Adult zebrafish hearts efficiently compensate for excessive forced overload cardiac stress with hyperplastic cardiomegaly.

Although human cardiomyocytes (CMs) are capable of some cell division, this response is neither sufficient to repair damaged cardiac tissue nor efficient to compensate for pathological stress. Danio rerio (zebrafish) CMs have been shown to have high proliferative capability to completely repair hearts after injury; however, no reports have focused on their physiological and cellular response to cardiac overload stress. We hypothesized that forced excessive long-term cardiac overload stress would elicit a proliferative response similar to regenerative cardiac repair in zebrafish. We completed a 10-week forced fast-speed swimming exercise regimen, comparing exercised hearts to nonexercised controls for physiological function and histological evidence of cell proliferation. Our results indicate that exercised heart ventricles are 33% larger, yet exhibit no significant changes in cardiac physiological function as evaluated by the heart rate and the percent shortening fraction. We found 8% more CM nuclei per cross-sectional area within exercised ventricular tissue, indicating that cardiomegaly was not due to individual cell hypertrophy, but due to hyperplasia. This novel zebrafish cardiac stress model may be used to identify genes or proteins with therapeutic potential for treating cardiac stress pathologies, as well as molecules that could be used as initiators of cardiac cell proliferation in humans.