Differences in ionic currents between canine myocardial and Purkinje cells

Abstract An electrophysiological analysis of canine single ventricular myocardial ( VM ) and P urkinje ( P ) cells was carried out by means of whole cell voltage clamp method. The following results in VM versus P cells were obtained. I N a3 was present, had a threshold negative to the fast activating–inactivating I N a1 , its slow inactivation was cut off by I N a1 , and contributed to Na + influx at I N a1 threshold. I N a1 was smaller and had a less negative threshold. There was no comparable slowly inactivating I N a2 , accounting for the shorter action potential. Slope conductance at resting potential was about double and decreased to a minimum value at the larger and less negative I K 1 peak. The negative slope region of I‐V relation was smaller during fast ramps and larger during slow ramps than in P cells, occurred in the voltage range of I K 1 block by Mg 2+ , was not affected by a lower V h and TTX and was eliminated by Ba 2+ , in contrast to P cells. I C a was larger, peaked at positive potentials and was eliminated by Ni 2+ . I to was much smaller, began at more positive values, was abolished by less negative V h and by 4‐aminopyridine, included a sustained current that 4‐aminopyridine decreased but did not eliminate. Steeper ramps increased I K 1 peak as well as the fall in outward current during repolarization, consistent with a time‐dependent block and unblock of I K 1 by polyamines. During repolarization, the positive slope region was consistently present and was similar in amplitude to I K 1 peak, whereas it was small or altogether missing in P cells. The total outward current at positive potentials comprised a larger I K 1 component whereas it included a larger I to and sustained current in P cells. These and other results provide a better understanding of the mechanisms underlying the action potential of VM and P cells under normal and some abnormal (arrhythmias) conditions.