(A,D,G) Representative confocal images of Lucifer yellow (LY, yellow) spread after needle puncture in TnT hearts after pacing. (B,E,H) Corresponding images of non-gap junction permeable Tetramethylrhodamine dextran (TMR, green) that was simultaneously injected. (C, F, J) Composite image of LY, TMR and Cx43-P0 (red). The scale bar length is 100 μm. (K) Summary data of LY spread analysis from 15 sites in TnT-WT (3 hearts) and 17 sites in TnT-I79N hearts (4 hearts). For TnT-I79N only sites were analyzed that were clearly classified as Cx43-P0 negative (I79N P0 neg, n=9) or showed clear Cx43-P0 accumulation along at least two sides (I79N P0 pos, n=8). Dye spread was analyzed along the fiber (longitudinal) or across the fiber (transverse). * p≤0.05 vs. as indicated