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 mmK(+)(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 μm) and lidocaine (lignocaine, 100 μm) 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 Ba(2+) (2 mm) 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.