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Published in final edited form as: Auton Neurosci. 2008 Mar 5;139(1-2):60–67. doi: 10.1016/j.autneu.2008.01.007

Effects of Fitness and Age on the Response to Vagotonic Atropine

Kichang Lee 1, Glen Picard 1, Stacy D Beske 1, Gyu-Sam Hwang 1, J Andrew Taylor 1
PMCID: PMC2408756  NIHMSID: NIHMS46777  PMID: 18321791

Abstract

Previous work indicates compromised cardiac vagal control plays a prominent role in reducing arterial baroreflex gain with age, however older fit individuals display cardiovagal baroreflex responses similar to young individuals. The purpose of this study was to test the hypothesis that chronic aerobic exercise mitigates against age-related declines in cardiac parasympathetic receptor function. In forty-four young and old (fit and unfit) individuals, we used the parasympathomimetic responses to low doses of atropine to probe cardiac cholinergic receptor responses. Data was collected before and after eight doses of atropine sulfate from 0.4 to 7.2 ug/kg. Chronotropic responses were assessed from average RR intervals and heart rate variabilities were derived in time and frequency domains. All subjects exhibited bradycardia with at least one dose of atropine and peak bradycardia occurred at a similar dose in each group. However, changes in heart rate variability did not consistently track the chronotropic responses within subjects (r-square from 0.90 down to 0). As expected, basal RR interval was longer in the fit groups and was unaffected by age. However, the degree of RR interval lengthening with parasympathomimetic atropine was unaffected by physical fitness and was significantly less in all older subjects. These data indicate there are certain prepotent age-related declines in the cardiac parasympathetic system that cannot be prevented by regular physical activity.

Keywords: atropine, vagal, heart rate variability, muscarinic receptor

Our previous work indicates a decline in cardiac parasympathetic neural control compromises baroreflex gain with age (Kaushal et al., 2002) and this decline can be offset by regular physical activity (Hunt et al., 2001). This suggests a reduced cardiac vagal control accompanying physiologic aging may result essentially from progressive deconditioning (Goldsmith et al., 1997). Greater resting cardiac vagal tone is a distinguishing characteristic of physically fit individuals (Ekblom et al., 1973; Frick et al., 1967; Shi et al., 1995; Smith et al., 1989) and a vagally mediated resting bradycardia is observed in younger as well as older humans after exercise training (Stratton et al., 1994). Moreover, our prior work shows that older fit individuals can display baroreflex-mediated cardiac vagal control similar to young individuals (Hunt et al., 2001). In these individuals, baroreflex responses are maintained despite age-related arterial stiffening similar to sedentary individuals, indicating that exercise training may be a powerful stimulus to prevent vagal declines with age (Hunt et al., 2001).

Atropine sulfate is routinely used to probe the cardiac parasympathetic system. For example, the chronotropic response to high, fully parasympatholytic doses of atropine has been used to quantify resting cardiac vagal tone (Robinson et al., 1966). However, very low doses are parasympathomimetic reducing heart rate (Averill et al., 1959; McGuigan, 1921; Morton et al., 1958) and increasing variability (Raczkowska et al., 1983). The responses to vagotonic atropine may provide insight to mechanisms influencing resting cardiac vagal tone. The preponderance of evidence suggests that low dose atropine preferentially blocks presynaptic cholinergic autoreceptors in the heart (Brodde et al., 2001; Epstein et al., 1990; Hellgren et al., 2000; Oberhauser et al., 2001), increasing the release of acetylcholine from parasympathetic nerves. If receptor function is decreased with age or is enhanced by exercise training, the parasympathomimetic response would change accordingly. The present cross-sectional study sought to determine influences of aging and chronic aerobic exercise on these responses. We used sequential, low doses of atropine to determine maximal parasympathomimetic effect and compared responses in young and older, fit and unfit individuals. We hypothesized chronic aerobic exercise would augment responses to vagotonic atropine and mitigate against age-related declines in the response.

METHODS

Subjects

Forty-four healthy volunteers (23 male and 21 female) participated in the study. All volunteers were free from cardiovascular diseases as determined by health history questionnaires, fasting blood chemistries, and resting and maximal exercise electrocardiograms. Volunteers were excluded if they presented a history of any cardiovascular disease, depression, neurological disorders, diabetes, and obesity or if they were taking any cardioactive medications. All volunteers were nonsmokers and normotensive. Volunteers were instructed to refrain from consuming caffeine and alcohol and from performing any vigorous activity 24 hours prior to testing.

