Takashi Kumae. Keywords Autonomic modulation Exercise Heart rate Rat Spectral analysis. Introduction

Transcription

1 Environ Health Prev Med (2012) 17: DOI /s z REGULAR ARTICLE Assessment of training effets on autonomi modulation of the ardiovasular system in mature rats using power spetral analysis of heart rate variaility Takashi Kumae Reeived: 29 Septemer 2011 / Aepted: 15 Feruary 2012 / Pulished online: 11 Marh 2012 Ó The Japanese Soiety for Hygiene 2012 Astrat Ojetive To larify the effets of fored or voluntary exerise on autonomi modulation of the ardiovasular system, we monitored hanges in autonomi nervous ativity in a mature rat y spetral analysis of the heart rate (HR) during a 10-week training period. Methods Male Wistar rats implanted with a radio-telemetry system were divided into three groups at 18 weeks of age: (1) Control group (n = 8); (2) Voluntary group (n = 6), whih were housed separately in a age with a running wheel; (3) Fored group (n = 6), whih were exerised on a treadmill (35 m/min, 15 min/day, 5 days/ week). The eletroardiogram was analyzed y the maximum entropy method into two main osillations, lowfrequeny (LF) and high-frequeny (HF) osillations, respetively. LF and HF are onsidered to e markers of oth sympatheti and parasympatheti modulations and parasympatheti modulation, respetively. Results Average running distanes of the Voluntary group were more than twofold higher than those of the Fored group. HR levels in the Fored group were lower than those in the Control group. LF and HF levels in the Control and the Fored groups were almost the same during the experiment, and those in the Voluntary group showed a tendeny to derease. Conlusion The results in the Voluntary and the Fored groups suggest that ardiovasular adjustments are not simply aused y the quantity of exerise. In the Voluntary group, oth sympatheti and parasympatheti ativity may T. Kumae (&) Department of Health Promotion and Exerise, National Institute of Health and Nutrition, Toyama, Shinjuku-ku, Tokyo , Japan kumae@nih.go.jp derease with a predominane of sympatheti ativity. Conversely, in the Fored group, the aroreflex may e hyper-ativated y the undesired treadmill running and handling stress. Keywords Autonomi modulation Exerise Heart rate Rat Spetral analysis Introdution The overall aim of the researh presented in this paper is to prevent hroni fatigue during physial training, espeially in sedentary adults starting on an exerise routine for the purpose of personal health promotion. Overtraining due to exerise stress is onsidered to e one type of hroni fatigue syndrome, and the amount of researh into this ondition is inreasing [1, 2]. Israel [3] differentiates etween parasympatheti and sympatheti overtraining, haraterized y inhiition or exitation, respetively. Sympatheti overtraining seems to affet primarily speed and power athletes, while parasympatheti overtraining seems to affet mostly endurane athletes [4]. Lehmann et al. [5], in agreement with other investigators [6], reported that hanges in lood hemistry are proaly not oligatory in parasympatheti overtraining syndromes. The eletroardiogram (ECG) is muh less noninvasive than lood sampling, and modern eletroni and omputer tehnology enales the rapid analysis of heart rate variaility (HRV) in a frequeny domain (spetral analysis) that allows the autonomi modulation of the ardiovasular system to e evaluated [7]. This noninvasive tehnique has gained onsiderale interest of researhers and liniians in the field of oth ardiology [8] and neurology [9]. It has sine een postulated that the two main osillations

2 416 Environ Health Prev Med (2012) 17: oserved in the power spetrum, one at a low frequeny and the other at a high frequeny, may, respetively, e markers of the sympatheti and parasympatheti modulation of the ardiovasular system in rats. Low-frequeny power (LF) is onsidered to e a marker of sympatheti modulation y some investigators [10, 11] and y others as a marker of oth sympatheti and parasympatheti modulation [7, 12]. High-frequeny power (HF) has een aepted to e a marker of parasympatheti modulation [13] and the LF to HF ratio (LF/HF) is often onsidered as an index of the ardia sympathovagal alane [14, 15]. To estimate parasympatheti overtraining, researhers have studied the power spetrum of heart rate (HR) in freely ehaving onsious rats [16]. The study reported here was designed to haraterize ardia autonomi neuropathy in rats sujeted to fored or voluntary exerise through the analysis of HRV, ased on the hypothesis that the quantitative information provided y this analysis reflets the interation etween sympatheti and parasympatheti regulatory ativity [17]. For this purpose, the ECG was reorded from