DIFFERENCES IN IONIC CURRENTS BETWEEN CANINE MYOCARDIAL AND PURKINJE CELLS.

An electrophysiological analysis of canine single ventricular myocardial (VM) and Purkinje (P) cells was carried out by means of whole cell voltage clamp method. The following results in VM versus P cells were obtained. I(Na3) was present, had a threshold negative to the fast activating-inactivating I(Na1), its slow inactivation was cut off by I(Na1), and contributed to Na(+) influx at I(Na1) threshold. I(Na1) was smaller and had a less negative threshold. There was no comparable slowly inactivating I(Na2), 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(K1) 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(K1) 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(Ca) 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(K1) peak as well as the fall in outward current during repolarization, consistent with a time-dependent block and unblock of I(K1) by polyamines. During repolarization, the positive slope region was consistently present and was similar in amplitude to I(K1) peak, whereas it was small or altogether missing in P cells. The total outward current at positive potentials comprised a larger I(K1) 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.