Subjects were classified into 4 groups based on age and habitual physical activity levels (Table 1). Young subjects were age 21–30 and older subjects were age 55–75. Unfit subjects did not regularly participate in aerobic exercise and were excluded if they performed aerobic exercise for at least 30 minutes, 3 times per week. Fit subjects performed aerobic exercise for a minimum of 30 minutes, 5 times per week and had maintained their current training level for at least 1 year. In addition, each subject underwent maximal oxygen consumption (VO2max) determination during graded treadmill exercise to document that all fit subjects had maximal aerobic powers above the 90th percentile for individuals of their age and gender (American College of Sports Medicine, 2000). Standard criteria were applied to ensure all subjects achieved maximal aerobic consumption (i.e., respiratory exchange ratio ≥1.10, heart rate at least 85% of age-predicted, plateau in oxygen consumption with increasing workload, and a maximal rating of perceived exertion).

Table 1.

Subject Characteristics

Young Fit Young Unfit Old Fit Old Unfit
n (females) 11 (6) 11 (5) 11 (4) 11 (6)
age, yrs 24.0±0.8 24.6±0.8 59.0±1.2 66.1±2.0
bmi, kg/m2 24.1±0.6 22.3±1.0 22.7±0.8 24.2±0.9
weight, kg 73.1±2.3 69.9±5.0 65.5±4.6 68.7±3.2
systolic pressure, mmHg 112±6 108±3 123±5 110±3
diastolic pressure, mmHg 66±5 61±2 76±3 70±2
maximum VO2, ml/kg/min 55.5±2.3 43.0±1.4 51.4±2.8 27.8±1.0
length of training, years 4.7±1.3 21.5±4.2
Weekly activity, minutes
moderate 193±53 305±44 285±95 294±53
hard 19±12 36±15 141±52 63±32
very hard 262±57 57±22 289±57 39±28
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All values are mean±S.E.M.

To verify activity levels, each subject completed a Stanford 7-Day Physical Activity Recall Questionnaire (Table 1) (Richardson et al., 2001). This is a well known validated instrument which assesses physical activity performed at different intensities over the previous seven days. The questionnaire divides physical activity into 4 categories; very hard (10 METs), hard (6 METs), moderate (4 METs) and light activity (1.5 METs).

This protocol was approved by the Institutional Review Board of the Hebrew Rehabilitation Center for Aged and conformed with The Declaration of Helsinki. Each subject was thoroughly acquainted with all aspects of the experiment prior to obtaining informed consent.

Experimental protocol

Each subject reported to the laboratory at least two days after the VO2max test, between 7:00 am and 11:00 am following a 12-hour overnight fast. Following instrumentation and insertion of an antecubital venous catheter for drug infusion, subjects rested in the supine position for at least 10 min prior to data collection. During all data collection periods, subjects controlled their breathing frequency at 15 breaths/minute (0.25 Hz) in response to an auditory cue. Each data collection period consisted of five minutes from which the last three minutes was extracted for analysis. Data was collected at baseline and after eight bolus doses of atropine sulfate (0.4, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, and 1.4 ug/kg; cumulative doses of 0.4, 0.8, 1.4, 2.2, 3.2, 4.4, 5.8, and 7.2 ug/kg). After each injection, at least two minutes was allotted for development of full drug effect prior to data acquisition. If subjects demonstrated a significant tachycardia (heart rate ~10 beats/min above baseline) before the eighth dose, atropine administration was stopped. Atropine has a half-life of approximately 4 hours; therefore, the duration of our protocol was sufficiently short that effects were sustained throughout all measurement periods.

Measurements

Arterial blood pressure was recorded every minute from the right arm by an automated brachial cuff (Dinamap, Critikon). RR interval was determined from lead II of an electrocardiogram. Respiration was determined by a respiratory transducer bands placed around the midchest.

