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. INa3was present, had a threshold negative to the fast activating-inactivating INa1, its slow inactivation was cut off by INa1, and contributed to Na+influx at INa1threshold. INa1was smaller and had a less negative threshold. There was no comparable slowly inactivating INa2, 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 IK1peak. 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 IK1block by Mg2+, was not affected by a lower Vhand TTX and was eliminated by Ba2+, in contrast to P cells. ICawas larger, peaked at positive potentials and was eliminated by Ni2+. Itowas much smaller, began at more positive values, was abolished by less negative Vhand by 4-aminopyridine, included a sustained current that 4-aminopyridine decreased but did not eliminate. Steeper ramps increased IK1peak as well as the fall in outward current during repolarization, consistent with a time-dependent block and unblock of IK1by polyamines. During repolarization, the positive slope region was consistently present and was similar in amplitude to IK1peak, whereas it was small or altogether missing in P cells. The total outward current at positive potentials comprised a larger IK1component whereas it included a larger Itoand 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.
Department of Physiology and Pharmacology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, NY 11203, U.S.A.
M
M
Vassalle
L
L
Bocchi
cardiac electrophysiology; ionic currents; single ventricular myocardial and cardiac Purkinje cells; whole cell patch clamp method