Viagra helps ailing hearts to recover in a surprising way - by making them less stiff, scientists have learned.

The impotency drug causes too-rigid heart chamber walls to become more elastic.

The research explains how Viagra might benefit patients with diastolic heart failure.

The effect was seen in dogs with diastolic heart failure within minutes of the drug being administered.

Study leader Professor Wolfgang Linke, from the Ruhr Universitat Bochum (RUB) in Germany, said: ''We have developed a therapy in an animal model that, for the first time, also raises hopes for the successful treatment of patients.''

Viagra has a similar effect on blood vessels, which is why it was originally developed as a treatment for high blood pressure and heart disease.

The drug's active ingredient, sildenafil, inhibits an enzyme involved in the mechanism that regulates blood flow.

However, the enzyme is slightly different in different parts of the body.

The British scientists behind Viagra found to their initial disappointment that it was not a great help to patients with high blood pressure. But it had a miraculous effect on men with erectile dysfunctin.

The drug successfully suppressed the enzyme phosphodiesterase (PDE) in the penis, increasing blood flow to the organ.

Prof Linke's team found that it worked on the same enzyme in heart cells. This had the effect of causing a cardiac muscle protein called titin to become more elastic.

''The titin molecules are similar to rubber bands,'' said the professor. ''They contribute decisively to the stiffness of cardiac walls.''

The research is published today in the journal Circulation.

Almost half of emergency patients admitted to hospital with heart failure have a diastolic condition.

Diastolic heart failure affects the ''diastole'' half of the cardiac cycle, when the heart's chambers have finished contracting and are re-filling with blood.

Various medical conditions can cause the ventricles to become ''stiff''. They include high blood pressure, blocked arteries, and cardiomyopathy heart disorders.

Sildenafil is already being tested on heart failure patients taking part in the Relax trial in the US.

Prof Linke added: "Of all the patients aged over 60 who are in hospital because of a weak heart, half suffer from diastolic heart failure.

"Although we know that the decreased distensibility of the cardiac walls is the cause, the disease cannot be treated properly with today's medicines.

"If, for the first time, the drug is found to have a positive effect on heart failure, we would already have a molecular mechanism on hand to explain the effect."