Two types of action potential configuration in single cardiac Purkinje cells of sheep

Membrane potentials and currents of isolated sheep Purkinje and ventricular cells were compared using patch-clamp and microelectrode techniques. In approximately 50% of Purkinje cells, we observed action potentials that showed a prominent phase 1 repolarization and relatively negative plateau (LP cells). Action potential configuration of the remaining Purkinje cells was characterized by little phase 1 repolarization and relatively positive plateau (HP cells). Microelectrode impalement of Purkinje strands also revealed these two types of action potential configuration. In LP cells, the density of L-type Ca(2+) current (I(Ca,L)) was lower, whereas the density of transient outward K(+) current was higher, than in HP cells. Action potentials of HP cells strongly resembled those of ventricular cells. Densities of inward rectifier current and I(Ca,L) were significantly higher in ventricular cells compared with densities in both LP and HP Purkinje cells. Differences in current densities explain the striking differences in action potential configuration and the stimulus frequency dependency thereof that we observed in LP, HP, and ventricular cells. We conclude that LP Purkinje cells, HP Purkinje cells, and ventricular cells of sheep each have a unique action potential configuration.