Structural Changes in Heart DiseaseDuring hibernation, a grizzly bear's heart beats at about 18 beats per minute--one fifth the rate it pumps during the rest of the year. A typical human heart beats between 60 and 80 times per minute, and would never get that low, but if it did slow significantly, the atria (upper heart chambers) would distend with the backflow of blood that was not properly circulating. Such structural changes, collectively termed "cardiac remodeling," happen frequently in the 5 million Americans who have heart failure, often with adverse consequences. Hence, researchers are eager to learn how bears evade remodeling in the hope that it may pave the way for new therapies for humans.

Lynne Nelson, a veterinary cardiologist at Washington State University, took four grizzly bear cubs born in her lab, bottle-fed them, routinely handled them, and acclimated them to a variety of noninvasive tests such as echocardiograms. Now that the bears are grown, she can observe the contractions of their hearts without using anesthesia, which could disrupt normal function. Her research (presented in August at the International Hibernation Symposium in Namibia) suggests that grizzlies' atria cease contracting during hibernation. That may also keep the atria from dilating, she says, because it stops them from having to work against a congested heart.

Nelson also found that during hibernation a protein (N2BA titin) in heart muscle is--in many instances--replaced by N2B, a more rigid form of the same protein. The protein switch may be another strategy bears use to preserve the heart's shape during hibernation.

This hand-reared grizzly is used to having heart ultrasounds (echocardiograms).