Fig. 7.

Effect of cytosolic Ca²⁺ on coupled RyR1 gating. Activity of a group of coupled RyR1s was measured at V_m = 0 mV in the presence of cytosolic ATP (5 mM), Mg²⁺ (5.5 mM ; free Mg²⁺ ∼ 1 mM), and luminal Ca²⁺ (50 mM) as the current carrier. Concentration of Ca²⁺ at the cytosolic surface of the channels was increased by consecutive additions of CaCl₂ (tested Ca²⁺ levels ranged between 100 nM and 1.5 μM). In this example, representative recordings taken at [Ca²⁺]_cyt = 100 nM, 300 nM, and 1.5 μM are shown in A, B, and C, respectively. CR values were 0.05, 0.07, and 0.09 for A, B, and C, respectively. D: global open probability, (nP_o)_global, during 4-min recordings (black circles) and (nP_o)_event (grey circles) ± SE. (of n > 300 events) as a function of the cytosolic Ca²⁺ level. Expected values of average (nP_o)_event for independent channels are shown as grey triangles (see appendix for details). E: mean duration of coupled events ± SE as a function of cytosolic Ca²⁺.