Figure 16 – Increasing surface/inner CRU CSQ expression ratio promotes SCRs but inhibits DADs and does not affect SAP.

A) Increasing surface/inner CRU CSQ expression ratio (i.e., increasing CSQ density in surface CRUs without changing whole-cell CSQ expression) monotonically decreases the rate threshold of SCRs (i) and increases the rate threshold of DADs (ii), whereas SAPs (iii) only appear in cells with sparse tubules and surface/inner CRU CSQ expression ratio of 0.75. The effect on SCRs is greater in cells with intermediate tubular structures vs cells with sparse and dense tubules, whereas the effect on DADs is enhanced in cells with sparse and intermediate tubular structures vs cells with dense tubules. B) Effect of altered surface/inner CRU CSQ expression ratio on voltage (i) and global cytosolic Ca²⁺ concentration (ii) in cells with sparse (left), intermediate (middle), and dense (right) tubules following pacing at 3 Hz to examine the occurrence of DADs and SCRs. C) Mechanism underlying increasing surface/inner CRU CSQ expression ratio promoting SCRs, inhibiting DADs with no effect on SAPs. Biomarkers were determined from the first 100 ms of no-stimulation period and normalized to those of cells with a retained surface/inner CRU CSQ expression ratio of 1.0. As similarly described when altering overall CSQ expression (Figure 13), increasing surface/inner CRU CSQ expression ratio is associated with increased RyR leak in inner uncoupled CRUs (i) and decreased RyR leak in surface CRUs (ii). Enhanced RyR leak in inner uncoupled CRUs leads to stronger SCRs and DADs. However, less RyR leak in surface CRUs reduces NCX contribution to Ca²⁺ extrusion (iii) and lowers SCR-induced voltage changes/SCR amplitude ratio (iv). As such, while SCRs typically lead to DADs, this transition is prohibited by the decreased SCR-induced changes in V_m, thus explaining the opposing effects of increasing surface/inner CRU RyR expression ratio on SCRs and DADs.