Single-channel analysis of the potassium permeability in HeLa cancer cells: evidence for a calcium-activated potassium channel of small unitary conductance.

Cell-attached and inside-out patch-clamp experiments (O.P. Hamill et al., Pfluegers Arch. 391:85-100, 1981) were undertaken in order to characterize the molecular mechanisms responsible for the calcium-dependent potassium permeability observed in HeLa cancer cells. Our result essentially indicate that the HeLa cell external membrane contains potassium channels whose activity can be triggered within an internal calcium concentration range of 0.1 to 1 microM. This particular channel was found to behave as an inward rectifier in symmetrical 200 mM KCl with a conductance of 50 and 10 pS at large negative and large positive membrane potentials, respectively. I/V curves were also measured in 10, 20, 75, 200 and 300 mM KCl and the data interpreted in terms of a one-site-two-barrier model. The channel activity appeared to be nearly voltage independent within the voltage range -100 to +100 mV, an increase of Po, the open channel probability, being observed at large negative potentials only. In addition, the results obtained from inside-out experiments on the relationship between Po and the cytoplasmic free-calcium concentration have led to conclude that four calcium ions are probably required in order to open the channel. In this regard it was found that an increase of the internal free-calcium level affects more the number of channel openings per second than the actual channel mean lifetime. Finally, it is concluded following a time interval distribution analysis, that this particular channel has at least three closed states and two open states.