Effect of calmodulin on ginseng saponin-induced Ca²⁺-activated Cl^- channel activation in Xenopus laevis oocytes

We previously demonstrated the ability of ginseng saponins (active ingredients of Panax ginseng) to enhance Ca²⁺-activated Cl ^- current. The mechanism for this ginseng saponin-induced enhancement was proposed to be the release of Ca²⁺ from IP₃-sensitive intracellular stores through the activation of PTX-insensitive Gα_q/11 proteins and PLC pathway. Recent studies have shown that calmodulin (CaM) regulates IP₃ receptor-mediated Ca²⁺ release in both Ca²⁺-dependent and -independent manner. In the present study, we have investigated the effects of CaM on ginseng saponin-induced Ca²⁺-activated Cl^- current responses in Xenopus oocytes. Intraoocyte injection of CaM inhibited ginseng saponin-induced Ca²⁺-activated Cl^- current enhancement, whereas co-injection of calmidazolium, a CaM antagonist, with CaM blocked CaM action. The inhibitory effect of CaM on ginseng saponin-induced Ca²⁺- activated Cl^- current enhancement was dose- and time-dependent, with an IC₅₀ of 14.9 ± 3.5 μM. The inhibitory effect of CaM on saponin's activity was maximal after 6 h of intraoocyte injection of CaM, and after 48 h the activity of saponin recovered to control level. The half-recovery time was calculated to be 16.7 ± 4.3 h. Intraoocyte injection of CaM inhibited Ca²⁺-induced Ca²⁺-activated Cl^- current enhancement and also attenuated IP₃-induced Ca ²⁺-activated Cl^- current enhancement. Ca²⁺/CaM kinase II inhibitor did not inhibit CaM-caused attenuation of ginseng saponin-induced Ca²⁺-activated Cl^- current enhancement. These results suggest that CaM regulates ginseng saponin effect on Ca ²⁺-activated Cl^- current enhancement via Ca ²⁺-independent manner.