Cardiac Clock Misalignment, Arrhythmias Focus of Study

By Dirk Hoffman

Titled “Role of Cardiac Clock Misalignment in Promoting Arrhythmias in Swine and Donor Human Hearts,” the five-year, $3.4 million study is funded by a R01 grant from the National Heart, Lung and Blood Institute.

SCA, as well as onset of adverse ischemic events, exhibit time-of-day dependence suggesting a role for circadian regulation of these adverse events, however the mechanisms are not well understood, notes Aras, the principal investigator on the study.

“Successful completion of this work will illuminate opportunities for pharmacological interventions to attenuate time-of-day incidence of cardiac arrhythmias and SCA,” he says.

The circadian clocks are cellular time-keeping mechanisms that govern the 24-hour (diurnal) rhythms of various physiological functions (including cardiac electrophysiology) by regulating the expression of many genes and proteins.

“For example, the heart beats faster during the active period during the day and slower during the rest period at night,” Aras says. “These diurnal rhythms are modulated by coordination between a central clock located in the hypothalamus in the brain and peripheral clocks (molecular clocks) found in almost every tissue, including the heart.”

Aras says SCA patients often manifest with significant coronary artery disease — characterized by restricted/lack of blood flow to certain regions of the heart resulting in healthy and ischemic or unhealthy regions.

“We hypothesize that the cardiac clock becomes misaligned between healthy and ischemic regions resulting in dyssynchrony in the cardiac electrophysiology diurnal rhythms between the two regions,” he says. “This can lead to increased susceptibility to cardiac arrhythmias and associated SCA.”

To test this hypothesis, the study will use a combination of donor human hearts and swine model of chronic left anterior descending (LAD) artery stenosis that develops ischemia with high frequency of SCA. The researchers will employ optical mapping of ex vivo cardiac slices and in vivo electrical mapping of intact hearts to establish whether a diurnal window of arrhythmia vulnerability exists in swine with chronic LAD stenosis.

Optical mapping is a technique that entails first loading the cardiac tissue with a voltage (and sometimes calcium) sensitive fluorescent dye, which is then excited through a light source of a specific wavelength.

The change in fluorescence — tracked via specialized cameras — due to dye excitation corresponds to change in the voltage or calcium activity in the tissue, which allows the researchers to measure and map the electrical activity of the tissue.  

“Since we use a light source, this modality is called optical mapping. The advantage of this technique is the ability to map electrical activity at high spatial and temporal resolution, which is essential when tracking susceptibility to arrhythmias,” Aras says.

Ex vivo cardiac slices mapping involves taking a block of cardiac tissue and using a specialized instrument called a vibratome to produce cardiac slices, which can then be used to map electrical activity. A tissue block can produce tens of slices, which can be kept viable in an incubator for multiple weeks.

“The combination of long-term viability of these cardiac slices as well as the volume of slices that can be produced from a swine/donor human heart allows us to characterize the cardiac electrophysiology circadian rhythm of healthy and ischemic tissue for comparison,” Aras says.

In vivo electrical mapping involves using an electrode array placed on the whole heart surface (in situ) and mapping the electrical activity of the heart.

“This approach allows us to map the electrophysiology of a working heart — and under the influence of central clock — and validate the findings from ex vivo cardiac slice studies at distinct times of the day,” Aras notes.

The study’s three main aims are:

• to advance knowledge of cardiac electrophysiology diurnal rhythms in swine and donor human hearts, under both physiological (normal) and pathological (chronic ischemia) conditions, which is currently not available
• to provide a better understanding about the role of local cardiac clock misalignment between healthy and ischemic tissue in promoting time-of-day dependent susceptibility to cardiac arrhythmias and SCA
• to validate whether therapeutic targeting of the cardiac clock to restore circadian synchrony (of cardiac electrophysiology) can attenuate susceptibility to cardiac arrhythmias and SCA

“Understanding the role of cardiac circadian disruption in cardiac arrhythmias and SCA can provide novel chronotherapy — both pharmacological and non-pharmacological approaches — and enable appropriate lifestyle modifications that conform to circadian rhythms to diminish risk of cardiovascular events in vulnerable patients,” Aras says.

Study co-investigators from the University at Buffalo are:

• John M. Canty Jr., MD, SUNY Distinguished Professor and Albert and Elizabeth Rekate Professor of medicine in the Division of Cardiovascular Medicine in the Jacobs School
• Jun Qu, PhD, professor of pharmaceutical sciences in the School of Pharmacy and Pharmaceutical Sciences

Martin Young, PhD, professor of medicine at the University of Alabama at Birmingham, is also a co-investigator.