Early events in development of electrical activity and contraction in embryonic rat heart assessed by optical recording.

Spontaneous action potential and contraction in the early embryonic heart of the rat have been monitored optically using a voltage‐sensitive merocyanine‐rhodanine dye together with a multiple‐element photodiode matrix array, and the onset of rhythmical action‐potential activity in the early phases of rat cardiogenesis was conclusively determined for the first time. Spontaneous rhythmical action potentials were first generated in the central part of the embryonic heart at the middle period of the 3‐somite stage of development, at 91/2 days after copulation. Subsequently, contractions coupled with the action potential also appeared at the end of the 3‐somite stage. Usually, at the 3‐somite stage, spontaneous action signals were synchronized among the different areas in the heart. From this result, it is evident that the paired right and left cardiac primordia are fused completely at the time of initiation of spontaneous electrical activity. In the 3‐somite embryonic heart, excitatory waves were conducted radially over the heart, at a uniform rate (0.4‐0.8 mm/s), from the pace‐making area. However, the regional priority of pace‐making activity is not rigid but is flexible.