Figure 2

(A) Simulated L-type currents (above) and SR release currents (below) generated by the full dynamic diffusion model (solid line) or the steady state approximation (dotted line, see text). In the presence of fixed calcium binding sites, the amplitude of the release flux is only half that predicted by the steady state approximation. Most of the discrepancy can be compensated by assuming a reduced “effective” value for the RyR unitary current when using the steady state approximation. (B) Back-flux of calcium from the diadic cleft into the SR lumen through RyRs, taken into account in the dynamic-diffusion simulations, contributes to the reduction in the effective value of the RyR unitary current. For this computation, it was assumed that the net unitary current through the open RyR is proportional to the SR transmembrane gradient of [Ca²⁺], rather than SR lumenal [Ca²⁺] as in the steady state simulations, thereby taking account of the possible (unidirectional) flux of Ca²⁺ from cleft to lumen. The horizontal line shows the 0.4-pA unitary current assumed for the RyR in isolation, as in lipid bilayers. The mean in situ unitary current was defined as the ensemble-average SR release current divided by the open probability of the RyRs. The in situ current is less than that in isolation because calcium can reflux from the cleft to the SR, either through the channel through which it entered the cleft, or through neighboring RyRs. The latter effect makes the effective unitary current time dependent, because of the varying number of open RyRs available for back flux. Note that the net flux through the RyR can never be negative (since most of the calcium in the cleft originates in the SR), but it can be reduced by a component of unidirectional back flux.