The effect of ionomycin on the human KCNQ4 channels expressed in Xenopus leavis oocytes was investigated. KCNQ4 channels expressed in Xenopus oocytes were measured using two-electrode voltage clamp. The activation of KCNQ4 current had slow activation kinetics and low threshold (similar to-50 mV). The expressed current of KCNQ4 showed the half-maxii-nal activation (V-1/2) was -17.8 mV and blocked almost completely by KCNQ4 channel blockers, linopirdine (300 mu M) or bepridil (200 mu M). The significant increase of KCNQ4 outward current induced by ionomycin (calcium salt) is about 1.7-fold of control current amplitude at +60 mV and shifted V-1/2 by approximately -8 mV (from -17.8 to -26.0 mV). This effect of ionomycin could be reversed by the further addition of BAPTA-ANI (0.3 mM), a membrane-permeable calcium chelator. Furthermore, the increased effect of ionomycin on KCNQ4 current is abolished by pretreatment of linopirdine or bepridil. In contrast, direct cytoplasmic injection of calcium medium (up to 1 mM calcium, 50 nl) did not mimic the effect of ionomycin. In conclusion, the effect of ionomycin on enhancement of KCNQ4 current is independent of intracellular calcium mobilization and possibly acts on intramembrane hydrophobic site of KCNQ4 protein expressed in Xenopus oocytes. (c) 2006 Elsevier Inc. All rights reserved.