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 mm [K+](o), during depolarising clamp steps, a slowly inactivating current appeared at similar to-58 mV, negative to the threshold for the fast Na+ current (I-Na; similar to-50 mv). During depolarising ramps, the current underwent inward rectification with a negative slope region that began at similar to-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 similar 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 muM) and lidocaine (lignocaine, 100 muM) and therefore are due to a slowly inactivating Na+ current, labelled here I-Na3. Lower [K+](o) (2.7 mm) 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 mm) and Ba2+ (2 mm) reduced the initial current during depolarising ramps but not I-Na3. In current-damp 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.