December 15, 2009

BIRMINGHAM, Ala. - The time of day can be an important factor in determining the amount of damage caused by a heart attack, according to University of Alabama at Birmingham (UAB) Division of Cardiovascular Disease research published online in the journal Circulation Research.

Circadian clock is the name given to the internal body clock that regulates the 24-hour cycle of human and animal biological processes. UAB Associate Professor of Cardiovascular Disease Martin Young, Ph.D., and colleagues studying mouse models found that the time of day could have up to a three-and-a-half-fold impact on how much tissue dies during a heart attack, due to this internal body clock.

"In our mouse model, the most damage was shown to occur in the early morning, at the sleep-to-wake transition," Young said. "This is the same time of the day at which previous studies have shown more heart attacks occur in humans."

Young said it is widely accepted that time of day, week or season of the year influences cardiovascular health and disease. For example, heart attacks occur with greatest incidence early on a Monday morning, in fall and winter. It is thought this is because of fluctuations in posture, physical exertion, food consumption and body temperature over the course of the day. Young said he and his colleagues, including UAB Physiology and Biophysics graduate student David Durgan, looked at the internal body clock as a predictor of damage as a natural progression from this previous research.

"What has become increasingly clear is a significant contribution of intrinsic mechanisms mediating temporal-dependence of cardiovascular physiology and pathophysiology," Young said. "For instance, travelers retain time-of-day oscillations in sudden cardiac death, in such a way that the peak incidence is equivalent to the early hours of the morning in their time zone of origin, not where they have traveled to.

"Although circadian rhythms in heart-attack timing are well established, time-of-day oscillations in heart-attack damage have not been reported previously."

Young said he and his colleagues also found that that this time-dependence in damage is absent in a mouse model in which the circadian clock is specifically disrupted in the cardiac muscle cells.

"From this we concluded that cardiac muscle cell circadian clock mediates time-of-day-dependent changes in heart-attack tolerance," he said. "Future efforts likely will focus on identifying ways to reset the heart clock as a means to improve heart-attack tolerance."

About the UAB Division of Cardiovascular Disease

The UAB Division of Cardiovascular Disease emphasizes excellence in patient care, teaching, and basic and clinical research. Clinical research is closely associated with individual clinical cardiology services and encompasses a variety of opportunities and interactions with faculty associated with numerous large federal and industry supported clinical trials.