Two Kinds of Calcium-Induced Release of Calcium from the Sarcoplasmic Reticulum of Skinned Cardiac Cells

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Abstract
This article compares the Ca2+-induced release of Ca2+ that is triggered by a rapid increase of free Ca2+ concentration at the outer surface of the sarcoplasmic reticulum of a skinned cardiac cell to the spontaneous release of Ca2+ that is produced by a steady-state high free Ca2+ concentration which overloads the sarcoplasmic reticulum with Ca2+ in a skinned cardiac cell. The first process, that is triggered by a rapid increase of free Ca2+ concentration at the outer surface of the sarcoplasmic reticulum, has a time- and Ca2+-dependent activation and inactivation, does not require any preload of the sarcoplasmic reticulum with Ca2+, and is not affected by the addition of inositol(1,4,5)-trisphosphate. The second process, i.e. the spontaneous release of Ca2+ from the sarcoplasmic reticulum, is not inactivated by a high free Ca2+ concentration, requires an overload of the sarcoplasmic reticulum with Ca2+ and is enhanced by inositol(1,4,5)-trisphosphate. The filling inside the sarcoplasmic reticulum with Ca2+ is critical for the triggering of the spontaneous release of Ca2+. On the other hand, the spontaneous release of Ca2+ has many similarities to the “Ca2+-induced release of Ca2+” that is observed for isolated sarcoplasmic reticulum vesicles incorporated into a lipid bilayer which is triggered by the increase of free Ca2+ concentration at the outer surface of the sarcoplasmic reticulum. Although the Ca2+-induced release of Ca2+with time-and Ca2+-dependent activation and inactivation and the spontaneous release of Ca2+ are regulated by difference mechanisms, they are both inhibited by ryanodine, which suggests that they may take place through the same channel and may even have some partial common pathway.