Spontaneous versus triggered contractions of "calcium-tolerant" cardiac cells from the adult rat ventricle.

Cardiac cells were isolated from the adult rat ventricle by an enzymatic treatment. The cells considered intact were quiescent in the presence of 2.5 mM free Ca2+ but responded to an electrical stimulation by an homogeneous and brief contraction. When the procedure failed, spontaneous cyclic contractions occurred. Often they propagated as a wave from an intercalated disk, and the tension recording showed several components in each contraction. Electrical stimulation at a frequency higher than that of the spontaneous contractions induced synchronous activation with a single component of the tension. Experiments in skinned cardiac cells suggested that the spontaneous cyclic contractions observed in enzymatically separated cardiac cells are caused by a spontaneous cyclic release of Ca2+ from the sarcoplasmic reticulum (SR). This spontaneous release requires a Ca2+ overload of the SR. Its mechanism is different from that of the Ca2+-induced release of Ca2+, which is elicited by a rapid increase of [free Ca2+] at the outer surface of the SR of a previously quiescent skinned cell.