Alternans produced in human but not in porcine model. Intracellular Ca2+ transient [Ca]i and membrane voltage (Vm) response at various pacing frequencies.
(A) Relative [Ca]i of isolated right ventricular (RV) myocytes is shown versus time as the pacing cycle length progressively decreased in steps. The full pacing protocol is shown in the upper left graph. Excerpts are displayed with the exact pacing cycle length indicated above each. (B) Computer simulation of pig cell ionic model following the experimental protocol, but also showing transmembrane voltage (red). For 200 ms pacing, the transmembrane voltage trace shifted by one pacing cycle (blue) is also shown. Voltage alternans was not seen. (C) Computer simulation of human cell ionic model as in B. For 200 ms pacing, the transmembrane voltage trace shifted in time by one pacing cycle is also displayed, showing very small voltage amplitude alternans (1.2 mV). All x-axes display time. Purple arrows in the full protocol indicate the times of the excerpts.