Abstract: Abstract Objective The heterogeneity of the action potential(AP)，transmural dispersion of repolarization (TDR) and Ica-L ion current in left ventricular three layers myocytes were investigated in rabbit by using patch-clamp technique. And the acute electrophysiological effects of amiodarone on both AP and TDR were also studied. Methods Rabbits myocytes were isolated by O2 & CO2 mixed-gas combined with type –II collagenase enzymatic method．And the sub-endocardial (Endo) and sub-epicardium (Epi) tissues were separated from the other regions (midmyocardium，M)．Separated ventricular myocytes were perfused with Tyrod’s solution and 10 umol/L amiodarone. And then the whole cell patch clamp technique was used to study the AP and Ica-L. Result 1. The action potential (AP) in the three myocardial layers were significantly different．The action potential duration (APD) in M cells was the longest, while the Endo cells were the shortest. The APD 90 of M cells was 488.13±22.88ms, and that of Endo and Epi cells were 326.36±14.51ms and 391.1±23ms, respectively (P<0.001). TDR was 185ms. The M cells had more pronounced spike-and-dome configuration，with significantly larger phase 1 magnitude and plateau voltage．2) After acute amiodarone perfusion, the APD 20, 50 and 90 in M cells were sharply shortened (P<0.003). On the contrary， Endo cells have a remarkably prolonged period of AP (P <0.0003). So the TDR obviously decreased. Amiodarone has no effect on AP configuration. 3) ICa-L of three myocardial layers was significance different in the current density. The current density of Epi and Endo cells were 6.21±1.35 and 7.20±0.83 pA/pF, respectively. The M cells had a larger current density, the value was 1.59±3.71pA/pF, P<0.05. The three layers myocytes had no significance change in the I-V curved and voltage-dependence. 4) Compared with the normal ventricular myocytes，the current density of ICa-L were all decreased in three layers myocytes perfused by 10μmol/L amiodarone，but the M cells were the significant ones, 11.59±3.71pA/pF vs 7.28±2.01 pA/pF, (P<0.05). And amiodarone had no effect the I-V curved and voltage-dependence of ICa-L. Conclusion 1) The action potential and the L-Ca ion currents in the three myocytes layers were significantly different．Action potential duration (APD) in M cells was longer than that in Epi or Endo cells．M cells had a more pronounced spike—and—dome configuration，with a significantly larger phase 1 magnitude an d plateau voltage．2) ICa-L in M cells were higher than those of Epi and Endo．The ICa-L in Epi and Endo were similar with each other. And they were no difference in I-V curved and voltage-dependence of ICa-L. 3) Amiodarone could prolong APD in Endo cells and decrease that in M cells, so the TDR was decreased. Amiodarone could remarkably inhibited ICa-L in M cells, but had no influence on configuration of ICa-L．

Key words: M cells of rabbits, Action potentials, heterogeneity, transmural dispersion of repolarization, L-Ca2+ channels, Amiodarone