Page last updated at 16:37 GMT, Wednesday, 12 November 2008

Small hours heart risk peak clue

Enlarged left ventricle
Specialist stem cells keep the blood vessels healthy

US scientists may have uncovered a reason why heart attacks and strokes occur most often in the early hours.

The Emory University team say daily rhythms in the activity of cells found in blood vessels may be key.

They found the activity of these cells was at its lowest ebb in the early hours of the morning.

The study, presented to an American Heart Association conference, suggests this might make blood vessels less able to relax, raising the risk of problems.

We all have circadian rhythms, an internal body clock, and that this causes alterations in the biochemistry of our body
Ellen Mason
British Heart Foundation

Some scientists believe cells dubbed endothelial progenitor cells (EPCs) from the bone marrow play an important role in maintaining the lining of the blood vessels.

They believe these cells are essentially stem cells that help replace the endothelial cells that line the blood vessels at the site of an injury, and build new vessels at sites deprived of adequate blood supply.

However, the role of the cells is hotly disputed in scientific circles - it may be that instead of having a direct role in keeping the blood vessels healthy, they work to aid other nearby cells to perform the function.

The Emory team looked to see whether cell activity, and the properties of the blood vessels changed at different times of the day.

They tested 12 healthy middle aged volunteers every four hours throughout a 24-hour period.

Midnight peak

Both the ability of the blood vessels to relax and EPCs' ability to grow peaked at midnight, while cell numbers peaked at 2000. After that all three measures began to fall away.

Lead researcher Dr Ibhar Al Mheid said: "The lining of our vessels appears to function better at night.

"Endothelial function is particularly depressed in the early morning hours."

Dr Al Mheid said the results suggested that control was linked to daily, or circadian, rhythms in the brain.

Ellen Mason, senior cardiac nurse at the British Heart Foundation, said: "It has been known for a long time that the number of people having heart attacks seems to peak in the mornings, and researchers continue to try to better understand the mechanisms in the body that could cause this to occur.

"What is known is that we all have circadian rhythms, an internal body clock, and that this causes alterations in the biochemistry of our body.

"As this study is small, it is important that continuing research looks into how the lining of blood vessels may be affected as we come into the day from the night."

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