive than the normal mice not previously exposed to SO2. The rate of desensitization was dependent on time rather than on concentration of SO2, but it seems that there is a certain limit in the magnitude of desensitization which depends on the concentration of SO2. REFERENCES
1) Alarie, Y.: Irritating Properties of Airborne Materials to the Upper Respiratory Tract, Arch. Environ. Health, 13, 443-449 (1966). 2) Alarie. Y., I. Wakisaka and S. Oka: Sensory Irritation by Sulfur Dioxide and Chlorobenzilidene Malononitrile, Environ. Physiol. Biochem., 3, 53-64 (1973). 3) Widdicombe, J. G.: Respiratory Reflex from the Trachea and Bronchi of the Cat, J. Physiol., 122, 55-70 (1954). 4) Nadel, J. A., Salem, H., Tamplin, B. and Tokiwa, Y.: Mechanism of Bronchoconstriction during Inhalation of Sulfer Dioxide, J. Apply. Physiol., 20, 164-167 (1965). 5) Widdicombe, J. G.: Receptor in the Trachea and Bronchi of the Cat, J. Physiol.,123, 71-104 (1954). 6) West, P. and Gaeke, G. C.: Fixation of Sulfur Dioxide as Disulfite-mercurate (II) and Subsequent Colorimetric Estimation, Anal. Chem., 28, 1816-1819 (1956). 7) Dautrebande, H. Beckmann and W. Walkanhorst : New Studies on Aerosols (III); Production of Solid, Smallsized Aerosols, Arch. Int. Pharmacodyn. CXVI, No. 1-2, 170-186 (1959). 8) Cauna, N., Hinderer, K. H, and Wentges, R. T.: Sensory receptor organs of the human nasal respiratory mucosa, Amer. J. Anat., 124, 187-193 (1969). 9) Ulrich, C. E., Haddock, M. P, and Alarie, A.: Airborne chemical irritants; role of the trigeminal nerve, Arch. Environ. Health, 24, 37-42 (1972). 10) Amdur, M. O.: The Physiological Response of Guinea Pigs to Atmospheric Pollutants, Int. J. Air Pollution, 1, 170-183 (1959). 11) Amdur, M. O.: Respiratory Absorption Data and SO2 Dose-Response Curves, Arch. Environ. Health, 12, 729-732 (1966). 12) Nadel, J. A., Salem, H., Tamplin, B., and Tokiwa, Y.: Mechanism of Bronchoconstriction, Arch. Environ. Health, 10, 175-178 (1965). 13) Frank, N. R., Amdur, M. O., and Whittenberger, J. L.: A Comparison of the Acute Effects of SO2 Administered Alone or in Combination with NaCl Particles on the Respiratory Mechanics of Healthy Adults, Int. J. Air Water Pollution, 8, 125-133 (1964). 14) Amdur, M. O. and Mead, J.: A method for studying the mechanical properties of the lungs of unanesthetized animals: Application to the study of respiratory irritants, Proceedings of 3rd National Air Pollution Symposium, Pasadena, California, p. 150-159 (1955). 15) Weedon, F. R.: Experimental Acute Gastric Ulcer Produced in Animals by Exposure to Sulfur Dioxide Gas, New York State J. Med., 42, 620-623 (1942). 16) Leong, K. J., MacFarland, H. A. and Sellers, E. A.: Acute SO2 Toxicity: Effects of Histamine Liberation, Arch. Environ. Health 3, 668-675 (1961).
亜硫酸 ガスに よる上部呼吸道 の刺激 鹿児島大学医学部公衆衛生学教室 脇 亜 硫 酸 ガ スに よ る呼 吸 反応 をddマ
阪
一
郎
ウス を用 い て研 究 した。 亜 硫 酸 ガ ス に よ る刺 激感 覚 は, 急 速 な 呼 吸数 減 少
を ひ き お こ し, 数分 後 には そ の極 に達 し, しか る後, ば くろ 中 に除 々に 回復 して い った。 最 大 反 応 の程 度 は, ば くろ 濃度 と直 接 関 係が あ る。 キ シ ロカ イ ンエ ロゾル は, 亜 硫酸 ガ スに 対 して の呼 吸 反 応 に変 化 を あ た え なか った 。 この こ とは, 亜 硫 酸 ガ スに 対 して の感 覚 受 容 器 は, 局 所 麻 酔 に よ って 影 響 を うけな い こ とを示 す 。 あ る濃 度 の 亜 硫 酸 ガス に よ って, そ の刺 激 に対 す る感 受 性 を失 っ た マ ウス は, 別 な よ り高 い濃 度 の 亜硫 酸 ガ スに 対 して 尚反 応 を示 した が, あ らか じめ亜 硫 酸 ガス へ の ば くろを うけて い な い正 常 マ ウ スよ りも感 受 性 が低 い。 感 受 性 喪失 の速 度 は, 亜 硫 酸 ガス の濃 度 よ りもむ しろ 時 間 に関 係 して い たが, 感 受性 喪 失 の 強 さに は, あ る限度 が あ り, それ は 亜 硫 酸 ガ ス の濃 度 に よ る ら しい。 (受付1975年10月2日)
