Change in heart rate keeps bears healthy while hibernating: Study

WASHINGTON - It seems hibernating is much more complicated than one might think.

A new study has provided crucial insight into the mysteries of the hibernating heart of a bear.

It found that a complex series of changes occur in bears’ hearts as they hibernate. These changes guard against complications that could arise from greatly reduced activity.

A grizzly bear hibernates five to six months a year. During that time, its heart rate slows drastically from around 84 beats per minute when active to around 19.

“If a human heart were to slow down like this, you’d see very detrimental things happening,” said Bryan Rourke, a professor at Cal State Long Beach who worked on the research with his graduate student, Nathan Barrows.

Such a slow beat causes blood to pool in the heart’s four chambers. In a human, the increased pressure would cause the chambers to stretch out. The dilated muscle would be weaker and less efficient, leading ultimately to congestive heart failure.

“Bears are able to avoid this and we’re interested in how they do it,” said Rourke.

Barrows and Rourke teamed up with Lynne Nelson and Charles Robbins, researchers at the Washington State University who have been studying bears for years.

Nelson and Robbins operate a facility at Washington State where grizzlies have been raised since birth and acclimated to echocardiogram testing.

They had previously shown that, during hibernation, the muscle of a bear’s left ventricle stiffens to prevent it from stretching as blood accumulates.

But the stiffening of the ventricle presents another problem. The left atrium, which pushes blood into the left ventricle, must then work against greater resistance.

“The atrium is pushing against a brick wall. We thought there must be some kind of mechanism to keep the atrial muscle from wearing itself out,” said Rourke.

Using echocardiogram data from the captive bears at Washington State and tissue samples from wild bears, the researchers found that the atrium protects itself by weakening its beat.

A protein called myosin heavy chain controls muscle contractions in the heart. The protein comes in two varieties, alpha and beta. The alpha version produces a quicker but slightly weaker contraction compared to the beta.

“We found that the muscle in the left atrium produces more alpha protein during hibernation, which results in a slightly weaker beat,” said Rourke, adding “the lower force keeps the atrium from being damaged as it pushes against the stiffer ventricle.”

When the bears emerge from hibernation, the protein ratio switches back and the atrial contraction returns to its original strength.

The researchers believe this is the first study to show a shift in myosin ratio in bears, and they hope the information might someday have an application for humans.

“Bears aren’t a perfect model for humans, but the way in which a bear’s heart can change could be helpful in understanding human disease,” said Rourke.

The study is published in the latest issue of the journal Physiological and Biochemical Zoology. (ANI)