Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes

Subcellular Ca2+ signalling during normal excitation-contraction (E-C) coupling and during Ca2+ alternans was studied in atrial myocytes using fast confocal microscopy and measurement of Ca2+ currents (I-Ca). Ca2+ alternans, a beat-to-beat alternation in the amplitude of the [Ca2+](i) transient, causes electromechanical alternans, which has been implicated in the generation of cardiac fibrillation and sudden cardiac death. Cat atrial myocytes lack transverse tubules and contain sarcoplasmic reticulum (SR) of the junctional (j-SR) and non-junctional (nj-SR) types, both of which have ryanodine-receptor calcium release channels. During E-C coupling, Ca2+ entering through voltage-gated membrane Ca2+ channels(ICa)triggers Ca2+ release at discrete peripheral j-SR release sites. The discrete Ca2+ spark-like increases of [Ca2+](i) then fuse into a peripheral 'ring' of elevated [Ca2+](i), followed by propagation (via calcium-induced Ca2+ release, CICR) to the cell centre, resulting in contraction. Interrupting I-Ca instantaneously terminates j-SR Ca2+ release, whereas nj-SR Ca2+ release continues. Increasing the stimulation frequency or inhibition of glycolysis elicits Ca2+ alternans. The spatiotemporal [Ca2+](i) pattern during alternans shows marked subcellular heterogeneities including longitudinal and transverse gradients of [Ca2+](i) and neighbouring subcellular regions alternating out of phase. Moreover, focal inhibition of glycolysis causes spatially restricted Ca2+ alternans, further emphasising the local character of this phenomenon. When two adjacent regions within a myocyte alternate out of phase, delayed propagating Ca2+ waves develop at their border. In conclusion, the results demonstrate that (1) during normal E-C coupling the atrial [Ca2+](i) transient is the result of the spatiotemporal summation of Ca2+ release from individual release sites of the peripheral j-SR and the central nj-SR, activated in a centripetal fashion by CICR via I-Ca and Ca2+ release from j-SR, respectively, (2) Ca2+ alternans is caused by subcellular alterations of SR Ca2+ release mediated, at least in part, by local inhibition of energy metabolism, and (3) the generation of arrhythmogenic Ca2+ Waves resulting from heterogeneities in subcellular Ca2+ alternans may constitute a novel mechanism for the development of cardiac dysrhythmias.