onsious and unrestrained sedentary rats whih were not sujeted to handling stress through the use of a radio-telemetry system [18]. The ECG reorded y the telemetry system is ale to provide more reliale data for the assessment of autonomi nervous ativity than data otained y a tethering system [17]. To larify the effets of fored or voluntary exerise on the autonomi modulation of the ardiovasular system, we examined hanges in autonomi nervous ativity and alane during the training period using a mature rat as a model of a sedentary adult [19, 20]. Methods Experimental animals and protool The researh protool was approved y the institutional ommittee for animal researh. Male Wistar JI strain [speifi pathogen free (SPF), 9 weeks old] rats were otained from Nippon Clea Breeding Laoratories (Tokyo, Japan). The rats were maintained in the SPF ondition using an isolated room in a arrier area, as previously reported [16], and kept on a 12/12-h light/dark yle (the dark yle was from 6:00 a.m. to 6:00 p.m.). Eah rat was implanted with a two-lead ECG radio frequeny transmitter (model CA-F40; weight 7.9 g, volume 4.2 m 3 ; Data Sienes International, Saint Paul, MN) at 15 weeks of age. After a reovery period of 3 weeks (age 18 weeks), the rats were divided into the following three groups: (1) ontrol group (Control, n = 8), housed under the usual reeding ondition for rats kept under sedentary onditions; (2) voluntary training group (Voluntary, n = 6), housed separately in a age with a running wheel (Mirote Niti- On, Chia, Japan); (3) fored training group (Fored, n = 6); housed as the Control group, ut fored to exerise on a treadmill (Suzuya Co, Tokyo, Japan). The Voluntary group had 24-h aess to the running wheel (irumferene 1 m, width 10 m). The daily running distane of the Voluntary group was measured as rotation ounts of the running wheel. To monitor rotation ounts without disturing the voluntary running, the rotation ounts were onverted to eletri signals and transmitted y a ale to another room. To maintain a noiseless environment in the isolated reeding room, the treadmill exerise (35 m/min, 15 min/day, 5 days/week) was performed also in another room at almost the same time on eah test day in the dark yle (around 1:00 p.m.). The duration of the experimental period was 10 weeks. The rats were given rodent feed (F-2; Funaashi Farms, Chia, Japan) and water ad liitum. The feed weight was measured to alulate feed onsumption, and ody weights were measured eah week. ECG reording The ECG was reorded during the dark yle in onsious and unrestrained rats at one-weekly intervals at almost the same time (hour of the day) on eah oasion. The ECG reording for the Fored group was arried out efore the exerise (treadmill running). Care was taken to keep the room noiseless during the ECG reording, and the original ECG signal was reeived y one set of reeiving poles (RLA3000; Data Sienes International) loated on oth sides of eah rat age. The ECG signal was amplified using a reeiver multiplexer (RMX10; Data Sienes International), transmitted to another room y a ale to maintain the noiseless state of the isolated reeding room, further amplified, and then sujeted to ut-off noise y a retiorder (RJG-4122, Nihon Koden, Tokyo, Japan) and an eletroenephalograph (EEG) (EEG1A94; San-ei Co, Tokyo, Japan). To ollet the ECG data for a ontinuous 15-min interval for eah rat without physial movements and/or positional hanges, the ECG was monitored with the EEG during the reording. The amplified ECG analogue signals were reorded into a magneti tape (SIT80; Sony Magnesale, Tokyo, Japan) y an instrumentation tape reorder (A-49; Sony Magnesale, Tokyo, Japan) [16]. Power spetral analysis of HRV R wave-to-r wave (RR) intervals in the ECG analogue signals reorded with the tape reorder were analyzed at high resolution [21] y the maximum entropy method (MEM) [22] using speial software (Tarawa Mouse & Rat; Suwa Trust, Tokyo, Japan) in onseutive periods of 10 s for 15 min [16]. An aurate estimation of the power

3 Environ Health Prev Med (2012) 17: spetral analysis is possile even with a small numer of samples, produing a signal whih is supposed to maintain stationary [23]. In this study, 1 min of the lowest HR region of the ECG reorded without physial movements was seleted as the stationary part for the power spetral analysis. HRV is represented as an entropy (ETY) in the frequeny domain. ETY is alulated y ontinuous eight pulses of R waves and expressed as the perentage ratio set at onstant RR (0%); most random variation of RR is 100%. Most researhers agree that there are two major omponents, i.e., LF and HF, in the power spetra; however, there is as yet no onstant definition of the frequeny of LF and HF in