RELATION BETWEEN THE SARCOLEMMAL CA2-CONTROL THEORIES FOR CARDIAC EXCITATION-CONTRACTION COUPLING( CURRENT AND CA2+ SPARKS AND LOCAL)

Ca2+ sparks, the elementary events underlying excitation-contraction ( E-C) coupling, occur when sarcoplasmic reticulum (SR) Ca2+ release cha nnels open. They are activated locally by Ca2+ influx through sarcolem mal (SL) Ca2+ channels. By measuring the probability of spark occurren ce under conditions in which their probability of occurrence is low, w e address two important questions raised by our recent work: (1) When a Ca2+ spark is triggered, how many SL Ca2+ channels (at a minimum) co ntribute to its activation? (2) What is the relation between the subce llular local [Ca2+](i) produced by the opening of SL Ca2+ channels and the consequent SR Ca2+ release? By comparing the voltage dependence o f Ca2+ sparks in rat ventricular myocytes with the Ca2+ current, we sh ow that the opening of a single SL Ca2+ channel can trigger a Ca2+ spa rk. Furthermore, we deduce that the probability of SR Ca2+ release dep ends of the square of the local [Ca2+](i) produced by SL Ca2+ channel openings. These results are discussed with respect to the properties o f Ca2+-induced Ca2+ release (CICR) and the local control theory of exc itation-contraction coupling.