Figure 3

Ca²⁺ microdomains reconstructed using varied Ca²⁺ flux densities derived from experimental Ca²⁺ transients in response to test voltage steps. (A, B) Persistent Ca²⁺ microdomains with [Ca²⁺] magnitudes graded with experimentally derived Ca²⁺ influx densities J_influx corresponding to varying experimental test voltage steps applied to amphibian muscle fibres⁴⁵. (A) [Ca²⁺] changes with time following onset of the imposed J_influx and (B) the resulting dependences of the steady state changes in [Ca²⁺] upon radial distances from the centre of the T-SR junction. The [Ca²⁺] across the T-SR junction falls with reducing depolarizing steps at all points in both the steady state and with time. For comparison, the y axis limit has been fixed at 25 μM. (C–E) The T-SR model recapitulates experimentally reported Ca²⁺ flux densities and resulting Ca²⁺ concentrations following the graded test voltage steps. Thus: (C) Measured cytoplasmic [Ca²⁺] (filled symbols) achieved following test voltage steps ⁴⁵ compared with [Ca²⁺]_edge (open symbols) in response to Ca²⁺ flux densities, J_influx determined from reported rates of increase in [Ca²⁺], d[Ca²⁺]/dt, corresponding to those test voltages (abscissa) illustrated in (A, B) at exit length λ = 9.2 nm. (D) Plot of [Ca²⁺]_edge from the reconstructed T-SR junction against reported cytoplasmic [Ca²⁺]⁴⁵. The points fit a linear regression model with a gradient close to 1 (1.02) and intercepts close to the origin. (E) Plot of Ca²⁺ influx across the SR membrane Φ_influx against computed [Ca²⁺]_edge expressed in SI units. The points fit a linear regression model with a gradient and zero intercept matching that expected from the conservation condition. The matching computed and observed [Ca²⁺] in (C) and the gradients and intercepts in (D) and (E) confirm match of the T-SR junction model to previous experimental results at different test voltages.