rats. In this study, rat LF and HF omponents were defined as and Hz, respetively [16]. The HRV omponents, LF and HF, were log-transformed to normalize their distriution for the statistial tests [24], and the LF/HF ratio was alulated as an index of sympatheti and parasympatheti ativity alane [14 16]. Statistis Two-fator analysis of variane (ANOVA) was employed to estalish the effet of the experimental manipulation (fator group), the experimental period (fator week), as well as the interation effets etween fators, on the measured items using StatView software (SAS Institute, Cary, NC). The statistial signifiane of the differenes among the data otained on eah weekly examination was determined y one-fator ANOVA. The statistial signifiant hanges in the data otained in eah group during the experiment were determined y repeated-measures ANOVA. Where a main effet was oserved, post ho test using Dunnett s t was performed for multiple omparisons. Correlation analysis was arried out to examine the relations among indies of the power spetral analysis of HRV in eah group during the experiment. Results Changes in ody weight, feed onsumption, and running distane Inreases in ody weight were suppressed y the implantation surgery at age 15 weeks; however, ody weight did inrease after the 3-week reovery period. As shown in Fig. 1, 3 weeks following the eginning of the experiment, the mean ody weight in the Voluntary group was slightly lower than that in the Control group, and that in the Fored group was lower than that in the Voluntary group. Twofator ANOVA revealed that this differene in ody weight among the groups at 3 weeks was not signifiant, although the differene etween ody weight during the experiment Body weight (g) Fig. 1 Changes in ody weight during the experiment. Vertial axis Body weight (g). Symols Mean values of the experimental groups: open irles Control group (n = 8), filled irles Voluntary group (n = 6), open triangles Fored group (n = 6). Vertial ars Standard error (SE). See setion Experimental animals and protool for desription of animal groups Feed weight (g) * Fig. 2 Changes in feed onsumption during the experiment. Vertial axis Weight of onsumed feed (g). Asterisk Statistial differene (p \ 0.05) etween the Control group and other groups at the same time-point, as revealed y analysis of variane (ANOVA). denotes statistial differene (p \ 0.05) in feed onsumption in the Voluntary group etween week 1 and orresponding weeks as revealed y repeated-measures ANOVA,, statistial differenes (p \ 0.05 and p \ 0.01, respetively) in feed onsumption in the Fored group etween week 1 and orresponding weeks as revealed y repeatedmeasures ANOVA. Symols and vertial ars are the same as for Fig. 1 was signifiant (p \ 0.05). There was no signifiant differene in mean ody weight among the groups, and no signifiant hange during the experiment within eah group (Fig. 1). Changes in the feed onsumption are shown in Fig. 2. Compared to the mean level of feed onsumption in the Control group at experimental week 2, mean feed onsumption was higher in the Voluntary group and lower in the Fored group. Two-fator ANOVA revealed a signifiant differene in the feed onsumption among the groups (p \ 0.001) without any signifiant differene during the experimental period. The Fored group onsumed signifiantly (p \ 0.05) small amounts of feed than the Control group at Week 3 when the mean ody weight in the Fored group showed a tendeny to derease (Fig. 1). The Voluntary group showed a signifiant inrease in mean feed onsumption etween week 1 and week 3 and etween

4 418 Environ Health Prev Med (2012) 17: Running distane (m/day) Fig. 3 Changes in mean running distane in the Voluntary group. Vertial axis Running distane (m/day). denotes statistially signifiant differene (p \ 0.05) in mean running distane in the Voluntary group etween in week 1 and weeks denoted y, as revealed y repeated-measures ANOVA. Symols and vertial ars are the same as in Fig. 1 week 1 and week 6 (p \ 0.05 for oth). In ontrast, the Fored group showed dereased feed onsumption etween week 1 and weeks 3, 4, 5, 6, and 10 (Fig. 2). The running distane of the Fored group was kept as 525 m/day (35 m/min, 15 min/day, 5 days/week) during the experiment. As shown in Fig. 3, the Voluntary group run almost the same distane as the Fored group during week 1 (524 m/day), ut signifiant inreases in the mean running distane were oserved etween week 1 and week 5 up to week 8 (p \ 0.05 for all). The Voluntary group run an average around 800 m/day/week during the experiment. Changes in the mean levels of HR and ETY Changes in the mean HR level are shown in Fig. 4. Compared to the Control group, the mean HR level was higher in the Voluntary