Studies from our laboratory have defined alterations in Ca i handling in the non-dialyzed subepicardial cells that have survived in the 5 day infarcted heart (IZs). To determine whether changes in the action potential profile contributed to the observed Ca i changes we have used a combined voltage clamp/epifluorescent technique to determine and compare changes in fura 2 ratios in IZs compared to those of epicardial cells from the non-infarcted canine hearts (NZs). We found that Ca i changes in voltage clamped IZs persisted. In NZs, Ca i transients showed the expected voltage dependence while IZs did not. To determine whether altered NaCa exchanger activity contributed to the observed changes in Ca i in IZs, we measured NaCa exchanger Ca 2+ fluxes (reverse and forward mode) and ionic currents in both cell types and under different Na i loads (10 and 20 m m). We found that there were no significant differences in resting, peak or magnitude of fura 2 ratio changes or in outward current densities between NZs and IZs even under the different Na i loads. Thus, we suggest that chronic up- or downregulation of the NaCa exchanger protein does not underlie observed Ca i changes in IZs. Additionally, Ca 2+ released with paced voltage steps represented 79% of that released by caffeine in NZs while, in IZs, caffeine releasable Ca 2+ was equivalent to that released with step depolarization. Thus, abnormalities in Ca i handling in IZs appear not to arise secondarily to changes in action potential configuration nor do they appear to be due to disease-induced alteations in NaCa exchanger function.