Woodchucks and sudden cardiac death
How much calcium could a hibernating woodchuck’s heart cells sequester, if a hibernating woodchuck’s heart cells could sequester calcium? More than enough, it turns out, to protect the animals from cardiac arrhythmias – abnormal heart rhythms such as ventricular tachycardia and ventricular fibrillation that can lead to sudden cardiac death – according to a new study of the hibernating animals that may provide insight into arrhythmia therapies. The findings will be presented at a poster session at the 56th Annual Meeting of the Biophysical Society (BPS), which will take place Feb. 25-29 in San Diego, Calif.
Bear and bats can be roused from their slumber by external stimuli. But woodchucks (Marmota monax), also known as groundhogs, are “true hibernators,” which means they can enter a profoundly altered physiological state: their body temperature drops to near-ambient levels (often as low as freezing) and heart and respiration rates slow dramatically. Despite – or perhaps because of – these changes, hibernating animals have been found to be more resistant to cardiac arrhythmias and sudden cardiac death.
Electrophysiologist Lai-Hua Xie, an assistant professor at the University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School in Newark, and his colleagues examined muscle cells, or myocytes, isolated in winter and in summer from woodchucks.
Using a charge-coupled device (CCD) camera, the researchers monitored the release and uptake of calcium ions when the cells were activated. The team found that in winter woodchucks, the myocyte sarcoplasmic reticulum – the membrane system in muscle cells that stores and releases calcium – had less spontaneous leakage of calcium, released more of it during excitation, and took it back up faster than that of summer woodchucks or non-hibernating animals. This is likely, he says, “to generate a stronger contraction and faster relaxation, and most importantly, to prevent abnormal changes in the heart’s electrical activities called afterdepolarizations.”
The overall effect, Xie says, is a “higher resistance to arrhythmia in woodchucks in winter. Understanding these cardiac adaptive mechanisms in hibernators may suggest new strategies to protect non-hibernating animals, especially humans, from fatal cardiac arrhythmias induced by hypothermic stresses and myocardial ischemia.”
Surveys of lone afibbers generally find that the ratio of men to women is about 80:20. It is by no means clear why this is indeed the case, but the proportion of afib induced by sustained, vigorous endurance exercise would likely be higher among men. If all cases of atrial fibrillation (AF) are considered, including those involving heart disease, men have a 50% higher risk of developing AF than do women. The overall prevalence of AF is, however, higher among women because they tend to outlive men.
Other significant differences between men and women when it comes to arrhythmias and the heart in general were summarized in a recent study carried out by cardiologists at the University of South Florida. Among the highlights are:
- The average resting heart rate in women is about 3-5 bpm faster than in men. This may be due to an intrinsic difference in the sinus node.
- Women have longer QT (corrected) intervals than men. NOTE: The QT interval is the duration of the activation (contraction) and recovery of the ventricular myocardium. A prolonged QT interval is associated with ventricular arrhythmias.
- Women are more likely to suffer from supraventricular tachycardia (SVT). Research has shown that SVT episodes are more common during the luteal phase of the menstrual cycle when progesterone levels are elevated.
- Inappropriate sinus tachycardia (inappropriately high heart rate at rest [over 100 bpm] and during stress) is also more common among women and is thought to involve abnormal autonomic regulation of the sinus node. Editor’s comment: Inappropriate sinus tachycardia is also fairly common after an ablation, perhaps indicating that an ablation can result in a temporary, abnormal regulation of the sinus node.
- Women with AF are more likely than men to suffer an embolic stroke; however, they are also more likely to experience a major bleeding event if taking warfarin, so stroke prevention in women is particularly challenging. Women with paroxysmal AF tend to have longer episodes and a higher average heart rate during an episode.
- Pulmonary vein isolation (ablation) procedures are equally effective in men and women.
- Women are more likely to experience Torsades de Pointes (a distinctive form of ventricular tachycardia associated with a prolonged QT interval). This means that class 1C antiarrhythmics (flecainide and propafenone) are the preferred antiarrhythmics for women since they do not increase the QT interval. Amiodarone, sotalol, dofetilide and disopyramide may, on the other hand, increase the QT interval and should be used with caution in women.
- In the United States sudden cardiac death (SCD) claims between 300,000 and 400,000 victims every year. The incidence among women is only half of that among men and occurs 10-20 years later in life.
- The risk of SVT increases during pregnancy and during the post-partum period.
Xie and his colleagues conducted the work with funding from the National Institutes of Health and in collaboration with Stephen Vatner, director of the UMDNJ-New Jersey Medical School Cardiovascular Research Center.
The presentation, “Calcium handling properties in a hibernating animal: Insights into antiarrhythmic mechanisms,” is at 1:45 on Sunday, Feb. 26, 2012, in the San Diego Convention Center, Hall FGH. ABSTRACT: http://tinyurl.com/7mtrp4a
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This news release was prepared for the Biophysical Society (BPS) by the American Institute of Physics (AIP).
ABOUT THE 2012 ANNUAL MEETING
Each year, the Biophysical Society Annual Meeting brings together over 6,000 research scientists in the multidisciplinary fields representing biophysics. With more than 4,000 poster presentations, over 200 exhibits, and more than 20 symposia, the BPS Annual Meeting is the largest meeting of biophysicists in the world. Despite its size, the meeting retains its small-meeting flavor through its subgroup meetings, platform sessions, social activities, and committee programs.
The 56th Annual Meeting will be held at the San Diego Convention Center (111 W. Harbor Drive, San Diego, CA 92101), located three miles from the San Diego International Airport and less than one mile from the Amtrak station. The San Diego Trolley has two stops directly in front of the Center at Harbor Drive/First Avenue and Harbor Drive/Fifth Avenue.
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Ellen R. Weiss
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American Institute of Physics