Analysis of the electrophysiological properties and arrhythmias in directly contacted skeletal and cardiac muscle cell sheets.

OBJECTIVE Autologous skeletal muscle cell (SM) transplantation into the in vivo heart sometimes induces serious arrhythmias. The purpose of this study was to investigate the electrophysiology of cardiomyocyte (CM) and SM in direct contact and to study the mechanism underlying the cause of arrhythmia using the recently developed cell sheet engineering technique. METHODS Primary cultured rat neonatal SM and CM were prepared, and cell sheets were fabricated using temperature-responsive culture dishes. The action potential was recorded by a conventional microelectrode. Intracellular calcium concentration and optical mapping image of the action potential were recorded using Fluo-3 and di-4-ANEPPS, respectively. A video motion-detecting system was used for the detection of arrhythmias. RESULTS SM myotubes occasionally displayed automaticity. SM sheets did not display synchronized contraction, but instead groups of myotubes contracted independently. The action potential of SM, induced by artificial pacing, did not expand to the entire sheet but was limited within a restricted, small area around the electrode, and it was unfeasible to generate an electrical connection or propagate an action potential between CM and SM sheets. SM sheets, in which some of the myotubes displayed automaticity, caused fibrillation-like contraction in the co-cultured CM sheets, and this arrhythmia was specifically blocked by the stretch-activated channel blocker GsMTx-4. CONCLUSIONS These findings show that SM sheets do not contract synchronously or generate functional syncytia with the surrounding CM sheets and that stretch-induced arrhythmias due to spontaneous contraction of SM may occur in the CM sheet.