A detailed characterization of the hyperpolarization-activated "funny" current (I f ) in human-induced pluripotent stem cell (iPSC)-derived cardiomyocytes with pacemaker activity.

Properties of the funny current (I _f ) have been studied in several animal and cellular models, but so far little is known concerning its properties in human pacemaker cells. This work provides a detailed characterization of I _f in human-induced pluripotent stem cell (iPSC)-derived pacemaker cardiomyocytes (pCMs), at different time points. Patch-clamp analysis showed that I _f density did not change during differentiation; however, after day 30, it activates at more negative potential and with slower time constants. These changes are accompanied by a slowing in beating rate. I _f displayed the voltage-dependent block by caesium and reversed (E _rev ) at - 22 mV, compatibly with the 3:1 K ⁺ /Na ⁺ permeability ratio. Lowering [Na ⁺ ] _o (30 mM) shifted the E _rev to - 39 mV without affecting conductance. Increasing [K ⁺ ] _o (30 mM) shifted the E _rev to - 15 mV with a fourfold increase in conductance. pCMs express mainly HCN4 and HCN1 together with the accessory subunits CAV3, KCR1, MiRP1, and SAP97 that contribute to the context-dependence of I _f . Autonomic agonists modulated the diastolic depolarization, and thus rate, of pCMs. The adrenergic agonist isoproterenol induced rate acceleration and a positive shift of I _f voltage-dependence (EC ₅₀ 73.4 nM). The muscarinic agonists had opposite effects (Carbachol EC ₅₀ , 11,6 nM). Carbachol effect was however small but it could be increased by pre-stimulation with isoproterenol, indicating low cAMP levels in pCMs. In conclusion, we demonstrated that pCMs display an I _f with the physiological properties expected by pacemaker cells and may thus represent a suitable model for studying human I _f -related sinus arrhythmias.