Ginsenoside Rg3 Activates Human EAG Family of K+ Channels via Allosteric Modification of Gating

The human ether-a-go-go (EAG) family of voltage-gated K+ channels, including hERG, hEAG, and hELK exhibit variable biophysical and physiological properties. Ginsenoside Rg3 (Rg3), a steroid glycoside is a potent hERG channel activator. Here we characterize the mechanisms of action of Rg3 on three EAG-family representative channels, including hERG1, hEAG1 and hELK1 heterologously expressed in Xenopus laevis oocytes. Rg3 enhanced hERG1 channel current via a negative shift in the half-point for the voltage dependence of activation (V0.5act) that saturated at −14 mV, and a profound slowing in the rate of deactivation (EC50 = 242 nM). Rg3 had no effect on the rate of activation or the voltage dependence of hERG1 inactivation gating. For hEAG1 channels, Rg3 induced a −28 mV maxiumal shift in V0.5act (EC50 = 1.18 µM) and accelerated the rate of activation (∼20-fold faster). The maximum shift in V0.5act for hELK1 channels was much greater (>-100 mV; EC50 = 176 nM). The onset of Rg3 effects upon extracellular application was very fast and rapidly reversible upon washout for all 3 channel types, indicating this large molecule (MW = 785) binds to an extracellular region. Rg3 at 3 µM had no effect on Kv1.5 channels and only affects other Kv channels at much higher concentrations. Understanding the mechanisms of action of Rg3 may facilitate the development of more potent and selective gensenosides for therapeutic use.