Dynamic remodeling of K+ and Ca2+ currents in cells that survived in the epicardial border zone of canine healed infarcted heart

Action potentials (APs) of the epicardial border zone (EBZ) cells from the day 5 infarcted heart continue to be altered by day 14 postocclusion, namely, they shortened. However, by 2 mo, EBZ APs appear normal, yet conduction of wave fronts remains abnormal. We hypothesize that the changes in transmembrane APs are due to a change in the distribution of ion channels in either density or function. Thus we focused on the changes in Ca2+ and K+ currents in cells isolated from the 14-day (IZ(14d)) and 2-mo ( IZ(2m)) EBZ and compared them with those occurring in cells from the same hearts but remote ( Rem) from the EBZ. Whole cell voltage-clamp techniques were used to measure and compare Ca2+ and K+ currents in cells from the different groups. Ca2+ current densities remain reduced in cells of the 14-day and 2-mo infarcted heart and the kinetic changes previously identified in the 5-day heart begin to, but do not recover to, cells from noninfarcted epicardium (NZ) values. Importantly, I-Ca,I-L in both the EBZ and Rem regions still show a slowed recovery from inactivation. Furthermore, during the remodeling process, there is an increased expression of T-type Ca2+ currents, but only regionally, and only within a specific time window postmyocardial infarction (MI). Regional heterogeneity in beta-adrenergic responsiveness of I-Ca,I-L exists between EBZ and remote cells of the 14-day hearts, but this regional heterogeneity is gone in the healed infarcted heart. In IZ(14d), the transient outward K+ current (I-to) begins to reemerge and is accompanied by an upregulated tetraethylammonium-sensitive outward current. By 2-mo postocclusion, I-to and sustained outward K+ current have completed the reverse remodeling process. During the healing process post-MI, canine epicardial cells downregulate the fast I-to but compensate by upregulating a K+ current that in normal cells is minimally functional. For recovering I-Ca,I-L of the 14-day and 2-mo EBZ cells, voltage-dependent processes appear to be reset, such that I-Ca,I-L window current occurs at hyperpolarized potentials. Thus dynamic changes in both Ca2+ and K+ currents contribute to the altered AP observed in 14-day fibers and may account for return of APs of 2 mo EBZ fibers.