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Figure 3 (A) Automaticity was associated with more depolarized membrane potential. Phase 4 depolarization was observed. The slow automatic rhythm (CL = 1,150 ms) failed to conduct into PS and LA. (B) Concentration-response relationship between percent of action potential duration (APD₉₀) abbreviation and acetylcholine (ACH) concentrations. The concentration that produces 50% of the maximal effect (EC₅₀) was 1.35 ± 0.53 x 10^–7 mol/l (n = 12). (C) Concentration-response relationship between the incidence of induced sustained PV tachycardia (>2 s) and ACH concentrations. The EC₅₀ was 3.80 ± 0.85 x 10^–7 mol/l (n = 15). Other abbreviations as in Figures 1 and 2.