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

Properties of the funny current (I) 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 in human-induced pluripotent stem cell (iPSC)-derived pacemaker cardiomyocytes (pCMs), at different time points. Patch-clamp analysis showed that I 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 displayed the voltage-dependent block by caesium and reversed (E) at - 22 mV, compatibly with the 3:1 K/Na permeability ratio. Lowering [Na] (30 mM) shifted the E to - 39 mV without affecting conductance. Increasing [K] (30 mM) shifted the E 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. Autonomic agonists modulated the diastolic depolarization, and thus rate, of pCMs. The adrenergic agonist isoproterenol induced rate acceleration and a positive shift of I voltage-dependence (EC 73.4 nM). The muscarinic agonists had opposite effects (Carbachol EC, 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 with the physiological properties expected by pacemaker cells and may thus represent a suitable model for studying human I-related sinus arrhythmias.