Characterization of Ca(2+)- and voltage-dependent nonselective cation channels in human HepG2 cells

Nonselective cation channels have been identified and linked to important cell functions in rat hepatocytes. In this study, we characterized inward rectifying nonselective cation channels in detail by the patch clamp technique in human HepG2 cells. Channel properties were studied with high resistance borosilicate pipettes in cell-attached and inside-out configurations. With Ringer's solution and KCl as pipette solutions, the conductances were 19.7 +/- 2.1 and 22.2 +/- 0.0 picosiemens (pS), and reversal potentials were 30.9 +/- 3.5 and 31.3 +/- 4.6 mV, respectively. The channel was permeable to Ba2+, and the sequence of permeability ratios was Na+ > K+ > Cs+ > Ba2+. In the cell-attached configuration, the channel had a higher opening probability at depolarizing potential than at hyperpolarizing. In the inside-out patches with symmetric Ringer's solution, the current voltage curve was linear with conductance of 19.8 +/- 0.9 pS. Reversal potential shifted from -0.2 +/- 1.0 mV to 23.2 +/- 1.0 mV when the bath solution was replaced by dilute Ringer's solution. In the inside-out configuration, the gating was Ca(2+)-dependent, and the opening probability increased with increasing intracellular calcium concentration ([Ca2+]i). An outward rectifying channel appeared when [Ca2+]i was less than 1 mumol/L. The nonselective channel was reversibly blocked by 10 mumol/L internal flufenamic acid. We conclude that Ca(2+)- and voltage-dependent nonselective cation channels are present in human HepG2 cells. The channels might be involved in the regulation of Ca2+ influx and are associated with activation of other ion channels.