Common muscle cells transplanted from rabbits' legs to their hearts boosted cardiac performance, possibly by having "learned" within days how to act like neighboring heart cells, a new study shows.
In addition, the transplanted leg cells were functioning well for as long as 16 weeks following their insertion into the heart muscle, far longer than expected, Duke University researchers reported.
Molecular biologists who conducted the study believe cells extracted from leg tissue even may have developed the special bio-electrical properties that enable the heart to contract and pump blood.
Precisely how the transplanted cells work isn't known, and other researchers warned that concluding the cells had changed was premature.
"We need to be very careful in concluding they turned into another type of cell," said Dr. David Diechek of the University of California, San Francisco.
Even if the transplanted cells didn't fundamentally transform themselves, scientists said the Duke procedure, reported in the August issue of the journal Nature Medicine, is a preliminary demonstration that one type of muscle cell can help another.
"Something is clearly happening when you put these cells in," said Loren Field, a molecular biologist at Indiana University.
Unlike most muscles, heart tissue does not regenerate after it is damaged because it does not contain a reserve of embryonic muscle cells, or myoblasts, that could quickly mature and restore the heart's normal function.
Instead, the dead cells form scar tissue that doesn't flex when the remaining healthy heart muscle pumps. Blood flow and pressure is diminished, with potentially fatal results.
In the Duke experiment, biologist Doris Taylor and others purposefully damaged the hearts of 17 rabbits to mimic the scar tissue from heart attacks. Five of the rabbits were control animals that received no further treatment.
The researchers took muscle biopsies from the legs of the 12 remaining rabbits and grew myoblasts in cell cultures. The rapidly dividing cells were transfused into the rabbits' damaged hearts.
Within five to 10 days, the cardiac performance of seven rabbits was noticeably improved.
Microscopic analysis of the rabbits' hearts later showed the transplanted cells had developed intercalated discs, "a distinctly cardiac feature," Taylor said, that is involved in transferring from cell to cell the electrical charge that powers the heartbeat.
"What's surprising is the cells can change in response to a new environment," Taylor said.
Other researchers, however, said additional experiments are necessary with higher ratios of successful transplants -- and possibly using other species in addition to rabbits.
"There is ambiguity as to what the mechanism is," Field said. "Is the improvement a result from actual contraction of the transplant cells, or are the transplant cells just helping to make a more flexible scar? Is there really a conversion of cells or not?"
In addition, cardiologists cautioned that several hurdles stand in the way of human clinical trials and noted that cardiovascular performance after a heart attack can already be improved using an array of drugs.
"It would have to be better that the current drugs to be clinically useful," Diechek said.