Enzyme causes clotting fatalities.

• Position: Viagra

Incidents of heart attack and stroke, some fatal, in a small number of men taking Viagra have remained a puzzle. After all, the drug, commonly prescribed for erectile dysfunction, originally was developed to prevent these conditions--not only by dilating blood vessels, but also by stopping platelets in the blood from clumping. However, it does just the opposite, claim researchers at the University of Illinois at Chicago College of Medicine. They found that Viagra, by elevating levels of a compound in cells called cyclic guanosine monophosphate, or cGMP, actually encourages platelets to aggregate, thus amending 20 years of scientific claims that the compound acts to prevent platelet aggregation.

While that process helps minimize the loss of blood when injury occurs, it also could lead to clotting that blocks a blood vessel--a life threatening condition called thrombosis that can cause heart attack and stroke. "Viagra, by itself, probably is not sufficient to cause a heart attack in healthy people, but our research suggests that it may present a risk for patients with preexisting conditions such as atherosclerosis," points out Xiaoping Du, associate professor of pharmacology and the study's lead author.

Du says he had not set out to investigate the cause of fatalities in Viagra patients when he began this research five years ago. Rather, he was hoping to illuminate the highly complex series of molecular steps that control platelet coagulation. Platelets are disk-shaped cells that freely circulate in the blood. When a blood vessel is injured, the platelets form sticky surfaces, adhering to one another and to the damaged area to plug the hole.

Using recombinant DNA techniques, Du and his research team forced standard laboratory cells to manufacture two proteins key to platelet aggregation: one that helps the platelets clump together and stick to the surface of broken blood vessels, and another that activates the first. The genetic...