The effects of Ca2+ on electrical propagation in early embryonic precontractile chick hearts were studied optically using a voltage-sensitive merocyanine-rhodanine dye. Spontaneous optical signals, corresponding to action potentials, were recorded simultaneously from 25 separate regions of the eight-to-nine-somite embryonic primitive heart, using a square photodiode array. Electrical propagation was assessed by analyzing the timing of the signals obtained from different regions. Electrical propagation in the heart was suppressed by either lowering or raising extracellular Ca2+. Similar effects were produced by a Ca2+ ionophore (A23187). We have also found that electrical propagation across the primordial fusion line at the midline of the heart was enhanced by increasing, and depressed by lowering, external Ca2+. One possible interpretation is that intercellular communication in the embryonic precontractile heart is regulated by the level of the intracellular Ca2+ concentration, and it is suggested that intercellular communication across the primordial fusion line strongly depends on external Ca2+.

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