(AP) -- Low doses of the main active ingredient in marijuana slowed the progression of hardening of the arteries in mice, according to a new study.
Experts stressed that the finding does not mean people should smoke marijuana in hopes of getting the same benefit.
"To extrapolate this to, 'A joint a day will keep the doctor away,' I think is premature," said Dr. Peter Libby, chief of cardiovascular medicine at Boston's Brigham and Women's Hospital.
The mouse work is presented in Thursday's issue of the journal Nature by Dr. Francois Mach of Geneva University Hospital in Geneva, Switzerland, and colleagues. He said in an e-mail that he believed future work will focus on finding drugs that mimic the benefit without producing marijuana's effects on the brain.
Hardening of the arteries sets the stage for heart attacks. Inflammation plays a key role in the condition, characterized by a progressive buildup on the inside walls of blood vessels. So Mach and colleagues explored the anti-inflammatory effects of marijuana's main active ingredient, delta-9-tetrahydrocannabinol, or THC.
They fed mice a high-cholesterol diet for 11 weeks. About halfway through that period, they started giving some of the mice very low, daily oral doses of THC -- too low to produce any marijuana-like changes in behavior. At the end of the experiment, mice that had gotten the THC showed less blood vessel clogging than did mice that got no THC.
Related work showed no additional benefit from higher THC doses, such as a person would get from smoking marijuana, Mach noted.
Researchers found that the benefit came from THC's effect on immune-system cells. It reduced their secretion of an inflammation-promoting substance and their migration to the vessel wall, researchers found.
It apparently did that by binding to proteins called CB2 receptors, which are found mostly on immune-system cells. THC also targets CB1 receptors, found mostly in the brain. So the work suggests scientists should try to develop a drug that works on CB2 receptors while ignoring the brain receptors, Mach said.
Libby, who did not participate in the study, said the work was valuable for identifying the CB2 receptor as a potential target for treatment in hardening of the arteries, and showing that a natural substance could help.
But he noted that controlling one's weight, exercising and eating right have already been proven to reduce a person's risk of heart attacks and strokes from clogged arteries.
Dr. Edward A. Fisher of the New York University School of Medicine said THC's impact on artery-clogging in the experiment was relatively modest, and that it's not clear that results would apply to people.