Study: Heart regenerates itself over a lifetime

In a finding that may open new approaches to treating heart disease, Swedish scientists have succeeded in measuring a highly controversial property of the human heart: the rate at which its muscle cells are renewed during a lifetime.

The finding upends what has long been conventional wisdom: The heart cannot produce new muscle cells and so people die with the same heart they were born with.

About 1 percent of the heart-muscle cells are replaced every year at age 25, and that rate gradually falls to less than half a percent a year by 75, a team of researchers led by Dr. Jonas Frisen of the Karolinska Institute in Stockholm concluded. The upshot is that about half of the heart's muscle cells are exchanged in the course of a normal lifetime, the Swedish group calculated. Its results are published in the new issue of the journal Science.

"I think this will be one of the most important papers in cardiovascular medicine in years," said Dr. Charles Murry, a heart researcher at the University of Washington. "It helps settle a long-standing controversy about whether the human heart has any ability to regenerate itself."

If the heart can generate new muscle cells, researchers can hope to develop drugs that might accelerate the process, because the heart fails to replace cells killed in a heart attack.

The dogma that the heart cannot generate new muscle cells has been challenged since 1987 by a somewhat lonely skeptic, Dr. Piero Anversa, now of the Harvard Medical School. Anversa maintains that heart-muscle cells are renewed so fast a person who dies at 80 has replaced the heart four times over. Many other researchers doubt this assertion.

Cell turnover rates can easily be measured in animals by making their cells radioactive and seeing how fast they are replaced. Such an experiment, pulse-labeling, could not ethically be done in people. But Frisen realized several years ago that nuclear weapons tested in the atmosphere until 1963 had labeled the cells of the world's population.

The nuclear blasts generated a radioactive form of carbon, carbon-14. The amount of carbon-14 in the atmosphere has gradually diminished since 1963, when aboveground tests were banned, as it has been incorporated into plants and animals or diffused into the oceans.

In the body, carbon-14 in the diet gets into the DNA of new cells and stays unchanged for the life of the cell. Because the level of carbon-14 in the atmosphere falls each year, the amount of carbon-14 in the DNA can serve to indicate the cell's birth date, Frisen found.

Four years ago he used his new method to assess the turnover rate of various tissues in the body, concluding the average age of the cells in an adult's body might be as young as 7 to 10 years. But there is a wide range of ages, from the rapidly replaced cells of the blood and gut to the mostly permanent cells of the brain.

Frisen has successfully applied his method to the heart-muscle cells but had to navigate a series of technical obstacles created by the special behavior of the cells. Many have two nuclei, instead of the usual one, and within these double nuclei the DNA may be duplicated again. "I was really impressed at the level of rigor they put into this analysis," Murry said, calling it a "scientific tour de force."