The pacemaker current in cardiac Purkinje myocytes

Area of Science:

• Cardiology
• Electrophysiology
• Ion Channels

Background:

• The pacemaker potential in cardiac Purkinje fibers is traditionally attributed to the hyperpolarization-activated inward current, i(f).
• Some experimental findings challenge this established view, necessitating further investigation into other contributing currents.

Purpose of the Study:

• To investigate and characterize ionic currents active during the diastolic depolarization phase in single canine Purkinje myocytes.
• To differentiate between the proposed pacemaker current i(f) and other time- and voltage-dependent currents in this specific cardiac cell type.

Main Methods:

• Whole-cell patch clamp electrophysiology was employed on isolated canine Purkinje myocytes.
• Ionic currents were studied in the voltage range relevant to diastolic depolarization, with varying extracellular potassium concentrations and ion channel blockers (Cs+, Ba2+).

Main Results:

• A time-dependent inward current (iKdd) was identified, reversing near the potassium equilibrium potential (EK) and distinct from i(f).
• iKdd exhibited a more positive activation threshold than i(f) and its properties were modulated by extracellular potassium and cesium ions.
• Barium ions blocked iKdd, unmasking i(f) with a more negative threshold.

Conclusions:

• A voltage- and time-dependent potassium current (iKdd) likely contributes to the pacemaker potential in Purkinje myocytes.
• This iKdd current is pharmacologically and electrophysiologically distinct from the hyperpolarization-activated inward current i(f).
• The findings suggest a more complex ionic mechanism underlying pacemaking in Purkinje fibers than previously assumed.