Panel A shows spontaneous cyto-iATP fluorescence transients during sinus rhythm in intact SA node preparation. Panel B shows cyto-iATP fluorescence transients synchronized with field stimulation at 3 H z (5 V). Panel C shows the absence of cyto-iATP transients during pacing after mitochondrial uncoupling with FCCP. Panel D shows ASAP5 voltage fluorescence signals during spontaneous sinus rhythm in intact SA node. Panel E shows ASAP5 voltage signals entrained with field stimulation at 3 Hertz (5 Volts). Panel F shows loss of ASAP5 voltage responses during pacing after mitochondrial uncoupling with FCCP.
Mitochondrial ATP synthesis is obligatory for SA node excitability and cannot be bypassed by electrical pacing. (A–F) Representative recordings of cytosolic ATP dynamics (A–C; cyto-iATP, green) and membrane voltage (D–F; ASAP5, blue) obtained from intact SA node preparations. (A and D) Beat-locked cyto-iATP transients with positive deflections and corresponding voltage signals recorded during spontaneous sinus rhythm. (B and E) Entrainment of cyto-iATP and voltage signals during field stimulation at 3 Hz (5 V). (C and F) Loss of excitation–metabolism coupling during mitochondrial uncoupling with FCCP (10 µM). Traces were acquired during continued pacing at 3 Hz with stepwise increases in stimulation amplitude (5–60 V). For ASAP5 recordings, upward deflections correspond to membrane depolarization. Traces are shown as normalized fluorescence (F/F0). The dashed line indicates baseline fluorescence (F/F0 = 1). Scale bars: 0.5 F/F0, 500 ms.