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.