Pacemaker current, If, in single LV-transduced CMPCs. (A) Voltage steps and typical If traces of CMPC/GFP and CMPC/HCN4-GFP cells. Inset: voltage pulse protocol to measure activation properties. (B) Average current-voltage (I–V) relationships of If of CMPC/GFP and CMPC/HCN4-GFP cells. (C) Voltage dependence of If activation. Solid line is the Boltzmann fit to the experimental data. (D) Typical If traces of CMPC/HCN4-GFP cells. Inset: voltage pulse protocol to measure deactivation properties. (E) I–V relationship of the fully-activated HCN4 current. Solid line is the linear fit to the experimental data. (F) Time constants of (de)activation. Solid line is the best fit curve to the equation τ = 1/[A1 × exp(−V/B1) + A2 × exp(V/B2)], where τ is the activation or deactivation kinetic time constant, and A1, A2, B1, and B2 are calculated fitting parameters [25].