Flexibility of regional pacemaking priority in early embryonic heart monitored by simultaneous optical recording of action potentials from multiple sites.

The propagation of spontaneous action potentials in 7-9 somite embryonic pre-contractile chick hearts was measured optically using a potential-sensitive merocyanine-rhodanine dye. Spontaneous optical signals, corresponding to action potentials, were recorded simultaneously from 8-16 different sites of the primitive embryonic heart. Short delays were observed in the time of occurrence of optical signals obtained from the different regions. We have found (i) switching phenomena: the site exhibiting pacemaking priority was first situated in the right pre-atrium, double pacemakers: two different pacemaking areas were situated independently in the right and left pre-atrial portions of the heart. On the basis of analysis of such behavior, it was concluded that the regional priority of the pacemaking activity is not rigid but is flexible, that and the direction of the spread of excitation is adaptable to the circumstances in the early embryonic heart.