Individual Variability in the Core Interthreshold Zone as Related to Body Physique, Somatotype, and Physical Constitution Naoshi Kakitsuba1), Igor B. Mekjavic2) and Tetsuo Katsuura3) 1) Department of Environment and Technology, School of Science and Technology, Meijo University 2) Jozef Stefan Institute, Ljubljana, Slovenia 3) Graduate School of Engineering, Chiba University
Abstract For evaluating the effect of body physique, somatotype, and physical constitution on individual variability in the core interthreshold zone (CIZ), data from 22 healthy young Japanese male subjects were examined. The experiment was carried out in a climatic chamber in which air temperature was maintained at 20–24°C. The subjects’ body physique and the maximum work load were measured. Somatotype was predicted from the Heath-Carter Somatotype method. In addition, factors reflecting physical constitution, for example, susceptibility to heat and cold, and quality of sleep were obtained by questionnaire. The subjects wore a water-perfused suit which was perfused with water at a temperature of 25°C and at a rate of 600 cc/min, and exercised on an ergometer at 50% of their maximum work rate for 10–15 min until their sweating rate increased. They then remained continuously seated without exercise until shivering increased. Rectal temperature (Tre) and skin temperatures at four sites were monitored by thermistors, and sweating rate was measured at the forehead with a sweat rate monitor. Oxygen uptake was monitored with a gas analyzer. The results showed individual variability in the CIZ. According to the reciprocal crossinhibition (RCI) theory, thermoafferent information from peripheral and core sensors is activated by Tre, mean skin ¯sk), and their changes. Since T ¯sk was relatively temperature (T unchanged, the data were selected to eliminate the influence of the core cooling rate on the sensor-to-effector pathway before RCI, and the relationship between the CIZ and the various factors was then analyzed. The results revealed that susceptibility to heat showed a good correlation with the CIZ, indicating that individual awareness of heat may change the CIZ due to thermoregulatory behavior. J Physiol Anthropol 28(6): 275–281, 2009 http://www.jstage.jst.go.jp/browse/jpa2 [DOI: 10.2114/jpa2.28.275] Keywords: body temperature regulation, core cooling rate, shivering, sweating, core interthreshold zone
Introduction Regulation of internal body temperature in humans is characterized by heat loss and heat production responses. The magnitude of the response is determined by the change in the core and skin temperature. Since the core interthreshold zone (CIZ) is defined as the range between core temperature at the onset of shivering and that at the onset of sweating, constant ¯sk) is required to be independent of mean skin temperature (T ¯sk in determining the thermal responses due to changes in T CIZ. For this particular purpose, Mekjavic et al. (1991) ¯sk at 28°C, proposed the water immersion method to maintain T and validated the methodology. Mekjavic and his colleagues collected data from human experiments using the water immersion method to demonstrate the effect of nonthermal factors on the CIZ because nonthermal factors such as nitrogen narcosis (Mekjavic et al., 1995), blood glucose (Passias et al., 1996), and motion sickness (Mekjavic et al., 2001) may act on heat production and heat loss pathways before or after the region of the reciprocal cross-inhibition (RCI). This RCI theory is based on a consideration of overlap between the sensor-to-effector pathways for heat production and heat loss, therefore supporting the idea that a thermoeffectors threshold can be established rather than a set point. Recently, Mekjavic and Eiken (2006) have summarized the effect of nonthermal factors on the CIZ. Despite their vigorous studies, the peripheral interthreshold ¯sk at the onset of zone that is defined as the range between T ¯sk at the onset of sweating and its relation to the shivering and T CIZ had not been focused on. Kakitsuba et al. (2007) employed a modified version of the protocol described by Mekjavic et al. (1991). Instead of being immersed in water, the subjects donned a water-perfused suit consisting of three segments: trousers, shirt, and hood. The segments were perfused in parallel with water maintained at 25°C at a rate of ¯sk at 28°C. This modified 600cc/min in order to maintain T method was confirmed to be effective in determining the CIZ.
