Development of sensitivity to tetrodotoxin in beating chick embryo hearts, single cells, and aggregates.

The spontaneous activity of intact embryonic heart becomes progressively more sensitive to tetrodotoxin block with increasing age of the embryo. The activity of isolated single heart cells in culture was relatively insensitive, independent of embryo age. Aggregates formed from single cells responded to tetrodotoxin in the same manner as intact hearts; aggregated cells from older hearts were sensitive.