A time- and voltage-dependent potassium current in the rabbit sinoatrial node cell

Area of Science:

• Electrophysiology
• Cardiac Physiology
• Ion Channel Function

Background:

• The sinoatrial (S-A) node is the heart's natural pacemaker, initiating electrical impulses.
• Understanding ion channel function is critical for elucidating cardiac rhythm generation.
• Previous studies suggested complex ionic mechanisms underlying S-A node automaticity.

Purpose of the Study:

• To characterize the outward current tail in rabbit S-A node cells.
• To identify the ionic basis of the slow diastolic depolarization.
• To determine the properties of the responsible ion current.

Main Methods:

• Utilized the double microelectrode voltage clamp technique in rabbit S-A node cells.
• Analyzed the kinetics and voltage-dependence of the outward current tail.
• Investigated the reversal potential's dependence on extracellular potassium concentration.

Main Results:

• Identified a single component of potassium current (pacemaker current component) responsible for the outward current tail.
• Demonstrated that this current's magnitude follows a sigmoid activation curve from -50 mV to +20 mV.
• Found the current's time course to be exponential and voltage-independent, with a U-shaped reciprocal time constant.

Conclusions:

• The outward current tail in S-A node cells is attributed to a distinct pacemaker current component.
• This pacemaker current component plays a primary role in generating the slow diastolic depolarization.
• These findings provide key insights into the electrophysiological mechanisms of cardiac pacemaking.