Voltage-dependent inward ion channel currents in immortalized HL-1 cells

HL-1 cells are adult mouse atrial myocytes induced to proliferate indefinitely by SV40 large T antigen. As described earlier (PNAS 95,2979,1998), these cells beat spontaneously when confluent and express several adult cardiac cell markers and the delayed rectifier outward K channel (IKr). Here, we examined the presence of voltage-dependent Ca and If channel currents in these cells using the whole cell patch-clamp technique on isolated cells enzymatically dissociated from the culture at confluence. Cell membrane capacitance (Cm) ranged from 5 to 35pF. If was detected in 33% of the largest cells tested (Cm=303 pF, n=17) and its maximal slope conductance was 20255 pS/pF. If showed typical characteristics of cardiac If: reversal potential at -16 mV, time dependent activation by hyperpolarization, blockade by 4 mM Cs, stimulation by cAMP. Ca currents (measured in 20 mM Ba) were present in 75% of a population of smaller cells (Cm=127 pF, n=25) and the maximal current density was 105 pA/pF at 0 mV. In most cells held at -80 mV, both a slowly and a fast inactivating component were found. The fast component was suppressed by raising the holding potential to -50 mV or by 50 µM Ni and was thus attributed to the T-type Ca current. The slow L-type component displayed a very fast rundown. In conclusion, HL-1 cells express voltage-dependent Ca and If channels but do not behave as a homogenous population with respect to the expressed inward ion channel currents.