group and lower in the Fored group. Twofator ANOVA revealed a signifiant differene in the HR level among the groups (p \ 0.001), with a signifiant differene during the experimental period (p \ 0.01). A signifiant differene in HR level was oserved etween the Control and Fored groups at week 1 (p \ 0.05) and etween the Control and the Voluntary groups at weeks 7 and 9 (p \ 0.05 and p \ 0.001, respetively) (Fig. 4). The mean HR level in the Control group signifiantly dereased etween week 0 and week 5 (p \ 0.05). Compared to week 0, the Fored group showed dereased levels of HR during weeks 5, 9, and 10 (p \ 0.05 for all) (Fig. 4). Contrary to the hanges in the HR, there was no onstant trend with respet to hanges in the ETY level (Fig. 5). In the Voluntary group, mean levels of ETY showed a tendeny to derease after week 6; however, no signifiant differene among the groups and during the experimental period was oserved y two-fator ANOVA. Repeatedmeasures ANOVA revealed a signifiant differene in the mean level of ETY etween week 0 and week 6 (p \ 0.05) in the Voluntary group. Heart rate (eats/min) * Changes in indies of power spetral analysis of HRV The LF levels in the Control and Fored groups remained almost at the same levels during the experimental period and those in the Voluntary group showed a tendeny to derease after week 4 (Fig. 6). No signifiant differene among the groups and during the experimental period was revealed y two-fator ANOVA. A signifiant differene in the mean LF was oserved etween week 0 and week 10 (p \ 0.05) in the Voluntary group y repeated-measures ANOVA. Changes in mean levels of HF are shown Fig. 7. The hanges of HF were similar to those of LF ut mean levels in the Voluntary group showed apparent dereased tendeny than the hanges of LF (Fig. 6). Two-fator ANOVA revealed signifiant differene in mean levels of HF among the groups (p \ 0.01) without signifiant differene during the experimental period. Mean level of HF a * *** Fig. 4 Changes in the mean heart rate (HR). Vertial axis Heart rate (eats/min). Single and triple asterisks denote statistial differene (p \ 0.05 and p \ 0.001, respetively) in mean HR etween the Control group and orresponding groups at that time-point as revealed y ANOVA. a, denote statistial differene (p \ 0.05 for all) in mean HR etween week 0 and the orresponding weeks, as revealed y repeated measures in the Control and the Fored groups, respetively. Symols and vertial ars are the same as in Fig. 1 Entropy (ETY) (%) Fig. 5 Changes in the mean level of the entropy (ETY) during the experiment. Vertial axis Entropy (%). denotes statistial differene (p \ 0.05) in mean ETY in the Voluntary group etween week 0 and week 6 as revealed y repeated-measures ANOVA. Symols and vertial ars are the same as in Fig. 1

5 Environ Health Prev Med (2012) 17: LF (ln (ms 2 )) LF/HF (ratio) Fig. 6 Change in mean low-frequeny (LF) osillations during the experimental period. Vertial axis Natural logarithms of spetral powers in the LF region (m s 2 ). denotes statistial differene (p \ 0.05) etween mean LF in the Voluntary group at week 0 and week 10 as revealed y repeated-measures ANOVA. Symols and vertial ars are the same as in Fig. 1 HF (ln (ms 2 )) Fig. 7 Change in mean high-frequeny (HF) osillations during the experiment. Vertial axis Natural logarithms of spetral powers in the HF region (m s 2 ). denotes statistial differene (p \ 0.05) etween mean HF in the Voluntary group at week 0 and orresponding weeks as revealed y repeated measures. Symols and vertial ars are the same as in Fig. 1 at week 0 was signifiantly deeased at week 5, week 6, and week 10 (p \ 0.05 for all) in the Voluntary group revealed y the repeated measures. As shown in Fig. 8, the mean level of LF/HF seemed to flutuate differently in eah group. However, two-fator ANOVA revealed no signifiant differene in the mean LF/ HF among the groups, although there was a signifiant differene during the experimental period (p \ 0.05). There was no signifiant differene in the mean LF/HF among the groups and no signifiant hange during the experiment in eah group (Fig. 8). Correlations among HR, ETY, and indies of power spetral analysis of HRV The orrelation oeffiients and statistial signifianes are summarized in Tale 1. There was no signifiant orrelation etween HR and ETY, etween HR and LF/HF, and etween ETY and LF/HF in all groups. To the ontrary, signifiant positive orrelations were oserved etween Fig. 8 Changes of mean levels of the LF/HF during the experiment. Vertial axis is the LF/HF (ratio). Symols and vertial ars are same as Fig. 1 ETY and HF and etween LF and HF in all groups. In the Control group, HR signifiantly positively orrelated to LF and HF (p \ 0.05 for