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 si milar to 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 chara cterized by little phase 1 repolarization and relatively positive plateau ( HP cells). Microelectrode impalement of Purkinje strands also revealed thes e two types of action potential configuration. In LP cells, the density of L-type Ca2+ current (I-Ca,I- L) was lower, whereas the density of transient outward K+ current was higher, than in HP cells. Action potentials of HP c ells strongly resembled those of ventricular cells. Densities of inward rec tifier current and I-Ca,I-L were significantly higher in ventricular cells compared with densities in both LP and HP Purkinje cells. Differences in cu rrent densities explain the striking differences in action potential config uration and the stimulus frequency dependency thereof that we observed in L P, HP, and ventricular cells. We conclude that LP Purkinje cells, HP Purkin je cells, and ventricular cells of sheep each have a unique action potentia l configuration.