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 approximately -58 mV, negative to the threshold for the fast Na+ current (INa; approximately -50 mV). During depolarising ramps, the current underwent inward rectification with a negative slope region that began at approximately -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 approximately -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 (Vh) of -60 mV than at -80 mV and were associated with a marked decrease in slope conductance. At Vh -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 microM) and lidocaine (lignocaine, 100 microM) and therefore are due to a slowly inactivating Na+ current, labelled here INa3. 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 INa3 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 INa3. 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 INa3. 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 (INa3) with slow inactivation kinetics.
Rota
Marcello
M
Department of Physiology and Pharmacology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Box 31, Brooklyn, NY 11203, USA.
Vassalle
Mario
M
D000465:Algorithms; D000779:Anesthetics, Local / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001683:Biological Clocks / Q000502:physiology*; D002586:Cesium / Q000494:pharmacology; D004285:Dogs; D004594:Electrophysiology; D005260:Female; D006321:Heart / Q000294:innervation / Q000502:physiology*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008012:Lidocaine / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000502:physiology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D011690:Purkinje Fibers / Q000502:physiology*; D012964:Sodium / Q000502:physiology; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
D000779:Anesthetics, Local; D026902:Potassium Channel Blockers; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D002586:Cesium; D001464:Barium; D013779:Tetrodotoxin; D008012:Lidocaine; D012964:Sodium; D011188:Potassium