KCa3.1-Dependent Hyperpolarization Enhances Intracellular Ca2+ Signaling Induced by fMLF in Differentiated U937 Cells

Formylated peptides are chemotactic agents generated by pathogens. The most relevant peptide is fMLF (formyl-Met-Leu-Phe) which participates in several immune functions, such as chemotaxis, phagocytosis, cytokine release and generation of reactive oxygen species. In macrophages fMLF-dependent responses are dependent on both, an increase in intracellular calcium concentration and on a hyperpolarization of the membrane potential. However, the molecular entity underlying this hyperpolarization remains unknown and it is not clear whether changes in membrane potential are linked to the increase in intracellular Ca2+. In this study, differentiated U937 cells, as a macrophage-like cell model, was used to characterize the fMLF response using electrophysiological and Ca2+ imaging techniques. We demonstrate by means of pharmacological and molecular biology tools that fMLF induces a Ca2+-dependent hyperpolarization via activation of the K+ channel KCa3.1 and thus, enhancing fMLF-induced intracellular Ca2+ increase through an amplification of the driving force for Ca2+ entry. Consequently, enhanced Ca2+ influx would in turn lengthen the hyperpolarization, operating as a positive feedback mechanism for fMLF-induced Ca2+ signaling.