Individual Variability in the Core Threshold Zone
In our previous study (Kakitsuba et al., 2007), we carried out human experiments to determine both the peripheral interthreshold zone and the core interthreshold zone, and then demonstrated individual variability in both zones and a proportional correlation between the peripheral interthreshold zone and the CIZ. Although we were able to demonstrate individual variability in the CIZ, data collection was not enough to evaluate the effect of various factors on individual variability in the CIZ. Mekjavic and Eiken (2006) suggested that many exerciserelated nonthermal factors may influence the heat loss response and the exercise-induced increment in core temperature may be affected by the fitness level of the subject. In addition, a recent study on the relationship between mitochondrial haplogroup and psychophysiological characteristics of the Japanese (Nishimura et al., 2009) alluded to the influence of psychophysiological characteristics on the thermoregulatory system. Therefore, body physique, somatotype, and the factors reflecting constitution were considered to be the explanatory variables in the analysis. In the present study, we collected the data of a homogenous subject population, i.e., healthy young Japanese male subjects, 22 subjects in total, to analyze individual variability in the CIZ.
Methods Subjects Twenty-two healthy Japanese male subjects participated in the study for 3 years. They all gave their informed consent to participate in the study, and were fully aware that they could withdraw from the study at any time without prejudice. The protocol of the study was approved by the institutional ethics review process. Following the comprehensive procedure outlined by Drinkwater (1980), anthropometric measurements of skinfold thickness at multiple sites, and girth, length, and bone breadth of the specific body compartments were made of each subject. The obtained values were then used to estimate regional weights of skin, adipose tissue, skeletal muscle, bone, and residual tissues. The mass percentages of the skin and adipose tissues were combined to obtain a value of adiposity. In order to estimate the subjects’ maximum work capacity during an incremental load exercise on a cycle ergometer, the subjects were asked to pedal at a rate of 60 rpm, and the work rate was increased incrementally by 10 W/min until exhaustion or until they could no longer maintain the required cadence. The three-component rating of endomorphy (ENDO), mesomorphy (MESO), and ectomorphy (ECTO) for each subject was calculated from the Heath-Carter Somatotype method (Carter et al., 1983). For example, the rating of an average mesomorphy is 4.0. According to the rating of each component, individual somatotype was evaluated. Susceptibility to heat and cold was subjectively evaluated. Subjects were asked to answer simply “yes (1)” or “no (0).” Quality of sleep was also subjectively evaluated from
answers on total sleeping and feeling after awaking. Total sleeping was rated according to the following categories: 1, always sleep very well; 2, mostly sleep very well; 3, generally sleep well; 4, sometimes do not sleep well; and 5, mostly do not sleep well. Feeling after awaking was rated according to the following categories: 1, always refreshed with no fatigue; 2, mostly refreshed with no fatigue; 3, generally refreshed but sometimes feel fatigue; 4, always not refreshed; and 5, often feel tired, with a slight headache. Rating of quality of sleep is represented by the sum of ratings of total sleeping and feeling after awaking. The components of a subject’s body physique, i.e., height (Ht), weight (Wt), body surface area (BSA), surface area-tomass ratio (BSA/Wt), adiposity, as well as the maximum workload (MWL), somatotype, and subjective evaluations are presented in Table 1. Subjects A–I participated in our previous study (Kakitsuba et al., 2007) and subjects J–V participated only in the present study.
Experimental protocol The details of the experimental protocol were described in our previous work (Kakitsuba, 2007). Briefly, we maintained ¯sk) at 28°C by perfusing a watermean skin temperature (T perfused suit with water at 25°C. The entire system is shown in Fig. 1. Subjects wearing a water-perfused suit commenced exercising at 50% of their maximum work rate on a cycle ergometer. The exercise was terminated at the onset of sweating, which occurred after 10 to 15 min of exercise. The subjects then remained seated on the cycle ergometer for an additional 100 min. The onset of shivering was observed when oxygen uptake started to increase during the last part of the ¯sk remained at 28°C. The thresholds were defined trial, while T as rectal temperature (Tre) at which sweating rate (Esk) and oxygen uptake were elevated above the median resting levels. Since Mekjavic and Bligh (1987) demonstrated that the onset of sweating overlapped with the offset of sweating as long as experiments were carried out with the experimental protocol described here, the onset of sweating was used in the present study, although the original work by Mekjavic et al. (1991) preferred offset of sweating in the determination of CIZ.
