sparks to waves and therefore the potential for arrhythmias. In the 6-week follow-up of the animals, mortality is very low and does not allow conclusions regarding arrhythmias but some information can be gained from the cellular data. In cells that showed spontaneous Ca2+ waves, the time to the first wave (measured from the end of the last conditioning transient) was significantly shorter in MI (8.661.7 s in MI, ncells = 22, vs. 12.561.6 s in CTRL, ncells = 23, p,0.05) and the frequency of wave activity was higher in MI cells (0.1560.03 waves/s in MI, vs. 0.0960.05 waves in CTRL). These results are in line with an increased tendency for wave initiation and propagation in MI cells. A reduced density of NCX may however mitigate the arrhythmogenic potential of such waves. Conclusions In ventricular myocytes with a sparse TT network, RyR cluster properties related to their position relative to TTs lead to References 1. 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