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. 2017 Sep 4;149(9):847–855. doi: 10.1085/jgp.201711814

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© 2017 Sobie et al.

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Figure 1. — Hypothetical changes in SR and cytosolic [Ca²⁺] during SR release. (A) An increase in the fraction of total SR Ca²⁺ released causes a decrease in SR free [Ca²⁺] and a corresponding increase in cytosolic free [Ca²⁺]. Curves are nonlinear because of the presence of Ca²⁺ buffers in each compartment. Calculations assumed: (1) SR [Ca²⁺] and cytosolic [Ca²⁺] were 1,000 µM and 0.1 µM, respectively, before release; (2) cytosolic volume was 22.66 times larger than SR volume; (3) cytosolic buffering sites had maximal occupancy of 2,500 µM and a K_D of 630 µM; (4) SR buffering sites had maximal occupancy of 220 µM and a K_D of 0.96 µM. (B) Hypothetical profiles of SR [Ca²⁺] during release if Ca²⁺ diffusion within the SR is either extremely fast (blue) or quite slow (red). Because of blurring by confocal microscopes, attempts to record these SR [Ca²⁺] profiles are likely to yield similar values for the apparent extent of depletion.