Measurements Rectal (Tre) and skin (arm, chest, thigh, and calf) temperatures were monitored with thermistors and the values were stored every ten seconds using a data logger system (Cadac2 Model 9200A; Cadac, Tokyo, Japan). Sweating rate was measured at the forehead with a sweat rate monitor (Model SKD-4000; Skinos Company, Nagoya. Japan). Oxygen uptake was monitored with a gas analyzer (Respiromonitor RM-300i; Minato Medical Science Company, Tokyo, Japan). ¯sk while simultaneously extracting 120 Maintenance of T W/m2 of heat was achieved by having the subjects wear a Cool TubesuitTM (Med-Eng Systems, Inc., Ottawa, Ontario, Canada) water-perfused suit. The water perfusing the suit was pumped
Kakitsuba, N et al. J Physiol Anthropol, 28: 275–281, 2009
Fig. 1 Diagram of the cooling system. This diagram was shown in the previous publication by Kakitsuba et al. (2007). For the purpose of providing details of the system, it is shown again here. A chiller cooled water in the bath and a pump supplied cool water into the vinyl tubes incorporated in the suit. After water was perfused through the tubes, it was returned to the water bath and cooled again.
at a rate of 600 cc/min (Water Pump Model Super Tepcon; Terada, Tokyo, Japan) from a bath in which the water temperature was maintained at 25°C by a Cool Mate Model TE-105M heat exchanger (Toyo Seisakusho Co., Tokyo, Japan).
Statistical analysis All physiological and psychological variables measured were presented as meanSD. A comparison of the CIZ between the group that answered “susceptible to heat” and the group that answered “not susceptible to heat” was made using an unpaired t test. The Spearman’s rank correlations were examined for correlations between the CIZ and the factors related to body physique and subject’s characteristics. Significant difference was set at p0.05.
Results Validity in the core interthreshold zone ¯sk (calculated from We designed the experiment to maintain T the equation suggested by Ramanathan, 1976) consistently at ¯sk changed slightly (from 32°C 28°C. However, in some cases, T during exercise to 30°C at the end of the recovery period) during the experimental period because of the cooling method using the water-perfused suit instead of water immersion. ¯sk and Tre are shown in Fig. 2. Typical responses of T
Contribution of body physique to core cooling rate Correlations between core cooling rate (CCR) and the 6 components listed in Table 1, i.e., Ht, Wt, BSA, MWL, adiposity, and MWL/Wt, were examined by using Spearman’s rank correlation. The results are indicated in Table 2. The Wt, adiposity, and MWL/Wt showed a significant correlation with the CCR.
Fig. 2 An example of changes in rectal temperature and mean skin temperature throughout the experiment. This example was shown in the previous publication by Kakitsuba et al. (2007). For the purpose of providing details of the determination ¯sk indicate rectal of CIZ, it is shown again here. The Tre and T temperature and mean skin temperature, respectively. The onset of sweating was observed during exercise. Following exercise, the subjects rested until shivering was observed at 70 to 90 min after the beginning of exercise.
Relationship between core cooling rate and the core interthreshold zone The relationship between the CCR and the CIZ is depicted in Fig. 3. No significant correlation was found between the CCR and the CIZ. However, it is interesting to note that the CCR around 1.0°C/h induces the smallest zone. However, as the CCR increases or decreases compared with those at 1.0°C/h, the CIZ has a tendency to be wider.
21 21 21 21 19 21 21 21 20
20 21 21 24 21 23 23 21 20 23 20 22 21
A B C D E F G H I
J K L M N O P Q R S T U V
174 172 168 174 178 163 174 173 164 180 173 172 178
180 172.4 170 162.4 160.5 174.4 179.5 172 174.5
58.8 53.2 55.4 61.6 79.2 55.2 53.8 55.6 52 82 58.2 61 63.8
58.5 60.3 57 47 55.8 82.5 62.2 60 70
1.71 1.64 1.63 1.73 1.94 1.59 1.64 1.66 1.56 1.98 1.69 1.71 1.79
1.74 1.71 1.66 1.49 1.58 1.94 1.78 1.71 1.83
2.91 3.08 2.94 2.81 2.45 2.88 3.05 2.99 3 2.41 2.9 2.8 2.81
2.97 2.84 2.91 3.17 2.83 2.35 2.86 2.85 2.61
BSA/Wt (m2 /kg102)
274 210 270 270 308 226 242 214 212 344 228 214 296
294 260 250 218 204 300 276 282 296
0.22 0.22 0.24 0.24 0.29 0.23 0.25 0.22 0.22 0.31 0.24 0.25 0.22
0.29 0.26 0.27 0.23 0.26 0.32 0.32 0.23 0.24
4.6 2.2 1.7 2.3 2.8 1.2 4.1 2.1 2 1.5 2 1.8 2.1
3.5 3 3.5 1.7 3.1 5.7 3.3 2.4 3.1
3.9 3.2 2.1 2.3 4 2.5 4.8 3.4 3.8 2.5 3 3.8 3.5
2.6 3.7 2.6 3 4.6 5.1 2 2.9 2.8
1.7 3.7 5.2 3.7 2.9 4.9 1.8 3.4 3.6 4.6 4 4.2 4.1
5.4 3.6 3.8 4.4 2.2 0.9 4.6 3.6 2.4
6 5 8 9 5 6 5 9 7 6 5 7 4
6 7 4 6 7 6 6 4 4
Quality of everyday sleep
N Y Y N Y N N N Y N Y N Y
Y Y Y N Y Y N Y Y
N N Y N Y N N Y N N N N Y
N Y N Y Y N Y N N
Susceptible Susceptible to heat** to cold**
* BSA: body surface area (m2); ** Y: yes (1); N: no (0); MWL: maximum work load (W); ENDO: rating of endomorphy; MESO: rating of mesomorphy; ECTO: rating of ectomorphy.
