Though exercise can be a key part of managing high blood pressure and heart disease, new animal research suggests there can be too much of a good thing.

In experiments with rats, researchers found that excessive exercise worsened high blood pressure and progression to heart failure in rats with high blood pressure.

Dr. Rebecca L. Schultz and colleagues at the University of South Dakota, Sioux Falls, report the results in the journal Hypertension. Regular physical activity has been linked to a lower risk of heart disease in numerous studies. Moreover, exercise therapy has been shown to improve both blood pressure and symptoms of heart failure -- a chronic condition in which the heart loses its ability to pump blood efficiently, causing symptoms like breathlessness and fatigue.

The new findings in rats are, therefore, unexpected, according to an editorial published with the study.

The implications for humans are not yet certain, according to the editorialists Dr. Paul Christian Schulze, of Boston University Medical Center, and Satyam Sarma, of Brown University Medical Center in Providence, Rhode Island. However, the findings "should raise our awareness" of the potential harm intense exercise might do to people with untreated high blood pressure.

Humans, as well as rats, develop high blood pressure that can progress to heart failure. In the current study, some of the animals were housed with a running wheel, while the others remained sedentary.

Schultz and her colleagues found that the rats that lived with a running wheel tended to exercise excessively. The results, over time, were structural abnormalities in the heart and a reduced pumping ability -- all of which were worse in the active animals than in the sedentary ones.

The reasons for the findings are unclear, according to the study authors, but it's likely that the rats "simply exercised too much."

The study raises the possibility that "uncontrolled and excessive exercise" may negatively affect the heart in people with high blood pressure, potentially speeding the onset of heart failure, according to Sarma and Schulze.

One of the questions for future studies, they note, is whether this is true among people whose blood pressure is under control with medication.

"Defining the fine line between beneficial and detrimental effects of exercise requires further study," the editorialists conclude.