REPOLARIZING K-CELLS( CURRENTS IN RABBIT HEART PURKINJE)

1. Electrophysiological experiments on single myocytes obtained from P urkinje 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 reduction s in extracellular potassium, [K+](o), from normal (5.4 mM) to 2.0 mM resulted in a large hyperpolarization and marked lengthening of the ac tion potential. In ventricular myocytes, these changes were much less pronounced. Voltage clamp measurements demonstrated that these differe nces were mainly due to a much smaller inward rectifier K+ current, I- K1, 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, I-t. 4-AP significantly broadened the early, rapid repolarization phase of the action potential. 4. Selective inhibitors of the fast component, I-K,I-r (MK-499, 200 nar) and the slow component I-K,I-s (L-735821 (pr openamide), 20 nM) of the delayed rectifier K+ currents both significa ntly lengthened the action potential, suggesting that these conductanc es are present, but very small (< 20 pA) in Purkinje cells. Attempts t o identify time-and voltage-dependent delayed rectifier K+ current(s) in Purkinje cells failed, although a slow delayed rectifier was observ ed in ventricular myocytes. 5. These results demonstrate significant d ifferences in action potential waveform, and underlying K+ currents in rabbit Purkinje and ventricular myocytes. Purkinje cells express a mu ch smaller I-K1, and a larger I-t than ventricular myocytes. These dif ferences in current densities can explain some of the most important e lectrophysiological properties of these two tissues.