Subjects’ physical and psychological characteristics
278 Individual Variability in the Core Threshold Zone
Kakitsuba, N et al. J Physiol Anthropol, 28: 275–281, 2009
Spearman’s rank correlation matrix between components of physique and core cooling rate (n22) CCR (°C/h)
CCR (°C/h) Height (cm) Weight (Wt, kg) Body surface area (m2) Adiposity (ND) Maximum work load (MWL, W) MWL/Wt (W/kg)
1.000 0.225 0.509* 0.389 0.477* 0.268 0.46*
1.000 0.731** 0.884** 0.404 0.786** 0.148
Weight (Wt, kg)
1.000 0.951** 0.525* 0.802** 0.145
Body surface area (m2)
1.000 0.491* 0.866** 0.027
1.000 0.441* 0.157
Maximum workload (MWL, W)
* p0.05, ** p0.01; CCR: core cooling rate (°C/h)
Fig. 3 Relationship between core cooling rate and core interthreshold zone. The results showed no correlation of core cooling rate (CCR) with core interthreshold zone (CIZ). However, as the CCR increases or decreases compared with those at 1.0°C/h, the CIZ has a tendency to get wider.
Fig. 4 Relationship between core interthreshold zone and temperature difference between initial Tre and the core shivering threshold (n14). The result shows a good correlation of core cooling rate with temperature difference between initial Tre and the core shivering threshold.
Contribution of body physique, somatotype and the factors reflecting physical constitution to the core interthreshold zone
between the CIZ and temperature difference between the initial Tre and the core shivering threshold, and showed a good correlation (R20.881; p0.01), as indicated in Fig. 4. Thus, the difference in the CIZ was due mainly to difference in the core shivering threshold. In other words, onset of shivering of the subjects who answered “susceptible to heat” becomes lower as compared with that of the subjects who answered “not susceptible to heat.”
The relationship between the CIZ and the components of body physique, somatotype, and the factors reflecting physical constitution was analyzed. The results showed that no components and factors were correlated with the CIZ. However, we continued to analyze the data by selecting them based on the CCR since the CIZ becomes wider when the CCR increases or decreases. The CIZs with the smallest and the largest CCR were first excluded from the analyses and this continued stepwise until a significant correlation between the CIZ and any component or factor was found. Finally, the data of 14 subjects were selected. The CCR was 0.90.02°C/h (meanSD). As indicated in Table 3, no components of body physique, physical strength, and somatotype were correlated with the CIZ with the exception of “susceptible to heat.” The CIZ of the subjects who answered “susceptible to heat” was 0.720.16°C (meanSD, n7); that was significantly wider (p0.05) than the 0.490.2°C (meanSD, n7) of the subjects who answered “not susceptible to heat.” The statistical analyses demonstrated a linear relationship
Discussion ¯sk on CIZ, we In order to minimize the effect of change in T ¯sk at 28°C as far as possible. Benzinger tried to maintain T ¯sk must be maintained within 0.2°C when (1969) reported that T individual variability in core sweating thresholds was ¯sk at compared. The results of the present study showed that T the onset of sweating varied from 30.7°C to 32.6°C. Therefore, ¯sk may contribute to the core it is possible that differences in T ¯sk changed in the same sweating threshold. However, since T ¯sk manner for all subjects, we thought that the contribution of T differences may be comparable in degree in all subjects. Therefore, the results demonstrated individual variability in
1.000 0.100 1.000 0.316 0.544* 1.000 0.089 0.039 0.022 1.00 0.584* 0.071 0.275 0.088 1.000 0.414 0.846** 0.178 0.059 0.280 * p0.05, ** p0.01; ENDO: rating of endomorphy; MESO: rating of mesomorphy; ECTO: rating of ectomorphy
1.000 0.017 0.138 0.219 0.071 0.197 0.184 1.000 0.203 0.522 0.101 0.262 0.089 0.353 0.616* 1.000 0.716** 0.076 0.132 0.438 0.099 0.018 0.275 0.603* 1.000 0.854** 0.354 0.100 0.065 0.430 0.210 0.107 0.138 0.296 1.000 0.961** 0.767** 0.231 0.225 0.099 0.430 0.266 0.018 0.078 0.267 1.000 0.688** 0.829** 0.803** 0.427 0.019 0.080 0.163 0.022 0.232 0.039 0.255 1.000 0.138 0.038 0.041 0.048 0.192 0.040 0.355 0.220 0.298 0.587* 0.079 0.041 CIZ (°C) Height (cm) Weight (Wt, kg) Body surface area (m2) Maximum work load (MWL, W) MWL/Wt (W/kg) Adiposity (ND) ENDO MESO ECTO Susceptibility to heat Susceptibility to cold Sleep quality
