Cardiac pacemaker funny current (I_f) (I_f)

The cardiac pacemaker funny current (I_f) is a hyperpolarization-activated mixed Na^+/K^+ inward current mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, primarily HCN4 in sinoatrial node cells. It activates slowly upon hyperpolarization to diastolic voltages following action potential repolarization, contributing to the initial phase of diastolic depolarization that drives spontaneous pacemaker activity and sets the heart rate. I_f exhibits unique properties including dual activation by voltage and cyclic nucleotides like cAMP, enabling autonomic modulation: sympathetic stimulation increases cAMP, shifting activation to more positive potentials and accelerating heart rate, while parasympathetic input does the opposite. Though operating at low fractional conductance (2-5%), I_f carries substantial net charge during both depolarization and repolarization phases due to persistent channel activity and slow kinetics, with bidirectional flow around its -20 to -30 mV reversal potential. In disease, I_f dysregulation contributes to sinus node dysfunction and age-related bradycardia via ionic remodeling. Therapeutically, I_f is targeted by inhibitors like ivabradine, which provide use-dependent block to selectively slow heart rate in conditions like chronic heart failure and angina without major effects on contractility.

Other names

funny currentI_f currentpacemaker current

Specific inhibition of HCN channels underlying I_f, use-dependent block reducing pacemaker rate