A novel activation of Ca2+-activated C1- channel in Xenopus oocytes by Ginseng saponins:: evidence for the involvement of phospholipase C and intracellular Ca2+ mobilization

1 The signal transduction mechanism of ginsenosides, the active ingredients of ginseng, was studied in Xenopus oocytes using two-electrode voltage-clamp technique. Ginseng total saponin (GTS), i.e., an unfractionated mixture of ginsenosides produced a large outward current at membrane potentials more positive than -20 mV when it was applied to the exterior of oocytes, but not when injected intracellularly. The effect of GTS was concentration-dependent (EC50: 4.4 mug ml(-1)) and reversible. 2 Certain fractionated ginsenosides (Rb-1, Rb-2, Rc, Rf, Rg(2) and Ro) also produced an outward current in a concentration-dependent manner with the order of potency of Rf > Ro > Rb-1 = Rb-2 > Rg(2) > Rc. Other ginsenosides (Rd, Re and Rgl) had little or no effect. 3 The GTS effect was completely blocked by bath application of the Ca2+-activated Cl- channel blocker niflumic acid and by intracellular injection of the calcium chelator BAPTA or the IP3 receptor antagonist heparin. Also, the effect was partially blocked by bath-applied U-73122, a phospholipase C (PLC) inhibitor and by intracellularly injected GTP gammaS, a non-hydrolyzable GTP analogue. Whereas, it was not altered by pertussin toxin (PTX) pretreatment. 4 These results indicate that: (1) interaction of ginsenosides with membrane component(s) at the extracellular side leads to Ca2+-activated Cl- channel opening in Xenopus oocyte membrane; and (2) this process involves PLC activation, the release of Ca2+ from the IP3-sensitive intracellular store and PTX-insensitive G protein activation.