Data Analysis

Data were continuously digitized and stored on a personal computer at a sampling rate of 500 Hz (DI-700 & WinDaq, DATAQ Instruments, OH). Beat-by-beat RR intervals were determined using peak detection algorithms (Mathworks Inc, Natick, MA) and manually inspected to confirm data quality. Average blood pressures and RR intervals were determined from three minutes of paced breathing for each atropine dose. Chronotropic responses were assessed from RR interval because it most linearly reflects changes in parasympathetic stimulation (Koizumi et al., 1985; Parker et al., 1984). Heart rates were also derived to confirm that the peak chronotropic responses occurred at the same atropine dose regardless of chronotropic measure. RR interval standard deviation and the root of the mean square of successive differences in RR intervals were taken as representative time domain measures. The power spectra of 4 Hz re-sampled RR interval time series were calculated via fast Fourier transforms based on Welch's periodogram algorithm. Total spectral power below 0.5 Hz (i.e., total power) and respiratory power at 0.25 Hz (i.e., respiratory sinus arrhythmia) were derived as representative frequency domain measures. These measures of heart rate variability have been proposed to be standard and valid measures of cardiac vagal tone (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996).

Statistics

Data were compared across four groups using a two-way ANOVA to examine effects of fitness and age. Possible gender differences and interactions were tested, but none were found (lowest P=0.16, for dose-6 RR interval, gender × fitness). Therefore, only results from age and fitness and their interactions are presented. To examine the relationships between parasympathetically mediated cardiac chronotropy and RR interval variability measures, linear regressions were performed between the mean RR interval and each HRV index across all atropine doses within each subject. All values are presented as mean ± SE and statistical significance was accepted at P<0.05.

RESULTS

The protocol dictated atropine dosing was stopped if a tachycardia of 10 or more beats/min above baseline occurred before the eighth dose. As a result, two subjects had six doses, 22 subjects had seven doses, and 20 subjects had eight doses of atropine. Figure 1 depicts the average changes in RR interval across atropine doses for each group. All subjects exhibited bradycardia in response to at least one dose of atropine. The dose at which peak bradycardia occurred was not different across groups and developed with the fourth dose, or 2.2 ug/kg of atropine in two-thirds of the subjects. On average, arterial pressures were unchanged by lower drug infusion levels, but increased slightly at the highest doses (i.e., At dose seven, 5.8 ug/kg atropine, systolic pressure increased 3.7±1.3 mmHg and diastolic pressure increased 2.8±0.9 mmHg, P<0.05). These responses did not differ between groups.

Figure 1.

Figure 1

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Changes in RR interval with progressive low dose atropine in young and older, fit and unfit individuals. Atropine dose 8 (7.2 ug/kg) appears inconsistent in its effect because only 20 of 44 subjects received this dose. Values are mean ± S.E.M.

Basal RR interval was longer in the fit groups and was unaffected by age (Figure 2). Likewise, the peak RR interval with parasympathomimetic atropine was longer in the fit groups and was unaffected by age. However, the magnitude of RR interval lengthening with parasympathomimetic atropine was unaffected by physical fitness and was significantly less in both older groups compared with the young. Thus, exercise training resulted in a resting bradycardia, but had no effect on the response to parasympathomimetic atropine and did not prevent the age-related decline in the response. These differences remained when heart rate was used to assess bradycardiac responses.

Figure 2.

Figure 2

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Average basal and peak RR intervals and peak changes in RR interval with low dose atropine in young and older, fit and unfit individuals. Values are mean ± S.E.M

Basal heart rate variabilities in both time and frequency domains were significantly reduced with age (P<0.05 for all). However, changes in the variabilities did not consistently track chronotropic responses to parasympathomimetic and parasympatholytic atropine. Figure 3 shows data from two subjects who exemplify the range of patterns demonstrated by the heart rate variability indices across doses of parasympathomimetic and parasympatholytic atropine. As a result of this inconsistency, the relationship between variabilities and mean RR interval across doses of atropine demonstrated wide variations for within subject correlations, for standard deviation from −0.35 to 0.93, for root means square of successive differences from −0.79 to 0.95, for respiratory sinus arrhythmia from −0.80 to 0.90, and for total spectral power from −0.23 to 0.93. The dose of atropine eliciting the largest bradycardia was infrequently the dose of atropine eliciting the largest increase in variability. In fact, the peak increase in variabilities only occurred at the peak vagotonic dose for RR interval less than one third of the time. In many cases at the peak vagotonic dose for RR interval, variability was reduced below basal levels. Figure 4 shows the responses averaged across subjects within each group.

Figure 3.

Figure 3

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Examples of the responses to low dose atropine. One subject had a consistent pattern in both of RR interval and heart rate variabilities and the other did not.

