BACKGROUND: Human lung and blood-derived mast cells express a Ca2+-activated K+ channel (KCA) that has electrophysiological properties resembling the intermediate conductance KCA (iKCA1). This channel is predicted to enhance IgE-dependent mast cell responses. OBJECTIVE: To confirm the identity of this channel as iKCA1 in human lung mast cells and to examine the effect of an iKCA1 opener, 1-ethyl-2-benzimidazolinone (1-EBIO), on Ca2+ influx and degranulation after IgE-dependent activation. METHODS: iKCA1 expression was examined by using RT-PCR. Ion currents were measured by using the patch clamp technique in human peripheral blood-derived mast cells, freshly isolated human lung mast cells (HLMCs), and long-term cultured HLMCs (LTHLMCs). Currents were manipulated with the specific iKCA1 opener 1-EBIO and the iKCA1 blockers clotrimazole and TRAM-34. Ratiometric Ca2+ imaging was performed on single fura-2-loaded cells, and histamine release was measured by radioenzymatic assay. RESULTS: Both fresh HLMCs and LTHLMCs expressed iKCA1 mRNA. The iKCA1 opener 1-EBIO induced iKCA1 currents in 89% of human peripheral blood-derived mast cells, 12% of fresh HLMCs, and 67% of LTHLMCs, which were blocked by the iKCA1 blockers clotrimazole and TRAM-34. After cell activation with a suboptimal concentration of anti-IgE, 1-EBIO enhanced the IgE-dependent rise in cytosolic-free Ca2+ and potentiated IgE-dependent histamine release. CONCLUSION: Opening of iKCA1 enhances IgE-dependent Ca2+ influx and histamine release in HLMCs. Inhibition of iKCA1 may provide a novel approach to the treatment of mast cell-mediated disease.