Susceptibility to cold Susceptibility to heat ECTO MESO ENDO Adiposity (ND) MWL/Wt (W/kg) Body Maximum surface area work load (m2) (MWL, W) Weight (Wt, kg) Height (cm) CIZ (°C)
Spearman’s rank correlation matrix between core interthreshold zone and components of body physique, somatotype, and physical constitution (n14) Table 3
Individual Variability in the Core Threshold Zone
thermoregulatory responses by the central thermoregulatory ¯sk within the expected drive since we were able to control T range. Regarding the CCR, many studies have demonstrated that there is a significant difference in thermoregulatory responses between groups with different morphological factors such as skinfold thickness, body size, and body weight during cold water immersion, primarily because of differences in total body thermal insulation (Keatinge, 1960; Kollias et al., 1974; Toner et al., 1986; Mekjavic et al., 1987; White et al., 1992). In the present study, all subjects were cooled in the same manner with the water-perfused suit. However, individual variability in the CCR was observed, probably due to differences in body physique. Therefore, we first examined correlations between the CCR and the 6 components listed in Table 1, i.e., Ht, Wt, BSA, MWL, adiposity, and MWL/Wt, using Spearman’s rank correlation. The result agreed well with those reported by White et al. (1992), who found that the CCR is primarily related to body mass and adiposity. We expected a higher CCR to induce a higher shivering threshold, i.e., a narrower CIZ, as demonstrated by Cabanac and Massonnet (1977). However, the results failed to meet our expectations. Mekjavic et al. (1991) suggested that the effect of the CCR on the shivering response may not be conclusive because different core cooling rates may give rise to different internal temperature fields. This may be the one of the reasons for the relationship between the CCR and the CIZ as indicated in Fig. 3. As indicated in Table 1, the subjects who answered “not susceptible to heat” answered “not susceptible to cold” except for subject Q. Therefore, the wider CIZ of the subjects who answered “not susceptible to heat” was anticipated. However, the results failed to meet our expectations. One possible reason may be the season during which the experiments were carried out. We carried out all the experiments in summer. According to the replies of the questionnaire on everyday life, the subjects who answered “susceptible to heat” preferred use of airconditioning devices more than the subjects who answered “not susceptible to heat.” In other words, the subjects who answered “susceptible to heat” may be exposed to hot and cool environments in their everyday life more repetitively than the subjects who answered “not susceptible to heat.” Since Bligh (1973) clearly demonstrated that the CIZ becomes wider when the subjects were exposed to hot and cold repetitively, the lower core shivering threshold of the subjects who answered “susceptible to heat” may be reflected in their thermoregulatory behavior. If so, a seasonal difference in the CIZ is to be expected. In this study, the effect of body physique, somatotype, and constitution on individual variability in the CIZ has been ¯sk on the investigated. In order to eliminate the influence of T ¯sk was controlled sensor-to-effector pathway before RCI, T relatively consistently. The analyses demonstrated no correlation between the CIZ and all the components. However, further analyses by selecting data to eliminate the influence of
Kakitsuba, N et al. J Physiol Anthropol, 28: 275–281, 2009
the CCR on the sensor-to-effector pathway before RCI revealed that susceptibility to heat showed a good correlation with the CIZ, indicating that individual awareness of heat may change the CIZ due to thermoregulatory behavior. Acknowledgements This study was supported in part by Grant-in-Aid #15107005 and #20370098 for Scientific Research, Japan.
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