Facilitation of Ca2+-activated K+ channels (IKCa1) by mibefradil in B lymphocytes

K⁺ channels play critical roles in the proliferation and activation of lymphocytes. Mouse B cells express large-conductance background K⁺ channel (LK_bg) in addition to the voltage-gated K⁺ channel (Kv) and Ca²⁺-activated K⁺ channel current (IKCa1). Mibefradil, a blocker of T-type Ca²⁺ channels, has been reported to affect the proliferation of immune cells. In this study, we investigated the effects of mibefradil on the membrane potential and ion channels in murine B cell lines, WEHI-231 and Bal-17. In the whole-cell patch clamp experiments, mibefradil blocked Kv and LK_bg current with half inhibitory concentration (IC₅₀), 1.9 and 2.3 μM, respectively. Interestingly, IKCa1 current was increased by mibefradil. In the inside-out patch clamp study with cloned murine IKCa1 (mIKCa1) in HEK-293, mibefradil increased both Ca²⁺ sensitivity and maximum activity of mIKCa1. At high concentrations (>10 μM), mibefradil inhibited mIKCa1 in a voltage-dependent manner. Application of anti-IgM antibody to stimulate B cell receptors (BCR-ligation) induced transient hyperpolarization of Bal-17 and WEHI-231 cells, which became persistent with 1 μM mibefradil. The hyperpolarizing response was abolished by charybdotoxin, a selective blocker for SK4/IKCa1. In summary, our study firstly reports the ion channel-activating effects of mibefradil. The selective potent activation of IKCa1 suggests that mibefradil-derived drugs might be useful in the control of cell responses related with IKCa1. HY Yoo, H Zheng, and JH Nam contributed equally to this study.