β-Adrenergic Stimulation Synchronizes Intracellular Ca 2+ Release During Excitation-Contraction Coupling in Cardiac Myocytes

Abstract —To elucidate microscopic mechanisms underlying the modulation of cardiac excitation-contraction (EC) coupling by β-adrenergic receptor (β-AR) stimulation, we examined local Ca 2+ release function, ie, Ca 2+ spikes at individual transverse tubule–sarcoplasmic reticulum (T-SR) junctions, using confocal microscopy and our recently developed technique for release flux measurement. β-AR stimulation by norepinephrine plus an α 1 -adrenergic blocker, prazosin, increased the amplitude of SR Ca 2+ release flux (J SR ), its running integral (∫J SR ), and L-type Ca 2+ channel current ( I Ca ), and it shifted their bell-shaped voltage dependence leftward by ≈10 mV, with the relative effects ranking I Ca > J SR >∫J SR . Confocal imaging revealed that the bell-shaped voltage dependence of SR Ca 2+ release is attributable to a graded recruitment of T-SR junctions as well as to changes in Ca 2+ spike amplitudes. β-AR stimulation increased the fractional T-SR junctions that fired Ca 2+ spikes and augmented Ca 2+ spike amplitudes, without altering the SR Ca 2+ load, suggesting that more release units were activated synchronously among and within T-SR junctions. Moreover, β-AR stimulation decreased the latency and temporal dispersion of Ca 2+ spike occurrence at a given voltage, delivering most of the Ca 2+ at the onset of depolarization rather than spreading it out throughout depolarization. Because the synchrony of Ca 2+ spikes affects Ca 2+ delivery per unit of time to contractile myofilaments, and because the myofilaments display a steep Ca 2+ dependence, our data suggest that synchronization of SR Ca 2+ release represents a heretofore unappreciated mechanism of β-AR modulation of cardiac inotropy.