Objective Background and Objects: Naturally occurring temporal variability of action potentialduration (APD) in isolated myocytes has been noted. Most of the studies have beenfocusing on analyzes of the differences in ionic channels and currents among theepicardial-, mid-myocardial-(M) and endocardial myocytes, and the rate-dependent (adaptation) characteristics of APD. We have found that the change in APD during achange in frequency of stimulation mostly reflects a change in rate of repolarization at distinct membrane potential levels. We assumed that in the myocytes, there is balancing mechanism, which is constantly adjusting the various ionic currents accommodating to the changing conditions. This intrinsic ability of adaptation is important and may offer some of the consequences of the transmural heterogeneity in adaptation of APD. This adaptive behaviors maybe equally important in maintaining the normal electrophysiological properties and in induction of arrhythmia in a case of error in normal adaptation. Though most studies of Na +/Ca 2+ exchange (NCX) has been emphasized on its reverse activaty during pathyological condition. Our hypothesis is that reverse activaty of NCX also plays an important role in adjusting the repolarization of AP during a physiological condition. A mismatch between action potential (AP) repolarization and relaxation of the contraction can be caused by intracellular Ca 2+ transport abnormalities. Ca 2+ influx via reverse activation of NCX can load the sarcoplasmic recticulum (SR), which has arrhythmogenic effect.Methods We studied the single myocytes from the left ventricle of adult mongrel dogs. During the cell separation, collagenase was perfused through LAD by Langandorff system. We use the patch-clamp system to determinded AP in current clamp mode. Myocyte contraction was imaged by a video camera, shortening of unloaded myocytes was detected by a video edge motion detector, using changes in light intensity at the edges of the myocyte. Results From 60 consecutive recorded APs at a constant 1.0 Hz stimulation under steady state conditions we found there is a variance in the repolarization between 10mV and-40mV. We also found the variance in the APD during the rate adaptation range of repolarization. Fluctuation in the transient may contribute to the APD variability. To test thishypothesis we block the transient by intracellular dialysis with 10 mM EGTA(n=19), this caused a significant reduction in the coefficient variability (CV=SD/mean APD%) from 2.3± 0.8 to 1.3± 0.3 P< 0.01. During a rate change of the stimulation from 0.6 Hz to 1.0 Hz. The AP duration increased from 278±8 msec to 320±9 msec, Mean+SD, n=5, 50 APs, P< 0.05. contraction is accompanied by an after-contraction(A-CON). The relaxation of contraction precedes the repolarization of the AP. We assumed that the enhancement of repolarization and the production of after-contraction can be possibly induced by reverse mode of NCX. Reducing [Na +] o by substitution of 40mM Na + with Li + favors NCX activating the reverse mode, which significantly decreased the dome of the AP from 4.8± 0.3 to -10.6± 1.2mV, P< 0.05, and increased the APD from 330±13 to 368±14 msec. P< 0.05.Conclusions Intracellular calcium transient most likely contributes to the beat-to-beat variance of action potential duration in canine ventricular myocyte. And it attributes to the voltage-dependent switch of NCX mode. Calcium concentration is high inmyocytes during the repolarization, and high intracellular Ca 2+ activates NCX in such a manner, that it generates an inward (positive, depolarizing) current. This current works against the repolarization, it is prolonging it, with other words it increases the duration of the action potential. The magnitude of calcium concentration during repolarization is very much dependent on calcium transport in the SR. The calcium transport in the SR is subject to adrenergic actions, and other physiologic and pathologic regulators. Under pathologic conditions (exces Objective Background and Objects: Naturally occurring temporal variability of action potential duration (APD) in isolated myocytes has been noted. Most of the studies have been focused on on differences in ionic channels and currents among the epicardial-, mid-myocardial- (M) and endocardial myocytes, and the rate-dependent (adaptation) characteristics of APD. We have found that the change in APD during achange in frequency of stimulation mostly conditioned a change in rate of repolarization at distinct membrane potential levels. We assumed that in the myocytes, there is balancing mechanism, which is often adjusting the various ionic currents accommodating to adaptation conditions APD. This adaptive behaviors may equally important in may provide some of the consequences of the transmural heterogeneity in adaptation of APD. the normal electrophysiological properties and in induction of arrhythmia in Although most studies of Na + / Ca 2+ exchange (NCX) have been emphasized on its reverse activaty during pathyological conditions. Our hypothesis is that reverse activaty of NCX also plays an important role in adjusting the repolarization of AP during a physiological condition. A mismatch between action potential (AP) repolarization and relaxation of the contraction can be caused by intracellular Ca 2+ transport abnormalities. Ca 2+ influx via reverse activation of NCX can load the sarcoplasmic recticulum (SR), which has arrhythmogenic effect. Methods We studied the single myocytes from the left ventricle of adult mongrel dogs. During the cell separation, collagenase was perfused through LAD by Langandorff system. We use the patch-clamp system to determine AP in current clamp mode. contraction was imaged by a video camera, shortening of unloaded myocytes was detected by a video edge motion detector, using changes in light intensity at the edges of the myocyte. Results From 60 consecutive recorded APs at a constant 1.0 Hz stimulation under steady state conditions we found there is a variance in the repolarization between 10mV and-40mV. We also found the variance in the APD during the rate To test this hypotheses we block the transient by intracellular dialysis with 10 mM EGTA (n = 19), this caused a significant reduction in the coefficient variability (CV = SD / mean APD%) from 2.3 ± 0.8 to 1.3 ± 0.3 P <0.01. During a rate change of the stimulation from 0.6 Hz to 1.0 Hz. The AP duration increased from 278 ± 8 msec to 320 ± 9 msec, Mean + SD, n = 5, 50 APs, P <0.05. Contraction is accompanied by an after-contraction (A-CON). The relaxation of contraction precedes the repolarization of the AP. We assume that the enhancement of repolarization and the production of after-contraction can be possibly induced by re verse mode of NCX. Reducing [Na +] o by substitution of 40 mM Na + with Li + favors NCX activating the reverse mode, which significantly decreased the dome of the AP from 4.8 ± 0.3 to -10.6 ± 1.2 mV, P <0.05, and increased the APD from 330 ± 13 to 368 ± 14 msec. P <0.05.Conclusions Intracellular calcium transient most likely contributes to the beat-to-beat variance of action potential duration in canine ventricular myocyte. And it attributes to the voltage-dependent switch of NCX mode. Calcium concentration is high in myocytes during the repolarization, and high intracellular Ca 2+ activates NCX in such a manner that that it generates an inward (positive, depolarizing) current. This current works against the repolarization, it is prolonging it , with other words it increases the duration of the action potential. The magnitude of calcium concentration during repolarization is very much dependent on calcium transport in the SR. The calcium transport in the SR is subject to adrener gic actions, and other physiologic and pathologic regulators. Under pathologic conditions (exces