Optical studies of excitation-contraction coupling in the early embryonic chick heart

Excitation-contraction coupling at the onset of beating in the 9-10-somite embryonic chick heart was studied by means of an optical method together with a voltage-sensitive merocyanine-rhodanine dye. Spontaneous optical signals were recorded simultaneously from many areas of the embryonic heart, using a square photodiode matrix array. At time of initiation of the heartbeat, spontaneous optical signals consisting of two components were often detected. The first component (1st signal) is a dye-related absorption change due to the action potential, and the second component (2nd signal) is a light scattering change due to contraction. When Ca2+ in the bathing solution was partly replaced by Mg2+, the peak size of both signals was reduced. The correlation between the 1st and 2nd signals corresponded to the relationship between excitation and contraction. The formation of excitation-contraction coupling exhibited a regional non-uniformity in the developing 9-10-somite embryonic hearts: contraction was first generated in the right ventricular region, and then the contractile area spread widely over the whole of the heart. The curves of the excitation-related 1st signal vs. the contraction-related 2nd signal obtained from different areas were not superimposable. Decoupling of excitation from contraction was produced by raising the Ca2+ concentration in the bathing solution, by lowering the Na2+ concentration or by inclusion of a Ca2+ ionophore (A23187). Replacement of the bathing solution with D2O or hypertonic solution also suppressed excitation-contraction coupling. The results suggest that in the early embryonic initial beating chick heart, the contractile system is activated by Ca2+ influx across the sarcolemma accompanying the action potential, and that a Na+-Ca2+ exchange mechanism participates in the relaxation phase of the heartbeat.