Characterization of tetrandrine-induced inhibition of large-conductance calcium-activated potassium channels in a human endothelial cell line (HUV-EC-C)

The effects of tetrandrine, a blocker of voltage-dependent Ca2+ channels, o n ionic currents were investigated in an endothelial cell line (HUV-EC-C) o riginally derived from human umbilical vein. In whole-cell configuration, t etrandrine (0.5-50 mu M) reversibly decreased the amplitude of K+ outward c urrents. The IC50 value of tetrandrine-induced decrease in outward current was 5 mu M. The K+ outward current in response to depolarizing voltage puls es was also inhibited by iberiotoxin (200 nM), yet not by glibenclamide (10 mu M) or apamin (200 nM). The reduced amplitude of outward current by tetr andrine can be reversed by the further addition of Evans' blue (30 mu M) or niflumic acid (30 mu M). Thus, the tetrandrine-sensitive component of outw ard current is believed to be Ca2+-activated K+ current. Pretreatment with thapsigargin (1 mu M) or sodium nitroprusside (10 mu M) for 5 h did not pre vent tetrandrine- mediated inhibition of outward current. In outside-out co nfiguration, bath application of tetrandrine (5 mu M) did not change the si ngle-channel conductance but significantly reduced the opening probability of large-conductance Ca2+-activated K+ (BKCa) channels. The tetrandrine-med iated decrease in the channel activity was independent on internal Ca2+ con centration. Tetrandrine (5 mM) can also shift the activation curve of BKCa channels to more positive potentials by approximately 20 mV. The change in the kinetic behavior of BKCa channels caused by tetrandrine is due to a dec rease in mean open time and an increase in mean closed time. The present st udy provides substantial evidence that tetrandrine is capable of suppressin g the activity of BKCa channels in endothelial cells. The direct inhibition of these channels by tetrandrine should contribute to its effect on the fu nctional activities of endothelial cells.