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

Abstract

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. © 1985 The Physiological Society