Fig. 4.

T-tubule morphology in rabbit and mouse ventricular cardiomyocytes. (A) Confocal images of rabbit (Left) and mouse (Right) t-tubules labeled with di-8-ANEPPS (Top) and corresponding binary images (Bottom). For illustrative purposes, the rabbit and mouse t-tubule diameters are set to their means of 360 and 170 nm (respectively) (16). (Scale bar: 5 μm.) (B) Proportion of 6- × 6-μm quadrants that are internally connected in the directions shown for rabbit (n = 20 quadrants, 6 cells) and mouse (n = 48 quadrants, 5 cells; **P < 0.01, two-way ANOVA with Bonferroni post hoc analysis). (C) Branch-point density of t-tubule network. **P < 0.01, Mann–Whitney U test. (D) Tortuosity (ζ) of quadrants calculated by TauFactor (23). ***P < 0.005, two-way ANOVA with Bonferroni post hoc test. (E) Local variations in t-tubule width measured using a shape-based analysis method (16). The colors indicate the local width in micrometers with the width color scale at the right. (Scale bar: 2 µm.) (F) Distribution of t-tubule widths in living rabbit (n = 11) and mouse (n = 12) cardiomyocytes (total t-tubule length analyzed was ∼1 mm). The continuous curve was randomly sampled for simulations. (G) Effect of t-tubule varicosities in rabbit (n = 50 simulations) and mouse (n = 50 simulations) on time for 50% solute exchange compared with a circular cylinder of the same length and mean diameter. ****P < 0.001, Mann–Whitney U test. The inset at the right shows an exemplar simulation model used in calculations.