Effect of corticosteroid agonists and antagonists on the t-system in cultured rat and human cardiomyocytes. A, Confocal images of Di8-ANEPPS-stained living rat myocytes of freshly isolated cells (fresh) and cells after 3 days of culture treated with vehicle (CTRL3d), 1μmol/L dexamethasone (DEX3d), 1μmol/L corticosterone (CORT3d), or 1μmol/L dexamethasone + 10μmol/L mifepristone (DEX+MIFE3d). B, Confocal images of freshly isolated and cultured cardiomyocytes from human failing hearts. Cells were cultured for 1 day and treated with vehicle (CTRL1d) or dexamethasone (DEX1d). C, T-tubule density (TT density), mean intracellular distance to nearest t-tubule (TT distance), and fraction of longitudinal t-tubule components (longitudinal TT) in fresh, CTRL3d and DEX3d cells from matched cell isolations. D, TT density and TT distance in CTRL and DEX human myocytes cultured for 1–3 days. E, Same parameters as shown in C in CTRL3d, DEX3d, CORT3d and DEX+MIFE3d cells from matched cell isolations. ***p<0.001, **p<0.01, *p<0.05, n (cells/animals) in C = 53/7, 158/20, 157/20 in fresh, CTRL3d, DEX3d, respectively, n (cells/patients) in D = 32/4, 34/4 in CTRL and DEX, respectively, n (cells/animals) in E = 79/10, 79/10, 80/10, 73/10 in CTRL3d, DEX3d, CORT3d, DEX+MIFE3d, respectively. Two-tailed Welch’s t-test was used for all tests, with multiple comparison correction where required.