HCN4 current during human sinoatrial node-like action potentials.

Area of Science:

• Cardiovascular Physiology
• Cardiac Electrophysiology

Background:

• The role of the hyperpolarization-activated funny current (If), mediated by HCN4 channels, in human sinoatrial node (SAN) pacemaker activity remains debated.
• Previous studies have not fully elucidated the precise contribution of If to SAN function.

Purpose of the Study:

• To investigate the contribution of If to diastolic depolarization in human SAN cells.
• To determine the dependence of If on heart rate, cAMP levels (simulated by forskolin), and atrial load.

Main Methods:

• HCN4 channels were expressed in human cardiac myocyte progenitor cells (CMPCs).
• Perforated patch-clamp technique was used in voltage-clamp and action potential clamp modes with human SAN-like waveforms (500-1500 ms cycle length).
• Experiments were conducted with and without forskolin and with a voltage offset to simulate atrial load.

Main Results:

• Forskolin increased HCN4 current density by 14% and shifted activation by +7.4 mV, accelerating activation and slowing deactivation.
• HCN4 current amplitude increased with longer cycle lengths and was enhanced by forskolin and a negative voltage offset.
• The If current did not fully deactivate during the diastolic depolarization phase.

Conclusions:

• The If current is active during human SAN action potential waveforms.
• If amplitude is dynamically modulated by heart rate, beta-adrenergic stimulation, and diastolic voltage, indicating delicate physiological control.