Copyright © 2008 by the American Physiological Society. Cardiac titin isoform switching in hibernating bears 2 The hibernation phenomenon captures biological as well as clinical interests to understand how organs adapt. Here we studied how hibernating grizzly bears (Ursus arctos horribilis) tolerate extremely low heart rates without developing cardiac chamber dilation. We evaluated cardiac filling function in unanesthetized grizzly bears by echocardiography during the active and hibernating period. Because both collagen and titin are involved in altering diastolic function, we investigated both in the myocardium of active and hibernating grizzly bears. Heart rates were reduced from 84 bpm in active bears to 19 bpm in hibernating bears. Diastolic volume, stroke volume and left ventricular ejection fraction were not different. However, left ventricular muscle mass was significantly lower in the hibernating bears 300 ± 12 compared to 402 ± 14 grams (p = 0.003), and as a result the diastolic volume to left ventricular muscle mass ratio was significantly greater. Early ventricular filling deceleration times were shorter during hibernation, 106.4 ± 14 compared to 143.2 ± 20 ms (p = 0.002) suggestin
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