Scientists have developed a cocktail of drugs that can repair potentially fatal tissue damage suffered during heart attacks, according to research published today.

Tests in rats show that an injection of two drugs can encourage heart tissues to regenerate themselves after an attack, by regrowing healthy muscle cells and new blood vessels. American researchers based at Children's hospital Boston are conducting further tests to see if the treatment is effective when the drugs are administered many hours after a heart attack, but believe they will ultimately be tested in humans.

About 180,000 people in the UK are admitted to hospital each year with heart attacks caused by clots in coronary arteries. Most occur in people over 50 and they are three times more common in men than women. Heart attacks cause tissue to die by starving it of oxygen. Even if a patient is treated quickly to remove a blood clot in an artery, the damage to heart muscle tissues may be so severe that the heart is scarred and becomes too weak to pump blood around the body.

Felix Engel and Mark Keating used injections of two chemicals to try to prevent damage during simulated heart attacks in rats. The first chemical overcomes a natural process that stops heart muscle cells from growing and dividing. The second encouraged blood vessels to regrow.

In tests on 120 rats, the scientists found that the injections prevented scar formation after heart attacks, and instead encouraged blood vessels and healthy new heart cells to grow. Scarring was reduced by up to 50%, they report in the Proceedings of the National Academy of Sciences.

Cardiograms of the rats three months after treatment showed that the hearts of those given both drugs were pumping almost as well as those of healthy rats. Rats injected with only one of the chemicals showed only a temporary improvement.

"If you have a blood clot and you don't get to hospital quick enough to have it dissolved, the heart muscle tissue will start to die within one to two hours, and if enough dies, the heart will be too weak and it will be fatal. But if you survive you can still suffer arrhythmias which can be fatal later," said Dr Engel. "If we can use this to prevent damage to the heart in humans, people will be more likely to survive." The treatment follows previous research which has identified two animals which are capable of regrowing damaged heart tissue. Zebra fish and newts are both able to recover from severe heart damage, regrowing fresh heart muscle tissue within a few months.

In the study, the scientists injected rats immediately after inducing heart attacks by constricting a coronary artery. "The next question is how long after the heart attack is this still useful," said Dr Engel.