Fig. 1.

A: schematic model of a cluster of coupled ryanodine receptors (RyRs) in a cardiac cell. The cell has a length of 150 μm, with 75 elements, giving 2-μm spatial resolution. Coupling of these elements is via Ca²⁺ diffusion between neighboring cytoplasmic and network sarcoplasmic reticulum (SR) spaces, as described in the appendix. B: schematic model of Ca²⁺ cycling for each element of the cluster illustrated in A. Three main Ca²⁺ cycling processes were modeled, which included 1) Ca²⁺ entry by L-type Ca²⁺ channel and Ca²⁺-induced Ca²⁺ release (CICR); 2) Ca²⁺ removal by Na/Ca exchanger and ATP-dependent Ca²⁺ pump; and 3) Ca²⁺ diffusion and Ca²⁺ binding to its buffers.