Why does Maximum Heart Rate Seem to Decrease with Age?

First Posted: Oct 14, 2013 06:08 PM EDT

Researchers from the University of Colorado set out to explain why the maximum heart rate seems to drop with age.

This decrease in maxHR not only limits the performance of many aging athletes who often times must give up their careers as it can also lead to early nursing home admittance for otherwise-healthy elderly individuals who no longer have the physical capacity required for independent living. However, the study authors wanted to find out exactly why and how this process works.

As aerobic activity decreases with age, this might simply be explained by the fact that older people have fewer heart beats per minute than younger individuals-allowing them less breath to complete physical activities like they once might have.

Yet lead study authors Catherine Proenza, Ph.D, and Roger Bannister, Ph.D., from the University of Colorado School of Medicine, reports that one of the primary reasons for the age-dependent reduction in maximum heart rate could be due to spontaneous electrical activity of the heart's natural pacemaker, known as the sinoatrial node.

"I utilized a method to record ECGs from conscious mice and found that maximum heart rate was slower in older mice, just as it is in older people," said Eric D. Larson, a graduate from Proneza's lab in the Department of Physiology and Biophysics who discussed his dissertation, via a press release. "This result wasn't unexpected. But what was completely new was that the slower maxHR was because the individual pacemaker cells - called sinoatrial myocytes, or 'SAMs' - from old mice just couldn't beat as fast as SAMs from young mice."

The researchers recorded tiny electrical signals from isolated calls and found that SAMS from old mice beat more slowly, even when they were fully stimulated by the fight-or-flight response that can be observed in the cells of these individuals. The slower beating rate was often due to a limited set of changes in the action potential waveform generated by the cells. Any changes seen were due to altered behavior from some ion channels in the membranes of older cells.

The study concludes with the following, courtesy of the release: "Like most initial discoveries in basic science, this study opens many more questions and avenues for further research. But the significance of the study is that it raises the possibility that sinoatrial ion channels and the signaling molecules that regulate them could be novel targets for drugs to slow the loss of aerobic capacity with age. In the meanwhile, Proenza notes that 'although maximum heart rate goes down for everybody equally, regardless of physical conditioning, people can improve and maintain their aerobic capacity at all ages by exercising.'"

This study will be published in the Oct. 14 Proceedings of the National Academy of Sciences of the United States of America.

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