THE STAIRCASE PHENOMENON AND THE STRENGTH-INTERVAL RELATIONSHIP OF CONTRACTIONS IN CULTURED-CELLS OF CHICKEN EMBRYONIC VENTRICLES

The staircase phenomenon and the relationship between contraction stre ngth and stimulation interval were investigated in single or aggregate d cells cultured from 5-, 12-, and 19-d-old chicken embryonic ventricl es. Within 24 h after start of cultivation was begun, the cells were s eparated and spontaneously inactive. However, they contracted when ele ctrical stimulation was applied. The modes of a staircase phenomenon a nd a strength-interval relationship varied, depending on a few conditi ons such as the ages of the embryos, the presence of spontaneous activ ities and the composition of the bathing solution. The 19-d-old cells, which formed an aggregate and exhibited spontaneous activities, showe d a negative staircase phenomenon and the negative strength-interval r elationship when they were bathed in a culture medium containing bovin e serum. The 12-d-old cells demonstrated a less distinct staircase and a flat strength-interval relationship. Both 19- and 12-d-old cells sh owed a positive relationship, if they were cultured for less than 48 h and were spontaneously inactive, or if they were bathed in Tyrode sol ution during the experiments. The 5-d-old cells showed the positive st aircase whether they were spontaneously active or not. The development al change in the interval-dependency of contraction can be explained b y the functions of the sarcoplasmic reticulum (SR), although Ca2+ curr ent and the Na+-Ca2+ exchange mechanism should be involved. The functi on of the SR develops progressively during the embryonic stage and pla ys an important role in regulation of the contraction-relaxation cycle at the late stage before hatching.