oth), and a signifiant negative orrelation was oserved etween HF and LF/HF. Signifiant positive orrelations were oserved etween ETY and LF in the Voluntary and Fored groups (p \ 0.01 and p \ 0.05, respetively). In the Voluntary group, LF signifiantly positively orrelated to LF/HF (p \ 0.05). Disussion In this study, running distanes were different etween the Fored and Voluntary groups due to the intrinsi diffiulty in the experimental set-up of regulating the voluntary running [25, 26]. As shown in Fig. 3, although the running distane of the Voluntary group was almost same that of the Fored group at the week 1 time-point, it showed a tendeny to inrease from week 2 onwards in the Voluntary group. It was expeted that the running distane would settle down to the level of week 1 with pratie; onsequently, the treadmill running time in the Fored group was not ale to inrease as rapidly in aordane with the running distane of the Voluntary group for the prevention of stragglers in the Fored group [20]. The mean running distane of the Voluntary group was almost twofold that of the Fored group on any one day per week. The asene of a tendeny for the Voluntary group to inrease ody weight (Fig. 1) suggests that the inrease in energy expenditure through the voluntary running is ompensated y inreased feed onsumption (Fig. 2). It has een reported that voluntary wheel running does not modify ody weight gain and inreased total alori intake [27]. The tendeny for ody weights to derease in the Fored group may e aused y dereased feed onsumption. These results are in aordane with those of our previous study [20]. It has also een reported that rats exerising on a treadmill eat less food than resting animals [28] and that ody weight losses that ould e aounted for entirely y a redution in feed intake [29].

6 420 Environ Health Prev Med (2012) 17: Tale 1 Correlations among heart rate, entropy, and indies of the power spetral analysis of heart rate variaility *p \ 0.05, **p \ 0.01, ***p \ ETY Entropy, HR heart rate, LF low-frequeny osillation, HF high-frequeny osillation LF and HF were logtransformed to normalize their distriution for statistial tests Variales/experimental group ETY LF HF LF/HF HR Control * 0.628* Voluntary Fored ETY Control ** Voluntary 0.767** 0.882*** Fored 0.645* 0.588* LF Control 0.670* Voluntary 0.911*** 0.697* Fored 0.772** HF Control ** Voluntary Fored It has een reported that when working with onsious unrestrained animals, the limiting fator in the seletion of the data to e analyzed was that the animal e quiet [23]. In previous studies, stationary sets of data of the appropriate time span have often een seleted for analysis without any lear definition [30, 31]. In this study, for a fair and impartial seletion of stationary datasets, 1 min of the lowest HR region was analyzed y high-resolution MEM in onseutive periods of 10 s [21, 22]. It is well known that endurane training indues radyardia and a redution in the resting HR. Many investigators have examined the sympatheti and parasympatheti effets on the heart to explain the radyardia resulting from training. Baroreeptor signaling has een reported to play a deisive role in driving treadmill running-indued ardiovasular adjustments using rats with a hroni asene of aroreeptor inputted y sino-aorti denervation [32]. However, aording to the results of this study, average running distanes of the Voluntary group were more than twofold higher than those of the Fored group without any oserved redution in the HR (Fig. 4). These results suggest that ardiovasular adjustments are not simply aused y exerise quantity alone. Body weight, HR, and systoli and diastoli lood pressure have all een found to derease in response to dietary restrition in male Sprague Dawley rats [33]. The eat-to-eat interval of HR has also een reported to e signifiantly redued y elevated plasma nonesterified fatty aids levels [34]. Both treadmill and voluntary wheel running exerise has een found to derease the plasma onentrations of free fatty aids in diaeti rats [35] and Wistar/ST rats [36]. Consequently, it is possile that the redued feed onsumption and dereased level of plasma free fatty aids due to the exerise regimen ontriuted to the oserved redution in HR in the Fored group. The asene of a signifiant orrelation etween HR and ETY in all groups (Tale 1) suggests that HR is independent of ETY representing HRV. Inreased levels of ETY mean that the HRV varied more randomly and give an impression that the power spetra are inreased; however, theoretially, ETY is independent of omponents of the power spetra [22]. In this study, signifiant positive orrelations were oserved etween ETY and omponents of the power spetra with the exeption of LF in