TRANSPLANT patients and people being treated for cancer are at risk from dangerous spikes in blood pressure due to their drug treatments, researchers warned.

A study by scientists at Edinburgh University has revealed new links between the immune system and high blood pressure, which could also offer clues about how to reverse the condition in patients who do not respond to existing treatments.

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Untreated, high blood pressure - which affects one in three Scots - can cause strokes, heart attacks and kidney disease.

Researchers made the breakthrough after feeding mice a high salt diet and then artificially lowering the rodents' concentration of macrophages - types of white blood cell which fight disease.

They found that this led to an increase in blood pressure.

However, when the macrophage level returned to normal, the rodents' blood pressure also normalised.

It is understood that the fluctuations occur because macrophages scavenge for and ‘eat’ molecules of a powerful hormone known as endothelin, which causes blood vessels to tighten when it becomes too elevated.

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A reduction in macrophages therefore results in a rise in endothelin, and a corresponding increase in blood pressure.

Professor Matthew Bailey, an expert in renal physiology who led the study, said: "I don't think any of us were expecting such a big effect.

"This has huge relevance for blood pressure control, particularly in people receiving immuno-suppressants."

The researchers went on analyse the white blood cells in patients taking medication for small cell vasculitis, an immune system disorder that attacks blood vessels.

Prof Bailey said the findings also have important implications for cancer patients whose immune systems are weakened by chemotherapy, and transplant patients who have to take immuno-suppressants to prevent organ rejection.

He said: "One of my clinical colleagues runs a [vasculitis] clinic in Edinburgh and he was noting that people who were going on these therapies, their blood pressure was going up.

"We linked the two things together and it's opened up a completely different way of thinking about how the immune system, blood vessels and blood pressure inter-relate. That's completely new physiology knowledge.

"There are a lot of people on immuno-suppressant drugs for one thing or another and the study suggests that we need to monitor them clinically because their blood pressure may well rise, and we need to take action to make sure that risk factor is dealt with properly."

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Prof Bailey added that while some existing treatments can encourage people to generate more macrophages, the effect was generally not sustainable and would be "incredibly expensive".

He said a better option for patients low on macrophages could be to give them drugs that block the manufacture of the hormone endothelin.

It also gives hope to individuals whose high blood pressure has so far been resistant to drugs.

Prof Bailey said: "We have very effective treatments for hypertension but in some individuals they don't work very well, so you might find ways to give medicine more precisely."

The study, published today in the European Heart Journal, was funded by the British Heart Foundation.

Jeremy Pearson, Associate Medical Director at BHF, said: “This study shows for the first time that macrophages can be involved in the control of blood pressure.

"More research is needed but these cells could be a new target for drugs to treat the condition.”