Sudden cardiac death -catastrophic and unexpected fatal heart stoppage -- is more likely to occur shortly after waking in the morning and in the late night. In a report in the journal Nature, an international consortium of researchers that includes Case Western Reserve University School of Medicine in Cleveland and Baylor College of Medicine explains the molecular linkage between the circadian clock and the deadly heart rhythms that lead to sudden death.

The answer begins with a controller of the circadian clock -- krüppel-like factor 15 (Klf15), which has been a long-time target of the laboratory of Dr. Mukesh Jain of Case Western, said Dr. Xander Wehrens, professor of molecular physiology and biophysics and cardiology at BCM, also an author.

Klf15, in turn, controls the level of a potassium（钾） channel-interacting protein (KChIP2), which affects how potassium flows out of heart muscle cells called cardiac myocytes（肌细胞） .

Changes affect potassium current

Because the level of this KChIP2 protein fluctuates（波动） during the circadian or daily cycle, it can change the size of the potassium current in cardiac myocytes. Changes in this subunit or Klf15 can affect the potassium current that governs repolarization of the cardiac myocyte. Overall, this can shorten or lengthen the time the heart muscle has to empty the heart's pumping chamber (ventricle) of blood. This time interval for repolarization（极化恢复） is critical. Too much or too little can result in abnormal heart rhythms called arrhythmias. As the heart loses the regularity of the beat, it cannot pump blood efficiently.

Studies of mice that lacked Klf15 and mice with a genetic change that caused them to make more Klf15 than normal increased the risk of deadly arrhythmias（心率失常） .

This was a proof of principle, said Wehrens.

"It is the first example of a molecular mechanism for the circadian change in susceptibility to cardiac arrhythmias," he said.

"If there was too much Klf15 or none, the mice were at risk for developing the arrhythmias," he said.