Studying heart function during hibernation leads to an understanding of how the heart can adapt to stressful conditions: During their annual 4-6 month hibernation, bears minimize activity and do not eat, drink, or urinate. As part of that process, they develop extremely slow heart rates (profound bradycardia), yet the heart remains healthy. For example, active bears have heart rates of 80-90 beats per minute, but hibernating bears have average heart rates of 15-18 beats per minute, with some as low as 5 beats per minute. In people, heart rates this low would cause congestion and heart failure within a matter of weeks, but the bears show no signs of congestion even after 5 months. Studies have shown that the heart muscle becomes stiffer during hibernation and that the bears can ‘turn off’ two of the four heart chambers. These studies are continuing in an effort to understand how these natural adaptations might benefit people or pets with heart disease.
Understanding how muscles can remain strong during the inactivity of hibernation: Hibernating bears maintain muscle strength and mass even though humans would lose 70% of their muscle strength during a similar period of inactivity. When human muscles are not exercised, slow twitch fibers that are important for posture (i.e., standing) and prolonged activity (long distance running) convert to fast twitch fibers that have minimal endurance capability. Thus, bed-ridden patients need extensive rehabilitation in order to resume normal lives. However, bears are very capable of running and resuming their normal life immediately after emerging from 5 months of hibernation. Our major findings thus far are that muscle types, sizes, and contractile properties of bears do not change during hibernation. It appears that bears prevent these changes by having a series of whole body, isometric contractions that start at the neck and extend as a wave over the entire body several times per day. Thus, it seems that they are exercising while they are sleeping.
How bears maintain bone integrity while hibernating may provide insight to treatments for osteoporosis: When people are bed-ridden, bone loss and reduced bone strength occurs. To fully recover, most patients need therapy lasting 2-3 times longer than the length of time that they were inactive. However, bears from northern climates experience annual periods of disuse (hibernation) and activity that are approximately equal in length, yet they do not lose bone mass or strength even after many years. We have found that bears maintain a balance between bone resorption and bone deposition such that bone strength is fully maintained throughout hibernation. These processes are controlled by hormones regulating calcium balance. By learning more about these physiological processes, we hope to develop therapies for people or pets with osteoporosis.