Ginseng gintonin activates the human cardiac delayed rectifier K⁺ channel: Involvement of Ca²⁺/calmodulin binding sites

Gintonin, a novel, ginseng-derived G protein-coupled lysophosphatidic acid (LPA) receptor ligand, elicits [Ca²⁺]_i transients in neuronal and non-neuronal cells via pertussis toxin-sensitive and pertussis toxin-insensitive G proteins. The slowly activating delayed rectifier K⁺ (I_Ks) channel is a cardiac K⁺ channel composed of KCNQ1 and KCNE1 subunits. The C terminus of the KCNQ1 channel protein has two calmodulin-binding sites that are involved in regulating I_Ks channels. In this study, we investigated the molecular mechanisms of gintonin-mediated activation of human I_Ks channel activity by expressing human I_Ks channels in Xenopus oocytes. We found that gintonin enhances I_Ks channel currents in concentration- and voltage-dependent manners. The EC₅₀ for the I_Ks channel was 0.05 ± 0.01 µg/ml. Gintonin-mediated activation 1 of the I_Ks channels was blocked by an LPA1/3 receptor antagonist, an active phospholipase C inhibitor, an IP₃ receptor antagonist, and the calcium chelator BAPTA. Gintonin-mediated activation of both the I_Ks channel was also blocked by the calmodulin (CaM) blocker calmidazolium. Mutations in the KCNQ1 [Ca²⁺]_i/CaM-binding IQ motif sites (S373P, W392R, or R539W)blocked the action of gintonin on I_Ks channel. However, gintonin had no effect on hERG K⁺ channel activity. These results show that gintonin-mediated enhancement of I_Ks channel currents is achieved through binding of the [Ca²⁺]_i/CaM complex to the C terminus of KCNQ1 subunit.