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. 2008 Apr;131(4):325–334. doi: 10.1085/jgp.200709907

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Copyright © 2008, The Rockefeller University Press

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Figure 4. — CSQ2 shifted the cytosolic Ca²⁺ sensitivity of single RyR2 channels. Holding potential was 0 mV and the luminal solution contained 100 mM Cs⁺. (A) Summary Po results from CSQ2-stripped (open circles; n = 8) and CSQ2-replaced (filled circles; n = 6) channels. The CSQ2-replaced channels were associated with CSQ2-WT (0.5 μg/ml in luminal chamber). Luminal free Ca²⁺ concentration was 1 mM and cytosolic Ca²⁺ was titrated from 0.1 to 100 μM. The curve fit to the filled circles has an EC₅₀ of 1.04 ± 0.17 μM and a 3.4 Hill coefficient. The curve fit to the CSQ2-stripped data has an EC₅₀ of 2.01 ± 0.34 μM and a 2.6 Hill coefficient. An unpaired t test was used to determine if the Po between CSQ2-replaced and stripped channels at each Ca²⁺ concentration was statistically different (**, P < 0.01; *, P < 0.05). Dotted curve represents the cytosolic Ca²⁺ sensitivity of CSQ2-stripped channels when 10 mM luminal Ca²⁺ was present. (B) Luminal Ca²⁺ and Mg²⁺ sensitivity of CSQ2-stripped channels. These stripped channels were maximally activated by high cytosolic Ca²⁺ (100 μM) and then luminal Ca²⁺ (open diamond; n = 13) or Mg²⁺ (open square; n = 16) was varied. The curve fit to the Ca²⁺ data has an EC₅₀ of 379 ± 247 μM and a 0.70 Hill coefficient. The curve fit to the Mg²⁺ data has an EC₅₀ of 972 ± 208 μM and a 0.77 Hill coefficient.