Abstract 3502: Origin of Subcellular Spatially Discordant Intracellular Calcium Alternans
Background: Subcellular spatially discordant alternans (SDA), in which the amplitude of intracellular Ca (Cai) release alternates out-of-phase in different regions of the cell, has been observed in both atrial and ventricular myocytes. Theoretically, SDA could arise from either intrinsic Cai cycling dynamics or local phase resetting by premature spontaneous SR Ca release in a region of the myocyte. Here we present experimental evidence supporting the latter mechanism.
Methods: Cai was imaged using a CCD-based system in fluo-4-AM loaded isolated rabbit ventricular myocytes. Myocytes were either field-stimulated, current-clamped or action-potential (AP)-clamped either at a constant or incrementally decreasing pacing cycle length (PCL).
Results: During pacing at a constant PCL of 500 or 400 ms, no Cai transient alternans was observed under control conditions, but SDA was induced by exposure to Bay K8644 (50 nM) + Isoproterenol (100 nM) in 8 of 10 myocytes. In 4 of the 10 myocytes, spontaneous SR Cai waves were observed in the absence of pacing. When a paced AP clamp was introduced as a Cai wave was propagating through the myocyte, a small Cai release occurred in the region through which the Cai wave had just propagated, and a large Cai release from the remainder of the myocyte. This pattern was reversed on the next AP clamp, and so forth, resulting in SDA over many beats. Although we were unable to unequivocally document spontaneous Cai waves causing phase reversal at the onset of SDA during rapid pacing, Cai waves were frequently observed between beats when a cell was paced at long PCL (e.g 1 sec). Furthermore, the myocytes with SDA typically exhibited spontaneous Cai waves when pacing was terminated. The Ca waves were associated with delayed afterdepolarizations in membrane potential.
Conclusions: In the presence of BayK8644 + Isoproterenol, SDA is caused by spontaneous SR Ca release initiating Cai waves that propagate only partially through the myocyte before the next AP arrives. Because the SR is still refractory in the region just excited by the Cai wave, the regional Cai transient induced by the next AP is small; in the region unexcited by the Cai wave, however, the AP induces a large Cai release, creating SDA.