Figure 4.

Figure 4

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Average responses of heart rate variabilities to progressive low dose atropine in young and older, fit and unfit individuals. Values are mean ± S.E.M.

DISCUSSION

We sought to explore interactions between exercise- and age-related effects on the cardiac parasympathetic system. We hypothesized chronic aerobic exercise augments responses to vagotonic atropine and mitigates against age-related declines in cholinergic receptor function. However, we were surprised to find that fitness level does not impact responses to low doses of atropine. Although our data support previous findings that both young and older trained individuals demonstrate a resting bradycardia (Spina et al., 1998; Stratton et al., 1994), they suggest it does not relate to the responses to vagotonic atropine. Moreover, our data indicate prepotent age-related declines in cardiac cholinergic receptor function cannot be prevented by regular physical activity, even in highly trained endurance athletes. Considering prior data demonstrating maintained baroreflex control of cardiac vagal effects in older fit individuals, it appears vagotonic atropine exerts its effects at a site in the reflex pathway which is unaffected by exercise training.

The case has made by some that the mechanism for atropine’s vagomimetic effects are of central neural origin (Raczkowska et al., 1983). For example, cardiac vagal efferent activity in anesthetized dogs increases after atropine doses of approximately 4.86 micrograms/kg and higher (Katona et al., 1977). The response in humans with transplanted hearts has also been cited as support of a central neural mechanism. The donor sinus node does not respond to any dose of atropine, either vagotonic or vagolytic, whereas the native innervated sinus node demonstrates bradycardia at atropine doses less than 2 microgram/kg and tachycardia at higher doses (Epstein et al., 1990). However, anesthetized animals may not provide the best model of human autonomic responses and data from human transplanted hearts only indicate vagal innervation is required to observe the chronotropic responses to atropine.

There is strong evidence to indicate that the bradycardia with low dose atropine originates from peripheral receptor effects. For example, intravenous dosages that produce bradycardia result in no measurable atropine in human cerebral spinal fluid (Virtanen et al., 1982). Moreover, low doses of an atropine derivative that cannot cross the blood brain barrier also induce bradycardia (Kottmeier et al., 1968), indicating effects of low dose atropine may develop from blockade of peripheral muscarinic receptors. However, bradycardia cannot be generated by antagonism of post-synaptic cardiac muscarinic receptors, but only by antagonism of pre-synaptic M1 muscarinic autoreceptors. Selective stimulation of M1 receptors increases heart rate (Shannon et al., 1994), and selective blockade decreases heart rate (Jakubetz et al., 2000; Pitschner et al., 1988; Wellstein et al., 1988). Since there are no M1 receptors on the myocardium itself, agonist or antagonist mediated changes in heart rate must derive from altered acetylcholine release from cardiac parasympathetic nerves. It seems likely low dose atropine generates vagomimetic effects similarly, by blocking presynaptic muscarinic autoreceptors, enhancing acetylcholine release, and causing negative chronotropic effects through activation of post-synaptic M2 receptors. There is direct evidence for this: pirenzepine, a hydrophilic, selective M1 antagonist abolishes the bradycardia with low dose atropine but does not affect the tachycardia with high dose atropine (Wellstein et al., 1988). Thus, the preponderance of data suggests low dose atropine generates a bradycardia via blockade of pre-synaptic muscarinic autoreceptors.

Considering the above, lesser heart rate slowing in older adults in response to parasympathomimetic atropine reflects impairments in pre-synaptic and/or post-synaptic muscarinic receptor function. Previous research demonstrated both atropine and pirenzepine result in lesser bradycardia in older subjects (Poller et al., 1997), however response to either drugs is the sum of pre-synaptic receptor blockade and post-synaptic receptor stimulation. There are no data specifically examining the effect of age on pre-synaptic M1 receptors, although lesser release of acetylcholine from atria in older humans has been interpreted as reflecting up-regulation of pre-synaptic autoreceptors (Oberhauser et al., 2001). If this were the case in our subjects, we would have observed greater, not lesser bradycardia in older subjects with low dose atropine. The only direct evidence of cardiac muscarinic receptor change with age indicates lesser density and lower affinity of post-synaptic M2 receptors in adult humans (Brodde et al., 1998) and lesser mRNA and protein levels of M2 cholinoceptors in older rats (Lo et al., 2001). However, some animal data suggest acetylcholinesterase activity is reduced with age (Kennedy et al., 1990) which would effectively increase the relative amounts of acetylcholine reaching post-synaptic receptors. If this applies to humans, it suggests aging causes profound declines in cholinergic receptor function.

