Scientists say it for the first time! These people are most affected by heart problems

A new study shows how the circadian rhythms of heart cells help change heart function during the day, and may explain why night shift workers are more affected by heart problems.

Scientists have shown for the first time that heart cells self-regulate their circadian rhythms through daily changes in the levels of sodium and potassium ions inside the cell. The different levels of sodium and potassium ions inside and outside the heart cells create the electrical impulse that causes them to contract and make the heart beat.

Until now, ion concentrations were thought to be constant. But now scientists have discovered that heart cells change their internal sodium and potassium levels during the day and night. This allows the heart to adapt better and maintain the increased heart rate when we are physically active.

It is already known that there are daily clocks in heart cells and other tissues; which are normally synchronized by hormonal signals that harmonize our internal daily rhythms with the day and night cycle. Daily rhythms of heart function have been known for years and are thought to exist due to greater stimulation of the nervous system during the day.

This new study, supported by the Medical Research Council and AstraZeneca's Blue Sky Initiative, shows that circadian rhythms within each heart cell can also affect heart rate.

The team, led by scientists at the Laboratory for Molecular Biology in Cambridge, UK, in collaboration with AstraZeneca, say that understanding how these changes in ion levels alter heart function during the day could help explain the reason why employees who work night shifts are more vulnerable to heart problems.

This is because the ionic rhythms driven by the heart's clocks go "out of sync" during the stimulation they receive from the brain's clocks. This new understanding could lead to better treatments and preventative measures to fight heart disease.

The study's lead author, Aleksandra Stangerlin, was surprised to find that sodium and potassium levels varied by up to 30 percent in isolated cells and heart tissue.

This changes the electrical activity in these cells. In mice, this change appears to be just as important for understanding daily changes in heart rate as neural control. Dr. John O'Neill, of the Laboratory of Molecular Biology, who led the study, says:

"The ways in which heart function changes over time turns out to be more complex than previously thought. The amount of ions that contribute to the heart rate varies during the daily cycle.

This is likely to help the heart cope with the increased demands during the day, when changes in cardiac activity and output are much greater than at night, when we are usually asleep. This creates the possibility of more effective treatments for cardiovascular diseases, for example to take the medicines at the right time during the day".