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The staircase phenomenon and the relationship between contraction strength and stimulation interval were investigated in single or aggregated cells cultured from 5-, 12-, and 19-d-old chicken embryonic ventricles. Within 24 h after start of cultivation was begun, the cells were separated and spontaneously inactive. However, they contracted when electrical stimulation was applied. The modes of a staircase phenomenon and a strength-interval relationship varied, depending on a few conditions such as the ages of the embryos, the presence of spontaneous activities and the composition of the bathing solution. The 19-d-old cells, which formed an aggregate and exhibited spontaneous activities, showed a negative staircase phenomenon and the negative strength-interval relationship when they were bathed in a culture medium containing bovine serum. The 12-d-old cells demonstrated a less distinct staircase and a flat strength-interval relationship. Both 19- and 12-d-old cells showed a positive relationship, if they were cultured for less than 48 h and were spontaneously inactive, or if they were bathed in Tyrode solution during the experiments. The 5-d-old cells showed the positive staircase whether they were spontaneously active or not. The developmental change in the interval-dependency of contraction can be explained by the functions of the sarcoplasmic reticulum (SR), although Ca2+ current and the Na(+)-Ca2+ exchange mechanism should be involved. The function of the SR develops progressively during the embryonic stage and plays an important role in regulation of the contraction-relaxation cycle at the late stage before hatching.

The Staircase Phenomenon and the Strength-Interval Relationship of Contractions in Cultured Cells of Chicken Embryonic Ventricles.