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Cardiovascular Research Advance Access originally published online on June 6, 2008
Cardiovascular Research 2008 80(1):62-68; doi:10.1093/cvr/cvn150
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

The circadian pacemaker generates similar circadian rhythms in the fractal structure of heart rate in humans and rats

Kun Hu1,*, Frank A.J.L. Scheer1,2, Ruud M. Buijs2,3 and Steven A. Shea1,*

1 Division of Sleep Medicine, Brigham and Women’s Hospital, Harvard Medical School, 221 Longwood Avenue, Boston, MA 02115, USA
2 Department of Hypothalamic Integration Mechanisms, Netherlands Institute for Brain Research, Amsterdam, The Netherlands
3 Department Physiology, Instituto de investigaciones Biomedicas, Universidad Nacional Autónoma de México, Mexico City, Mexico

* Corresponding authors. Tel: +1 617 667 0346; fax: +1 617 667 0351. E-mail address: khu{at}bidmc.harvard.edu (K.H.); Tel: +1 617 732 5013; fax: +1 617 278 0683. E-mail address: sshea{at}hms.harvard.edu (S.A.S.)

Aims: Adverse cardiovascular events in humans occur with a day/night pattern, presumably related to a daily pattern of behaviours or endogenous circadian rhythms in cardiovascular variables. Healthy humans possess a scale-invariant/fractal structure in heartbeat fluctuations that exhibits an endogenous circadian rhythm and changes towards the structure observed in cardiovascular disease at the circadian phase corresponding to the time of the broad peak of adverse cardiovascular events (at about 10 AM). To explore the relationship between the rest/activity cycle, endogenous circadian rhythmicity, and cardiac vulnerability, we tested whether the fractal structure of heart rate exhibits a similar circadian rhythm in a mammalian species that is nocturnally active (Wistar rats) compared with diurnally active humans, and how this fractal structure changes after lesioning the circadian pacemaker (suprachiasmatic nucleus, SCN) in rats.

Methods and results: Analysis of heart rate data collected over 10 days in eight intact and six SCN-lesioned Wistar rats during constant darkness revealed that: (i) as with humans, rats exhibit an endogenous circadian rhythm in the scaling exponent characterizing the hourly fractal structure of heart rate (P = 0.0005) with larger exponents during the biological day (inactive phase for rats; active phase for humans); (ii) SCN lesioning abolished the rhythm in the fractal structure of heart rate and systematically increased the scaling exponent (P = 0.01).

Conclusion: Rats possess a circadian rhythm of fractal structure of heart rate with a similar temporal pattern as previously observed in humans despite opposite rest/activity cycles between the two species. The SCN imparts this endogenous rhythm. Moreover, lesioning the SCN in rats results in a larger scaling exponent, as occurs with cardiovascular disease in humans.

KEYWORDS Heart rate control; Cardiac vulnerability; Circadian rhythms; Scale-invariant/fractal structure; Behaviour; Rest/activity cycle; Suprachiasmatic nucleus

Time for primary review: 29 days


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