Repolarizing K⁺ currents in rabbit heart Purkinje cells

1. Electrophysiological experiments on single myocytes obtained from Purkinje fibres and ventricular tissue of adult rabbit hearts were done to compare the contributions of three potassium (K+) currents to the action potentials in these two tissues. 2. In Purkinje cells reductions in extracellular potassium, [K+]o, from normal (5.4 mM) to 2.0 mM resulted in a large hyperpolarization and marked lengthening of the action potential. In ventricular myocytes, these changes were much less pronounced. Voltage clamp measurements demonstrated that these differences were mainly due to a much smaller inward rectifier K+ current, IK1, in Purkinje cells than in ventricular myocytes. 3. Application of 4-aminopyridine (4-AP, 2 mM) showed that all Purkinje cells exhibited a very substantial Ca2+-independent transient K+ outward current, It. 4-AP significantly broadened the early, rapid repolarization phase of the action potential. 4. Selective inhibitors of the fast component, IK, r (MK-499, 200 nM) and the slow component IK,s (L-735821 (propenamide), 20 nM) of the delayed rectifier K+ currents both significantly lengthened the action potential, suggesting that these conductances are present, but very small (< 20 pA) in Purkinje cells. Attempts to identify time- and voltage-dependent delayed rectifier K+ current(s) in Purkinje cells failed, although a slow delayed rectifier was observed in ventricular myocytes. 5. These results demonstrate significant differences in action potential waveform, and underlying K+ currents in rabbit Purkinje and ventricular myocytes. Purkinje cells express a much smaller IK1, and a larger It than ventricular myocytes. These differences in current densities can explain some of the most important electrophysiological properties of these two tissues.