the Control group (Tale 1). The ETY levels showed no onstant trend in hanges and no signifiant differene among the groups (Fig. 5). These results suggest that the oserved positive orrelations are aused y time-ourse effets of exaggerated ardiovasular responses to exerise, oth the voluntary and fored exerise, on ETY and omponents of the power spetra. The LF omponents of HRV in rats are redued y ardia parasympatheti or sympatheti lokade [14, 17], and whereas sympatheti lokade has no signifiant effet on the HF omponents [17], ardia parasympatheti lokade aolishes the HF omponents [37]. Aording to these studies, LF in rats an therefore e onsidered to e a marker of oth sympatheti and parasympatheti modulations and HF as a marker of parasympatheti modulation. In this study, no signifiant differene among the groups and during the experimental period in mean LF was revealed y two-fator ANOVA. To the ontrary, mean levels of HF in the Voluntary group showed a tendeny to derease after week 4 (Fig. 7), when a signifiant

7 Environ Health Prev Med (2012) 17: differene among the groups was oserved. Aording to these results, ardia parasympatheti modulation is suppressed in the Voluntary group. However, there is no signifiant differene among the groups in mean levels of LF/HF (Fig. 8) whih is the onvenient index of ardia sympathovagal alane [14 16]. In this study, ECG was reorded during the dark yle when rats were expeted to e awake and ative. It has een reported that, in rats, LF and HF in the dark yle are lower than those in the light yle and that LF/HF in the dark yle is signifiantly higher than that in the light yle [38]. The lower HF levels oserved in the Voluntary group suggest that the Voluntary group is awake longer and more ative than other groups in the dark yle. There is another interpretation for the lower HF levels. It has een shown that HF is dependent on respiratory rate [37]. The Voluntary group was housed separately in a age with a running wheel. Respiratory rate was not measured in this study; however, ompared to animals housed in pairs, isolated rats have een shown to have a similar HR and to reathe faster [39]. The relative role of parasympatheti, sympatheti, and ventilatory influenes on HRV is ontroversial [40]. In rats, mild stress-eliited inreases in HR have een assoiated with spetral modifiations refleting sympatheti hyperativity, i.e., an inrease in LF and a derease in HF [41]. In one study, a signifiant inrease in LF was oserved y treadmill exerise in a dog [42], and in another study the LF/HF ratio was augmented whenever the sympathovagal alane was shifted toward sympatheti predominane with mild physial exerise or mental stress [11]. In this study, ECG was measured efore the treadmill exerise, whih was assumed to the reasons for there eing no signifiant differene in the mean LF and LF/HF ratio among the groups. It also has een suggested that vagal ativation is independent of sympatheti ativation [43]. LF and HF annot e interpreted as a reiproal or push pull relationship etween sympatheti and parasympatheti ontrol [44]. Signifiant positive orrelations were oserved etween LF and HF in all groups (Tale 1), suggesting that the relationship etween LF and HF is not a simple reiproal relationship and that hanges of the LF and HF levels our simultaneously in the time-ourse of the experiment. In the Voluntary group, the HF levels signifiantly dereased, ut the HR and LF/HF remained at almost Control group levels, indiating that oth sympatheti and parasympatheti ativity dereased, sympatheti ativity predominating. To the ontrary, radyardia was oserved in the Fored group without any signifiant differene in the power spetral analysis, indiating that oth sympatheti and parasympatheti ativity inreased, with parasympatheti ativity predominating. Although the mean running distane in the Fored group was almost half that in the Voluntary group, the Fored group is thought to e in the overtraining situation due to the fored training and handling stress [18]. The aroreflex in the Fored group may e hyper-ativated to adapt to an aute inrease of lood pressure aused y the undesired treadmill running. Inhiitory vagal effets on food intake have een demonstrated using vagotomized rats [45]; therefore, dereased levels of feed intake in the Fored group may e aused y inreased levels of parasympatheti ativity. More detailed studies with an inreased numer of animals are needed to larify the effets of exerise on the autonomi modulation of the ardiovasular system. It is diffiult to extrapolate the present findings in rats to humans eause of the marked differenes in the physiologial harateristis etween rodents and humans. 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