Characterization of pacemaker current If in immortalized HL-1 cardiomyocytes.

HL-1 cells are adult mouse myocytes induced to proliferate indefinitely by SV40 large T antigen. As described earlier (PNAS 95,2979,1998), these cells express several adult cardiac cell markers, the delayed rectifier K+ channels (IKr) and they show spontaneous contracting groups, becoming more frequent at confluence. Several reports (J Physiol (Lond) 454,503,1992; Pflugers Arch 433,533,1997; Cardiovas Res 42,416,1999) demonstrated that spontaneously beating cardiac myocytes show the presence of the hyperpolarization activated If current. Due to these reasons, we examined the presence of If current in HL-1 cells, using the whole cell patch-clamp technique on isolated cells enzymatically dissociated from the culture. Moreover, the reverse-transcription polymerase chain reaction amplification (RT-PCR) has been used to identify the f channel isoforms present in HL-1 cells. Cells membrane capacitance (Cm) ranged from 7 to 53 pF and a similar dispersion of values was present over the passages of the cultures. If current was detected in 31% (n=33) of cells at complete confluence. The current showed typical characteristics of cardiac If: reversal potential of -21.0 ±1.3 mV, maximal slope conductance of 160.0 ± 0.08 pS/pF, time dependent activation by hyperpolarization, blockade by 4mM cesium, stimulation by cAMP. By RT-PCR the cDNA relative to HCN1, HCN2, HCN3 and HCN4 isoforms of the f channels has been detected. In conclusion, HL-1 cells express voltage dependent If channels, functionally present as a constant characteristic of the culture. The comparison with the brain allows us to expect HCN1 and HCN2 isoform to be expressed in larger amount with respect to HCN3 and HCN4 isoforms and to contribute predominantly to the functional characteristics of If current. However, HL-1 cells do not behave as a homogeneous population with respect the expressed If current.