β-adrenergic stimulation synchronizes intracellular Ca2+ release during excitation-contraction coupling in cardiac myocytes

To elucidate microscopic mechanisms underlying the modulation of cardiac excitation-contraction (EC) coupling by beta -adrenergic receptor (beta -AR) stimulation, we examined local Ca2+ release function, ie, Ca2+ spikes at individual transverse tubule-sarcoplasmic reticulum (T-SR) junctions, using confocal microscopy and our recently developed technique for release flux measurement. beta -AR stimulation by norepinephrine plus an alpha (1)-adrenergic blocker, prazosin, increased the amplitude of SR Ca2+ release flux (J(SR)), its running integral (integralJ(SR)), and L-type Ca2+ channel current (I-Ca), and it shifted their bell-shaped voltage dependence leftward by approximate to 10 mV, with the relative effects ranking I-Ca> J(SR)>integralJ(SR). Confocal imaging revealed that the bell-shaped voltage dependence of SR Ca2+ release is attributable to a graded recruitment of T-SR junctions as well as to changes in Ca2+ spike amplitudes. beta -AR stimulation increased the fractional T-SR junctions that fired Ca2+ spikes and augmented Ca2+ spike amplitudes, without altering the SR Ca2+ load, suggesting that more release units were activated synchronously among and within T-SR junctions. Moreover, beta -AR stimulation decreased the latency and temporal dispersion of Ca2+ spike occurrence at a given voltage, delivering most of the Ca2+ at the onset of depolarization rather than spreading it out throughout depolarization. Because the synchrony of Ca2+ spikes affects Ca2+ delivery per unit of time to contractile myofilaments, and because the myofilaments display a steep Ca2+ dependence, our data suggest that synchronization of SR Ca2+ release represents a heretofore unappreciated mechanism of beta -AR modulation of cardiac inotropy.