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Articles in PresS, published online ahead of print November 27, 2002
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00870.2002
Submitted on October 15, 2002
Accepted on November 22, 2002
1 Department of Cell Biology and Anatomy, Medical University of South Carolina, Charleston, SC, USA
2 Klinik und Poliklinik fur Nuklearmedizin, University of Munster, Munster, Germany
3 Institute of Physiology, University of Lausanne, Lausanne, VD, Switzerland
* To whom correspondence should be addressed. E-mail: sedmerad{at}musc.edu.
Zebrafish and Xenopus have become popular model organisms for studying vertebrate development of many organ systems, including the heart. However, it is not clear whether the single-ventricular hearts of these species possess any equivalent of the specialized ventricular conduction system found in higher vertebrates. Isolated hearts of adult zebrafish (Danio rerio) and Xenopus laevis were stained with voltage-sensitive dye and optically mapped in spontaneous and paced rhythms followed by histological examination focused on myocardial continuity between the atrium and the ventricle. Spread of the excitation wave through the atria was uniform with average activation times of 20±2 and 50±2 ms for zebrafish and Xenopus, respectively. After a delay of 47±8 and 414±16 ms, the ventricle became activated, first in the apical region. Ectopic ventricular activation was propagated significantly more slowly (total ventricular activation times: 24±3 vs. 14±2 in zebrafish and 74±14 vs. 35±9 ms in Xenopus). Although we did not observe any histologically defined tracts of specialized conduction cells within the ventricle, there were trabecular bands with prominent PSA-NCAM staining forming direct myocardial continuity between the atrio-ventricular canal and the apex of the ventricle - i.e., the site of the epicardial breakthrough. We thus conclude that these hearts are able to achieve the apex-to-base ventricular activation pattern observed in higher vertebrates in the apparent absence of differentiated conduction fascicles, suggesting that the ventricular trabeculae serve as a functional equivalent of the His-Purkinje system.
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