Regardless of pre- or post- synaptic deficit, our data provide support that human aging markedly reduces cardiac muscarinic receptor function regardless of physical fitness. We reasoned that the resting, training-related bradycardia in both young and older fit individuals would relate to greater slowing of heart rate with vagotonic atropine. However, we did not find this; in fact, even though our older fit subjects tended to be slightly younger, their bradycardiac response was no different from their unfit peers. This suggests that, regardless of age, increased parasympathetic muscarinic receptor function does not contribute to the training bradycardia. However, resting heart rate reflects both intrinsic rate and autonomic influences and data suggest that the resting bradycardia in trained individuals derives in part from a lower intrinsic rate (Katona et al., 1982) but with a parasympathetic predominance (Smith et al., 1989). Thus, decreases in intrinsic heart rate (Randall et al., 2003; Scott et al., 2004) may play a more vital role in the resting bradycardia that results from regular aerobic conditioning. In addition, there may be a ceiling for maximal vagal activity and vagal effects on the heart may be greatest in young adult humans and may not be able to be enhanced. Both cross-sectional (Smith et al., 1988) and longitudinal data (Kingwell et al., 1992) suggest that cardiac vagal baroreflex control is not enhanced by endurance training in young individuals. In contrast, our prior work showed that baroreflex gain is preserved in older active individuals and this was achieved primarily through enhanced neural control (Hunt et al., 2001). Our current data suggest that cholinergic receptor declines are a primary, age-associated change that are unaffected by exercise training. This type of receptor decline may not be surprising since it has also been shown that reduced cardiac beta-adrenergic responsiveness is also a primary age-associated decline that is unaffected by physical activity (Stratton et al., 1992). Considering our prior and current results, exercise training may result in central neural adaptations that are sufficient to overcome age-related cardiac autonomic receptor deficits.

We derived time and frequency domain estimates of heart rate variability widely accepted as valid indices of cardiac vagal tone (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996). We presumed these indices would provide additional insight to the vagotonic effects of low dose atropine. Although absolute RR interval is the best reflection of vagal effects (Carlsten et al., 1957; Koizumi et al., 1985; Parker et al., 1984; Stuesse et al., 1981), there exists a conventional wisdom that heart rate variability indices provide useful metrics for cardiac vagal effects. However, we found that none of the variability indices we derived tracked the progressive, cholinergically mediated, chronotropic effects of atropine. Respiratory pattern is unlikely the culprit for our finding. All subjects adequately maintained eupneic breathing at a frequency of 0.25 Hz for measurement of variabilities. Although our finding may be analogous to data demonstrating the relationship between heart rate variability and parasympathetic effect is best described by a nonlinear function with marked interindividual variation (Goldberger et al., 2001), it underscores the latter aspect. If variability reflects sinusoidal vagal modulation of heart rate and not vagal tone, our data show this modulation can change in an unpredictable manner in some individuals and provide a cautionary note on heart rate variabilities.

One limitation of this study is that we did not obtain the heart rate dose response curve up to a fully parasympatholytic dose of atropine. We used successive incremental doses of atropine to induce a parasympathomimetic effect and then a partial parasympatholytic effect. Reaching the plateau phase of the heart rate response would have allowed calculation of parasympathetic tone in the subject groups and provided additional information on vagal control. However, to use this full atropine blockade data for inferences on vagal tone, we would have needed to block cardiac sympathetic effects first and this would have obscured the responses to the low doses of atropine. Thus, this approach would not only require a different study design, but would provide no information on possible cholinergic receptor effects of aging and physical activity.

Our results indicate chronic aerobic exercise does not augment responses to vagotonic atropine. From the available evidence, this suggests bradycardia in both young and older trained individuals is not a receptor-mediated phenomenon. Moreover, our data suggests exercise training does not mitigate against age-related declines in cardiac muscarinic receptor function. Therefore maintained baroreflex gain in older endurance athletes must derive from vagal neural adaptations which overcome declines in cardiac muscarinic receptor function that may be a fundamental characteristic of human aging.

