Figure 4. Calcium signals arising from the ionic fluxes via the external and t-tubule membrane in the presence of 100 µM Fluo-3.

(A–B) The voltage-clamp protocol and whole-cell L-type Ca²⁺ current. (C–E) Predicted global Na⁺/Ca²⁺ and Ca²⁺ leak currents and global Ca²⁺ transient when no detectible differences in [Ca²⁺]_i are found (see panel F). (F–H) Calcium concentrations visualized as line-scan images in transverse cell direction. (I–K) Local Ca²⁺ transients taken at three featured spots along the scanning line of interest: 0.17 µm – blue lines; 3.09 µm – green lines; 5.45 µm – red lines. In (F) and (I) the L-type Ca²⁺ current density followed heterogeneous distribution along the length of t-tubule as shown in Fig. 2A. In (G) and (J) the L-type Ca²⁺ current density was uniform along the t-tubule and six times higher than in external membrane. In (H) and (K) the L-type Ca²⁺ current density was homogeneous throughout the cell surface. In (F–G) Na⁺/Ca²⁺ flux density was three times higher in the t-tubule and Ca²⁺ leak homogeneously distributed. In (H) Na⁺/Ca²⁺ exchanger and Ca²⁺ leak were homogeneously distributed via the sarcolemma. (L) Local Ca²⁺ time-courses with re-plot from experimental data [5]. The re-plots were taken along the scanned line at 0µm (blue), 3.96 µm (green) and 5.65 µm (red) from the near surface location. (M) Estimated SCH values with respect to the three flux distribution choices. In this numerical experiment the line-scan was positioned at 200nm away from the t-tubule membrane at the angle 120°. The scanned line in Cheng et al. experiment was located at 200nm from the surface of the t-tubule.