THE CONTROL OF CALCIUM-RELEASE IN HEART-MUSCLE

The control of calcium release from intracellular stores (the sarcopla smic reticulum) in cardiac muscle was examined with the use of a confo cal microscope and voltage clamp techniques. Depolarization evoked gra ded calcium release by altering the extent of spatial and temporal sum mation of elementary calcium release events called ''calcium sparks.'' These evoked sparks were triggered by local L-type calcium channel cu rrents in a stochastic manner, were similar at different potentials, a nd resembled spontaneous calcium sparks. Once triggered, the calcium r elease from the sarcoplasmic reticulum during a calcium spark was inde pendent of the duration of the triggering calcium influx. These result s were used to develop a unifying model for cardiac excitation-contrac tion coupling that explains the large (but paradoxically stable) ampli fication of the trigger calcium influx by a combination of digital and analog behavior.