Acknowledgements

We wish to thank the subjects for their generous participation. This research was supported by Grant AG014376 from the National Institute on Aging (J.A.T.). There are no conflicts of interest.

Footnotes

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References

  1. Franklin BA, Whaley MH, Howley ET, Balady GJ. American College of Sports Medicine. ACSM's guidelines for exercise testing and prescription. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2000. p. 368. xix. [Google Scholar]
  2. Averill KH, Lamb LE. Less commonly recognized actions of atropine on cardiac rhythm. Am J Med Sci. 1959;237:304–318. doi: 10.1097/00000441-195903000-00004. passim. [DOI] [PubMed] [Google Scholar]
  3. Brodde OE, Bruck H, Leineweber K, Seyfarth T. Presence, distribution and physiological function of adrenergic and muscarinic receptor subtypes in the human heart. Basic Res Cardiol. 2001;96:528–538. doi: 10.1007/s003950170003. [DOI] [PubMed] [Google Scholar]
  4. Brodde OE, Konschak U, Becker K, Ruter F, Poller U, Jakubetz J, Radke J, Zerkowski HR. Cardiac muscarinic receptors decrease with age. In vitro and in vivo studies. J Clin Invest. 1998;101:471–478. doi: 10.1172/JCI1113. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Carlsten A, Folkow B, Hamberger CA. Cardiovascular effects of direct vagal stimulation in man. Acta Physiol Scand. 1957;41:68–76. doi: 10.1111/j.1748-1716.1957.tb01510.x. [DOI] [PubMed] [Google Scholar]
  6. Ekblom B, Kilbom A, Soltysiak J. Physical training, bradycardia, and autonomic nervous system. Scand J Clin Lab Invest. 1973;32:251–256. doi: 10.3109/00365517309082468. [DOI] [PubMed] [Google Scholar]
  7. Epstein AE, Hirschowitz BI, Kirklin JK, Kirk KA, Kay GN, Plumb VJ. Evidence for a central site of action to explain the negative chronotropic effect of atropine: studies on the human transplanted heart. J Am Coll Cardiol. 1990;15:1610–1617. doi: 10.1016/0735-1097(90)92834-o. [DOI] [PubMed] [Google Scholar]
  8. Frick MH, Elovainio RO, Somer T. The mechanism of bradycardia evoked by physical training. Cardiologia. 1967;51:46–54. doi: 10.1159/000165849. [DOI] [PubMed] [Google Scholar]
  9. Goldberger JJ, Challapalli S, Tung R, Parker MA, Kadish AH. Relationship of heart rate variability to parasympathetic effect. Circulation. 2001;103:1977–1983. doi: 10.1161/01.cir.103.15.1977. [DOI] [PubMed] [Google Scholar]
  10. Goldsmith RL, Bigger JT, Jr, Bloomfield DM, Steinman RC. Physical fitness as a determinant of vagal modulation. Med Sci Sports Exerc. 1997;29:812–817. doi: 10.1097/00005768-199706000-00012. [DOI] [PubMed] [Google Scholar]
  11. Hellgren I, Mustafa A, Riazi M, Suliman I, Sylven C, Adem A. Muscarinic M3 receptor subtype gene expression in the human heart. Cell Mol Life Sci. 2000;57:175–180. doi: 10.1007/s000180050507. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Hunt BE, Farquhar WB, Taylor JA. Does reduced vascular stiffening fully explain preserved cardiovagal baroreflex function in older, physically active men? Circulation. 2001;103:2424–2427. doi: 10.1161/01.cir.103.20.2424. [DOI] [PubMed] [Google Scholar]
  13. Jakubetz J, Schmuck S, Wochatz G, Ruhland B, Poller U, Radke J, Brodde OE. Human cardiac beta1- or beta2-adrenergic receptor stimulation and the negative chronotropic effect of low-dose pirenzepine. Clin Pharmacol Ther. 2000;67:549–557. doi: 10.1067/mcp.2000.105989. [DOI] [PubMed] [Google Scholar]
