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© The Rockefeller University Press, 0021-9525/2000//731 $5.00
The Journal of Cell Biology, Volume 149, Number 3, , 2000 731-740

Electromechanical Coupling between Skeletal and Cardiac Muscle

Hans Reinecke^a, Glen H. MacDonald^b, Stephen D. Hauschka^c, and Charles E. Murry^a

^a Department of Pathology, University of Washington, Seattle, Washington 98195
^b Department of Otolaryngology, University of Washington, Seattle, Washington 98195
^c Department of Biochemistry, University of Washington, Seattle, Washington 98195
University of Washington, Department of Pathology, Box 357470, Room E-520 HSB, Seattle, WA 98195-7335.(206) 543-3644(206) 616-8685

murry{at}u.washington.edu

Skeletal myoblasts form grafts of mature muscle in injured hearts, and these grafts contract when exogenously stimulated. It is not known, however, whether cardiac muscle can form electromechanical junctions with skeletal muscle and induce its synchronous contraction. Here, we report that undifferentiated rat skeletal myoblasts expressed N-cadherin and connexin43, major adhesion and gap junction proteins of the intercalated disk, yet both proteins were markedly downregulated after differentiation into myo-tubes. Similarly, differentiated skeletal muscle grafts in injured hearts had no detectable N-cadherin or connexin43; hence, electromechanical coupling did not occur after in vivo grafting. In contrast, when neonatal or adult cardiomyocytes were cocultured with skeletal muscle, 10% of the skeletal myotubes contracted in synchrony with adjacent cardiomyocytes. Isoproterenol increased myotube contraction rates by 25% in coculture without affecting myotubes in monoculture, indicating the cardiomyocytes were the pacemakers. The gap junction inhibitor heptanol aborted myotube contractions but left spontaneous contractions of individual cardiomyocytes intact, suggesting myotubes were activated via gap junctions. Confocal microscopy revealed the expression of cadherin and connexin43 at junctions between myotubes and neonatal or adult cardiomyocytes in vitro. After microinjection, myotubes transferred dye to neonatal cardiomyocytes via gap junctions. Calcium imaging revealed synchronous calcium transients in cardiomyocytes and myotubes. Thus, cardiomyocytes can form electromechanical junctions with some skeletal myotubes in coculture and induce their synchronous contraction via gap junctions. Although the mechanism remains to be determined, if similar junctions could be induced in vivo, they might be sufficient to make skeletal muscle grafts beat synchronously with host myocardium.

Key Words: skeletal myocytes • cardiomyocytes • electromechanical coupling • N-cadherin • connexin43

© 2000 The Rockefeller University Press

Abbreviations used in this paper: BDM, 2,3-butanedione monoxime; MT, myotubes; OMDR, optical memory disk recorder.

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