Effects of fitness and age on the response to vagotonic atropine

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. Keywordsatropine; 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 Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Atropine sulfate is routinely used to probe the cardiac parasympathe...

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