  14. Katona PG, Lipson D, Dauchot PJ. Opposing central and peripheral effects of atropine on parasympathetic cardiac control. Am J Physiol. 1977;232:H146–H151. doi: 10.1152/ajpheart.1977.232.2.H146. [DOI] [PubMed] [Google Scholar]
  15. Katona PG, McLean M, Dighton DH, Guz A. Sympathetic and parasympathetic cardiac control in athletes and nonathletes at rest. J Appl Physiol. 1982;52:1652–1657. doi: 10.1152/jappl.1982.52.6.1652. [DOI] [PubMed] [Google Scholar]
  16. Kaushal P, Taylor JA. Inter-relations among declines in arterial distensibility, baroreflex function and respiratory sinus arrhythmia. J Am Coll Cardiol. 2002;39:1524–1530. doi: 10.1016/s0735-1097(02)01787-4. [DOI] [PubMed] [Google Scholar]
  17. Kennedy RH, Seifen E. Aging: effects on chronotropic actions of muscarinic agonists in isolated rat atria. Mech Ageing Dev. 1990;51:81–87. doi: 10.1016/0047-6374(90)90164-b. [DOI] [PubMed] [Google Scholar]
  18. Kingwell BA, Dart AM, Jennings GL, Korner PI. Exercise training reduces the sympathetic component of the blood pressure-heart rate baroreflex in man. Clin Sci (Lond) 1992;82:357–362. doi: 10.1042/cs0820357. [DOI] [PubMed] [Google Scholar]
  19. Koizumi K, Terui N, Kollai M. Effect of cardiac vagal and sympathetic nerve activity on heart rate in rhythmic fluctuations. J Auton Nerv Syst. 1985;12:251–259. doi: 10.1016/0165-1838(85)90065-7. [DOI] [PubMed] [Google Scholar]
  20. Kottmeier CA, Gravenstein JS. The parasympathomimetic activity of atropine and atropine methylbromide. Anesthesiology. 1968;29:1125–1133. doi: 10.1097/00000542-196811000-00008. [DOI] [PubMed] [Google Scholar]
  21. Lo SH, Liu IM, Huang LW, Cheng JT. Decrease of muscarinic M2 cholinoceptor gene expression in the heart of aged rat. Neurosci Lett. 2001;300:185–187. doi: 10.1016/s0304-3940(01)01580-4. [DOI] [PubMed] [Google Scholar]
  22. McGuigan H. The effect of small doses of atropine on heart rate. J. Am. Med. Assoc. 1921;76:1338–1340. [Google Scholar]
  23. Morton HJ, Thomas ET. Effect of atropine on the heart-rate. Lancet. 1958;2:1313–1315. doi: 10.1016/s0140-6736(58)90580-4. [DOI] [PubMed] [Google Scholar]
  24. Oberhauser V, Schwertfeger E, Rutz T, Beyersdorf F, Rump LC. Acetylcholine release in human heart atrium: influence of muscarinic autoreceptors, diabetes, and age. Circulation. 2001;103:1638–1643. doi: 10.1161/01.cir.103.12.1638. [DOI] [PubMed] [Google Scholar]
  25. Parker P, Celler BG, Potter EK, McCloskey DI. Vagal stimulation and cardiac slowing. J Auton Nerv Syst. 1984;11:226–231. doi: 10.1016/0165-1838(84)90080-8. [DOI] [PubMed] [Google Scholar]
  26. Pitschner HF, Wellstein A. Dose-response curves of pirenzepine in man in relation to M1- and M2-cholinoceptor occupancy. Naunyn Schmiedebergs Arch Pharmacol. 1988;338:207–210. doi: 10.1007/BF00174872. [DOI] [PubMed] [Google Scholar]
  27. Poller U, Nedelka G, Radke J, Ponicke K, Brodde OE. Age-dependent changes in cardiac muscarinic receptor function in healthy volunteers. J Am Coll Cardiol. 1997;29:187–193. doi: 10.1016/s0735-1097(96)00437-8. [DOI] [PubMed] [Google Scholar]
  28. Raczkowska M, Eckberg DL, Ebert TJ. Muscarinic cholinergic receptors modulate vagal cardiac responses in man. J Auton Nerv Syst. 1983;7:271–278. doi: 10.1016/0165-1838(83)90080-2. [DOI] [PubMed] [Google Scholar]
  29. Randall DC, Brown DR, McGuirt AS, Thompson GW, Armour JA, Ardell JL. Interactions within the intrinsic cardiac nervous system contribute to chronotropic regulation. Am J Physiol Regul Integr Comp Physiol. 2003;285:R1066–R1075. doi: 10.1152/ajpregu.00167.2003. [DOI] [PubMed] [Google Scholar]
