Steady-state calcium concentration in the JSR that would result from holding open different numbers of RyRs, based on Eq. A1.18, as a function of the JSR refilling rate constant D_up. The computation assumes that a single RyR has a unitary current of 0.35 pA (Mejía-Alvarez et al., 1999) at a resting [Ca²⁺]_JSR of 1 mM, and takes into account the series resistance of the dyadic cleft. The parameter D_up is the reciprocal of the time constant for JSR refilling by diffusion from the FSR that would pertain in the absence of Casq. The relationship of D_up to refilling time in the presence of Casq is given in Eq. 1.4. For comparison, D_up = 1 would give a refilling time of ∼10 ms, much faster than observed. The observed rate of spark restitution in rat (Sobie et al., 2005; Ramay et al., 2011) would correspond to D_up of ∼0.1, and blink recovery time in rabbit (Picht et al., 2011) would be even lower. The implication is that opening of even a single RyR is capable of depleting JSR calcium by >50% under any conceivable assumption about intra-lumenal diffusion. Note that this steady-state calculation does not depend on the presence or any assumptions about Casq.