Functional expression of the hyperpolarization‐activated, non‐selective cation current I f in immortalized HL‐1 cardiomyocytes

HL‐1 cells are adult mouse atrial myocytes induced to proliferate indefinitely by SV40 large T antigen. These cells beat spontaneously when confluent and express several adult cardiac cell markers including the outward delayed rectifier K + channel. Here, we examined the presence of a hyperpolarization‐activated I f current in HL‐1 cells using the whole‐cell patch‐clamp technique on isolated cells enzymatically dissociated from the culture at confluence. Cell membrane capacitance ( C m ) ranged from 5 to 53 pF. I f was detected in about 30 % of the cells and its occurrence was independent of the stage of the culture. I f maximal slope conductance was 89.7 ± 0.4 pS pF −1 ( n = 10). I f current in HL‐1 cells showed typical characteristics of native cardiac I f current: activation threshold between −50 and −60 mV, half‐maximal activation potential of −83.1 ± 0.7 mV ( n = 50), reversal potential at −20.8 ± 1.5 mV ( n = 10), time‐dependent activation by hyperpolarization and blockade by 4 m m Cs + . In half of the cells tested, activation of adenylyl cyclase by the forskolin analogue L858051 (20 μ m ) induced both a ≈6 mV positive shift of the half‐activation potential and a ≈37 % increase in the fully activated I f current. RT‐PCR analysis of the hyperpolarization‐activated, cyclic nucleotide‐gated channels (HCN) expressed in HL‐1 cells demonstrated major contributions of HCN1 and HCN2 channel isoforms to I f current. Cytosolic Ca 2+ oscillations in spontaneously beating HL‐1 cells were measured in Fluo‐3 AM‐loaded cells using a fast‐scanning confocal microscope. The oscillation frequency ranged from 1.3 to 5 Hz and the spontaneous activity was stopped in the presence of 4 m m Cs + . Action potentials from HL‐1 cells had a triangular shape, with an overshoot at +15 mV and a maximal diastolic potential of −69 mV, i.e. more negative than the threshold potential for I f activation. In conclusion, HL‐1 cells display a hyperpolarization‐activated I f current which might contribute to the spontaneous contractile activity of these cells.