FIGURE 8.

Rapid intracellular Ca²⁺ release at ATs triggered by instantaneous action potential propagation. (A) Combined confocal imaging of AT membranes via Chol-PEG-KK114 and [Ca²⁺]_i via Fluo-4 in mouse, rat and rabbit AM during 0.5 Hz electrical field stimulation. Top row: AM imaging plane and position of the laser line (yellow triangles) used to detect intracellular Ca²⁺ release at AT structures (see also section “Materials and Methods”). Bottom row: Transversal line scans showing two ATs localized by Chol-PEG-KK114 (Chol) between the surface sarcolemma (SS) boundaries in representative AMs from each species. Note the earliest local Ca²⁺ transient onset and amplitude dynamics upon electrical field excitation (yellow triangles). N, nucleus. Scale bars top row 10 μm. (B) Left: Exemplar 2-photon recording from a mouse AM. TAT components were labeled with the voltage-sensitive dye di-4-ANE(F)PTEA (2 μM). Right: Normalized fluorescence traces (ΔF/F₀) recorded from representative scan regions indicated in color. Simultaneous action potential activation upon stimulation (black arrowheads) is clearly visible in TT and AT structures. (C) Grouped bar graphs show no significant differences between AT and other membrane locations for AP amplitude (ΔF/F), AP onset (the time interval between the end of the stimulus and the rise of the fluorescence signal above a threshold of 4% ΔF/F), and maximum slope (ΔF/dt). Sample numbers: n = 13 SS, 44 TT, and 8 AT from 13 AMs and 4 mouse hearts.