Background Elevated intracellular calcium activity is thought to play an important role in arrhythmia induction, particularly during ischemia and reperfusion. Delayed afterdepolarization-induced triggered activity and intracellular communication problems are thought to be responsible.Methods and Results Increased extracellular calcium levels and rapid pacing are interventions known to elevate intracellular calcium activity. The present study, conducted using standard microelectrode techniques, was designed to compare the effects of increased (Ca2+)o(1.8 to 5.4 mmol/L) in isolated canine ventricular epicardial and endocardial tissues and to test the hypothesis that elevated intracellular calcium activity contributes to arrhythmogenesis in working ventricular myocardial tissues by promoting electrical heterogeneity. High (Ca2+)ocaused a slight abbreviation of action potential duration (APD90) in endocardium but more dramatic rate-dependent and dynamic changes in epicardium. Under steady-state conditions, epicardium displayed a marked abbreviation of APD sub 90 at fast rates but no significant changes at slow rates. A significant augmentation of phase 1 was evident at the faster stimulation rates. Vmaxand conduction velocity were only slightly reduced. The marked abbreviation of the epicardial response at the faster rates was due to loss of the action potential dome. Recovery of the dome after deceleration was not synchronous throughout the preparation. As a consequence, a sudden slowing of rate caused marked dispersion of repolarization among neighboring epicardial sites, giving rise to ectopic activity via a phase 2 reentry mechanism. These effects of high (Ca2+)owere mimicked by exposure of the preparations to low (Na sup +)o. Electrical homogeneity was restored and arrhythmias were abolished after addition of the Itoblocker 4-aminopyridine 1 mmol/L. 4-Aminopyridine also eliminated the differential response of epicardium and endocardium to high (Ca2+)o.Conclusions Our data demonstrate the induction of marked electrical heterogeneity and reentrant activity by high (Ca sup 2+) sub o and rapid stimulation, conditions known to elevate (Ca sup 2+) sub i. The results suggest that increased intracellular calcium activity, as occurs during ischemia and reperfusion, may contribute to the development of electrical inhomogeneity in the ventricle and thus to the genesis of ventricular arrhythmias through a mechanism other than triggered activity, namely, phase 2 reentry. Our data point to an increase in net outward current as the underlying mechanism for the calcium-induced changes. Our results also suggest that the presence of a prominent transient outward current (Ito) in epicardium sensitizes that tissue to the effects of high calcium. Finally, the results suggest that Itoblockers can reverse high calcium-induced electrical heterogeneity and thus can exert antiarrhythmic actions. (Circulation. 1994;89:1839-1850.)