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Department of Physiology, Tokyo Medical and Dental University School of Medicine, Japan.

1. Regional gradient of pacemaker activity in the early embryonic precontractile chick heart was quantitatively assessed by means of simultaneous multiple-site optical recordings of changes in membrane potential, using a measuring system with a 10 X 10-element photodiode array which had a spatial resolution of 30 microns. 2. Absorption changes related to spontaneous electrical activity were recorded simultaneously from many contiguous regions in the area in which the pacemaker site was located in seven- to nine-somite embryonic hearts stained with a voltage-sensitive merocyanine-rhodanine dye (NK 2761). 3. The absorption changes related to slow diastolic depolarization were detected, and they were concentrated in and near the pacemaking area. The area in which the absorption changes related to slow diastolic depolarization were detected increased in size as development proceeded. 4. The slope of the absorption change related to diastolic depolarization was measured as an indicator of the pacemaker activity. It was largest in the pacemaking area, and gradually decreased towards the periphery. 5. The maximum slope of the optical change related to slow diastolic depolarization also increased as development proceeded and was related to early development of the heart rate. Thus, these results suggest that formation of a regional gradient of pacemaker activity results in the functional architecture of the pacemaking area in the early phases of cardiogenesis.