Abnormal electrical properties of myocytes from chronically infarcted canine heart. Alterations in Vmax and the transient outward current.

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Abstract
BACKGROUND Reentrant ventricular arrhythmias can occur in the surviving muscle fibers of the epicardial border zone of the canine heart 5 days after coronary artery occlusion. To understand the cellular basis of these arrhythmias, we developed a method of dispersing myocytes (IZs) from the epicardial border zone. METHODS AND RESULTS We compared the electrophysiological properties of IZs with those of cells dispersed from the epicardium of control noninfarcted (NZs) and of sham-operated animals (NZsham). Transmembrane action potentials of IZs are reduced in total action potential amplitude and maximum upstroke velocity compared with NZs. However, resting potential of IZs is no different from that of NZs. Action potential duration at -10 mV is significantly reduced in IZs compared with control, and IZ potentials do not show the typical "spike and dome" morphology that is evident in all NZs. Using Vmax as an indirect measure of the peak inward current available for the upstroke of the action potential, we found that the availability curve for IZs is significantly different from the NZ curve. Furthermore, the time course of recovery of Vmax after a depolarizing voltage clamp step was significantly altered in IZs. Using whole-cell voltage clamp techniques, we determined that the voltage-dependent, Ca(2+)-independent, 4-aminopyridine-sensitive transient outward current (ito1) occurred in all NZs (n = 16) but existed in only 37% of IZs (n = 16). There was a significant reduction in the density of ito1 elicited by depolarizing steps in those IZs showing ito1 compared with ito1 density in NZs. CONCLUSIONS We have developed a single-cell model of cells that survive in the infarcted heart. Our studies indicate that there are changes in Vmax in IZs. In addition, there is no prominent phase 1 of repolarization in IZ action potentials. This is consistent with the dramatic loss in the function of the ionic channel responsible for the voltage-dependent transient outward current, ito1.