Heart cells’ 24-hour clock may explain prevalence of morning heart attacks
New research has discovered a distinct circadian mechanism within heart cells that influences how cardiac activity shifts across 24-hour periods. The findings offer clues to why shift workers are at higher risk of heart problems.
Our bodies are acutely programmed into a 24-hour cycle known as our circadian rhythm. Beyond simply sleeping and waking, this circadian programming influences everything from immune system activity to gene expression.
Disruption to the body’s natural circadian rhythm is known to be linked to greater incidences of cancer and heart disease. But exactly how individual cells alter their activity over the course of a 24-hour cycle is still unclear.
We know hormonal signals play a big role in this process, plus other signaling from the central nervous system helps regulate circadian rhythms, but this new research has uncovered a previously unknown mechanism within individual heart cells that highlights how heart function changes across the day.
Before this study it was thought cellular ion concentrations were relatively stable over a 24-hour period. But it turns out, in heart cells at least, there are some pretty dramatic shifts that take place across a day. The new research found levels of sodium and potassium ions in heart cells drop during the day and increase at night.
These fluctuations were not insignificant, with ion levels dropping by as much as 30 percent during the day. And these alterations were seen to influence changes in heart rate activity indicating a newly discovered cardiovascular circadian mechanism.
“The ways in which heart function changes around the clock turn out to be more complex than previously thought,” says lead researcher on the project, John O’Neill. “The ion gradients that contribute to heart rate vary over the daily cycle. This likely helps the heart cope with increased demands during the day, when changes in activity and cardiac output are much greater than at night, when we normally sleep.”
The findings help explain what was seen in another study, published earlier this year, demonstrating how abrupt circadian disruptions can directly disturb heart rhythms. These studies help explain how lifestyles that clash with circadian cycles are at greater risk of adverse cardiac effects such as arrhythmias.
“Many life-threatening problems with the heart happen at specific times of day, and more often in shift workers,” says O’Neill. “We think that when the circadian clocks in the heart become desynchronised from those in the brain, as during shift work, our cardiovascular system may be less able to deal with the daily stresses of working life. This likely renders the heart more vulnerable to dysfunction.”
The researchers also propose this mechanism potentially plays a role in age-related heart disease. It is commonly cited that heart attacks in the elderly are most likely to occur in the morning. Based on these new findings it is hypothesized age-related dysfunction in this cellular circadian mechanism could make the heart less responsive to increases in morning activity. This opens the door to investigations into new kinds of cardiovascular therapies to help prevent unexpected heart attacks.
The new research was published in the journal Nature Communications.