TETRODOTOXIN SENSITIVITY OF CULTURED EMBRYONIC HEART CELLS DEPENDS ON CELL INTERACTIONS

Tetrodotoxin (TTX) is a potent and specific inhibitor of the sodium current in nerve and muscle (3, 4, 10) This current is responsible for the rapid rising phase of the action potential in many excitable cells (5) During development of the chick heart the action potentials (12) and pulsatile activity (6) undergo a transition from a TTXinsensitive state at day 4 of incubation to a TTXsensitive state by day 7: Isolated embryonic myocardial cells and small monolayer sheets are relatively insensitive to TTX at all ages, whereas aggregates of such cells reflect the differentiation taking place in the parent hearts (6). ~¥e here demonstrate that aggregates formed from isolated cells are TTX-sensitive, whereas single cells derived by trypsin-dissociaUon of such aggregates are TTX-insensitive Further, exposure to dilute trypsin renders intact aggregates TTXinsensitive. The process of resensitization is dependent on protein synthesis.