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

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Abstract
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 different regions. Apparently switching phenomena, the site exhibiting pacemaking priority was first situated in the right pre-atrium, thereafter it switched over to the left pre-atrium, or vice versa, and double pacemakers, 2 different pacemaking areas were situated independently in the right and left pre-atrial portions of the heart. The regional priority of the pacemaking activity evidently is not rigid but is flexible, and the direction of the spread of excitation is adaptable to the circumstances in the early embryonic heart.