Patch-clamp analysis in canine cardiac Purkinje cells of a novel sodium component in the pacemaker range

A putative Na + component playing a role in the initiation and maintenance of spontaneous discharge in Purkinje fibres was studied by means of the whole-cell patch-clamp technique in canine cardiac single Purkinje cells. In 4 m m [K + ] o , during depolarising clamp steps, a slowly inactivating current appeared at ∼−58 mV, negative to the threshold for the fast Na + current ( I Na ; ∼−50 mV). During depolarising ramps, the current underwent inward rectification with a negative slope region that began at ∼−60 mV. The current underlying the negative slope increased during faster ramps, decreased as a function of time when the initial depolarising ramp was over, decreased during depolarisations positive to ∼−35 mV and was much larger than the current during the symmetrical repolarising ramp. Increasing biphasic (‘oscillatory’) voltage ramps required much smaller currents at a holding potential ( V h ) of −60 mV than at −80 mV and were associated with a marked decrease in slope conductance. At V h −50/−40 mV, the oscillatory ramp currents and superimposed pulse currents reversed direction. The negative slope in the I-V relation as well as the change in current direction at −50/−40 mV were markedly reduced by tetrodotoxin (15 μ m ) and lidocaine (lignocaine, 100 μ m ) and therefore are due to a slowly inactivating Na + current, labelled here I Na3 . Lower [K + ] o (2.7 m m ) reduced the steady state slope conductance as well as the current in the diastolic range, and increased as well as shifted I Na3 in a negative direction. High [K + ] o had the opposite effects. Cs + (2 m m ) and Ba 2+ (2 m m ) reduced the initial current during depolarising ramps but not I Na3 . In current-clamp mode, current-induced voltage oscillations elicited action potentials through a gradual transition between diastolic depolarisation and upstroke, consistent with the activation of I Na3 . Thus, the initiation and maintenance of spontaneous discharge in Purkinje strands appear to involve a voltage- and K + -dependent decrease in K + conductance as well as the activation of a voltage- and time-dependent inward Na + current ( I Na3 ) with slow inactivation kinetics.