Figure 2

Sarcoplasmic reticulum Ca²⁺ release produces Ca²⁺ microdomains in the T-SR model. (A-C) Heat map with overlaid 12 nm finite element mesh demonstrating steady state radial [Ca²⁺] gradients following application of Ca²⁺ influx density, J_influx, expected from a depolarizing step from the resting to a 0 mV test voltage in voltage clamped amphibian muscle fibre as described in previous reports⁴⁵. Highest [Ca²⁺] is at the T-SR centres with ~ fivefold concentration reductions between the centre and edge in both (A), (C) T-tubular membrane and (B) SR membrane face. Edge concentrations are close between the two membranes. (D–F) The Ca²⁺ microdomains extend the entire axial T-SR distance. This is demonstrated in heat maps of: (D) radial [Ca²⁺] (vertical, z-axis) over the T-tubular membrane surface (xy-plane), more closely quantified by (E) midline axial section map of [Ca²⁺] taken along the y-axis. [Ca²⁺] plotted against radial distance along the T-tubular membrane in both x and y axes in (D) and in the y axis in (E). The jagged base of the surface plot is the result of interpolation of results from a cylindrical geometry. (F) [Ca²⁺] heatmap through midline axial section through the T-SR junction. Vertical axis plots axial (z) distance from the T-tubular membrane within the T-SR junction. Horizontal (x) axis plots radial distance from the T-SR junction centre. Colours represent the resulting steady state [Ca²⁺]. Note uniformity in concentration along the z axis and fivefold gradients along the x axis. Colour bars in (D) and (E), and (F), show [Ca²⁺] as represented in the heat maps. (G–I) Time course of microdomain [Ca²⁺] solutions at selected positions within the T-SR junction. (G) Solutions for [Ca²⁺] at the edge (black), centre (blue) of the T-SR junction and midway between these two (red lines) plotted against time following initiation of the Ca²⁺ influx, J_influx from the SR face of the T-SR junction. Initial instabilities in plots observed in the edge traces reflect stiff-stable properties of the solutions. (H) Variations in steady state [Ca²⁺] with radial distance from the centre of the T-SR junction parallel to the radial axis at the T-tubular membrane, the SR membrane, or halfway between these. (I) [Ca²⁺] plotted with time following the onset of Ca²⁺ influx, but with its termination after 0.5 ms.