The quantal nature of Ca$^2+$ sparks and in situ operation of the ryanodine receptor array in cardiac cells.
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Proc. Natl. Acad. Sci. U. S. A. 101 (11): 3979--3984 (March 2004)

Intracellular Ca$^2+$ release in many types of cells is mediated by ryanodine receptor Ca$^2+$ release channels (RyRCs) that are assembled into two-dimensional paracrystalline arrays in the endoplasmic/sarcoplasmic reticulum. However, the in situ operating mechanism of the RyRC array is unknown. Here, we found that the elementary Ca$^2+$ release events, Ca$^2+$ sparks from individual RyRC arrays in rat ventricular myocytes, exhibit quantized Ca$^2+$ release flux. Analysis of the quantal property of Ca$^2+$ sparks provided a view of unitary Ca$^2+$ current and gating kinetics of the RyRC in intact cells and revealed that spark activation involves dynamic recruitment of small, variable cohorts of RyRCs. Intriguingly, interplay of RyRCs in multichannel sparks renders an unusual, thermodynamically irreversible mode of channel gating that is unshared by an RyRC acting solo, nor by RyRCs in vitro. Furthermore, an array-based inhibitory feedback, overriding the regenerative Ca$^2+$-induced Ca$^2+$ release of RyRCs, provides a supramolecular mechanism for the microscopic stability of intracellular Ca$^2+$ signaling.
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