Vol. 292, Issue 1, 188-195, January 2000

Characterization of Tetrandrine-Induced Inhibition of Large-Conductance Calcium-Activated Potassium Channels in a Human Endothelial Cell Line (HUV-EC-C)¹

Sheng-Nan Wu, Hui-Fang Li and Yi-Ching Lo

Department of Medical Education and Research, Veterans General Hospital-Kaohsiung (S.-N.W., H.-F.L.); and Department of Pharmacology, Kaohsiung Medical University, Kaohsiung, Taiwan, Republic of China (Y.-C.L.)

The effects of tetrandrine, a blocker of voltage-dependent Ca²⁺ channels, on ionic currents were investigated in an endothelial cell line (HUV-EC-C) originally derived from human umbilical vein. In whole-cell configuration, tetrandrine (0.5-50 µM) reversibly decreased the amplitude of K⁺ outward currents. The IC₅₀ value of tetrandrine-induced decrease in outward current was 5 µM. The K⁺ outward current in response to depolarizing voltage pulses was also inhibited by iberiotoxin (200 nM), yet not by glibenclamide (10 µM) or apamin (200 nM). The reduced amplitude of outward current by tetrandrine can be reversed by the further addition of Evans' blue (30 µM) or niflumic acid (30 µM). Thus, the tetrandrine-sensitive component of outward current is believed to be Ca²⁺-activated K⁺ current. Pretreatment with thapsigargin (1 µM) or sodium nitroprusside (10 µM) for 5 h did not prevent tetrandrine-mediated inhibition of outward current. In outside-out configuration, bath application of tetrandrine (5 µM) did not change the single-channel conductance but significantly reduced the opening probability of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. The tetrandrine-mediated decrease in the channel activity was independent on internal Ca²⁺ concentration. Tetrandrine (5 µM) can also shift the activation curve of BK_Ca channels to more positive potentials by approximately 20 mV. The change in the kinetic behavior of BK_Ca channels caused by tetrandrine is due to a decrease in mean open time and an increase in mean closed time. The present study provides substantial evidence that tetrandrine is capable of suppressing the activity of BK_Ca channels in endothelial cells. The direct inhibition of these channels by tetrandrine should contribute to its effect on the functional activities of endothelial cells.