Continuously active sodium channels in osteoblastic ROS 17/2.8 cells.

Cell-attached patch clamp experiments revealed 13-20 pS Na(+)-conducting channels active at normal resting potentials (-28 +/- 1 mV; +/- SEM; 7 cells) in the rat osteosarcoma cell line, ROS 17/2.8. These channels were not blocked by tetrodotoxin, Cd2+, verapamil, or nifedipine. Replacing all cations in the patch pipette except Ca2+ with tetraethylammonium (TEA+) abolishes channel activity; but adding TEA+ to a pipette solution containing only Na+ does not. Depolarization was not necessary to activate these channels, and the open times were much longer than the millisecond open times characteristic of Na+ channels in excitable cells. Current-voltage curves reconstructed from mean single channel currents and mean channel open times resemble L-type Ca2+ current-voltage curves obtained from whole-cell experiments, with current peaks shifted to resting or more hyperpolarized potentials. The voltage sensitivity of these channels has implications on membrane potential stability and on the hyperpolarizing membrane potential spiking activity exhibited by ROS 17/2.8 cells.