  30. Richardson MT, Ainsworth BE, Jacobs DR, Leon AS. Validation of the Stanford 7-day recall to assess habitual physical activity. Ann Epidemiol. 2001;11:145–153. doi: 10.1016/s1047-2797(00)00190-3. [DOI] [PubMed] [Google Scholar]
  31. Robinson BF, Epstein SE, Beiser GD, Braunwald E. Control of heart rate by the autonomic nervous system. Studies in man on the interrelation between baroreceptor mechanisms and exercise. Circ Res. 1966;19:400–411. doi: 10.1161/01.res.19.2.400. [DOI] [PubMed] [Google Scholar]
  32. Scott AS, Eberhard A, Ofir D, Benchetrit G, Dinh TP, Calabrese P, Lesiuk V, Perrault H. Enhanced cardiac vagal efferent activity does not explain training-induced bradycardia. Auton Neurosci. 2004;112:60–68. doi: 10.1016/j.autneu.2004.04.006. [DOI] [PubMed] [Google Scholar]
  33. Shannon HE, Bymaster FP, Calligaro DO, Greenwood B, Mitch CH, Sawyer BD, Ward JS, Wong DT, Olesen PH, Sheardown MJ, et al. Xanomeline: a novel muscarinic receptor agonist with functional selectivity for M1 receptors. J Pharmacol Exp Ther. 1994;269:271–281. [PubMed] [Google Scholar]
  34. Shi X, Stevens GH, Foresman BH, Stern SA, Raven PB. Autonomic nervous system control of the heart: endurance exercise training. Med Sci Sports Exerc. 1995;27:1406–1413. [PubMed] [Google Scholar]
  35. Smith ML, Graitzer HM, Hudson DL, Raven PB. Baroreflex function in endurance-and static exercise-trained men. J Appl Physiol. 1988;64:585–591. doi: 10.1152/jappl.1988.64.2.585. [DOI] [PubMed] [Google Scholar]
  36. Smith ML, Hudson DL, Graitzer HM, Raven PB. Exercise training bradycardia: the role of autonomic balance. Med Sci Sports Exerc. 1989;21:40–44. doi: 10.1249/00005768-198902000-00008. [DOI] [PubMed] [Google Scholar]
  37. Spina RJ, Turner MJ, Ehsani AA. Beta-adrenergic-mediated improvement in left ventricular function by exercise training in older men. Am J Physiol. 1998;274:H397–H404. doi: 10.1152/ajpheart.1998.274.2.H397. [DOI] [PubMed] [Google Scholar]
  38. Stratton JR, Cerqueira MD, Schwartz RS, Levy WC, Veith RC, Kahn SE, Abrass IB. Differences in cardiovascular responses to isoproterenol in relation to age and exercise training in healthy men. Circulation. 1992;86:504–512. doi: 10.1161/01.cir.86.2.504. [DOI] [PubMed] [Google Scholar]
  39. Stratton JR, Levy WC, Cerqueira MD, Schwartz RS, Abrass IB. Cardiovascular responses to exercise. Effects of aging and exercise training in healthy men. Circulation. 1994;89:1648–1655. doi: 10.1161/01.cir.89.4.1648. [DOI] [PubMed] [Google Scholar]
  40. Stuesse SL, Wallick DW, Zieske H, Levy MN. Changes in vagal phasic chronotropic responses with sympathetic stimulation in the dog. Am J Physiol. 1981;241:H850–H856. doi: 10.1152/ajpheart.1981.241.6.H850. [DOI] [PubMed] [Google Scholar]
  41. Task Force of the European Society of Cardiology and the North American Society of Pacing and ElectrophysiologyHeart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93:1043–1065.
  42. Virtanen R, Kanto J, Iisalo E, Iisalo EU, Salo M, Sjovall S. Pharmacokinetic studies on atropine with special reference to age. Acta Anaesthesiol Scand. 1982;26:297–300. doi: 10.1111/j.1399-6576.1982.tb01770.x. [DOI] [PubMed] [Google Scholar]
  43. Wellstein A, Pitschner HF. Complex dose-response curves of atropine in man explained by different functions of M1- and M2-cholinoceptors. Naunyn Schmiedebergs Arch Pharmacol. 1988;338:19–27. doi: 10.1007/BF00168807. [DOI] [PubMed] [Google Scholar]

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