Luminal Ca²⁺ action on single RyR2 function. (A) Representative single-channel currents from WT (top) and K4750Q (bottom) recorded at 20 and −20 mV. (B) Summary of the luminal Ca²⁺ dependence of Po in WT (20 mV, black open circles), R2473S (20 mV, green filled circles), K4750Q (20 mV, red filled circles), and K4750Q (−20 mV, blue filled circles). n = 6 for WT and mutant channels. *, P < 0.05 versus WT. **, P < 0.01 versus WT. The right panel replots K4750Q Po as a function of unitary RyR2 Ca²⁺ flux amplitude (pA). Values shown are mean ± SEM.