According to the results of a new study by researchers at Johns Hopkins, Viagra, a drug used to treat Erectile dysfunction (ED) in millions of men, reduces the stimulatory effects of hormonal stress on the heart by half.

The drug sildenafil, (Viagra), which is more widely known for helping genital blood vessels expand to maintain an erection and, recently, as a treatment for pulmonary hypertension, has previously been thought to have little direct effect on the human heart.

According to the study’s senior author and cardiologist David Kass, M.D., a professor at The Johns Hopkins University School of Medicine and its Heart Institute, sildenafil blunts the strengthened heart beat caused by chemically induced stress, thereby lessening the excess amount of blood and force used to pump it to the body.

Kass says that in effect the drug puts a ‘brake’ on chemical stimulation of the heart.

The findings are believed to be the first confirmation in humans that sildenafil has a direct effect on the heart.

In previous research by Kass and his team it was shown that sildenafil had such effects in mice, blocking the short-term effects of hormonal stress in the heart.

Related studies by the group also showed that sildenafil prevents and reverses the long-term effects in the heart from chronic High Blood Pressure.

Kass says the latest results confirm that sildenafil helps control heart function only when the heart is under duress, but has little impact under normal conditions.

In separate research by Kass and his team earlier this year it was seen in mice that sildenafil could reverse the negative effects on heart muscle weakened by heart failure and enlargement, a condition called hypertrophy.

But at that time the team had no firm evidence as to whether or how this therapy might work in the human heart.

Their latest research provides firm evidence that the drug does have an important impact on the heart.

In the study 35 healthy men and women, with an average age of 30 and no previous signs of coronary artery disease, participated in a six-month study.

Within a three-hour timeframe, each participant received two separate injections of dobutamine , a synthetic, adrenaline-like chemical that increases heart rate and pumping strength.

Between injections, study participants were randomly assigned to a group that was treated with sildenafil or to a group given a sugar pill placebo.

All participants were then given the second dobutamine injection to see what effects sildenafil or placebo had on the heart.

Measurements of heart function were made before and after each injection. This included blood pressure readings, electrocardiograms and echocardiograms, as well as blood samples to confirm relatively equal levels of sildenafil and other enzymes.

The results showed that each dobutamine injection stimulated heart function, increasing heart rate and the force of each heartbeat used to pump blood throughout the body, similar says Kass to the increased heart function triggered by emotional or exercise stress, or in diseases such as heart failure.

It appears that after the first injection of dobutamine, the force of heart contraction increased by 150 percent in both groups, and in the placebo group, this increase repeated itself after the second injection.

However, in the group treated with sildenafil, the increased heartbeat was slowed by 50 percent, resulting in a smaller increase in blood flow and blood pressure generated by the heart in response to chemical stimulation.

Between injections, heart function was not altered in the sildenafil group, demonstrating the absence of adverse side effects on the resting human heart.

Kas says knowledge about the effects of sildenafil on heart function allows safe evaluation of its use as a treatment for heart problems, as until now, it was thought drugs such as sildenafil had no effects on the human heart and that its only purpose was vasodilation in the penis and the lungs.

Kass believes their results could encourage further studies of sildenafil’s immediate and long-term effects on the heart and its ability to modify other neurohormonal and stress stimuli, including adrenaline and hypertension.

Apparently though the precise biological actions of sildenafil on the heart are not fully understood, the drug is known to work by stopping the action of an enzyme involved in the breakdown of a key molecule which controls stress and limits overgrowth in the heart.

The research appears in the journal Circulation online Oct. 24.