Panel A. Representative calcium transients from normal and HF hearts from alternans resistant and susceptible cells are shown. Tau (τ) , the time constant of decay of the calcium transient, is slower in alternans susceptible cells in both normal and HF hearts. Panel B. Summary data of τ in alternans resistant (R, gray bars) and alternans susceptible (S, black bars) in both HF (right) and normal (left) hearts at a baseline pacing HR of 100 bpm. Importantly, τ is slowest in the most alternans susceptible HF cells, suggesting that HF-induced slowing of calcium reuptake promotes alternans. Similar results were also observed at a baseline pacing HR of 30 bpm (data not shown).