The mystery of how cocaine triggers heart attacks may have been partly solved by scientists who found the drug can induce spasms in dog and rabbit arteries.

"Up until recently, there's been a missing link about how cocaine caused the heart attacks," especially in people whose arteries aren't clogged with fatty plaque, said Dr. Jeffrey Isner, a cardiovascular medicine professor at Boston's Tufts University."There's now overwhelming evidence cocaine may cause spasm of vascular smooth muscle, the principal component of the normal artery," he said, adding that how the drug starts spasms isn't known.

Studies presented Tuesday during the American College of Cardiology's annual meeting indicated how cocaine can kill by narrowing arteries and how it also impairs the heart's electrical system.

Smoked "crack" cocaine is more dangerous because it is absorbed into blood more quickly, but inhaled powder cocaine "is clearly enough to achieve blood levels that can cause these cardiac complications," Isner said.

Dr. Robert Kloner, heart research director at Hospital of the Good Samaritan in Los Angeles, said his study found that high dosages of cocaine given rapidly cause narrowing of coronary arteries and depression of the function of the heart.

"Recreational doses also have been shown to have effects," he said. "You don't need to be on a binge to have a heart attack."

A heart attack is the death of part of the heart muscle. It occurs when arteries that are narrowed or blocked keep a section of the heart from getting enough oxygen, which is carried in blood.

Researchers believe cocaine triggers a spasm, narrowing an artery and allowing a clot to form at the narrowing to cause the heart attack.